UNIVERSITY OF THE WITWATERSRAND 

       

Development of eco-friendly building bricks 
derived from carbon nanotube-reinforced coal 

ash and low-density polyethylene waste 
materials 

 

By 

 

Boitumelo Makgabutlane 

Student number:- 2380830 

 

A thesis submitted to the Faculty of Science, University of the 
Witwatersrand, Johannesburg, in fulfilment of the requirements for 

degree of 
 

 
Doctor of Philosophy in Chemistry 

 
 

                     Supervisors:- Dr M.S. Maubane-Nkadimeng  

                                         :- Prof N.J. Coville  

http://www.google.co.za/url?sa=t&rct=j&q=&esrc=s&source=images&cd=&cad=rja&ved=0CAQQjRw&url=http://rufreshmeat.blogspot.com/2008_10_01_archive.html&ei=3KoPU6KsM8iVhQfBi4GIAw&usg=AFQjCNEhYnvUQrrdyEv1yGgozXveZu27iw&bvm=bv.61965928,d.ZG4


                                         :- Prof S.D. Mhlanga 



i 
 

DECLARATION 
 

I declare that this is my own work, performed under the supervisor of Dr Maubane-

Nkadimeng, Prof Coville and Prof Mhlanga. This work is being submitted at the University of 

the Witwatersrand for a degree of Philosophy Doctor. It has not been submitted for another 

degree qualification.  

 

 

Boitumelo Makgabutlane 

 

31/05/2024 

Date 

 

 

 

 

 

 

 

 

 

 

 

 



ii 
 

ABSTRACT 
 

This study reports on the incorporation of carbon nanotubes (CNTs) into the all-waste derived 

building bricks. The focus was on waste management and beneficiation of plastic waste and 

coal ash, which are generated in large volumes without sufficient recycling. The waste 

materials were characterized using a range of techniques to ascertain their properties for 

application. Multiwalled carbon nanotubes (MWCNTs) were synthesized using a facile floating 

chemical vapour deposition method (CVD) and their physicochemical properties were tested. 

Bricks with dimensions of 220 x 105 x 70mm were developed with an optimum 85:15 coal ash 

to plastic waste ratio respectively using a specialized reactor. The bricks were tested for 

compressive strength, split tensile strength, water absorption, strain, thermal stability and 

durability using oxygen permeability index, chloride conductivity index and water sorptivity 

index as indicators. Furthermore, environmental and financial sustainability and 

ecotoxicology were tested. At optimum conditions, high quality MWCNTs with a diameter of 

83 nm, length of 414 µm and a carbon yield of 73% were obtained. The ID/IG ratio of 0.44, an 

oxidation temperature of 649 °C, a purity of 94% and surface area of 50.9 m2/g were achieved. 

Coal fly ash with a spherical shape, particle size of below 10 micron and a thermal stability of 

680 °C was used as an aggregate for the bricks. The bricks (without CNTs) developed their 

maximum compressive strength of 11.9 MPa at 14 days.  The incorporation of the CNTs 

improved the microstructure of the bricks by filling the voids and providing a bridging effect 

as reinforcement mechanisms. The optimum CNT loading of 0.05 wt.% produced bricks with 

a compressive strength of 22 MPa and tensile strength of 8.7 MPa, which exceeded the South 

African National Standards (SANS227:2007) requirements for building bricks by 450% and 

625% respectively. The durability properties were improved as the CNT dosage was increased 

from 0-10 wt.%. The 0.05 wt.% bricks were categorized as “good” for all the durability indexes. 

The CNT containing bricks showed improved thermal stability and maintained their structural 

integrity. The chemical resistance also improved and the efflorescence was minimal on all the 

bricks. The utilization of waste in the bricks enabled resource conservation, reduced pollution 

and reduced cost compared to conventional bricks. When only considering the raw materials 

used, the cost of production per brick was $0.091. The ecotoxicology of the powdered brick 

samples was tested on Raphidocelis subcapitata (microalga) and Daphnia magna (aquatic 

organism) using leachates from neutral, acidic and basic mediums. Some heavy metals were 



iii 
 

leached above the threshold limit especially in acidic medium. The leachates were toxic to the 

test species at low concentrations and resulted in growth inhibition of the microalga and 

immobization of the aquatic organisms. The toxicity of the CNTs was inconclusive and 

dedicated tests are required to study their effect. With appropriate treatment of CFA, the 

waste derived CNT bricks have a great potential of being a sustainable alternative to the 

conventional bricks based on cost, properties and environmental impact. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



iv 
 

DEDICATION 
 

I would like to dedicate this work to: 

The Almighty God who gave me the wisdom and strength to complete this work despite the 

hardships faced.  

My late mother, Naniky Makgabutlane: she saw greatness in me way before I could recognise 

it in myself. She is the reason I pursed this degree. Her love, care, support and prayers carried 

me through. I am forever grateful for her teachings that contributed in shaping me to be the 

person I am today.  “mama I made it”. 

My husband, Siyabonga Mhlongo: he is the light that shines so bright in my life. My biggest 

supporter and cheerleader. I appreciate the love, support and kindness. 

My sister, Mpho Makgabutlane: thanks to my “deputy mother” for always believing in me, 

wishing the best for me and celebrating every success no matter how small.  

 

 

 

 

 

 

 

 

 

 

 

 



v 
 

ACKNOWLEDGEMENTS 
 

I would like to greatly appreciate my supervisors (Dr Maubane-Nkadimeng, Prof Coville and 

Prof Mhlanga) for their support and guidance for the completion of this degree. Their 

expertise, knowledge and mentorship shaped me to being a confident researcher. 

Special thanks to SabiNano Pty Ltd and Prof Mhlanga (CEO) for allowing me to utilize the 

premises to undertake some of my synthesis and tests. I really appreciate the assistance that 

was offered. 

My sincere gratitude to Prof Mike from Civil Engineering (Wits) who allowed me to use their 

facilities. I am immensely indebted to Mr Masindi from Civil Engineering (Wits) who taught 

and assisted me with the operation of the equipments. He would skip his lunch and leave late 

to ensure that I complete my tests. He went above and beyond for me and I am very grateful. 

To my research group CARBOCAT (carbon catalysis) and CATMAT (Catalysis and Materials); 

thank you for the valuable suggestions and constructive criticisms that helped improve my 

work.  

I appreciate the Microscopy and Microanalysis Unit (MMU) staff members (Dr Maubane-

Nkadimeng, Dr Tetana, Dr Linganiso, Prof Ziegler) for the instrumental analysis especially 

during COVID where I was not trained and relied on them to run the tests for me. I again 

appreciate Zaynab Cader from Metallurgy Department (Wits) for the Scanning Electron 

Microscopy (SEM) analysis. 

Warmest gratitude to Mbuyi Moloi from the Council of Scientific and Industrial Research 

(CSIR) for the toxicology tests. The dedication and effort he put on my test analysis with his 

busy schedule is highly appreciated.  

In addition, I would like to thank the Post Graduate Merit Award (University of the 

Witwatersrand) for tuition funding and DSI-CSIR bursary for funding my living expenses and 

travel grants that ensured my conference attendances in order to share my work and network 

with other researchers.  

 



vi 
 

CONFERENCES 
 

• 11th International Conference of the African Materials Research Society 2023 (oral 

presentation) 

• 32nd Annual International Conference of Catalysis Society of South Africa (CATSA 

2022) (poster presentation) 

• 8th International conference on Nanoscience and Nanotechnology in Africa 

(NanoAfrica 2022) (poster presentation) 

• The 8th Collaboration Symposium 2022 (oral presentation) 

• WITS 13th Cross-Faculty Postgraduate Symposium 2022 (oral presentation) 

 

INVITED TALKS 

 

• 2nd Scale Up Women in Business Summit: SMMES in construction and facilities 

management 2024 

• 14th Annual Green Building Convention 2021  

 

PUBLICATIONS 
 

• Fungal synthesis of copper nanoparticles and their applications in agri-food, 

environmental, and biomedical sectors, LN Nthunya, S Mbakop, B Makgabutlane, G Matlou, 

S Mhlanga, H Richards, Fungal Cell Factories for Sustainable Nanomaterials Productions and 

Agricultural Applications, Elsevier, 91-114 (2023) 

• Lab-to-market guide for commercialisation of nanomaterials: A South African 

university perspective, S Dikweni, B Makgabutlane, SD Mhlanga, South African Journal of 

Science, ISBN: 979-8-218-00238-1 (2023)  

• Facile floating catalyst chemical vapour deposition synthesis of carbon nanotubes for 

application in sustainable construction composites, B Makgabutlane, M Maubane-

Nkadimeng, N Coville, S Mhlanga, TechConnect, 75-78 (2022)  



vii 
 

• Plastic-fly ash waste composites reinforced with carbon nanotubes for sustainable 

building and construction applications: A review, B Makgabutlane, MS Maubane-

Nkadimeng, NJ Coville, SD Mhlanga, Results in Chemistry, 100405 (2022)  

• Green synthesis of carbon nanotubes to address the water-energy-food nexus: A 

critical review, B Makgabutlane, LN Nthunya, MS Maubane-Nkadimeng, SD Mhlanga, 

Journal of Environmental Chemical Engineering 9 (1), 104736 (2021)  

 

AWARDS 
 

• South African Women in Science Award (2023) 

• Tutor and Teaching Assistant Award (2022) 

• 1st place PhD oral presentation: 8th Collaboration Symposium (2022) 

• 2nd place PhD oral presentation: WITS 13th Cross-Faculty Postgraduate Symposium 
(2022) 

• 3rd place PhD oral presentation: NYRS (2021)  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



viii 
 

TABLE OF CONTENTS 
 

DECLARATION .......................................................................................................................................... i 

ABSTRACT ................................................................................................................................................ ii 

DEDICATION ........................................................................................................................................... iv 

ACKNOWLEDGEMENTS ........................................................................................................................... v 

CONFERENCES ........................................................................................................................................ vi 

INVITED TALKS........................................................................................................................................ vi 

PUBLICATIONS ........................................................................................................................................ vi 

AWARDS ................................................................................................................................................ vii 

LIST OF FIGURES .................................................................................................................................... xii 

LIST OF TABLES ...................................................................................................................................... xv 

ABBREVIATIONS ....................................................................................................................................xvi 

CHAPTER 1 .............................................................................................................................................. 1 

1. Introduction .................................................................................................................................... 1 

1.1 Background of the study ......................................................................................................... 1 

1.1.1 Plastics ............................................................................................................................. 2 

1.1.2 Coal fly ash and coal bottom ash (coal ash) .................................................................... 3 

1.2 Purpose of the study ..................................................................................................................... 7 

1.2.1 Research questions ................................................................................................................ 7 

1.2.2 Aim of the study ..................................................................................................................... 8 

1.2.3 Objectives ............................................................................................................................... 8 

1.2.4 Hypothesis of the study ......................................................................................................... 9 

1.3 Thesis layout ................................................................................................................................. 9 

1.4 References .................................................................................................................................. 11 

CHAPTER 2 ............................................................................................................................................ 15 

PLASTIC-FLY ASH WASTE COMPOSITES REINFORCED WITH CARBON NANOTUBES FOR SUSTAINABLE 

BUILDING AND CONSTRUCTION APPLICATIONS: A REVIEW ................................................................. 15 

2. Introduction .................................................................................................................................. 15 

2.1 Application of waste materials in construction composites ..................................................... 18 

2.1.1 Waste Plastics ................................................................................................................... 18 

2.1.1.1 Processing plastics for application in construction composites .................................... 21 

2.1.1.2 Properties of construction composites with waste plastics .......................................... 22 

2.1.2 Coal fly ash ........................................................................................................................ 26 

2.1.2.1 Properties of coal fly ash ............................................................................................... 27 

2.1.2.3 Application of coal fly ash in construction composites ................................................. 30 



ix 
 

2.1.3 Other waste materials....................................................................................................... 33 

2.2 Hybrid construction composites with engineered nanomaterials ........................................... 36 

2.2.1 Carbon Nanotubes ............................................................................................................ 41 

2.2.2 Dispersion methods of CNTs in composite matrix ......................................................... 43 

2.2.3 Application of CNT- composites in construction materials ........................................... 45 

2.2.4 CNT-fly ash nanopolymer hybrid ................................................................................... 47 

2.3 Release of CNTs to the environment ........................................................................................ 49 

2.4 Summary and conclusion .......................................................................................................... 51 

2.5 References ................................................................................................................................ 53 

CHAPTER 3 ............................................................................................................................................ 66 

FACILE FLOATING CATALYST CHEMICAL VAPOUR DEPOSITION SYNTHESIS OF MULTIWALLED- 

CARBON NANOTUBES ........................................................................................................................... 66 

3. Introduction .................................................................................................................................. 66 

3.1 Experimental ............................................................................................................................... 68 

3.1.1 Materials and methodology ................................................................................................. 68 

3.1.2 Characterization ................................................................................................................... 69 

3.2 Results and discussion ................................................................................................................ 70 

3.2.1 Carbon Yield ......................................................................................................................... 70 

3.2.2 Effect of temperature .......................................................................................................... 72 

3.2.3 Effect of injection rate ......................................................................................................... 80 

3.2.4 Effect of flow rate ................................................................................................................ 85 

3.2.5 Properties of importance of MWCNTs for application in brick composites ........................ 93 

3.2 Conclusion ................................................................................................................................... 94 

3.3 References .................................................................................................................................. 95 

CHAPTER 4 ............................................................................................................................................ 99 

CHARACTERIZATION OF RAW MATERIALS: COAL FLY ASH (CFA), COAL BOTTOM ASH (CBA) AND LOW-

DENSITY POLYETHYLENE (LDPE) ............................................................................................................ 99 

4. Introduction .................................................................................................................................. 99 

4.1 Experimental ....................................................................................................................... 101 

 ........................................................................................................................................................ 102 

4.2 Results and discussion ........................................................................................................ 102 

4.2.1 Coal fly ash .................................................................................................................. 102 

4.2.2 Coal Bottom Ash ......................................................................................................... 108 

4.2.3 Comparison of CFA and CBA ....................................................................................... 113 

4.2.4 Low density polyethylene (LDPE) ................................................................................ 114 

4.3 Conclusion ........................................................................................................................... 117 



x 
 

4.4 References .......................................................................................................................... 118 

CHAPTER 5 .......................................................................................................................................... 122 

APPLICATION OF CARBON NANOTUBES IN WASTE-DERIVED BRICKS ................................................ 122 

5. Introduction ................................................................................................................................ 122 

5.1 Methodology ....................................................................................................................... 124 

5.2 Tests performed on the bricks ............................................................................................ 126 

5.2.1 Workability .................................................................................................................. 126 

5.2.2 Compressive strength ................................................................................................. 126 

5.2.3 Split tensile strength ................................................................................................... 127 

5.2.4 Density ........................................................................................................................ 127 

5.2.5 Water absorption ........................................................................................................ 127 

5.3 Results and discussion ........................................................................................................ 128 

5.3.1 The effect of mixing ratios on the workability ............................................................ 128 

5.3.2 Compressive strength ................................................................................................. 129 

5.3.3 Split tensile strength ................................................................................................... 134 

5.3.4 Density ........................................................................................................................ 136 

5.3.5 Microstructure analysis ............................................................................................... 137 

5.3.6 Mechanism of interaction and reinforcement ............................................................ 139 

5.3.7 Water absorption and deformation ............................................................................ 141 

5.3.8 Current study and comparative literature data .......................................................... 143 

5.4 Conclusion ........................................................................................................................... 144 

5.5 References .......................................................................................................................... 146 

CHAPTER 6 .......................................................................................................................................... 150 

DURABILITY OF WASTE-DERIVED CARBON NANOTUBE REINFORCED BRICKS ................................... 150 

6. Introduction ................................................................................................................................ 150 

6.1 Method ............................................................................................................................... 151 

6.1.1 Preparation of test specimens .................................................................................... 151 

6.1.2 Tests on brick specimens ............................................................................................ 152 

6.2 Results and discussion ........................................................................................................ 159 

6.2.1 Oxygen permeability ................................................................................................... 159 

6.2.2 Water sorptivity and porosity ..................................................................................... 160 

6.2.3 Chloride conductivity .................................................................................................. 162 

6.2.4 Thermal resistance ...................................................................................................... 163 

6.2.5 Compressive strength after thermal exposure ........................................................... 166 

6.2.6 Chemical attack ........................................................................................................... 167 

6.2.7 Efflorescence ............................................................................................................... 169 



xi 
 

6.2.8 Research gaps in durability tests for waste-bricks ...................................................... 170 

6.3 Conclusion ........................................................................................................................... 170 

6.4 References .......................................................................................................................... 172 

CHAPTER 7 .......................................................................................................................................... 175 

ENVIRONMENTAL AND ECONOMIC SUSTAINABILITY OF CARBON NANOTUBE-REINFORCED BRICKS

 ............................................................................................................................................................ 175 

7. Introduction ................................................................................................................................ 175 

7.1 Methodology ........................................................................................................................... 177 

7.1.1 Sample preparation and leachate analysis ................................................................. 177 

7.1.2 Toxicity studies ............................................................................................................ 177 

7.2 Results and discussion ........................................................................................................ 179 

7.2.1 Environmental impact ................................................................................................. 179 

7.2.2 Economic sustainability ............................................................................................... 181 

7.2.3 Elemental analysis of heavy metals in leachates ........................................................ 184 

7.2.4 Ecotoxicology .............................................................................................................. 187 

7.2.5 CNT leaching................................................................................................................ 193 

7.3 Conclusion ........................................................................................................................... 196 

7.4 References .......................................................................................................................... 197 

CHAPTER 8 .......................................................................................................................................... 201 

CONCLUSIONS AND RECOMMENDATIONS ......................................................................................... 201 

8.1 Conclusions ............................................................................................................................. 201 

8.1.1 Synthesis of carbon nanotubes ................................................................................... 201 

8.1.2 Application of CNTs in bricks ....................................................................................... 201 

8.1.3 Durability of the CNT-bricks ........................................................................................ 202 

8.1.4 Environmental and economic sustainability of CNT-bricks ........................................ 203 

8.2 Recommendations .............................................................................................................. 204 

 

 

 

 

 
 



xii 
 

LIST OF FIGURES 
 

Figure 1.1: Waste management hierarchy [4] ........................................................................................ 2 

Figure 1.2: Classification of solid waste .................................................................................................. 2 

Figure 2.1: Estimated quantities of waste plastic generated in a year (PET= polyethylene 

terephthalate, HDPE= high density polyethylene, LDPE= low density polyethylene, PP= 

polypropylene, PS= polystyrene) [34] ................................................................................................... 19 

Figure 2.2:The benefits of using waste in concrete, C= concrete, FA= fly ash, P= plastic [36] ............. 21 

Figure 2.3:  SEM image of a representative sample of coal fly ash particles in the range of 10–100 µm 

[63] ........................................................................................................................................................ 28 

Figure 2.4: Particle size and surface area comparison of nanofillers and other aggregates in concrete 

[123] ...................................................................................................................................................... 39 

Figure 2.5: a) Schematic diagram of graphene/fly ash geopolymer composite under compression 

load and toughening mechanism and b) SEM image of graphene composite under stress [137] ....... 40 

Figure 2.6: Schematic diagram of a) graphene sheet b) SWCNTs and c) MWCNTs [143] .................... 42 

Figure 2.7: dispersion of CNTs a) ultrasonication and b) ultrasonication and SDS surfactant [159] .... 44 

Figure 2.8: SEM images of a) fly ash, (b) CNT and (c–d) CNT-fly ash nanocomposites [177] ............... 48 

Figure 2.9: Schematic diagram of the proposed mechanism of interaction between HDPE, 

functionalized CNTs and alkali activated coal fly ash [14,114] ............................................................. 49 

Figure 2.10: Schematic diagram of the life-cycle stages of engineered nanomaterials [183] .............. 50 

Figure 3.1: FCCVD reaction setup a) horizontal furnace, b) syringe pump, and c) temperature control 

system ................................................................................................................................................... 69 

Figure 3.2: carbon yield at various a) temperature b) injection rate and c) N2 flow rate .................... 72 

Figure 3.3:SEM images and particle size distribution histograms of CNTs synthesized at a) 750 b) 800 

and c) 850 °C, while d) is a lower magnification of image CNTs synthesized at 850 °C ....................... 74 

Figure 3.4:TEM images and EDS of CNTs synthesized at a) 750, b) 800 c) 850 °C and d) high-

magnification HRTEM of CNTs synthesized at 850 °C. .......................................................................... 76 

Figure 3.5: Raman spectra of CNTs synthesized at a) 750, b) 800 and c) 850 °C. ................................. 77 

Figure 3.6: XRD spectra of CNTs synthesized at a) 750 b) 800, and c) 850 °C ...................................... 78 

Figure 3.7: TGA and derivative plots of CNTs synthesized at various temperatures. ........................... 79 

Figure 3.8: SEM images and particle size distribution histograms of CNTs synthesized at various 

injection rates a) 0.2 ml/min, b) 0.49 ml/min and c) 1 ml/min ............................................................ 81 

Figure 3.9: TEM images of CNTs synthesized at various injection rate a) 0.2 ml/min, b) 0.49 ml/min 

and c) 1 ml/min ..................................................................................................................................... 82 



xiii 
 

Figure 3.10: Raman spectra of CNTs made at varied injection rates .................................................... 83 

Figure 3.11: XRD spectra of CNTs at synthesized varied injection rates .............................................. 84 

Figure 3.12: TGA and derivative plots synthesized at varied injection rates ........................................ 85 

Figure 3.13: SEM images of CNTs and particle size distribution histograms at varied N2 flow rate a) 

120 ml/min, b) 140 ml/min and c) 160 ml/min .................................................................................... 86 

Figure 3.14: TEM images and EDS of CNTs synthesized at various N2 flow rate a) 120 ml/min, b) 140 

ml/min and c) 160 ml/min .................................................................................................................... 88 

Figure 3.15: Raman spectra of MWCNTs synthesized at varied flow rate............................................ 89 

Figure 3.16: XRD spectra of CNTs synthesized at varied flow rates ..................................................... 90 

Figure 3.17: TGA and derivative plots of MWCNTs synthesized at varied flow rate ............................ 90 

Figure 3.18: FTIR spectra of pristine and functionalized CNTs ............................................................. 92 

 

Figure 4. 1: Image of Lethabo thermal power plant ash collection sites. ........................................... 102 

Figure 4.2: As received a) coal fly ash, b) coal bottom ash, and c) low density polyethylene............ 102 

Figure 4.3: XRD profile of coal fly ash. ................................................................................................ 105 

Figure 4.4: SEM images and EDS spectra of coal fly ash. .................................................................... 106 

Figure 4.5: FTIR spectrum of coal fly ash. ........................................................................................... 107 

Figure 4.6: TGA and derivative plots of coal fly ash. ........................................................................... 108 

Figure 4.7: XRD profile of coal bottom ash. ........................................................................................ 110 

Figure 4.8: Images and EDS spectra of coal bottom ash. .................................................................... 111 

Figure 4.9: FTIR spectrum of coal bottom ash .................................................................................... 112 

Figure 4.10: TGA and derivative plot of coal bottom ash. .................................................................. 113 

Figure 4.11: FTIR spectra of LDPE ....................................................................................................... 115 

Figure 4.12: XRD profile of LDPE. ........................................................................................................ 116 

Figure 4.13: TGA plot and derivative plots of LDPE. ........................................................................... 117 

Figure 5.1: Schematic of the method used to make the bricks .......................................................... 125 

Figure 5.2:  Split tensile strength testing setup .................................................................................. 127 

Figure 5.3: Brick samples using different coal fly ash: LPDE ratios. a) 80:20, b) 85:15 and c) 90:10. . 129 

Figure 5.4: Compressive strength development of CFA and CBA bricks with time ............................ 131 

Figure 5.5: Compressive strength of coal fly ash bricks with different CNT dosages ......................... 133 

Figure 5.6: Bricks after compressive stress was applied ..................................................................... 133 

Figure 5.7: Stress beyond load of failure applied on the 0.05 wt.% CNT brick ................................... 134 

Figure 5.8: Split tensile strength of coal fly ash bricks with different CNT dosages ........................... 135 

Figure 5.9: Bricks after the split tensile stress test ............................................................................. 136 



xiv 
 

Figure 5. 10: SEM images of CFA bricks reinforced with different CNT dosages a) control, b) 0.01 c) 

and 0.05 wt.% d) 0.1 wt.% .................................................................................................................. 139 

Figure 5.11: FTIR spectra of the brick a) 0.0 wt.% (control) and b) 0.05 wt.% CNT dosage ............... 140 

Figure 5.12: Schematic representation of the possible mechanism of interactions between the 

different materials applied in the preparation of bricks .................................................................... 141 

Figure 5.13: a) mass variation during water absorption and b) deformation due to water absorption 

of bricks ............................................................................................................................................... 143 

Figure 6.1: a) water-cooled diamond-tipped core barrel, b) water-cooled diamond cutter and c) 

cored specimen ................................................................................................................................... 152 

Figure 6.2: Specimen assembly parts for the permeability cell [10] .................................................. 153 

Figure 6.3: a) Permeameter setup, (b) a close-up of specimen assembly, and (c) cell without 

specimen assembly [10] ...................................................................................................................... 154 

Figure 6.4: Sorptivity tray with specimens and Ca(OH)2 solution ....................................................... 155 

Figure 6.5: Chloride conductivity test setup ....................................................................................... 156 

Figure 6.6: Oxygen permeability index of bricks containing varied amounts of CNTs ....................... 160 

Figure 6.7: a) Water sorptivity index and b) porosity of bricks at various CNT dosages .................... 161 

Figure 6.8: Chloride conductivity index of bricks with varied CNT dosages ....................................... 163 

Figure 6.9: Control bricks after thermal exposure a) 120 °C, b) 220 °C, and c) 320 °C ....................... 165 

Figure 6.10: 0.05 wt.% CNT bricks after thermal exposure a) 120 °C, b) 220 °C, and c) 320 °C ......... 165 

Figure 6.11: Compressive strength of bricks after thermal exposure ................................................ 167 

Figure 6.12: Compressive strength of bricks after chemical attack .................................................... 169 

Figure 6.13: Control, 0.01, 0.05 and 0.1 wt.% CNT bricks before and after efflorescence tests ........ 170 

Figure 7.1: Compressive strength of the bricks at different CNT loading vs cost per brick ................ 183 

Figure 7.2: Growth inhibition of algae exposed to pristine brick leachates in a) neutral, b) acidic, and 

c) basic medium .................................................................................................................................. 188 

Figure 7.3: Growth inhibition of algae exposed to CNT brick leachates at a) neutral, b) acidic, and c) 

basic medium ...................................................................................................................................... 190 

Figure 7. 4: Immobilised neonates exposed to pristine brick leachates in a) neutral, b) acidic, and c) 

basic medium ...................................................................................................................................... 192 

Figure 7.5: Immobilised neonates exposed to CNT brick leachates in a) neutral, b) acidic, and c) basic 

medium ............................................................................................................................................... 193 

Figure 7.6: TEM image and EDS of CNT brick a) neutral, b) acidic, and basic leachate ...................... 195 

 

 



xv 
 

LIST OF TABLES 
 

Table 2.1: Properties of different plastics commonly found as waste in the environment ................. 19 

Table 2.2: Physical properties of recycled plastics and their possible construction applications ........ 23 

Table 2.3: Recent applications of waste plastic in bricks ...................................................................... 25 

Table 2.4: Classification of coal fly ash according to ASTM .................................................................. 29 

Table 2.5: Recent applications of coal fly ash in bricks ......................................................................... 31 

Table 2.6: Recent applications of waste additives in bricks ................................................................. 34 

Table 2.7: Nanomaterials in cementious construction composites ..................................................... 37 

Table 2.8: Carbon nanomaterials applied in cementious composites .................................................. 46 

Table 3.1: Synthesis conditions of MWCNTs…………………………………………………………………………………….72 

Table 3.2: BET surface area and pore volume of MWCNTs synthesized at various conditions............ 93 

Table 4.1: Physical properties of coal fly ash………………………………………………………………………………….102 

Table 4.2: ICP analysis of coal fly ash .................................................................................................. 103 

Table 4.3: Physical properties of coal bottom ash .............................................................................. 109 

Table 4.4: ICP-OES analysis of coal bottom ash .................................................................................. 110 

Table 4.5: Differences in the properties of coal fly ash and coal bottom ash .................................... 114 

Table 5.1: Mix design of the developed bricks……………………………………………………………………………….126 

Table 5.2: Mixing ratios of raw materials ........................................................................................... 129 

Table 5.3: Crack measurements after split tensile stress ................................................................... 136 

Table 5.4: Dimensions, mass and density of the coal fly ash bricks ................................................... 137 

Table 5.5: Studies on CNT-reinforced composites .............................................................................. 144 

Table 6.1: Porosity measurement of bricks…………………………………………………………………………………… 162 

Table 6. 2: Linear expansion of bricks ................................................................................................. 166 

Table 6.3: Chemical resistance of bricks ............................................................................................. 168 

Table 7. 1: Inputs for brick manufacturing……………………………………………………………………….. ………….. 181 

Table 7. 2: Cost analysis of raw materials per brick in $ ..................................................................... 182 

Table 7.3: Leaching of heavy metals from pristine bricks under different conditions ....................... 186 

Table 7.4: Leaching of heavy metals from CNT bricks under different conditions ............................. 187 

 

 

 



xvi 
 

ABBREVIATIONS 
 

ASTM: American Society for Testing Material  

BET: Brunauer-Emmett-Teller   

CBA: Coal bottom ash 

CCI: Chlorine conductivity index 

CFA: Coal fly ash 

CNTs: Carbon nanotubes 

D. magna: Daphnia magna 

DI: Durability index 

EDS: Energy dispersive X-ray spectroscopy  

FFCVD: Floating catalyst chemical vapour deposition 

FTIR: Fourier-transform infrared spectroscopy   

HNO3: Nitric acid  

LDPE: low density polyethylene  

LOI: Loss on ignition 

MWCNTs: Multiwalled carbon nanotubes 

NaOH: Sodium hydroxide 

OPC: Ordinary Portland Cement  

OPI: Oxygen permeability index 

 R. subcapitata: Raphidocelis subcapitata 

SANS: South African national standards 

SEM: scanning electron microscopy  

TGA: Thermal gravimetric analyzer   

TEM: Transmission electron microscopy  

WSI: Water sorptivity index 

XRD: X-ray diffraction  

 



1 
 

CHAPTER 1 

 

1. Introduction 
 

1.1 Background of the study 
 

The large volumes of waste (e.g., paper, plastics, rubble, coal fly ash (CFA), coal bottom ash 

(CBA)) generated by growing industries and increasing human activities puts pressure on the 

waste management systems [1]. Unfortunately, not all industries or people can depend on 

the local municipalities for a solution to the waste collection and management issues. This 

ends up leading to the generation of unauthorized dumping sites that pose both 

environmental and health risks. The accurate cost of waste not only involves the cost of 

thrown away materials, but includes ineffective utilization of raw materials, futile usage of 

water and energy, destruction of excess products, disposal of by-products, waste collection 

and treatment, as well as wasted labour [2]. The cost of waste disposal is always on the rise 

and also recycling incentives are provided to encourage the repurposing of the waste for 

other applications. For example, in recent years the cost of plastic bags in South Africa 

increased drastically to encourage consumers to reuse their bags or choose sustainable 

alternative [3]. However, “reuse” is a lesser preferred measure, as is disposal, as they are 

reactive approaches rather than preventative approaches to waste issues (see Figure 1.1) [4]. 

One method to achieve this aim is to replace the use of plastics with more sustainable 

materials (i.e., cellulose from biomass) as a preventative measure [5]. Furthermore, greener 

energy generation methods (i.e. hydro, solar and wind) are on the rise as alternatives to 

substitute coal powered energy processes which produce high amounts of waste (i.e. carbon 

dioxide and coal ash). 

 



2 
 

 

Figure 1.1: Waste management hierarchy [4] 

 

It is still a long journey to reduce the generation of waste. However, its production should be 

minimized and managed effectively. There are many types of waste and can be categorized 

in various ways for effective recycling as shown in Figure 1.2. The focus of this study is on solid 

waste i.e., coal fly ash, coal bottom ash and plastic waste (low density polyethylene) and its 

re-use.  

 

 Figure 1.2: Classification of solid waste 

 

1.1.1 Plastics  
 

Plastics are a significant part of our everyday lives owing to their versatility in application. 

Industries use plastics for many applications that include packaging of food items and 



3 
 

electronics to the manufacturing of plastic-based products such as chairs, toys, pipes, 

automobile parts etc. [7]. One of the most widely used plastics in households are plastic bags 

to carry groceries and clothing; they can be reused, for example to carry garbage for disposal 

purposes [8].  

Plastics have a greater global market due to lower processing costs, durability, strength and 

chemical resistance [9]. However, these excellent stability properties of plastic also create an 

environmental threat and is detrimental to the ecosystem. These plastics are chemically non-

biodegradable, making their disposal a major concern. This is due to the fact that plastic waste 

lasts a long time—natural decomposition can take up to 500 years [10]. The thermoplastic 

waste, which is primarily composed of poly-olefins, comprises over 32% of low-density 

polyethylene (LDPE) and accounts for over 78% of the total plastic waste produced annually 

[10]. This waste is generally placed in land fill sites. The plastic can persist in the form of 

microplastics, finding its way into the ground water, tap water, river banks and ultimately the 

sea [11]. Plastics are the largest contributor to aquatic life harm and have also become a 

health risk to human life [11].  

Recycling of plastics can reduce their negative environmental impact. Researchers have 

focused on various recycling and reutilization strategies to transform plastic waste to 

generate oil/gas products and energy [12,13], to produce carbon precursors applicable in the 

synthesis of carbon nanotubes (CNTs) [14] etc.  In order to create products with high 

compressive and flexural strength and mechanical resistance comparable to typical concrete 

products, the plastic waste can also be turned into aggregates for concrete mixtures [15].  It 

is well known that plastic has a high linear burning rate and is highly flammable [16]. Fillers 

are therefore needed to enhance its mechanical and physical qualities and lessen its 

flammability. 

 

1.1.2 Coal fly ash and coal bottom ash (coal ash)  

 

Coal ash is another type of noxious combustion-waste, produced in large quantities by 

thermal power plants. The waste is especially a challenge in a developing economy like South 

Africa that heavily relies on coal combustion for energy generation due to the high availability 



4 
 

of coal. The power utility, (ESKOM, in South Africa) reported in 2018 that it generates 34.4 Mt 

of coal fly ash annually. Currently, only 2.41 Mt is beneficiated while the rest is unutilized and 

stored in ponds, landfills and ash dams [17]. The coal powered energy generation value chain 

which involves coal mining, transportation, preparation, combustion and by-product storage 

results in numerous unsustainable environmental impacts [17]. Furthermore, the CFA and 

CBA both contribute to air contamination and can be considered health hazards. They consist 

of fine particles (0.5-400 µm) that can simply infiltrate the air passages and cause breathing 

problems [18].  

Research towards the use of CBA has been slower compared to that of CFA. The reasons are 

that it makes up considerably less than CFA, at just 25% of the total amount of coal ash 

generated [19]. Additionally, it has been shown that CFA is the more dangerous industrial 

waste, whereas CBA is said to have fewer toxic elements. Lastly, CBA is inhomogeneous, 

which makes it harder to recycle and limits its applicability [19].  

Although CFA and CBA are both regarded as a waste product, they have been studied as 

aggregate materials for various applications.  They have valuable properties such as being 

light weight, easy availability, low density, chemical inertness and thermal stability [20]. The 

largest application of CBA is as an aggregate in construction composites because its particle 

size distribution is similar to that of sand [21]. The largest consumers of CFA are the cement 

industry, utilizing more than 40% of the fly ash produced annually worldwide [22]. However, 

a specific grade of coal ash, with low loss on ignition (LOI), is required for application in 

construction, thus leaving the unrequired grades in ash dumps [23].  

The residual carbon in coal ash can absorb water and chemical admixtures (such as 

superplasticizers and air-entraining agents) in cement composites, decreasing their 

effectiveness or perhaps leading to an insufficient air-void system in the concrete. The 

pozzolanic reaction is inhibited by the lower fineness brought on by the coarser carbon 

particles [26]. Thus, it is commonly anticipated that low-quality coal ash with a high LOI and 

low fineness may hinder the development of concrete's strength [27].  Due to their ceramic-

like properties, ash-based composites generally exhibit quasi-brittle behaviour together with 

poor tensile strength and fracture toughness [28].   

 



5 
 

The limitation of the current studies with the use of waste in building bricks is their low 

inclusion due to the properties of the bricks being compromised at higher waste dosages. Coal 

ash is usually used as a partial replacement of cement/ clay in bricks and it is mostly applied 

in concentrations of up to 40 wt.% [29]. This is because cement/clay are the primary strength 

developing aggregates used in conventional bricks and their reduction affects the mechanical 

properties. In a recent study, Islam et al., found that higher amount of CFA (20% and 25%) 

replacement showed lower compressive strength of the bricks at 28 and 90 days. This was 

attributed to the increase in silica (contained in CFA) and the gradual reduction of calcium 

oxide (contained in OPC) in the concrete, leading to unavailability of CaO for pozzolanic 

reaction. Furthermore, it was found that the workability of fresh concrete reduced when the 

inclusion of CFA exceeded 10% of the cement [30]. Siddique, used higher loadings of CFA and 

observed that the inclusion of 35%, 45%, and 55% CFA as a replacement for cement resulted 

in reductions of 23%, 36%, and 49% in the split tensile strengths of concrete bricks, 

respectively [31]. Other researchers found that the flexural strength of 10%, 20%, 30% and 

40% CFA concrete decreased by 5.26%, 7.89%, 10.53% and 21.05% respectively, compared to 

the control at 28 days [32].  

 

Plastic waste is currently used as a partial replacement of fine and coarse aggregates such as 

sand and stones at dosages of up to 40 wt.% [33]. Vanitha et al., used plastic waste to partially 

(0-10 wt.%) replace coarse aggregates in concrete blocks. It was found that above 4 wt.% 

inclusion, the compressive strength decreased by 9% [34]. Osei et al., found that the use of 

plastic aggregate in structural concrete at dosages higher than 36% was not suitable as it 

reduced the compressive strength. The only desired property obtained by the higher dosages 

of plastic waste was lighter composites [35]. Jaivignesh et al., replaced fine and coarse 

aggregates in concrete with 10%, 15% and 20 % plastic waste and also added steel fibers to 

counter the impact of plastic on the compressive strength. The composites still suffered a 

reduction in compressive strength and as such it was concluded to use plastic waste as a green 

aggregate despite its impact on the compressive strength [36].   

 

The use of coal ash as a 100% replacement of conventional strength developing aggregates 

such as cement and clay is novel to this study. The coal ash is solely used as an aggregate and 

does not impart its pozzolanic properties which are known to facilitate the hydration reaction 



6 
 

and strength development due to the absence of water. Furthermore, plastic waste was used 

as a binder (liquified), unlike it is conventionally used (shredded solid particles) as a partial 

replacement of fine and coarse aggregates. The use of CNTs as a reinforcement filler in the 

all-waste derived bricks is unique to the study. The CNTs are incorporated to counter the 

negative impact of the waste materials on the properties of the bricks as previously reported 

in literature.   

 

Recent studies have focused on the use of nanomaterials for composite reinforcement, 

including graphene-based materials, nanosilica, nanotitanium dioxide (TiO2), and CNTs [37-

39]. CNTs have a high surface area and high aspect ratio in addition to having exceptional 

mechanical, electrical, thermal, and optical properties [40]. With a tensile strength of 150 

GPa, which is 100 times higher than steel [41], CNTs are lightweight materials that greatly 

enhance the mechanical properties of composites. The filler-effect caused by their nanosize 

and hydration chemistry improves the behaviour of coal ash binders both fresh and hardened, 

as well as reducing shrinkage-related micro and macrocracks [42]. The mechanical properties 

of cement are significantly improved by the inclusion of small amounts of multi-walled CNTs 

(MWCNTs), between 0.05 wt.% and 0.5 wt.% [43]. When CNTs are added to concrete, it is 

anticipated that the compression strength would rise to 200 MPa, enabling the building of 

skyscrapers that reach mile-high heights [42]. 

The disposal of plastic and coal ash (CFA and CBA) is unsustainable and is too costly to 

manage; it is already forbidden in certain countries [44,45]. Thus, alternative applications are 

required for the utilization of these waste materials. In this study, a green sustainable 

approach was developed to utilize waste materials (plastic and coal ash) for the 

manufacturing of CNT reinforced bricks. This encourages a closed resource loop by 

transforming waste to high value products.  

 

1.1 Motivation of the study (Problem Statement) 

Plastic waste is the largest contributor of global solid waste [46].  This is due to its high 

production rate for application in many industries. However, plastic reuse and recycling 

processes remain a challenge since the waste is not adequately categorized for further 



7 
 

processing. Another waste generated in large quantities is coal ash (CFA and CBA) [47]. This 

waste has limited applications compared to the rate of its production.  

The application of plastic waste to produce materials for building is a conceived and tested 

idea [48-50]. Although this has also been achieved using coal ash, the process often involves 

the use of other materials such as cement. Also, the use of coal ash for making bricks using 

the standard processes is limited by water scarcity, as the process involves the usage of high 

volumes of water [51]. Furthermore, the application of waste materials in bricks has been 

shown to compromise their properties; as such it is added in limited amounts to maintain the 

desired properties that are acceptable for building bricks. This restriction further reduces the 

amount of waste that is recycled.  Thus, alternative ways of making useful products from coal 

ash and plastic waste are desired.   

Our approach was the novel simultaneous incorporation of CNTs, coal ash and plastic waste 

to develop bricks. Due to their excellent properties, CNTs provide strength, binding capacity 

and thermal stability to the final product. The incorporation of the CNTs in the bricks ensures 

that higher volumes of waste are used in the bricks without compromising their properties. 

This is due to the superior properties they impart as a reinforcement to the bricks.  The 

synthesis of these bricks does not require cement and thus, no water was used during 

production. This approach also reduced pollution while creating useful products from waste 

that are generally not used in the conventional synthesis of concrete mixtures and bricks. 

Moreover, the bricks were eco-friendly, sustainable and low-cost as the method used waste 

materials and greener processes. Ultimately the successful use of such nano-enhanced bricks 

will potentially lead to the creation of new products, jobs while solving environmental waste 

problems.   

 

1.2 Purpose of the study 

 

1.2.1 Research questions 

 

The study sought to answer the following research questions. 

• Will the waste materials be suitable to develop the eco-friendly bricks? 



8 
 

• What concentration of CNTs will be required to reinforce the composite for their 

application as building materials? 

• Will surface treatment of CNTs significantly improve their dispersion in the 

composite? 

• Will the CNTs and coal ash improve the thermal stability of the composites? If not, 

what modification will be done? 

• Will CNTs significantly improve water resistance and durability of the composites?  

• Will the composite materials be strong enough (mechanical strength) for structural 

application in building and construction?  

• Will the developed bricks be more environmentally and financially sustainable 

compared to the conventional bricks? 

 

1.2.2 Aim of the study 

 

The aim of the study is to develop building and construction eco-friendly bricks derived 

from CNT-reinforced waste materials (plastic and coal ash).  

1.2.3 Objectives 

 

• To collect, classify, pre-treat and characterize CFA, CBA and plastic waste materials 

(LDPE) using a range of techniques. 

• To functionalize and characterize the synthesized CNTs in order to increase their 

dispersion in the bricks.  

• To study the physicochemical properties of the bricks developed from different raw 

materials and the effect of the inclusion of CNTs. Furthermore, test their applicability 

according to the building and construction standards.  

• To study the durability properties of the developed bricks. 

• To assess the environmental impact and financial sustainability of the developed 

bricks as compared to the conventional bricks. 



9 
 

1.2.4 Hypothesis of the study  

 

The expected properties of the engineered building bricks are summarized: 

• High strength desired for building. The bricks can be made stronger than concrete 

bricks but not too strong to prevent them from being cut and chiseled with the usual 

tools. 

• The bricks should possess hydrophobic properties as a result of the incorporated CNTs. 

This means they should not absorb excess water into their internal structure. Thus, 

possess high durability. 

• The bricks must be thermally stable to withstand high temperatures desired for 

buildings.  

The proposition of this study is that the CNT-reinforced bricks will have a great potential with 

comparable and even improved properties relative to conventional concrete/ clay bricks. 

 

1.3 Thesis layout  

 

This thesis does not have a dedicated methodology chapter, but rather details the methods 

and procedures undertaken in each chapter.  

Chapter 1 gives a background, motivation, aim and objectives of the study. 

Chapter 2 presents a detailed literature review of the work that has been done and highlights 

the identified gaps that are bridged by the study. This chapter has been published: B 

Makgabutlane, M Maubane-Nkadimeng, N Coville and S Mhlanga, ‘Plastic-fly ash waste 

composites reinforced with carbon nanotubes for sustainable building and construction 

composites: A review’, Results in Chemistry, 2022, 4, 100405. 

Chapter 3 details the synthesis of carbon nanotubes using a facile approach of a floating 

catalyst vapour deposition. Parts of this work were published in a conference proceeding: B 

Makgabutlane, M Maubane-Nkadimeng, N Coville and S Mhlanga, 'Facile floating catalyst 

chemical vapour deposition synthesis of multiwalled carbon nanotubes for application in 

sustainable construction,' TechConnect Briefs. 2022, ISBN: 979-8-218-00238-1. 



10 
 

Chapter 4 reports on the characterization of all the raw materials (coal fly ash, coal bottom 

ash and low-density polyethylene) used in this study.  

Chapter 5 presents the application of carbon nanotubes as a reinforcement filler in the waste 

derived bricks.  

Chapter 6 outlines the durability of the bricks under various conditions. 

Chapter 7 focuses on the environmental and economical sustainability of the bricks. 

Chapter 8 concludes the study and gives recommendation on the future prospects. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



11 
 

1.4 References 

 

[1] S.P. Raut, R. v Ralegaonkar, S.A. Mandavgane, Development of sustainable construction material 

using industrial and agricultural solid waste : A review of waste-create bricks, Constr Build Mater. 25 

(2011) 4037–4042. https://doi.org/10.1016/j.conbuildmat.2011.04.038. 

[2] S. Perera, A. Arulrajah, Y. Choy, S. Horpibulsuk, F. Maghool, Utilizing recycled PET blends with 

demolition wastes as construction materials, Constr Build Mater. 221 (2019) 200–209. 

https://doi.org/10.1016/j.conbuildmat.2019.06.047. 

[3] J. Dikgang, A. Leiman, M. Visser, Analysis of the plastic-bag levy in South Africa, Resour Conserv 

Recycl. 66 (2012) 59–65. https://doi.org/10.1016/j.resconrec.2012.06.009. 

[4] S. Singh, S. Ramakrishna, M.K. Gupta, Towards zero waste manufacturing: A multidisciplinary review, 

J Clean Prod. 168 (2017) 1230–1243. https://doi.org/10.1016/j.jclepro.2017.09.108. 

[5] S. Janaswamy, M.P. Yadav, M. Hoque, S. Bhattarai, S. Ahmed, Cellulosic fraction from agricultural 

biomass as a viable alternative for plastics and plastic products, Ind Crops Prod. 179 (2022). 

https://doi.org/10.1016/j.indcrop.2022.114692. 

[6] H. Limami, I. Manssouri, K. Cherkaoui, A. Khaldoun, Study of the suitability of unfired clay bricks with 

polymeric HDPE & PET wastes additives as a construction material, Journal of Building Engineering. 

27 (2020) 100956. https://doi.org/10.1016/j.jobe.2019.100956. 

[7] B.G. Mwanza, C. Mbohwa, Drivers to Sustainable Plastic Solid Waste Recycling : A Review, Procedia 

Manuf. 8 (2017) 649–656. https://doi.org/10.1016/j.promfg.2017.02.083. 

[8] J.N. Hahladakis, E. Iacovidou, An overview of the challenges and trade-offs in closing the loop of 

post- consumer plastic waste ( PCPW ): Focus on recycling, J Hazard Mater. 380 (2019) 120887. 

https://doi.org/10.1016/j.jhazmat.2019.120887. 

[9] G. Faraca, T. Astrup, Plastic waste from recycling centres : Characterisation and evaluation of plastic 

recyclability, Waste Management. 95 (2019) 388–398. 

https://doi.org/10.1016/j.wasman.2019.06.038. 

[10] R. Dhawan, B. Mohan, S. Bisht, R. Kumar, S. Kumari, Recycling of plastic waste into tiles with reduced 

flammability and improved tensile strength, Process Safety and Environmental Protection. 124 

(2019) 299–307. https://doi.org/10.1016/j.psep.2019.02.018. 

[11] L. Bradney, H. Wijesekara, K. Niroshika, M.B. Kirkham, Particulate plastics as a vector for toxic trace-

element uptake by aquatic and terrestrial organisms and human health risk, Environ Int. 131 (2019) 

104937. https://doi.org/10.1016/j.envint.2019.104937. 

[12] A. Ahamed, A. Veksha, K. Yin, P. Weerachanchai, A. Giannis, Environmental impact assessment of 

converting flexible packaging plastic waste to pyrolysis oil and multi-walled carbon nanotubes, J 

Hazard Mater. (2019) 121–449. https://doi.org/10.1016/j.jhazmat.2019.121449. 

[13] P. Dwivedi, P.K. Mishra, M. Kumar, N. Srivastava, Heliyon Non-biodegradable polymeric waste 

pyrolysis for energy recovery, Heliyon. 5 (2019) e02198. 

https://doi.org/10.1016/j.heliyon.2019.e02198. 



12 
 

[14] J. Wang, B. Shen, M. Lan, D. Kang, C. Wu, Carbon nanotubes ( CNTs ) production from catalytic 

pyrolysis of waste plastics : The influence of catalyst and reaction pressure, Catal Today. (2019) 1–8. 

https://doi.org/10.1016/j.cattod.2019.01.058. 

[15] Z.Z. Ismail, E.A. Al-hashmi, Use of waste plastic in concrete mixture as aggregate replacement, Waste 

Management, 28 (2008) 2041–2047. https://doi.org/10.1016/j.wasman.2007.08.023. 

[16] L. Delva, S. Hubo, L. Cardon, K. Ragaert, On the role of flame retardants in mechanical recycling of 

solid plastic waste, Waste Management. 82 (2018) 198–206. 

https://doi.org/10.1016/j.wasman.2018.10.030. 

[17] Beneficiation of Eskom Ash Fact Sheet (2018). https://www.eskom.co.za/wp-

content/uploads/2021/03/ENV0009BeneficiationEskomAshRev1.pdf 

[18] S.J.K. Suhas V. Patil, Suryakant C. Nawle, Industrial Applications of Fly ash : A Review, International 

Journal of Science, Engineering and Technology Research (IJSETR). 2 (2013) 1659–1663. ISSN 2278-

7798. 

[19] J.C. Hower, K.R. Henke, S. Dai, C.R. Ward, D. French, S. Liu, U.M. Graham, Generation and nature of 

coal fly ash and bottom ash, in: Coal Combustion Products (CCPs): Characteristics, Utilization and 

Beneficiation, Elsevier, 2017: pp. 21–65. https://doi.org/10.1016/B978-0-08-100945-1.00002-2. 

[20] E.R. Teixeira, A. Camões, F.G. Branco, J.B. Aguiar, R. Fangueiro, Recycling of biomass and coal fly ash 

as cement replacement material and its effect on hydration and carbonation of concrete, Waste 

Management. 94 (2019) 39–48. https://doi.org/10.1016/j.wasman.2019.05.044. 

[21] U. Kant, V.K. Saxena, A. Sarkar, A.K. Varma, K.K. Mishra, Characterization of Coal ash from a Captive 

Power plant for Potential End uses, n.d. http://www.evipar.org/. 

[22] M.D.A. Thomas, Optimizing the Use of Fly Ash in Concrete, Portland Cement Association. J Mater Sci 

Chem Eng, Vol.5 No.12, (2007) 24. 

https://www.scirp.org/reference/referencespapers?referenceid=2182357 

[23] D.F. Velandia, C.J. Lynsdale, J.L. Provis, F. Ramirez, A.C. Gomez, Evaluation of activated high volume 

fly ash systems using Na 2 SO 4 , lime and quicklime in mortars with high loss on ignition fly ashes, 

Constr Build Mater. 128 (2016) 248–255. https://doi.org/10.1016/j.conbuildmat.2016.10.076. 

[24] V. Gupta, D.K. Pathak, S. Siddique, R. Kumar, S. Chaudhary, Study on the mineral phase 

characteristics of various Indian biomass and coal fly ash for its use in masonry construction 

products, Constr Build Mater. 235 (2020) 117413. 

https://doi.org/10.1016/j.conbuildmat.2019.117413. 

[25] S.H. Ngo, T.P. Huynh, T.T. Thi Le, N.H. Thi Mai, Effect of High Loss on Ignition-Fly Ash on Properties of 

Concrete Fully Immersed in Sulfate Solution, IOP Conf Ser Mater Sci Eng. 371 (2018). 

https://doi.org/10.1088/1757-899X/371/1/012007. 

[26] A.J. Babafemi, B. Šavija, S.C. Paul, V. Anggraini, Engineering properties of concrete with waste 

recycled plastic: A review, Sustainability. 10 (2018) 3–26. https://doi.org/10.3390/su10113875. 

[27] H. Chen, N. Shih, C. Wu, Effects of the Loss on Ignition of Fly Ash on the Properties of High-Volume 

Fly Ash Concrete, Sustainability. 11 (2019). https://doi.org/10.3390/su11092704.  



13 
 

[28] P. Timakul, W. Rattanaprasit, P. Aungkavattana, Improving compressive strength of fly ash-based 

geopolymer composites by basalt fibers addition, Ceram Int. 42 (2016) 6288–6295. 

https://doi.org/10.1016/j.ceramint.2016.01.014. 

[29]     J.C. Hower, K.R. Henke, S. Dai, C.R. Ward, D. French, S. Liu, U.M. Graham, Generation and nature of 

coal fly ash and bottom ash, in: Coal Combustion Products (CCPs): Characteristics, Utilization and 

Beneficiation, Elsevier, 2017: pp. 21–65. https://doi.org/10.1016/B978-0-08-100945-1.00002-2. 

[30]     M.N. Islam, M.A. Noaman, K.S. Islam, M.A. Hanif, Mechanical properties and microstructure of brick 

aggregate concrete with raw fly ash as a partial replacement of cement, Heliyon 10 (2024). 

https://doi.org/10.1016/j.heliyon.2024.e28904. 

[31]     R. Siddique, Properties of concrete incorporating high volumes of class F fly ash and sand fibers, 

Cement Concr. Res. 34 (1) (2004) 37–42, https://doi.org/10.1016/s0008-8846(03)00192-3. 

[32]     R. Upadhyay, V. Srivastava, A. Herbert, P.K. Mehta, Effect of fly ash on flexural strength of portland 

pozzolona cement concrete, J. Academia Industr. Res. (JAIR) 3 (5) (2014) 218–220. ISSN: 2278-5213. 

[33]     K. Musab Manzoor, S. Ehtisham Uddin, Utilization of a mixture of waste plastic materials and granite 

particles as a partial replacement of coarse aggregate in concrete blocks, Mater Today Proc (2023). 

https://doi.org/10.1016/j.matpr.2023.05.095. 

[34]     S. Vanitha, V. Natrajan, M Praba ‘‘Utilization of waste plastic as a partial replacement of coarse 

aggregates in concrete Blocks” Indian Journal of Science and Technology, 8 (2015) 12. ISSN (Print) 

0974-6846 ISSN (Online) 0974-5645. 

[35]     D. Osei, ‘‘Experimental investigation on recycled plastics as aggregate in concrete”, International 

Journal of Structural and Civil Engineering Research, 3 (2014) 2. ISSN 2319-6009. 

[36]     B Jaivignesh, A Sofi. Study on Mechanical Properties of Concrete Using Plastic Waste as an Aggregate. 

IOP Conference Series: Earth and Environmental Science. 80(2017). 012016. 10.1088/1755-

1315/80/1/012016 

[37] H. Korucu, B. Şimşek, T. Uygunoğlu, A.B. Güvenç, A. Yartaşı, Statistical approach to carbon based 

materials reinforced cementitious composites: Mechanical, thermal, electrical and sulfuric acid 

resistance properties, Compos B Eng. 171 (2019) 347–360. 

https://doi.org/10.1016/j.compositesb.2019.05.017. 

[38] K.M. Cantor, P. Watts, Plastics Processing, Elsevier, 2011. https://doi.org/10.1016/B978-1-4377-

3514-7.10012-1. 

[39] S. Rizal, H.M. Fizree, M.S. Hossain, Ikramullah, D.A. Gopakumar, E.C. Wan Ni, H.P.S.A. Khalil, The role 

of silica-containing agro-industrial waste as reinforcement on physicochemical and thermal 

properties of polymer composites, Heliyon. 6 (2020) e03550. 

https://doi.org/10.1016/j.heliyon.2020.e03550. 

[40] I. Muz, M. Kurban, A comprehensive study on electronic structure and optical properties of carbon 

nanotubes with doped B, Al, Ga, Si, Ge, N, P and As and different diameters, J Alloys Compd. 19 

(2019) 32295–9. https://doi.org/10.1016/j.jallcom.2019.06.210. 

[41] J. Hone, M.C. Llaguno, M.J. Biercuk, A.T. Johnson, B. Batlogg, Z. Benes, J.E. Fischer, Thermal 

properties of carbon nanotubes and nanotube-based materials, Appl Phys A Mater Sci Process. 74 

(2002) 339–343. https://doi.org/10.1007/s003390201277. 

https://doi.org/10.1016/j.ceramint.2016.01.014
https://doi.org/10.1016/B978-0-08-100945-1.00002-2
https://doi.org/10.1016/j.heliyon.2024.e28904
https://doi.org/10.1016/s0008-8846(03)00192-3
https://doi.org/10.1016/j.matpr.2023.05.095


14 
 

[42] O. V. Kharissova, L.M. Torres Martínez, B.I. Kharisov, Recent Trends of Reinforcement of Cement 

with Carbon Nanotubes and Fibers, in: Advances in Carbon Nanostructures, 2016: pp. 139–159. 

https://doi.org/10.5772/64307. 

[43] B. Makgabutlane, M.S. Maubane-Nkadimeng, N.J. Coville, S.D. Mhlanga, Plastic-fly ash waste 

composites reinforced with carbon nanotubes for sustainable building and construction applications: 

A review, Results Chem. 4 (2022). https://doi.org/10.1016/j.rechem.2022.100405. 

[44] X. Liu, X. Zhao, H. Yin, J. Chen, N. Zhang, Intermediate-calcium based cementitious materials 

prepared by MSWI fly ash and other solid wastes: hydration characteristics and heavy metals 

solidification behavior, J Hazard Mater. 349 (2018) 262–271. 

https://doi.org/10.1016/j.jhazmat.2017.12.072. 

[45] Y. Liang, Q. Tan, Q. Song, J. Li, An analysis of the plastic waste trade and management in Asia, Waste 

Management. 119 (2021) 242–253. https://doi.org/10.1016/j.wasman.2020.09.049. 

[46] O. Olaitan, S. Dlova, O. Jeremiah, F. Ntuli, W. Kehinde, P. Facal, A. Blencowe, P. Apata, Challenges of 

plastic waste generation and management in sub-Saharan Africa: A review, Waste Management. 110 

(2020) 24–42. https://doi.org/10.1016/j.wasman.2020.04.017. 

[47] C. Grifa, C. Germinario, A. De Bonis, M. Mercurio, F. Izzo, F. Pepe, P. Bareschino, C. Cucciniello, V. 

Monetti, V. Morra, P. Cappelletti, G. Cultrone, A. Langella, N. Hlabangana, G. Danha, V. Moyo, N.G. 

Mguni, N. Hlabangana, G. Danha, Use of coal fly ash to manufacture a corrosion resistant brick, 

Procedia Manuf. 35 (2019) 252–266. https://doi.org/10.1016/j.promfg.2019.05.072. 

[48] S. Kofi, E. Kofi, F. Kotoka, D. Mensah, Plastic wastes to pavement blocks : A significant alternative 

way to reducing plastic wastes generation and accumulation in Ghana, Constr Build Mater. 241 

(2020) 118044. https://doi.org/10.1016/j.conbuildmat.2020.118044. 

[49] P.O. Awoyera, A. Adesina, Materials Plastic wastes to construction products: Status, limitations and 

future perspective, Case Studies in Construction Materials. 12 (2020) 00330. 

https://doi.org/10.1016/j.cscm.2020.e00330. 

[50] S. Bahij, S. Omary, F. Feugeas, A. Faqiri, Fresh and hardened properties of concrete containing 

different forms of plastic waste – A review, Waste Management. 113 (2020) 157–175. 

https://doi.org/10.1016/j.wasman.2020.05.048. 

[51] J. Mendes, A. Walter, D. Gallipoli, Optimization of bricks production by earth hypercompaction prior 

to fi ring, J Clean Prod. 214 (2019) 475–482. https://doi.org/10.1016/j.jclepro.2018.12.302. 

  

 

 

 

 

 

 

 



15 
 

CHAPTER 2 

PLASTIC-FLY ASH WASTE COMPOSITES REINFORCED WITH CARBON NANOTUBES FOR 

SUSTAINABLE BUILDING AND CONSTRUCTION APPLICATIONS: A REVIEW 
 

2. Introduction  
 

Bricks are a large and important part of the building and construction industry and have been 

used as masonry units due to the strength and durability they provide in structural 

applications [1].  The properties of each brick produced depend on the raw materials used 

and their preparation methods [2]. The commonly used bricks are those produced from clay, 

cement concrete and recently fly ash, with clay bricks being the most ancient and widely used 

[3]. Clay is made up of alumina and silica, with the latter being the promoter of particle fusion 

at low temperatures and determines the colour of the brick produced [4]. A typical brick 

manufacturing process involves: i) raw material preparation and mixing, ii) brick formation, 

iii) drying, iv) firing and v) cooling [4].  Clay bricks are either burnt (fired) or unburnt (unfired) 

during the production process. Burnt clay bricks obtain their strength through the sintering 

process, where the crystalline mineralogical phases in clay are fused into the pores to 

strengthen the microstructure of the brick. Their mechanical and durability properties are 

also dependent on the type of clay, water ratio, burning and curing process [5]. Unburnt clay 

bricks are normally incorporated with lime stabilizers to foster the pozzolanic reaction, which 

ensures effective particle bonding and thus aids the stiffness and mechanical strength in the 

bricks [6]. Furthermore, the size and geometry of the bricks have an effect on the mechanical 

performance of wall panels [7]. 

Concrete bricks are another type of conventional brick made of Ordinary Portland Cement 

(OPC), water, aggregates and additives [8]. Aggregates in concrete bricks comprise of 65-70% 

of the total volume of a brick [9]. Thus, the construction industry is considered to be among 

the largest consumer of natural resources. The process of cement production is an energy 

intensive one using about 4 GJ energy per ton of cement produced [10]. Furthermore, 622 kg 

of CO2 per ton of cement is produced as well as other hazardous greenhouse gases during 

fuel combustion and limestone-calcination [10]. Similarly, clay bricks use great amounts of 

natural resources which causes ecological degradation due to excessive movement/removal 



16 
 

of the brick constituent from the environment. The firing process to make burnt clay is 

normally performed at temperatures between 1000-1200 °C and can take up to 5 days [11]. 

This process is almost 300% more energy demanding as compared to the manufacturing of 

concrete bricks [10].   

It is evident that the conventional brick production methods are not environmentally benign 

as they contribute a great deal to carbon emissions and are also not economically sustainable. 

The building and construction industry is the backbone of the economy for many countries, 

and as such, alternative ways of producing bricks in the future that are eco-friendly and 

affordable is crucial. This is also important in addressing the sustainable development goals 

(SDGs), such as SDG 13 of climate action; which has an agenda of reaching net zero carbon by 

2050 [12]. Furthermore, sustainable building materials speaks to SDG 9 & 11 of industry 

innovation and infrastructure and sustainable cities and communities respectively [12]. The 

application of sustainable building materials has a positive impact on both the quality of 

people’s lives and the environment.  

The incorporation of waste into bricks as sustainable construction materials is among the 

studies that have gained considerable attention. This is due to rapid industrialization that 

results in the production of large volumes of waste which causes disposal and handling 

problems. Moreover, significant volumes of construction materials are necessary for the 

creation of infrastructure, such as used in buildings for housing and industries as well as 

facilities water and sewage storage [13]. Thus, sustainable approaches are required to meet 

this ever-growing demand. Zero-cement products known as alkali-activated bricks have been 

developed as sustainable construction materials [14]. These products utilize waste such as 

rice husk, fly ash, bottom ash, cement kiln dust, blast furnace slag and mine tailings [15–17]. 

These bricks are produced by alkali activation (sodium hydroxide (NaOH) or potassium 

hydroxide (KOH)) of the aluminosilicate source material. Alkali-activated bricks are developed 

through a process of geopolymerization. The process relies on the particle size, loss on 

ignition and the alumina-silica content of the source materials [18].  The process results in 

creation of high strength eco-friendly products with a compressive strength between 5-60 

MPa and a density of 1400-1500 kg/m3 [19]. For example, Gavali et al., produced alkali 

activated bricks using co-fired blended ash and stone with a compressive strength of 5–15 

MPa, water absorption of 6–14% and thermal conductivity of 0.35–0.40 W/(m.K) [17]. 



17 
 

Similarly, in another study Gavali et al., used the same industrial waste to make sustainable 

bricks. The alkali activated bricks resulted in bricks with a reduced cost (7%), embodied energy 

(22%) and lower operation energy (13%) compared to bricks used in conventional buildings 

[20].  

Reusing construction concrete waste materials (i.e. ceramics, bricks, blocks, pillars) as 

pozzolanic admixtures in the production of new products is another sustainable approach to 

minimize the depletion of natural aggregates, reduce energy usage and environmental 

pollution. These waste materials mainly come from demolitions, renovations and products 

with defects. They effectively substitute or even replace cement and aggregates in 

construction materials [21]. The waste is crushed and used as either small or large aggregates. 

The replacement ratio is normally between 1- 50 wt.% of cement or aggregates [21]. Gencel 

et al., replaced clay with up to 15 wt.% concrete waste and obtained a compressive strength 

of about 7 MPa, which met the required standard  [22]. There are other waste materials that 

have been incorporated into bricks to reduce the reliance on non-renewable resources. These 

include paper processing residues, cigarette butts, textile sludge, mill scale, marble slurry, 

polystyrene foam, straw, polystyrene fabric, cotton waste, dried sludge, rubber, wood 

sawdust and waste plastic [23,24].  

When hybrid composites, consisting of reinforcement and matrix materials are made, the 

former impart their distinct mechanical and physical properties to the brick to enhance the 

matrix properties [25]. The incorporation of waste materials in construction composites can 

compromise some of the brick properties. For example, plastic waste is known to have a high 

linear burning rate which renders it flammable [26]. Its incorporation in construction 

materials compromises the thermal stability of the product, which is a set standard 

requirement.  

This review seeks to evaluate the application of waste materials, in particular, coal fly ash and 

plastics, their typical replacement ratios and their physicochemical properties, in construction 

composites (bricks). The influence of nanomaterials, especially CNTs, as a reinforcement filler 

in the brick composite made of waste, is also described. As will be seen, the success of hybrid 

composites is based on strong linkages between the individual materials to ensure the 

nanomaterials do not agglomerate in the composite and actually facilitates load transfer. 

Thus, a mechanism is proposed with the aim of understanding the interaction of waste 



18 
 

materials and CNTs within the composite. Lastly, the potential release of CNTs to the 

environment is review. This is to ensure safe usage of CNTs from application to end of life 

cycle of its products.  

 

2.1 Application of waste materials in construction composites 
 

The generation of high volumes of waste is both an environmental and economic challenge. 

It has become unsustainable to manage this waste. Thus, recycling of waste into valuable 

products has become part of the sustainable development goals for many countries. Plastic 

waste and coal fly ash are among the largest contributors of solid waste generated by the 

plastics (polymers) and mining industries respectively [27]. The rate at which these waste 

materials are generated requires large industrial applications, such as in the building and 

construction industries, to utilize them. The application of plastic waste and coal fly ash in 

construction composites is reviewed below.   

 

2.1.1 Waste Plastics 
 

Plastics are among the largest contributors of  the total global solid waste generated, with 

low-density polyethylene (LDPE) being the most produced [28] (Figure 2.1). LDPE plastics 

mainly include household items such as plastic bags, squeezable plastic bottles, frozen food 

containers, and cosmetic and detergent bottles [24].  These items are generally used once 

and discarded, typically in disposal sites. They also later find their way into the ground water, 

riverbanks and ultimately the sea. South Africa ranks as the 11th country (out of 192 countries) 

with the highest level of mismanaged plastic waste entering the ocean [29]. Plastic waste 

pollution  in the costal environment has an adverse effect on aquatic life and poses a health 

risk to humans [30].  Thus, recycling and reusing plastic is a sustainable solution for reducing 

the negative impact the waste has on both the environment and the quality of life. The 

recyclability of plastics is dependent on their type and properties. Plastics are categorized as 

either thermosets or thermoplastics according to their characteristic properties [31]. 

Thermoset plastics have polymers that combine to form irreversible chemical bonds during 



19 
 

curing process. Their strength is reinforced when heated, however, following the heating they 

cannot be reshaped or reheated [32]. Thermoplastics are the most common type of plastic 

and are easier to recycle and reuse due to their ability to be molded and remolded. Current 

thermoplastics soften in the range of  65°C-200°C [33]. The prominent characteristics of 

typical thermoplastics are shown in Table 2.1. These properties inform their suitability in 

application and their recyclability. Polyethylene terephthalate (PET) has the highest melting 

point and would thus require higher energy to recycle compared to the other types of plastics.  

 

 

Figure 2.1: Estimated quantities of waste plastic generated in a year (PET= polyethylene 
terephthalate, HDPE= high density polyethylene, LDPE= low density polyethylene, PP= 

polypropylene, PS= polystyrene) [34] 

 

Table 2.1: Properties of different plastics commonly found as waste in the environment [35] 

Properties of plastics Types of plastics 

Low-density 

polyethylene  

High-density 

polyethylene 

Linear low-

density 

polyethylene  

Polypropylene Polyethylene 

terephthalate 

Density (g/cm3) 0.92 0.94-0.96 0.92 0.905 1.37 

Melting point (°C) 105-111 126-135 130 160 225 



20 
 

Max service temperature 

(°C) 

80 80 50 80 70 

Tensile strength (MPa) 0.2-0.4 0.2-0.4 0.2-0.4 0.95-1.3 0.25 

Notched impact strength 

(kJ/m2) 

No break No break No break 3-30 1.5-3.5 

 

The application of plastics is vast due to their inherent characteristics such as high impact 

resistance, their light weight and flexibility to be molded into different shapes. They are also 

resistant to bacteria [34]. However, they are not biodegradable. Notably, there are benefits 

to incorporating plastic waste in construction materials (Figure 2.2). According to Figure 2.2, 

the usability lifespan of plastics is shorter than its non-usability life span and thus, the plastic 

persists for years as unusable waste rather than as a valuable product. Similar to coal fly ash, 

it is a waste by-product which persist longer as waste than a valuable product due to its 

limited applications.  In contrast, concrete has a longer usable lifespan than its non-usable 

lifespan [36]. Therefore, the insertion of waste such as plastic and fly ash in concrete materials 

is a sustainable approach for recycling and safely disposing waste products. Importantly, the 

limitations of labour intensive sorting, which is a strict requirement in other industries, is 

exempted in some construction applications [27]. This makes the recycling process easier and 

less costly. Furthermore, the utilization of waste in construction materials not only reduces 

pollution but also the number of natural aggregates used, which are responsible for 

environmental degradation and the depletion of natural resources.  

 



21 
 

 

Figure 2.2:The benefits of using waste in concrete, C= concrete, FA= fly ash, P= plastic [36] 

 

2.1.1.1 Processing plastics for application in construction composites 
 

Like any composite material, the controlling factor towards the successful application of the 

composite is the interfacial transition zone between different materials. The substitution of 

natural aggregates such as sand with plastic reduces the density and compressive strength of 

a composite at high substitution ratios, resulting in a loss of strength [37]. This may be as a 

result of the hydrophobic surface of the plastic leading to excess water, thus, increasing voids 

and also of poor bonding between the plastic and the matrix.  Appropriate mix design as well 

as the choice of the type of plastic waste can limit the reduction of strength [8]. Furthermore, 

reduction in compressive strength can be minimized by improving the chemical and physical 

bonding in the concrete mix. Different physical and chemical approaches exist to achieve this 

in construction mixtures.  

Physical approaches, such as shredding plastic to smaller particle sizes is normally the first 

treatment step. During shredding, the size of the plastic is reduced to a range of 2.36 mm-

4.75 mm.  This process has been shown to improve blending in concrete mixes and to 

minimize the loss of compressive strength [38]. Tulashie et al., used shredder blades to obtain 

3 mm plastic waste. The shredded plastic, together with the pit sand or sea sand, were placed 



22 
 

in an extrusion machine in batches. The batches were melted at 175 °C until there were no 

longer flakes [38]. In another study, LDPE was grinded after being melted at 120 °C into a 

semi-liquid form using a pestle and mortar. Then a 4.75 mm sieve was used to achieve the 

desired particle size. Additionally, a rotary grinder was used for shredding plastics to fine 

aggregates of less than 500 µm [39]. Chen et al., proposed a simple method of processing 

waste plastic in concrete mixes. The waste plastics were directly placed in a screw extruder 

and pelletized without any mechanical size reduction. Together with the filler and coupling 

agents, the pellets were again passed through the extruder machine. The initial size of the 

plastics was not important as the plastics melted and different segments stuck together to 

form homogeneous pellets [40]. It is noteworthy that mechanical size reduction, shredding or 

pelletization makes it easier for the fine plastic particles to mix with solid material before 

heating. When the plastic composition, on melting, becomes viscous, it becomes a challenge 

to mix with other aggregates such as concrete, stone and sand [35]. Thus, the mix design is 

important to ensure plastics mix homogeneously throughout the composite to give a good 

matrix and effectively improve the mechanical strength.  

Chemical compounds such as sodium hydroxide and sodium hypochlorite have been used to 

treat the surfaces of plastics prior to mixing with construction mixtures [41]. Plastics are 

expected to mix physically with concrete mixtures rather than form chemical bonds. 

However, oxidising agents such as sodium hydroxide enables a chemical reaction to occur 

between the polymer chains and the concrete by modifying the polymer surface functional 

groups.  The polymers can also contain ionic groups (hydroxide, oxide) that will bind to the 

concrete [41]. Choi et al., treated shredded PET bottles between 5-15 mm with ground 

granulated blast furnace slag (GGBS). The aim was to solidify the aggregate with GGBS and 

foster a reaction which resulted in formation of a pozzolanic material due to the 

strengthening of the interfacial zone between a cement paste and the aggregate [42].  

 

2.1.1.2 Properties of construction composites with waste plastics  
 

The plastic wastes are incorporated as aggregates, binders, or molds in construction materials 

(Table 2.2). As aggregates, plastic wastes were found to be favorable in reducing the cost of 

concrete, improving resistance to impact loading and changing the modes of failure of 

concrete from brittle to ductile [8]. Furthermore, waste plastics can be incorporated in bricks 



23 
 

to improve their porosity while firing and to reinforce bricks under ambient temperatures 

[24]. PET waste plastic was reported to improve resistance to corrosion especially against 

sulfuric acid and also found to make the concrete lighter [43]. In plastic-related studies waste 

materials were prepared by cutting or shredding them into narrow strips or little fibers in a 

range of a few millimeters [24]. Limami et al., used HDPE and PET as fine aggregates (1-6 mm) 

in unburnt clay bricks [44]. Xu et al., used expandable polystyrene (PS) (3 mm) to manufacture 

lightweight concrete and bricks [45].  

In brick and concrete production, waste plastics partially replace one or more of the 

conventional materials. The replacement ratios are typically from 0.1 to 15%, although in 

some studies up to 80% waste plastic has been used as a binder material [46].  For example, 

Hannawi et al., replaced sand with 3%, 10%, 20% and 50% of PC and PET waste plastic in 

mortar [47]. Natural aggregates were partially replaced with PET in burnt clay bricks [48].  It 

was observed that concentrations above 15% of PET led to disintegration of the bricks at high 

temperature. Thus, concentrations less than 15% PET were optimum for fired clay bricks using 

controlled conditions [48].  

 

Table 2.2: Physical properties of recycled plastics and their possible construction 
applications [34] 

Plastic  Physical properties Possible construction application 

High-density polyethylene (HDPE) Rigid Plastic lumber, table, chairs 

Low-density polyethylene (LDPE) Flexible Bricks, blocks, stepping stones, pillars 

Polypropylene (PP) Hard and flexible Aggregate in asphalt mixtures 

Polystyrene (PS) Hard and brittle Insulation material 

Polyethylene terephthalate (PET) Hard and flexible  Fibres in cementitious materials 

Polycarbonate (PC) Hard and rigid Aggregates in cementitious materials 

 

The incorporation of plastic waste into construction composites has environmental benefits 

of reducing pollution and producing new construction materials with distinct properties 

compared to conventional materials.  Belmokaddem et al., studied the mechanical and 

physical properties of concrete containing plastic waste aggregates. Three types of plastic 

waste were used (i.e. HDPE, PET and polyvinyl chloride (PVC)) to substitute natural aggregates 



24 
 

in different amounts (25, 50 and 75%) [49]. It was noted that the use of plastic waste reduced 

the density of the composite and the lowest dynamic elastic modulus was recorded for 

composites containing 75% HDPE, which corresponds to a more ductile composite. The 

lowered dynamic elastic modulus resulted in more deformable concrete composites which 

can be used in flexible structures. An improved thermal insulation was detected using 

composites with 75% PVC and all the composites containing plastic aggregates showed a low 

heat capacity. This suggests the use of these materials as durable concrete pavements that 

alleviate the night-time island effects due to high temperature differences between daytime 

and nighttime. However,  a decline in the compressive strength upon an increase in plastic 

aggregate substitution was noted [49]. It is noteworthy that the concrete and bricks with 

waste plastics still showed a relatively high compressive strength which is acceptable 

according to building and construction standards. Mondal et al., observed a decline in the 

compressive strength of bricks from 34 MPa to 17 MPa upon addition of 0-10 wt. % 

polycarbonates, polystyrenes and mixed plastics [50]. Interestingly, Herki et al., detected no 

change in the concrete compressive strength up to 30 wt. % waste using expanded 

polystyrene replacement [51]. In another study, 3 wt. % polypropylene waste plastic was 

found to be the maximum for sand replacement in burnt clay bricks [52]. 

Plastic replacement optimization studies are required to ensure that the properties of the 

construction composite meet those of the desired application. Table 2.3 shows the 

application of waste plastic in bricks using various replacement ratios. In sand bricks, waste 

plastics are used as binders without the use of cement or pozzolanic aggregates.   

Bricks are classified according to their properties, which informs their application. First-class 

bricks refer to bricks with a compressive strength of about 14 MPa, with a water absorption 

rate of not more than 20% when immersed in water for 24 h. These bricks are great for 

flooring and external walls applications [52]. Bricks with compressive strength of not less than 

7 MPa and water absorption of not more than 22% are classified as second class. This type is 

not necessarily preferred for flooring, but good for walls that do not require plastering. The 

third class of bricks has a compressive strength that ranges between 3.5-7 MPa, with water 

absorption of not more than 26% and are used in ordinary construction. Class four bricks have 

low porosity and  strength of about 15 MPa, this type is not good for building purposes and 

usually applied in road construction [52].  



25 
 

Alaloul et al., investigated the effect of polyethylene terephthalate (PET) and polyurethane 

(PU) on the mechanical strength and thermal stability of interlocking bricks. A compressive 

strength of 5.3 MPa was obtained for a PET/PU brick with a 60:40 ratio, suitable for a non-

load bearing masonry brick wall such as a partition wall. A low thermal conductivity of 

0.153 W/mK was achieved for a PET/PU brick with a 80:20 ratio which signifies a good thermal 

insulating material [53]. Bharathi et al., obtained a compressive strength of 55.91 MPa which 

is 88.59% higher than the standard clay bricks, upon employment of 60 wt. % waste plastic in 

sand bricks. The water absorption was also reduced from 19.8% of conventional bricks to 

0.452%. The properties of waste plastics thus show their potential application in different 

types of innovative building and pavement bricks.  

 

Table 2.3: Recent applications of waste plastic in bricks 

Polymer  Replacement  

% 

Type of 

replacement 

Other 

materials 

Application Brick 

dimensions 

(mm)   

Test performed  Ref 

PP 1-5 Shreds Crushed 

glass, clay 

Aggregate 150 × 100 × 

63  

Water 

Absorption, 

Shrinkage, 

Mechanical 

strength 

[52] 

PET& 

PU 

60:40 Shreds - Binder 64 × 13 × 3.2 compressive 

strength, 

impact, 

flexural 

strength and 

thermal 

conductivity 

[53] 

LDPE 20 Pellets  Sand, 

cement 

Aggregate 40 × 20 × 10  Compressive 

strength 

Water 

absorption 

Drag strength 

[31] 

PET 30, 20 Shreds  Sand Binder 222 × 106 × 

73  

Durability 

through acid, 

[54] 



26 
 

Water 

absorption 

Compressive 

Tensile 

strength 

PP & PE 10-30 Shreds  Sand Binder 200 × 195 × 

100 

compressive 

strength, 

efflorescence, 

water 

absorption, 

soundness, 

fire resistance 

test 

[55] 

LDPE 33.33- 66.67 Shreds Sand Binder 200 × 150 × 

100  

Thermal 

conductivity 

Bending stress 

[56] 

HDPE 100 Shreds - - 240 × 120 × 

60  

Compre