3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The influence of two South African fly ashes on the engineering properties of concrete.(1991) Grieve, Graham Robert Hugh.The South African cement producers would like to utilise cement extenders in cement to increase manufacturing capacity at a relatively low cost. A" understanding of the implications such a step might have for the cement user was required. A study was made of the performance of portland cement blends made with fly ash derived from the two commercial sources in South Africa that were being utilised by 1991. The performance of the blends was compared against the performance of ordinary portland cement, the cement type to which the majority of users in South Africa were exposed in the past. An extensive literature study was provided which covered the latest research aspects .ot the use of fly ash in many other parts or the world as well as mformation from local South African research reports. The ash concrete or mortar properties studied in the laboratory tests were as follows: • workability • rheology • setting time bleeding • heat of hydration The following properties of hardened concrete were studied in laboratory and simulated site exposure tests: • compressive strength flexural strength elastic modulus • shrinkage • creep • permeability • abrasion resistance • carbonation Some preliminary investigations were made into the effect that the use of fly ash had on the effectiveness of chemical admixtures, and into the compressive strength development of ternary (three component) blends. As far as possible, use was made of standard SABS, BS, ASTM, DIN or RILEM test methods although it was necessary to use other non-standard or modified test methods in some cases to provide the necessary information. Appropriate references or details have been provided where methods have deviated from standard methods. Recommendations were made regarding the level of fly ash replacement which was believed to be practical for site use for two categories of users, nameiy unsophisticated users with a low !evel of understanding of concrete technology and relatively crude methods of site batching and mixing, and sophisticated users with access to higher levels of technology and equipment.Item Effect of fly ash composition on the synthesis of carbon nanomaterials(2016-05-10) Matshitse, Refilwe Manyama StephinaFly ash is a by-product generated during the combustion of coal for electricity gen- eration. Previous studies have shown that various waste fly-ashes (Japanese, Saudi Arabian, and Australian) contain trace quantities of transition metal elements which can be used in the synthesis of shaped carbon nanomaterials. A survey of the litera- ture has shown that no attempts to correlate the composition of a particular coal fly ash and the type or quantity of carbon nanomaterials (CNMs) that can be synthesized has been made. Neither has the effect of leached fly ash been tested for the synthesis of CNMs. Hence a study on the effect of the chemical composition of South African fly ash (collected from ESKOM’s Duvha power station in Mpumalanga) upon the chemical vapour deposition (CVD) synthesis of carbon nanostructures is justified. Untreated and chemically treated fly ash samples were used as catalysts in the CVD method to synthesize CNMs. In the latter case selective leaching experiments were conducted on the fly ash samples under acidic, basic and neutral conditions. Op- timal CNM synthetic conditions were achieved by initially flowing H2 gas to re- duce the metal oxides within the fly ash catalyst followed by the introduction of the carbon source (C2H2) at a temperature range of 600 - 800 ◦C. All samples were quantitatively and/or qualitatively characterized. Inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray fluorescence (XRF) techniques were used to quantify the metal ions which were removed from the fly ash samples. Fur- thermore, qualitative studies were conducted with (PXRD, and laser Raman spec- troscopy), morphological and surface area characterization techniques (SEM, TEM and BET) were used to investigate the synthesis of CNMs from the untreated and chemically treated fly ash samples. Results have shown that carbon nanofibers (CNFs) of different geometric morpholo- gies were synthesized at an optimal yield temperature of 700◦C. A combination of smooth, thin, wide, spiral platelet-like, stacked cup, and fishbone morphologies were reported when the untreated fly ash catalyst was used. Fly ash catalysts under acidic, basic and neutral treatments showed CNFs of varying sizes and specific morpholo- gies. Smooth graphitic platelet-like, stacked cup and platelet-like CNFs were re- ported when the fly ash catalyst was leached with neutral, basic and acidic solutions. Carbon nanofibre sizes with the IG ID ratios were reported as follows 115 nm (1.092), 52 nm (0.799), and 200 nm (0.960) under neutral, basic and acidic mediums respec- tively. Surface areas (41, 14 and 7) m2/g for the CNFs that were synthesised from the neutral, basic and acidic treated fly ash catalysts were related to the selective leaching of metals. The quality and quantity of CNFs obtained under acidic medium were associated with the leaching of iron (5.6%), cobalt (1.7%), calcium (20.4%), copper (12.5%), chromium (4.6%), magnesium (23.3%), manganese (15.2%) and nickel (2%) from the fly ash catalyst. Under a basic medium only chromium (0.2%), calcium (0.3%) and copper (7.4%) were removed. Significantly the best quality of CNFs was ob- tained when fly ash was treated under neutral conditions. Metal ions such as: cal- cium (3.7%), copper (3.8%), chromium (0.1%), and magnesium (1.3%) were mod- erately removed from the ash matrix. Therefore, composition and quantity of the fly ash catalyst had an effect on the synthesis of CNFs.