Assessment of the Performance of Corn Cob Ash as a Partial Replacement for Portland Cement in Concrete

Abstract

The production of Portland cement is associated with the release of greenhouse gases especially carbon dioxide which is estimated to be about a ton per every ton of clinker produced contributing to climate change. Several mitigation strategies have been proposed but the most viable remains the use of supplementary cementitious materials as partial replacement for Portland cement. There have been considerable success with the use of some industrial by-products (fly ash and slag) and natural materials (calcined kaolin clay) as supplementary cementitious materials. However, the non-availability of these by-products in countries like Nigeria calls for the investigation of locally available substitutes. Supplementary cementitious materials are either pozzolanic or possess latent hydraulic properties making them choice materials as partial Portland cement replacement. The classification and choice of a material as supplementary cementitious material lies in the understanding of their characteristic properties (chemical composition and mineralogy) and subsequent performance in cementitious systems. The performance of corn cob ash calcined at 700°C and 800°C as partial replacement for Portland cement (PC) compared to Portland cement and fly ash (FA) was studied with the following objectives: to determine the influence of calcination temperature on the reactivity of corn cob ash; investigate the effects of corn cob ash content at varying w/b ratio on the i) hydration reaction of Portland cement; ii) the compressive strength of concrete iii) drying shrinkage strain of mortar iv) penetrability of concrete v) microstructure of concrete The laboratory investigation involves using corn cob ash to partially replace Portland cement at two levels of 15% and 30% by mass using two w/b ratios of 0.4 and 0.6 at a water content of 205 kg/m3. The corn cob ashes in binary combination with either Portland cement or fly ash were used to prepare concrete samples which were used for the determination of compressive strength, durability index tests (namely oxygen permeability, water sorptivity and chloride conductivity) to assess the durability of concrete, and microstructural development. The concrete was designed using the South African Cement and Concrete Institute method of mix design. Also, mortar samples made from one part of cement to three parts of sand were prepared for the investigation of drying shrinkage and estimation of strength activity index while paste samples were prepared for determining reactivity of the ashes and effect on Portland cement hydration. iv Reactivity of the ashes was measured using both strength activity index and R3 reactivity test. Strength activity index was estimated from the compressive strength of 50 mm cube mortars at the ages of 28, 56 and 90 days of curing in order to better understand the mechanism of reaction of the ash, while R3 test was performed on model paste using the bound water approach at the age of 7 days. The amorphous content of ash calcined at 700°C and 800°C is 1.9% and 2.4% respectively while the gain in strength of mortar cubes prepared with only Portland cement, Portland cement/fly ash, Portland cement/corn cob ash calcined at 700°C and 800°C between 28 and 90 days are 14%, 24%, 10% and 9% respectively. The surface area of the Portland cement, fly ash, corn cob ash calcined at 700°C and at 800°C is 2.38, 2.224, 3.122 and 2.751 m2/g respectively. The results indicate that the corn cob ashes (CCA) calcined at 700°C (C700) and 800°C (C800) are low reactive materials with limited pozzolanic reactivity while the mechanism of reaction is largely influenced by filler effect due to finer particle size than plain PC. The compressive strength of concrete containing 15% CCA calcined at 700°C and 85% Portland cement ranges between 40 to 58 MPa between 3 and 90 days of curing at w/b ratio of 0.4 compared to 56 to 83 MPa for Portland cement concrete and 48 to 82 MPa for fly ash/Portland cement concrete at the same replacement level. The porosity of concrete containing 15% C700 and C800 at w/b ratio of 0.4 is 9.66 and 6.9% respectively at 28 days of curing compared to 8.37% for PC and 6.52% for fly ash at the same age and replacement level. The presence of CCA affects the heat of hydration of plain PC by prolonging the induction phase by about 12 hours which delayed the evolution of main heat peak. The use of CCA lead to a reduction in strength compared to PC/FA system with compressive strength decreasing with increasing w/b ratio and increasing PC replacement level. CCA has a high potassium oxide content which is highly soluble with a high concentration in the pore solution of concrete. CCA influences volume change leading to a high drying shrinkage strain compared to plain PC and FA. CCA also affects the durability of concrete by increasing the penetrability of concrete which increases with increasing ash content. In terms of the studied properties of cementitious systems, there is no marked difference in the effect of C700 compared to C800 while the effects recorded becomes significant with increasing PC replacement level. In comparison to FA, the effect of CCA on the properties studied was inferior due to the largely crystalline nature resulting in limited pozzolanic activity