An investigation into the use of weathered fly ash (WFA) and coal gasification ash (CGA) as partial replacements for portland cement in concrete

Abstract

The cement industry has recognised the need to reduce the output of CO2 emissions in the atmosphere. This investigation considers the use of alternative binders. Three supplementary cementitious materials (SCMs), namely, fly ash (FA), coal gasification ash (CGA), and weathered fly ash (WFA) were used to replace 10%, 15%, and 30% of Portland cement (PC) by mass. Each mix used water/binder (w/b) ratios of 0.50 and 0.60. 100% PC concrete was the primary control and FA concrete the secondary control. Samples were subjected to slump, compressive and tensile strength, durability and shrinkage tests. The compressive strength results of WFA blends at 28-days of testing were observed to be equal to or greater than those of FA blends. At 56-days the compressive strength results of WFA blends measured below those of 28-days. The compressive strength results of 15% CGA blend at 28-days of testing was equal to that of 15% FA with 0.50 w/b and greater than that of 15% FA with 0.60 w/b ratio. The 56-days strength of 30% CGA-0.50 and 30% CGA-0.60 were greater than the corresponding 30% FA blends at 28-days of testing. Indicating an increase in strength with age for CGA blends. Due to the loss of compressive strength at 56-days for WFA, CGA was concluded to be the Sasol ash with the higher potential to be used as a partial replacement of PC in concrete for strength purposes. The durability index tests conducted had varying overall results, CGA blends were less permeable to oxygen, FA blends had the lowest absorption rates and WFA blends were the most resistant to chloride ingress. In conclusion, from the observed results, CGA and WFA can be used as partial replacements of PC in concrete, but further research is required. The research into WFA and CGA as supplementary cementitious materials will assist in gaining knowledge on their impact on the concrete characteristics to allow for greater exploitation