EFFECTS OF PARTIAL REPLACEMENT OF PORTLAND CEMENT WITH FLY ASH ON THE STRENGTH AND DURABILITY CHARACTERISTICS OF CONCRETE

Abstract

The high cost and environmental damage resulting from cement production, such as the emission of carbon dioxide (CO2) into the atmosphere, has led to the search for alternative binders that are friendly and also contribute to waste management. In this research, the effect of fly ash blended with Ordinary Portland Cement (OPC) on selected properties of concrete was assessed. The effect of fly ash (FLA) on concrete was investigated for replacement levels of 5% to 35% by weight of OPC at 5% intervals. The slump height and compacted weight of fresh concrete for a 1:2:4 mix-ratio and 0.55 water-cement ratio were determined. A total of ninety-six 150mm x 150mm x 150mm cubes of hardened concrete were tested for compressive strength at 7, 14, 21, and 28 days for all specimen types using standard procedure. A further test on a total number of sixty-four concrete cylinders was carried out to determine the depth of penetration of chloride ion to the concrete samples, at different curing ages of 14, 21 and 28 days. The results obtained showed a gentle but consistent increase in the slump of concrete as FLA replacement level increased up till 25%. However, the Analysis of Variance (ANOVA) result showed that such increase was not statistically significant at 5% level of significance. Compressive strength test also showed that at each replacement level of OPC by fly ash, the compressive strength increases as the curing age increases. Furthermore, for all replacement levels, the compressive strength was lower than that of the control concrete. However, there was no significant difference between the strength of the control and that of concrete with partial replacement of OPC by fly ash up till 25% replacement level after which the difference in compressive strength became significant. The research thus, established FLA as capable of successfully replacing OPC partially in concrete production up till 25% without loss both in the workability and in the compressive strength. The result of the chloride ion penetration test revealed that at 28 days, the penetration depth of chloride ions remained essentially the same with increase in the percentage replacement of fly ash until at 20% level when there was an abrupt reduction in the depth of penetration. Furthermore, the ANOVA and the follow-up Least Significant Difference tests confirmed that there was a significant difference between the control and the 20% replacement level. The implication of the observed trend in the depth of chloride ion penetration is that the durability of concrete in aggressive environment is enhanced by the addition of fly ash at an optimal level of 20% by mass of OPC.