Effects of fly ash on compressive strength of structural concrete

A systematic study of fly ash effects on the compressive strength of concrete is presented, where an experimental program has been designed to examine the role of fly ash in influencing the compressive strength of concrete. Various percentages of fly ash are added to concrete mixtures as partial replacements of both cement and fine aggregate or sand. The compressive strengths of concrete aged 7 and 28 days are measured on the specimens with replacement ratios of 15%, 30% and 45% fly ash as well as on fly ash free concrete specimens. These observed compressive strengths are compared in terms of replacement method, i.e. cement or sand replacement, and replacement ratio. It is found that while for both replacement methods, an optimum fly ash ratio exists where the maximum compressive strengths of fly ash containing concrete can be achieved, the sand replacement method produces superior concrete in terms of compressive strength. The experimental results also demonstrate that in comparison with their fly ash free counterparts, the compressive strength of fly ash containing concrete improves more slowly but more strongly with aging. Furthermore, the observed phenomena are discussed in terms of hydration reaction and interface behaviours