Effect of fly ash on Portland cement systems

A typical low-calcium fly ash was used as additive in mortar, replacing part of the volume either of Portland cement or aggregate. The development of the strength, heat, porosity, bound water, and calcium hydroxide content was measured. In aggregate replacement higher strengths were observed after 14 days, whereas in cement replacement higher strengths were observed after 91 days. The final strength gain was found to be roughly proportional to the content of active silica in the concrete volume. Bound water content and porosity results showed that fly ash reacts with calcium hydroxide, binding small amounts of water. On the basis of the experimental results, a simplified scheme describing the chemical reactions of the low-calcium fly ash in hydrating cement is proposed. Using the reaction stoichiometry, quantitative expressions for the estimation of the chemical and volumetric composition of a fly ash concrete are proposed. The model expressions can be applied in mix design and concrete performance prediction.