Evaluation of blends of high and low calcium fly ashes for use as supplementary cementing materials

The present paper outlines the results of a research attempt aimed at developing and evaluating the performance of ternary blended cements, incorporating mixtures of two different types of fly ash (of high and low calcium content). The main target of this study was to investigate whether and by what means, the introduction of a certain type of fly ash into a fly ash-cement (FC) matrix containing a different type of ash, can improve the performance of the initial binary system. For achieving this, new pozzolans were prepared by mixing, in selected proportions, a high lime fly ash with an ash of lower calcium content. The efficiency of the new materials was examined in terms of active silica content, pozzolanic activity potential, strength development, k-values and progress of the pozzolanic action by means of fixed lime capabilities. The results obtained demonstrated that the mixtures containing equal amounts of each fly ash were the most effective for moderate cement substitution, whilst for higher replacements the intermixture possessing the highest active silica content shows supremacy at almost all hydration ages. The superior performance of the ternary fly ash blends was mainly attributed to synergistic effects detected for all the ashes utilized. These were quantified in each case and almost linear correlations were obtained with the k-values of the most efficient ternary mixes.