Influence of mineral admixtures on the alkali-aggregate reaction

Different mineral admixtures were used as cement replacements to study the alkali-aggregate reaction between quartzite and cement paste. The experimental program included expansion measurement of the mortar-bar specimens, observation of the microstructure, and analysis of the gel composition. Two Portland cements with different alkali contents were used to prepare mortar bars containing different amounts of natural pozzolan, fly-ash, and slag. Three natural pozzolans with different alkali contents were also utilized to assess the effect of their alkalies on the mortar expansion. The results indicate that not all gels are equally expansive and that mineral admixtures change the composition of the gel. The double layer theory is used to explain the expansion results of the mortar bars containing different levels and types of mineral admixtures. As this theory predicts, the experimental results show a strong negative correlation between expansion and the charge fraction of bivalent cations in the gel.