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The effect of different local pozzolans, as a supplementary cementitious material (SCM), on the expansion of mortar bar due to Alkali Silica Reaction (ASR) were reported in this paper. Accelerated test on specimens using local volcanic aggregates, rhyolite, was used to investigate and to compare the effects of fly ash (FA) and metakaolin (MK) on the suppression of the length change. In this study, three different percentages of FA, namely, 20, 35 and 50 and three of MK, namely, 10, 15 and 20 were used as cement replacement. The results at 14 days showed slight decrease expansions for 20% FA replacement of 0.073% but the lowest expansions were 0.025% and -0.001%, respectively for 35% FA and 10% MK, compared to 0.176 % of the control mix. Increase the SCMs to 50% FA and 1520% MK yielded reduction of expansion to the innocuous level. Results from 28 days acceleration registered a slight increase in expansion for FA mixes of 35 and 50% and all MK mixes, but still less than 0.10%, the threshold for innocuous level. Chemical composition analysis revealed decrease in CaO/SiO2 and CaO/Al2O3 ratio of the cementitious systems. This correlated with the expansion reduction. But alumina in Al2O3/SiO2 ratio showed the dominant effect on ASR suppression. Microstructures of all materials and casted specimens were also studied in detail. Considering all aspects, these SCMs, used in suitable amount were considered good candidates for ASR prevention in new concrete structures for both short and long term

Influence of Type and Compositions of SCMs on Expansion of Mortar Bars from Alkali Silica Reaction