AbstractThis study investigates the mechanisms of alkali-silica reaction (ASR) mitigation by supplementary cementitious materials (SCMs) under accelerated mortar bar test (AMBT) conditions. The study compares the effect of traditional SCMs (fly ash, slag, metakaolin, and silica fume) on ASR expansion, calcium silicate hydrate (C-S-H) composition, and portlandite consumption as well as on the availability of silicon and aluminum in solution. Results show that at typical SCM replacement levels for effective ASR mitigation (15% metakaolin, 25% fly ash, and 65% slag), the Si/Ca and Al/Si ratios of C-S-H are increased to comparable values, suggesting that at these dosages the SCMs contribute almost equivalent amounts of silicon and aluminum in solution. Studies of blended cement + SCM pastes show that the order of pozzolanicity is as follows: silica fume > metakaolin > fly ash > slag, which is consistent with the order of efficacy of SCMs in mitigating ASR expansion and the measured concentrations of silicon in solution. Solubility studies of the SCMs showed formation of sodium aluminum silicate hydrate (N-A-S-H) in fly ash and metakaolin and formation of calcium aluminum silicate hydrate (C-A-S-H) in slag after 28 days of exposure to AMBT conditions. This highlights the role of alkali activation of SCMs in ASR mitigation under AMBT conditions.