How does fly ash mitigate alkali–silica reaction (ASR) in accelerated mortar bar test (ASTM C1567)?

ASTM C1567 is a commonly used accelerated test method to determine the required dosage of supplementary cementitious materials (SCMs) to mitigate alkali–silica reaction (ASR) in mixtures containing reactive siliceous aggregates. Past research suggested that fly ash and other SCMs inhibit ASR, primarily through alkali dilution and binding. In ASTM C1567, however, the alkalinity of the pore solution is largely influenced by the penetration of NaOH from the external soak solution; and this could erase the beneficial effects of alkali dilution and binding. To better understand why fly ash inhibits ASR in this test, the present study performs a quantitative evaluation of six potential ASR mitigation mechanisms: (1) alkali dilution, (2) alkali binding, (3) mass transport reduction, (4) increasing tensile strength, (5) altering ASR gel, and (6) reducing aggregate dissolution rate. The results suggest that (2), (3), (4), and (6) are the primary mitigation mechanisms, while (1) and (5) show a negligible impact.