Short-term thermal activation of clay-bearing aggregates for alkali-silica reactivity mitigation

This study investigates why aggregates from İstanbul’s Thrace Formation exhibit alkali-silica reactivity (ASR) and demonstrates that their behavior is governed by the fine reactive silica dispersed within a phyllosilicate-rich matrix. Mineralogical and thermal analyses show that a short 800 °C/7-min treatment induces partial dehydroxylation of muscovite and clinochlore, producing structural disorder similar to the early stages of calcined-clay reactivity, but without full amorphization. This controlled modification alters the clay-rich matrix configuration, reduces alkali accessibility, and limits the dissolution of micro-scale silica. ASTM C1260 results confirm a substantial reduction in expansion, with all treated aggregates remaining below critical limits. SEM-EDS analyses further show that ASR gels formed with treated aggregates contain lower alkali-to-silica ratios, corresponding to reduced swelling capacity. Mechanical performance is preserved or improved. Overall, the findings reveal that aggregate-level mineralogical modification-analogous in principle to pozzolanic activation-offers an effective and practical pathway to mitigate ASR in clay-bearing sandstone aggregates.