Equilibrium constraints on alkali-silica reaction phase stability across CaO–SiO₂–Al₂O₃ binder compositions

Alkali–silica reaction (ASR) mitigation by binder design requires comparison at a fixed total alkali inventory and under clearly defined calcium-availability conditions. This study applies a two-step equilibrium modelling approach in the CaO–SiO₂–Al₂O₃ space to quantify how binder chemistry influences hydrate assemblage, pore-solution composition, and ASR phase stability. Using GEMS-Selektor v3.7.0 with Cemdata18 at 25 °C, 79 binder points were computed at w/b = 0.45 with a fixed total alkali inventory of 1.25% Na₂Oeq. Step 1 provides equilibrium hydrate and pore-solution chemistry. Step 2 adds reactive SiO₂ (0.25–1.00 mol per 100 g binder) and evaluates CNASH and shlykovite under CH-buffered and portlandite-suppressed conditions. CH-buffered systems favour larger Ca-bearing ASR product volumes, whereas Ca-limited systems reach high shlykovite supersaturation at small precipitated volumes. Concrete prism test data compiled from the literature show lower average expansions for SiO₂–Al₂O₃-enriched binders. The results identify regions of binder composition associated with lower expansion in the compiled dataset, but they should be interpreted as comparative trends rather than as mix-design limits. • Equilibrium modelling maps ASR phase stability in CaO–SiO₂–Al₂O₃ binders. • Fixed alkali basis isolates binder chemistry effects on hydrate assemblage. • Calcium-buffered and calcium-limited conditions yield distinct ASR regimes. • High ASR supersaturation can occur at low precipitated phase volumes. • Literature CPT data align with lower expansion for SiO₂–Al₂O₃-rich binders.