Alkali-carbonate reaction in concrete - Microstructural consequences and mechanism of expansion

A detailed microstructural investigation of a concrete expanding due to alkali‑carbonate reaction (ACR) shows that the cement paste adjacent to reactive aggregate particles is carbonated, which leads to a sulfur redistribution resembling internal sulfate attack. Simultaneously to dedolomitization, partial dissolution of illite occurs in aggregate particles leading to the formation of brucite, hydrotalcite, magnesium-silicate-hydrate (M-S-H) and calcium‑aluminum-silicate-hydrate (C-A-S-H), in addition to calcite and thus to a substantial increase in solid volume. Thermodynamic modelling indicates that the simultaneous presence of illite and dolomite can accelerate the reactions within the aggregates. No alkali-silica reaction (ASR) products are observed. Dolomite, illite and all reaction products display a negative ζ-potential at high pH generating repulsive forces during dedolomitization. Together with the substantial increase in molar volume, the concrete expansion can be mainly attributed to the solidification pressure of hydrotalcite and M-S-H formation.