Simultaneous formation and decomposition of calcium silicate hydrate under carbonation in water

Carbonation curing captures CO₂ while rapidly gaining strength, as calcium silicate hydrate (C-S-H) precipitates and carbonates simultaneously. The simultaneous formation and decomposition of C-S-H under carbonation was investigated systematically in C-S-H suspensions at an initial pH of 13.5. Four distinct periods were observed: initial carbonation, rapid decomposition, slow ongoing and quasi-stable period. During the initial time, large quantities of C-S-H formed at maintained high pH, Subsequently, calcium in the liquid and C-S-H interlayer was consumed. With Ca/Si decreased to below 0.78, the rapid decomposition period proceeded with the decomposition of main layer, while silicate chains dissolved. Silica gel is formed in the slow ongoing period with pH < 11.0, which dominates with calcium carbonate in the quasi-stable period at a stable pH of 10, remaining small amount of residual C-S-H. Understanding the simultaneous formation and carbonation process of C-S-H contributes to understanding the microstructural evolution of cements during carbonation curing.