Carbonation behaviors of calcium silicate hydrate (C-S-H): Effects of aluminum

The cement systems containing supplementary cementitious materials (SCMs) usually present lower resistance to carbonation compared with ordinary Portland cement (OPC) systems. However, the contribution of the main component in these systems, calcium aluminum silicate hydrate (C-A-S-H), to carbonation still remains unclear. In this study, the carbonation behaviors of C-S-H and C-A-S-H were compared by accelerated carbonation experiments. FTIR, XRD, TGA, NMR, and electron microscope were integrated to reveal the kinetics and thermodynamics of C-(A)-S-H carbonation. It is concluded that the carbonation of C-(A)-S-H experiences three periods including dissolution, diffusion, and stationary period, turning into calcium carbonate with different polymorphs, calcium and aluminum modified silica gel. In the end product of carbonation, aluminum modified silica gel is observed to have a similar Al/Si to the initial C-A-S-H. But such small incorporation of aluminum into C-S-H structure (maximum Al/Si ∼ 0.1) can hardly have a significant effect on its resistance to carbonation.