Elucidating the accelerated carbonation products of calcium silicates using multi-technique approach

This article features an investigation of the accelerated carbonation products of four calcium silicate phases, namely monoclinic-tricalcium silicate (3CaO·SiO2 or C3S), γ-dicalcium silicate (2CaO·SiO2 or γ-C2S), tricalcium disilicate (3CaO·2SiO2 or C3S2, rankinite), and monocalcium silicate (CaO·SiO2 or CS, wollastonite). During the carbonation reaction, the calcium silicate minerals form calcium carbonate and Ca-modified silica gel. These reaction products were examined using thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), 29Si magic angle spining (MAS) nuclear magnetic resonance (NMR), 13C {1H} cross polarized (CP)/MAS NMR, and dynamic vapor sorption (DVS) techniques. Along with the crystalline forms of CaCO3 (i.e., calcite, vaterite, and aragonite), amorphous calcium carbonate (ACC) was also found to be present in the carbonated calcium silicate systems. Based on the experimental results, it is proposed that the ACC particles were stabilized by the deposition of silica layers on their surfaces. Presence of ACC also affected the pore size distribution of the matrixes, which eventually influenced the diffusion based carbonation rate of the matrixes. Moreover, the degree of carbonation of CS was found to be most effective compared to other calcium silicates in terms of the percentage of the phase reacted for the experimental conditions used in this study.