Aqueous Carbonation of Calcium Silicates With Different Ca/Si Ratios Studied by Solid-State NMR Spectroscopy.

Calcium silicates react readily with CO under aqueous conditions, forming CaCO and silica gel. This is utilized to produce new cement binders and to sequester CO, thereby contributing to a lowering of the CO footprint for the cement industry. The present work investigates aqueous carbonation of three hydraulic and three non-hydraulic calcium silicates with the aim of analyzing the impact of the Ca/Si ratio on the structure of the amorphous silica gel and on the extent and rate of carbonation. This information is obtained from Si NMR experiments, whereas C NMR and FT-IR are used to characterize the polymorphic forms of CaCO formed upon carbonation. The structure of the silica gel is not dependent on the type of carbonated calcium silicate or their Ca/Si ratio. In addition, the amounts of CaCO from TGA analysis match well the theoretical maximum values. Si and Si{H} CP/MAS spectra of a commercial silica gel are very similar to those observed for the carbonated calcium silicates, which strongly suggests that a hydroxylated silica gel without incorporated Ca ions constitutes the silica gel in carbonated calcium silicates. From C NMR and FT-IR, it is found that calcite is the principal CaCO polymorph for all samples carbonated for 6 h. However, aragonite and calcite do co-exist during the initial carbonation (20 min) of γ-CaSiO. Comparison of the carbonation evolution for the hydraulic and non-hydraulic calcium silicates strongly suggests that an early hydration and formation of C-S-H is not a required initial step in the aqueous carbonation process.