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

ABSTRACT Calcium silicates react readily with CO 2 under aqueous conditions, forming CaCO 3 and silica gel. This is utilized to produce new cement binders and to sequester CO 2 , thereby contributing to a lowering of the CO 2 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 29 Si NMR experiments, whereas 13 C NMR and FT‐IR are used to characterize the polymorphic forms of CaCO 3 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 3 from TGA analysis match well the theoretical maximum values. 29 Si and 29 Si{ 1 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 13 C NMR and FT‐IR, it is found that calcite is the principal CaCO 3 polymorph for all samples carbonated for 6 h. However, aragonite and calcite do co‐exist during the initial carbonation (20 min) of γ‐Ca 2 SiO 4 . 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.