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

Calcium silicates react readily with CO₂ under aqueous conditions, forming CaCO₃ and silica gel. This is utilized to produce new cement binders and to sequester CO₂, thereby contributing to a lowering of the CO₂ 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 ²⁹Si NMR experiments, whereas ¹³C NMR and FT-IR are used to characterize the polymorphic forms of CaCO₃ 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 CaCO₃ from TGA analysis match well the theoretical maximum values. ²⁹Si and ²⁹Si{¹H} 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 ¹³C NMR and FT-IR, it is found that calcite is the principal CaCO₃ polymorph for all samples carbonated for 6 h. However, aragonite and calcite do co-exist during the initial carbonation (20 min) of γ-Ca₂SiO₄. 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.