Experimental study on the stability of C-S-H nanostructures with varying bulk CaO/SiO2 ratios under cryogenic attack

The stability of C-S-H nanostructures with varying bulk CaO/SiO2 (C/S) ratios under cryogenic attack (-170 degrees C) was experimentally studied by means of XRD, TG, SEM-EDS, Si-29 MAS NMR, and FTIR. An efficient hydro-thermal method was adopted to synthesize C-S-H by accelerating the thermodynamic equilibrium. The results show that the C-S-H atomic structure with a lower bulk C/S ratio is more stable under cryogenic attack. Ca2+ is more likely to leach out from C-S-H with a high C/S ratio. An increase in the bound water fraction was also observed in C/S1.5. At the atomic level, the number of interlayer Ca2+ per tetrahedral site in C/S1.5 significantly decreased from 0.201 to 0.083. The number of vacant bridging sites in C/S2.0 declined from 0.208 to 0.178, probably indicating a slight polymerization. Based on the deconvolution results of Si-29 MAS NMR spectra, three reaction equations were proposed to characterize these observed variations.