Insights into the deterioration of C-S-H gels in hardened cement pastes with different NaNO3 concentrations

Abstract Portland cement is widely used for constructions and facilities in underground environments. The durability of hardened cement pastes and concrete is closely related to the long-term stability of C-S-H gels. This study focused on the deterioration of C-S-H gels with different structures, due to different NaNO3 concentrations. The 29Si NMR and FTIR were used to characterize the change in [SiO4]4− in the C-S-H gels and EDS-Mapping was used to detect the change in Ca/Si ratios. The results indicated that Ca2+ preferred to leach out from the hardened cement pastes with high NaNO3 concentrations. The most occurring silicates after accelerated leaching were those of Q3 site and Q4 sites, due to the decalcification of C-S-H gels. Δ(Q3 + Q4), associated with the high polymerization degree, increased with increasing NaNO3 concentrations after accelerated leaching, except that in the matrix with 100 g/L NaNO3 solution. The Ca/Si ratios in the hardened cement pastes with NaNO3 decreased with increasing NaNO3 concentrations. It can be asserted that the C-S-H gels with a low polymerization degree more easily decalcified and polymerized in the deterioration process.