Dissolution kinetics of C–S–H gel: Flow-through experiments

• The kinetics of the C–S–H gel dissolution was studied at room T.
• The gel dissolution evolves to congruent dissolution of tobermorite’s Ca/Si ratio.
• A dissolution rate law for C–S–H gel is proposed.
• The rate law does not include dependence on pH.

C–S–H gel dissolution kinetics was studied by means of flow-through experiments under CO2-free atmosphere at room temperature (23 ± 2 °C). The starting C–S–H gel was obtained by hydration of commercial C3S (3CaO·SiO2) and consisted of 67 wt% C–S–H with a Ca/Si ratio of 1.7 ± 0.1 and 33 wt% portlandite. Based on the changes in aqueous chemistry and in the C–S–H solid, it was observed that the C–S–H gel dissolves incongruently when the Ca/Si ratio is high and congruently as the Ca/Si ratio decreases to the tobermorite stoichiometric Ca/Si ratio of 0.83. First, a dissolution rate law for C–S–H gel with Ca/Si ratio equal to 0.83 is proposed based on the dissolution rates normalized to the final BET surface area. Additionally, reactive transport modeling of the changes in aqueous chemistry allowed the fitting of the rate constants for C–S–H with Ca/Si ratio ranging from 1.7 to 0.83. Solid examination by SEM-EDX and EPMA before and after the experiments showed some variability of the Ca/Si ratios of the analyzed particles, suggesting the existence of compositional domains with variable Ca/Si ratios. 29Si MAS NMR spectra showed an increase in polymerization of the reacted C–S–H, and also the formation of Si-rich domains in some cases, mainly under slow flow conditions. Inhibition of the rates in these cases will have to be further investigated.