Dissolution of Poorly Crystalline Magnesium Silicate Hydrate: Mechanistic Insights from Solution-Based Measurements

Cement dissolution is important for understanding the service life and durability of concrete. Magnesium silicate hydrate (M-S-H)-based cement has emerged as a potential substitute for ordinary Portland cement (OPC), but its dissolution kinetics remains poorly understood. In this study, M-S-H precipitates with Mg/Si molar ratios between 0.51 and 1.18 were dissolved in a batch reactor, and the time-resolved solution composition was measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Calcium silicate hydrate (C-S-H), the main binding phase in OPC, was studied in parallel experiments for comparison. Dissolution rates of M-S-H based on Mg and Si release generally increased with the Mg/Si ratio, consistent with the decreasing extent of silica polymerization. The Si-based dissolution rate of C-S-H is higher than that of M-S-H across the Mg/Si ratios examined in this study, and the Ca-based rate for C-S-H is higher than the Mg-based rate for M-S-H when Mg/Si ratios are equal to or lower than the Ca/Si ratio. Furthermore, time-dependent dissolution rates were mapped to the corresponding solution saturation states and modeled using an affinity-based rate law. Comparison with atomic force microscopy (AFM)-derived dissolution rates revealed rates substantially higher than those obtained from the solution-based measurements. The systematic differences suggest that apparent dissolution rates may include contributions from both molecular-scale bond breaking and interfacial processes involving particle detachment. These observations improve our understanding of silicate hydrate dissolution mechanisms and provide insight into the long-term durability and viability of M-S-H as an alternative to C-S-H-based OPC.