Formation of magnesium silicate hydrate (M-S-H) at pH 10 and 50°C in open-flow systems

Magnesium silicate hydrate (M-S-H) is a poorly crystalline Mg-silicate phase formed under alkaline conditions at low temperatures (T < 100 °C). Its formation has been studied in closed systems but not in open-flow systems, which better represent natural surface/subsurface environments. Here, MgO powder was used in reactions at pH ∼10 and 50 °C in flow-through experiments to study the formation of M-S-H as a function of aqueous Si concentration (1.5, 0.15, and 0 mM). Consumption of aqueous Si during precipitation of M-S-H resulted in an increase in the dissolution rate of the primary material (Mg hydroxide). Steady-state Si concentrations were used to calculate the dissolution rate of Mg hydroxide and the precipitation rate of M-S-H. Analyses of retrieved solids by electron microscopy and nuclear magnetic resonance spectroscopy confirmed the formation of M-S-H, although X-ray diffraction patterns provided no clear evidence of the presence of M-S-H because of the small amount precipitated and its nano-crystallinity. The chemical composition (Mg/Si ratio) of the M-S-H varied with the aqueous Si concentration of the injected solution. Mg/Si ratios of 1.00 ± 0.09 and 1.59 ± 0.15 were obtained with Si concentrations of 1.5 and 0.15 mM, respectively. Results indicate that the formation of M-S-H is feasible under Earth surface conditions, with dissolved silica coexisting with Mg-bearing minerals at alkaline pH.This work has been financially supported by Grants-in-Aid for Scientific Research A (No. 19H00878) from the Japan Society for the Promotion of Science (JSPS) to T.S. and T.O, and SPRING (No. JPMJSP2119) from the Japan Science and Technology Agency (JST) to Y.N. IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S funded by MCIN/AEI/10.13039/501100011033). The TEM and STEM studies were conducted in the Faculty of Engineering, Hokkaido University and supported by the Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We thank Jordi Bellés (IDAEA-CSIC) for experimental assistance, Maite Romero (Scientific and Technical Services of the University of Barcelona) for ICP–OES analyses, Ryo Ota (High Voltage Electron Microscope Laboratory, Faculty of Engineering, Hokkaido University) for STEM–EDS analyses, Naoya Nakagawa (The Frontier Chemistry Center, Faculty of Engineering, Hokkaido University) for NMR analyses, Kosuke Nakamura (Thin-Section Lab., Faculty of Science, Hokkaido University) for preparation of polished sections, and Tatsuya Fujimura (Graduate School of Engineering, Hokkaido University) for SEM observations. We also thank an anonymous reviewer for their constructive comments that helped us to improve the quality of the manuscript.Peer reviewe