Influence of seawater on the microstructure of calcium-silicate-hydrate (C-S-H) gels with varying Ca/Si ratios based on alite-silicon dioxide system

Seawater sea-sand concrete has emerged as a promising sustainable construction material, leveraging abundant marine resources to mitigate freshwater dependency and address global sand scarcity while offering economic and ecological advantages. Despite its potential, the hydration mechanisms governing ordinary Portland cement (OPC) blended with supplementary cementitious materials (SCMs) in seawater remain unresolved, impeding predictions of long-term durability. This study investigated the influence of seawater on the alite (C₃S, the primary clinker in OPC)-silicon dioxide (SiO₂, the main composition of SCMs) hydration system, employed as a simplified model for OPC and SCMs, with a particular emphasis on hydration kinetics, hydration products, and the microstructural evolution of calcium-silicate-hydrate (C-S-H) gels with varying calcium-to-silicon (Ca/Si) ratios. Comparative analyses of specimens mixed with seawater or deionized water for 1 and 28 days revealed that seawater significantly accelerated the hydration rate of C₃S-SiO₂ system, an effect amplified by the incorporation of silica and sustained throughout hydration. Hydration in seawater led to the formation of unique products, including sodium chloride and gypsum, with increased silica content enhancing their amounts. Furthermore, seawater exposure altered the C-S-H gel microstructure, enhancing the overall amount. However, higher silica contents (lower Ca/Si ratios) promoted an increased adsorption of sodium and chloride ions, and consequently decreased the mean molecular chain length and polymerization degree of C-S-H. The high silica content also degraded the micromechanical properties, which was reflected by the shifts towards lower-density C-S-H and a greater decrease in average modulus in the seawater system compared to the deionized water system.