Strength degradation of seawater-mixed alite pastes: an explanation from statistical nanoindentation perspective

Interest in investigating seawater concrete has been increasing in recent years, but most studies of the strength degradation of seawater mixed concrete have only been carried out on the macro-scale. In this study, alite, the main phase in ordinary Portland cement, was hydrated with various salt solutions (NaCl, MgCl₂, CaCl₂ and simulated seawater) with a solution-to-binder ratio of 0.5 to explore the effects of common cations in seawater on the micro-mechanical performance of alite pastes. Nanoindentation-energy dispersive X-ray spectroscopy analysis, porosity analysis, and phase evolution tests were carried out to reveal the microstructure evolution of salt-added alite systems. The results showed that in addition to the acceleration of alite hydration by the salt solutions, more low-density (LD) C-S-H in the hydration products was formed. All the cations used in this study were able to enter the mesopores in the C-S-H particles, which improved the micro-mechanical performance of alite pastes. However, the deleterious effects of simulated seawater on the modulus and hardness of C-S-H were noticed, mainly attributed to the presence of Mg²⁺ in seawater.