Ceramics-Silikáty

As global marine strategies advance, using seawater and sea sand concrete has become a vital solution to the shortage of natural aggregates. In addition, because of high temperatures in certain regions, understanding the mechanisms by which seawater temperature and ions affect the hydration of cement-based materials is crucial for research and practical applications in this field. This study employed high-temperature ultrasonic testing, X-ray diffraction, environmental scanning electron microscopy, and low-field nuclear magnetic resonance to investigate the effects of seawater and its principal ions on hydration. This study used high-temperature ultrasonic testing, X-ray diffraction, environmental scanning electron microscopy, and low-field nuclear magnetic resonance to investigate the effects of seawater and its major ions on the hydration, microstructure evolution, and phase evolution of cement-based materials at different curing temperatures. Results show that seawater and NaCl solutions exert similar effects on cement paste: they accelerate early hydration, enhance early micromorphology, and reduce post-hardening porosity. However, these effects diminish at curing temperatures above 60 °C. In addition, sodium sulfate (Na₂SO₄) solutions enhance the early strength of cement paste; however, at curing temperatures above 60 °C, the porosity of hardened cement paste increases, leading to lower strength compared to freshwater-mixed cement paste of the same age. Moreover, the formation of Mg(OH) ₂ in magnesium chloride (MgCl₂) solutions at curing temperatures of 20 °C and 40 °C inhibits cement hydration. However, at curing temperatures of 60 °C and 80 °C, the promoting effect of Cl^⁻ outweighs the inhibiting effect of Mg^²⁺, resulting in higher strength than that of cement paste mixed with fresh water.