Evidence of an amorphous intermediate phase during non-classical calcium hydroxide crystallization from cementitious pore solutions

Calcium hydroxide, a primary hydration product of Portland cement, is undetectable by conventional scattering methods or thermal analysis during the dormant period of cement hydration. Its absence suggests it is either dissolved in the pore solution or present in a non-crystalline state, prompting us to investigate its native state in cementitious pore solutions. Here, we show that conventional transmission electron microscopy detects large (100 nm) amorphous calcium hydroxide spheres, whereas cryo-high-resolution transmission electron microscopy, combined with small-angle X-ray scattering and pair distribution function analysis, reveals that these spheres are absent from the native pore solution after 15 min of hydration. Instead, calcium hydroxide exists as fully solvated ions or ion pairs, alongside tiny (1 nm) metastable gypsum prenucleation clusters. In addition to silicates, sulfate ions inhibit calcium hydroxide nucleation, keeping isolated pore solutions metastable for days at sub-ambient temperatures under inert gas, whereas heating or evaporation triggers the multistage “non-classical” crystallization of calcium hydroxide. Our findings confirm “non-classical” crystallization pathways during early Portland cement hydration, the binder of concrete, the most widely used man-made material.