A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl2) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C3S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl2. It appears that CaCl2 accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.