The Hydration of β- and α′H-Dicalcium Silicates: An X-ray Spectromicroscopic Study

Dicalcium silicate (C2S) is an important clinker mineral in Portland and belite-calcium sulfoaluminate cement. However, there is still a lack of information on the local degree of silicate polymerization and calcium coordination in C2S hydration products. This study aimed to fill this gap by characterizing the hydration of two C2S polymorphs, the β- and α′H- types, using scanning transmission X-ray microscopy coupled with ptychographic imaging. The results showed that the coordination of the Ca species in β- and α′H-C2S had a distorted, cubic-like symmetry, whereas the Ca coordination of calcium silicate hydrate (C-S-H), the main hydration product, was structurally similar to that of tobermorite. The outer hydration product (Op) of both polymorphs exhibited an increasing degree of silicate polymerization over time. The inner hydration product (Ip) of β-C2S polymerized slower than Op; however, the degree of silicate polymerization of both Ip and Op in the α′H-C2S hydration system was comparable. The polyme...