The Hydration of β- and αH′-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 beta- and alpha(H)'- types, using scanning transmission X-ray microscopy coupled with ptychographic imaging. The results showed that the coordination of the Ca species in beta- and alpha(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 beta-C2S polymerized slower than Op; however, the degree of silicate polymerization of both Ip and Op in the alpha(H)'-C2S hydration system was comparable. The polymerization degree of Op was relatively heterogeneous, whereas, in alpha(H)'-C2S, the polymerization degree was more homogeneous. Ptychographic imaging shows that the Op of alpha(H)'-C2S exhibits coarser fibrils than the Op of beta-C2S, and a clear Op Ip interface of hydrated beta-C2S is observed.