Heating-induced transformations in calcium silicate hydrate (C-S-H): In-situ investigations of composition, structure, and morphology

Transformation of C-S-H is crucial in the deterioration of concrete at high temperatures. This study investigates the composition, structure, and morphology of C-S-H from 30 °C to 1000 °C using in-situ heating XRD, TGA/TG-IR, in-situ heating XPS, and in-situ heating TEM combined with image recognition. The results reveal that during heating, C-S-H undergoes weakly and strongly bound water loss, dehydroxylation, and transformation into CaSiO₃. During heating, the Si-O-Si bonds within C-S-H silicate chains remain highly stable. The primary change observed is the conversion of Si-OH groups into Si-O-Ca/Na following dehydroxylation. TEM morphology exhibits shrinkage and densification similar to ceramic sintering, with the overall process divided into five stages. The first three stages are dominated by dehydration and dehydroxylation, while the final two stages are governed by phase changes and liquid-phase sintering. The dehydration of C-S-H in the first stage has the greatest impact on shrinkage, while the fourth stage transforms the C-S-H morphology from foil-like to drop-like, having the largest effect on densification. Although the Ca/Si ratio of C-S-H remains constant during the heating, the crystallinity decreases. This study offers new insights into the mechanisms driving the transformation of C-S-H under heating.