This article presents the thermal transitions of a tobermorite-bearing sample when heated from 30°C up to 1200°C, both in vacuum and in static air, including tobermorite transforming to wollastonite, aragonite to calcite and calcite to lime. Characteristics obtained by in situ high temperature X-ray diffraction, field emission scanning electron microscopy and scanning transmission electron microscopy analyses jointly indicate that the investigated tobermorite is anomalous. The variations along the a, b, c axes and the volume changes of tobermorite with increasing temperature are described, and its thermal shrinkage coefficients therefore determined. The comparison between the refined structures at 30°C and 800°C demonstrates that the shrinkage degree (Δa/a0) along the a axis is higher than those (Δb/b0, Δc/c0) along the b and c axes. The wollastonite is formed in two ways: Tobermorite converting to wollastonite and lime reacting with quartz to form wollastonite.