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The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca,SiO,) were studied by TEM. Three types of superlattice structures were observed in the ol; and p phases. Almost all p grains were twinned and strained. Symmetry-related domain structures inherited from previous high-temperature transformations were observed in f3 grains. Both the a-ol; and olL-.f3 transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the f3--y transformation is proposed to be strains and cation charge repulsions in the p structure. This mechanism can be displacive, but it needs to overcome a comparatively high energy barrier.