Dicalcium Silicate and Belite Phase

Polymorphism Ca2[SiOJ and stractuies of phases stable at different range of temperature are relatively well known. On Fig. 2.51 the range of temperature of individual polymorphic phases stability after Niesel and Thormann [127] are shown. Dicalcium silicate forms four phases which are stable at different temperature ranges, however, fifth phase P is unstable. Ntmse [128] gives the hypothetical system for C2S in which phase P is thermodynamically stable, at high pressme. 

All C2S phases are formed of independerrt SiO tetrahedra linked by calcium atoms, but the stractural arrangement is changing in individual polymorphs (Fig. 2.52). 

Phase a appears in two forms ot and which are stable at temperature range 

1420-1160 °C and 1160-675 °C respectively. Their structures were determined for the samples stabilized with additions of SrO, BaO or B2O3 [130], The distances Si-0 in tetrahedron differ significantly (175, 177 and 167 pm) which is caused by the belonging of each oxygen atoms to the surrounding of two kinds of calcium 

Phase 21 Structure type 4J r—y f — Space group Number of molecules in unit cell Caleium coordination Lattice constants, nm 

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