PHYSICAL STABILIZATION OF THE BETA-]GAMMA TRANSFORMATION IN DICALCIUM SILICATE

It has been shown that the monoclinic beta-phase of dicalcium silicate (Ca2SiO4) can be stabilized against transformation to the orthorhombic gamma-phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high-temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high-temperature hexagonal (alpha) to orthorhombic (a(H)') transformation at 1425-degrees-C, which is accompanied by a -4.7% volume decrease, causes periodic fracture of beta-twins due to accumulated strains. Chemical doping with K2O or Al2O3 Promotes the formation of amorphous phases which mold themselves around beta-Ca2SiO4 grains. Annealing treatments cause crystallization of the glass and subsequent transformation to the gamma-phase.