Effect of Na2O and heat-treatment on crystallization of glass–ceramics from phosphorus slag

Na2O–CaO–Al2O3–SiO2–MgO glass was prepared from phosphorus slag as the raw materials and subsequently converted to glass–ceramics. The effects of both heat-treatment system and Na2O content on crystallization and microstructure were investigated by XRD and SEM. For the glass–ceramics with 15 mass% Na2O (A), Na2Ca2Si3O9 appears initially at 850 °C, then the dominant β-CaSiO3 from 850 °C to 1050 °C, which further transforms into α-CaSiO3 from 1050 °C to 1150 °C. The existing phases are α-CaSiO3 and Na2Ca2Si3O9 at above 1150 °C. For the glass–ceramics with 17 mass% Na2O (B), the primary and second phases are β-CaSiO3 and Na2Ca2Si3O9 in the range of 750 °C–950 °C, respectively. The transformation from β-CaSiO3 to Na2Ca2Si3O9 occurs from 950 °C to 1050 °C. Na2Ca2Si3O9 gradually transforms into Na2Ca2Si3O9 solid solution from 1050 °C to 1150 °C, and Na2Ca2Si3O9 solid solution becomes the predominant crystal phase above 1150 °C. Surface crystallization is dominant mechanism in glass–ceramics A, while volume crystallization mechanism is predominant in glass–ceramics B.