Effects of CaO/SiO2 ratio and heat treatment parameters on the crystallization behavior, microstructure and properties of SiO2-CaO-Al2O3-Na2O glass ceramics

The SiO2-CaO-Al2O3-Na2O glass ceramics were prepared by the melting-sintering method. For samples with a CaO/SiO2 ratio of 0.23, wollastonite was the only crystal phase after heat treatment for 2h in the temperature range of 800-1000 degrees C, and a maximum flexural strength of 99.39 MPa can be achieved at 900 degrees C. When the sintering time was extended from 2 h to 40 h at 800 degrees C, the metastable phase Na2CaSiO4 first appeared, but then disappeared as the heat treatment continued. The maximum hardness and flexural strength were achieved at the sintering time of 20 h in this case. Furthermore, when the CaO/SiO2 ratio was increased from 0.23 to 0.45 under the same heat treatment conditions (800 degrees C for 20 h), the main crystalline phase transformed from wollastonite to Na2CaSiO4, besides nepheline (NaAlSiO4) and SiO2. The existence of the nepheline enhanced the mechanical properties of glass ceramics.