Synthesis of glass ceramics from kaolin and dolomite mixture

Cordierite- and anorthite-based binary glass ceramics of the CaO–MgO–Al2O3–SiO2 (CMAS) system were synthesized by mixing local and abundant raw minerals (kaolin and doloma by mass ratio of 82/18). A kinetics study reveals that the activation energy of crystallization (E a) calculated by the methods of Kissinger and Marotta are 438 kJ·mol−1 and 459 kJ·mol−1, respectively. The Avrami parameter (n) is estimated to be approximately equal to 1, corresponding to the surface crystallization mechanism. X-ray diffraction (XRD) analysis shows that the anorthite and cordierite crystals are precipitated from the parent glass as major phases. Anorthite crystals first form at 850°C, whereas the μ-cordierite phase appears after heat treatment at 950°C. Thereafter, the cordierite allotropically transforms to α-cordierite at 1000°C. Complete densification is achieved at 950°C; however, the density slightly decreases at higher temperatures, reaching a stable value of 2.63 kg·m−3 between 1000°C and 1100°C. The highest Vickers hardness of 6 GPa is also obtained at 950°C. However, a substantial decrease in hardness is recorded at 1000°C; at higher sintering temperatures, it slightly increases with increasing temperature as the α-cordierite crystallizes.