Preparation and properties of glass-ceramics from kaolin clay refining waste (Kira) and paper sludge ash

Glass-ceramics were prepared from mixtures of wastes generated from refining of silica sand and kaolin clay (Kira) and paper sludge ash. Kira and paper sludge ash were mixed in mass ratios of 55/45. The mixtures were melted at 1400 °C and quenched in water to obtain glasses. The quenched glasses were ground to <48 mesh and used to prepare glass-ceramics. Crystallization of the parent glass occurred above 950 °C, producing quartz solid solution at 1000 °C and cristobalite at 1100 °C as the major crystalline phases. The color of the glass-ceramics is white or slightly pale yellow. The four-point bending strengths of the as-fired glass-ceramics range from 63 to 66 MPa and the Vickers microhardness values from 6.0 to 6.4 GPa. The average linear thermal expansion coefficients of the glass-ceramics range from 6.3 × 10^-6 /°C to 8.1 × 10^-6 /°C (30-380 °C) and show non-linearity around the phase transition temperatures of quartz and cristobalite. The chemical durability of the resulting glass-ceramics is excellent in alkali but poor in acid solution. The glass-ceramics show rather better performance than commercial glass-ceramics even when prepared solely from wastes starting materials.