Glass-ceramics prepared from sludge generated by a water purification plant

Glass-ceramics were prepared from sludge generated in a water purification process. The main constituents of this sludge were SiO2 and Al2O3, with considerable amounts of Fe2O3 and MnO also present. The batch composition of the parent glass was adjusted to be close to the composition of the eutectic point in the CaO–Al2O3–SiO2 system by mixing the sludge with CaCO3 and waste soda-lime glass. The samples were melted at 1300 °C for 2 h and quenched in water. The resulting glass cullet was ground to <100 mesh in a ball mill, uniaxially pressed to form green bodies and fired at 700–1250 °C to densify and crystallize. Crystallization occurred at >850 °C, forming gehlenite (Ca2Al2SiO7) and wollastonite (CaSiO3) above 900–950 °C. The color of the sample changed from black to yellow-brown upon crystallization. The average four-point bending strength and Vickers microhardness of the glass-ceramics heated at 1000 °C (99 MPa and 5.2 GPa) was comparable with the values of commercial glass-ceramics. The chemical durability in acid and alkali solutions was 17.7 and 0.23 mg/cm2, respectively, showing excellent durability in alkali solution.