Sintered glass-ceramics from Municipal Solid Waste-incinerator fly ashes—part I: the influence of the heating rate on the sinter-crystallisation

A glass composition, made by mixing 70% of MSW ashes and 30% of waste from feldspar production, was studied. The batch was melted at 1350 °C and the melt was quenched in water. The chemical stability of the glass was investigated by TCLP leaching test. DTA experiment, at 10 °C/min heating rate in the 20–1200 °C range, showed that the glass has a high crystallisation trend with a gehlenite (2CaO.Al2O3.SiO2) formation at 935 °C. The percentage of crystal phase formed as a function of the heat treatment was measured by density variation and XRD. In order to obtain sintered glass ceramics, the frit was heat treated in Al2O3 moulds at 950, 1000 and 1050 °C by 2 and 20 °C/min heating rates. At high heating rate predominantly surface crystallisation occurred with fibre-like crystals growing perpendicularly from the surface; at low heating rate bulk crystallisation took place. At low rate, the sintering was inhibited by the crystallisation process. Non porous sintered samples were obtained by a 30 °C/min heating rate and a 40 min isothermal step at 1120 °C, near the liquidus temperature.