Synergistic preparation of glass ceramics from sewage sludge incineration ash and blast furnace slag

Sewage sludge incineration ash is comprised of large amounts of SiO₂, a certain amount of P and many kinds of heavy metals. By utilizing sewage sludge incineration ash as the composition adjustment agent, nucleation agent and flux, glass ceramics with good mechanical performance and chemical stability were successfully synthesized by synergistic preparation with blast furnace slag without adding any chemicals. The influence of heat treatments on the glass-ceramic properties and the crystallization process of the glass ceramics were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), mechanical properties test, and chemical stability test. It is found that the optimum heat treatment condition for preparing the glass ceramics is heating at 850~C for 1 h as a nucleation and sintering process and then heating at 980~C for 2 h as a crystallization process. Under this heat regime, the glass ceramics have the best performance with the rupture strength of 45 MPa, the compressive strength of 200 MPa and the mass lost rate of acid or alkali resistant of less than 0.2%. The initial crystallization temperature of the glass ceramics is 880~C, and crystalline phases in the glass ceramics mainly contain anorthite and a small amount of gehlenite. However the proportion of gehlenite in the glass ceramics increases with the crystallization temperature raising and the crystallization time prolonging. The gehlenite shows a radial needle-like structure, which can contribute to the improvement in bending strength of glass ceramics; but too long crystallization time is harmful to their properties due to grain growth and coarsening.