Preparation of High-Porosity and High-Strength Ceramisites from Municipal Sludge Using Starch and ${\mathrm{CaCO}}_3$ as a Combined Pore-Forming Agent

A mixed pore-forming agent composed of starch and calcium carbonate (${\mathrm{CaCO}}_3$) is proposed to assist the fabrication of high-porosity and high-strength ceramisites from solid wastes of municipal sludge and fly ash. The maximum porosity of the sintered body without adding a pore-forming agent was only 38.5% under a premised strength higher than 8 MPa. In comparison, the porosity and compressive strength of the sintered body reached 68.7% and 13.2 MPa under the optimized conditions of ${\mathrm{CaCO}}_3$ and starch content at 10% and 15% by weight. The performance comparison with those in the literature indicates that the proposed strategy is promising for fabrication of high-performance lightweight ceramisites from solid municipal sludge and fly-ash wastes. A schematic diagram for the sintering and pore-forming mechanism was proposed to attribute the high porosity of the sintered ceramisites to a low-temperature combustion of starch and a high-temperature decomposition of ${\mathrm{CaCO}}_3$. Simultaneously, the volume shrinkage caused by starch and expansion by ${\mathrm{CaCO}}_3$ counteracted each other, thus preventing cracking of the ceramisites after sintering and ensuring the high strength of the sintered compacts.