Preparation of High-Porosity and High-Strength Ceramisites from Municipal Sludge Using Starch and CaCO3 as a Combined Pore-Forming Agent

A mixed pore-forming agent composed of starch and calcium carbonate (CaCO3) 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 CaCO3 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 CaCO3. Simultaneously, the volume shrinkage caused by starch and expansion by CaCO3 counteracted each other, thus preventing cracking of the ceramisites after sintering and ensuring the high strength of the sintered compacts.