Preparation of green and durable magnesium oxysulfate cement using sewage sludge ash: Physical properties, microstructure, and leaching behavior

The aim of this study was to explore the application of sewage sludge ash (SSA) as a substitute for light-burnt magnesium oxide (LBM) in the preparation of magnesium oxysulfate cement (MOSC), in order to achieve the harmless treatment of SSA and improve the performance of MOSC. By adjusting the SSA dosage, a series of MOSC samples were prepared, and their setting time, compressive strength, water resistance, and heavy metal leaching rates were then systematically evaluated. Additionally, their crystal phase composition, relative content, and microstructure were characterized using XRD, TGA, FTIR, and SEM in order to elucidate the mechanism by which SSA affects the properties of the MOSC. The results indicate that the addition of SSA prolongs the setting time of MOSC and reduces its compressive strength. However, pozzolanic SiO2 in SSA can hydrate with MgO to form magnesium-silicate-hydrate gel, effectively improving the water resistance of MOSC. In particular, when SSA replaces 30 % of LBM, MOSC not only meets the heavy metal leaching standards for Class IV surface water in China, but also has a compressive strength of up to 67 MPa and a significant improvement in water resistance by 44 %. This study not only opens up new avenues for the harmless treatment of SSA, but also provides scientific basis and practical reference for optimizing the water resistance of MOSC.