Influence of high-sulfur fly ash on the microstructure and properties of magnesium phosphate cement

Using fly ash (FA) to modify magnesium phosphate cement (MPC) not only extends setting time and enhances MPC performance but also reduces costs. Currently, high-quality FA is predominantly used, whereas the accumulation of low-quality FA poses challenges for waste management and environmental protection. This study focuses on low-quality FA with excessive sulfur content. Initially, X-ray fluorescence spectroscopy was employed to select two types of FA with SO₃ contents of 1.06% and 6.07%. FA replaced magnesium oxide in equal mass to prepare FA/MPC composite cementitious materials. The composite materials were studied for hydration heat, phase composition, infrared spectroscopy, mechanical properties, and microscopic morphology to explore SO₃’s impact on MPC hydration processes, microstructure, and properties. The findings indicate that adding FA effectively dilutes reactants, extending setting time from 7 to 17 min and 18 min, aiding construction operations. In the FA/MPC composite cementitious materials, MPC hydration, along with FA's synergistic hydration, forms a new hydration product MgSiO₃, improving material density and mechanical strength post-hardening. Excessive SO₃ in high-sulfur FA accelerates MgO decomposition, providing ample Mg²⁺ for K-struvite synthesis, promoting MPC hydration, and transforming K-struvite from amorphous to crystalline, enhancing material strength.