Evolution and mechanism of high-temperature performance of magnesium ammonium phosphate cement

Although magnesium ammonium phosphate cement exhibits promising thermal resistance, the effects of mix proportions on its phase evolution and performance at high temperatures remain unclear. This study investigated the effects of magnesia-to-phosphate, water-to-cement, and boric acid to-magnesia ratios on the performance of magnesium ammonium phosphate cement (MAPC) after exposure to temperatures up to 1000°C. Some engineering properties and residual strength were tested and phase evolution was revealed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicated that boric acid-to-magnesia ratio (B/M) most strongly controlled high-temperature degradation. Although a lower B/M ratio reduced strength at room temperature (20°C), it enhanced exposure to high temperatures residual strength by mitigating boric acid dehydration and adverse phase transformations. Notably, MAPC specimens with 5–10% boric acid achieved an optimal residual strength of 19.9 MPa at 1000°C. In addition, increasing the magnesia-to-phosphate ratio (M/P) and decreasing the water-to-cement ratio (W/C) improved high-temperature strength and reduced porosity by ensuring sufficient residual MgO. At 1000°C, increasing the M/P ratio from 1.5 to 3.0 elevated the residual compressive strength from 4.8 MPa to 7.8 MPa, while concurrently reducing the porosity from 42.80% to 41.38%. Reducing the W/C ratio from 0.20 to 0.15 bolstered the residual compressive strength from 7.8 MPa to 9.2 MPa, while simultaneously decreasing the porosity from 41.38% to 30.20%. MgNH4PO4∙6H2O transformed into Mg3(PO4)2 after exposure to 800°C. However, with 20% boric acid, the reaction between Mg3(PO4)2, MgO, and B2O3 formed plate-like Mg3(BO3)(PO4) clusters at 1000°C. The associated mass loss and inter-layer microcracks were identified as the primary causes of strength reduction. This study systematically investigates the influence of M/P, W/C, and B/M ratios on the high-temperature performance of MAPC, with a focus on the mechanisms underlying the substantial impact of the B/M ratio.