SiO2-Induced Performance Deterioration in Magnesium Phosphate Cement: Chemical Consumption and Physical Deactivation of Reactive Magnesia

This study investigates the dual mechanisms by which SiO2 deteriorates magnesium phosphate cement (MPC) performance. MgO-SiO2 clinkers were prepared using lightly calcined magnesia (MgO) with SiO2 additions ranging from 1% to 9%, followed by calcination at temperatures between 1100 °C and 1500 °C. Through XRD–Rietveld refinement, workability, compressive strength, and hydration heat analyses, the damaging effects of SiO2 were systematically evaluated. Results reveal that SiO2 degrades MPC through two concurrent mechanisms: chemical consumption and physical deactivation of reactive MgO. Chemically, SiO2 reacts with MgO during calcination to form inert forsterite (Mg2SiO4), irreversibly reducing reactive MgO content. Physically, SiO2 and its reaction products lower the crystallinity and reactivity of remaining MgO while diluting reactive components. A calcination temperature of 1200 °C was optimal, yielding the highest compressive strength (3 d strength > 30 MPa). Increasing SiO2 dosage monotonically reduced strength; at 1200 °C, 9% SiO2 reduced 3 d strength by ~40% compared to 1%. Hydration heat analysis showed that both heat flow rate and cumulative heat release increased with SiO2 content due to enhanced heterogeneous nucleation from Mg2SiO4. Critically, this increased heat output did not translate into higher strength, indicating that microstructural quality—not reaction extent—governs mechanical performance. Rietveld quantification confirmed that Mg2SiO4 formation increased linearly with SiO2 dosage and temperature (reaching 72.24% at 1500 °C with 9% SiO2), providing the material basis for dual damage. This work offers mechanistic insights and experimental support for utilizing low-grade magnesite and optimizing MPC performance.