Synergistic regulation of performance and mechanism of magnesium oxysulfate cement via a potassium dihydrogen phosphate and sodium malonate composite modifier

Abstract

The engineering application of low-carbon magnesium oxysulfate (MOS) cement is strictly limited by its intrinsic low mechanical strength and poor water resistance, originating from the preferential formation of metastable Mg(OH)₂. To resolve these limitations, this study constructs a dual-functional PD-SM composite strategy by coupling Potassium Dihydrogen Phosphate (PD) with the pioneering novel modifier Sodium Malonate (SM). Systematic investigations reveal that the optimal formulation (2.0% PD + 1.5% SM) achieved a 28-day compressive strength of 88.8 MPa—representing a 179.4% increase over the unmodified control—along with a superior softening coefficient of 0.96 and minimized drying shrinkage (0.411%). Multi-scale characterization (XRD, XPS, SEM, etc.) elucidates a novel “Activation-Control” synergistic mechanism: PD acts as a selective activator by creating an acidic environment and providing Mg-P heterogeneous nucleation templates specifically for the 5·1·7 phase; simultaneously, SM functions as a kinetic controller by chelating free Mg²⁺ and adsorbing onto reactive surfaces to inhibit the rapid, chaotic precipitation of Mg(OH)₂. This synergy effectively decouples the inhibition of harmful phases from the promotion of beneficial phases, shifting the hydration pathway toward a dense, interwoven network of high-aspect-ratio 5·1·7 whiskers. This work provides a theoretical basis and a scalable technical route for the production of high-performance, durable magnesium-based cementitious materials.