Production of Rapid-Hardening Magnesium Oxysulfate Cement Containing Boric Acid

Rapid-hardening magnesium oxysulfate (MOS) cements were prepared using a high-activity light-burned magnesia (HA-LBM) powder and boric acid (BA). The early hydration, compressive strength, phase composition, microstructure, water resistance, and solution chemistry have been studied. HA-LBM powder rapidly hydrates to form Mg(OH)2, and this causes rapid setting and poor mechanical properties. The addition of BA delays hydration, decreases the hydration temperature, increases setting time, promotes formation of Phase 517, and improves the physical properties. MOS cement pastes containing 1.0% by weight of BA had an initial setting time at 73 min, a final setting time at 101 min, a compressive strength after 12 h of 33.6 MPa, 28-day compressive strength of 91.4 MPa, and 28-day Rf of 0.87. The research highlights the excellent properties of MOS cements manufactured using HA-LBM and BA.