<p>The effects of liquid-solid ratio and reaction time on the leaching rate of magnesium at room temperature were investigated, as well as the effects of the molar ratio of MgO/MgCl₂, the amount of water added, and the amount of acid-impregnated slag dosed on the compressive strength and water resistance of LR-MOC. The results showed that the magnesium element in the boron mud could be maximally leached under the conditions of 1:1 concentration of hydrochloric acid at room temperature, liquid-solid ratio of 2.5 mL·g⁻¹, and reaction time of 5 h, and the main products were amorphous SiO₂ as well as a small amount of magnesium olivine which had not been completely reacted. The LR-MOC prepared using the acid-soaked mixture could reach a softening coefficient of 0.85 for 28 d of water immersion when the molar ratio of MgO/MgCl₂ was 2.2, the amount of water added was 0 g, and the acid-soaked slag dosing was 40 wt%, which also led to an appreciable late-strength, with an increase of 19.4% in compressive strength at 28 d compared to that at 7 d. Unlike previous studies, LR-MOC prepared in this way has a final strength phase that is not the more easily hydrolysed 3-phase but the lath-like 5-phase. For this phenomenon, we analyzed the mechanism and found that, during the acid leaching process, a part of amorphous SiO₂ dissolved in the acid leaching solution formed a silica sol, in which Mg²⁺ played a bridging role to make the silica sol more stable. With the addition and hydrolysis of MgO, the silica sol gel coagulation slows down, providing a capping layer to inhibit the hydrolysis of the 5-phase crystals and providing some strength after coagulation. The amorphous SiO₂ in the other part of the acid-impregnated slag generated M-S-H gel with Mg²⁺ and OH⁻, which synergised with the dense structure composed of interlocking crystals to improve the water resistance of LR-MOC.</p>