Preparation, classification, hydration mechanism and durability of magnesium-based cementing material

Magnesium-based cementing material is a new type of cementitious material based on active MgO. It features rapid solidification, early strength and fire resistance, exhibiting significant advantages in repair and emergency repair projects. This study divided magnesium-based cementitious materials into three types according to the differences in calcination temperature of MgO and blending solution : magnesium oxychloride cement, magnesium oxysulfate cement, magnesium phosphate cement. We conducted detailed discussion and review of their hydration mechanism and durability. The hydration of magnesium oxychloride cement lies in the hydration of ternary system of MgO, MgCl₂ and H₂O. The hardened system of magnesium oxysulfate cement shows low strength due to the existence of free MgSO₄ in the hydration process. The hydration rate of magnesium phosphate cement delayed owing to the overlapping of exothermic of MgO solution and sharp reaction between MgO and phosphate, which leads to excessive hydration and over-concentrated heat release. Carbonation reduces the pores contents, optimizes the internal pore structure, and improves the strength and durability of magnesium-based cementitious materials. The magnesium-based cementitious materials have poor water resistance, among which no unified understanding has been reached as to the reasons for magnesium oxychloride cement. For magnesium oxysulfate cement, the unreacted MgO reacts with water to form Mg(OH)₂, and the volume expansion leads to the cracking of the hardened matrix. In the case of magnesium phosphate cement, the phosphate can lead to the dissolution of hydration products and unreacted MgO.