Volume Stability of Magnesium Slag-Based Building Materials: A Critical Review of Mechanisms and Mitigation Strategies

Magnesium slag (MS), an important industrial solid waste generated during the magnesium production process, is recognized for its considerable potential to reduce carbon emissions and advance sustainability in the building materials industry. However, its widespread application is severely hindered by volume instability, which is primarily attributed to the delayed hydration expansion of free calcium oxide and free magnesium oxide. In this review, the physical and chemical properties of magnesium slag are systematically summarized, and the underlying mechanisms responsible for the volume instability of MS-based cementitious materials are critically elucidated. Furthermore, targeted strategies for improving volume stability are focused on and evaluated, including the acid treatment of magnesium slag for oxide passivation and the collaborative utilization with complementary solid wastes such as ground granulated blast furnace slag, steel slag, and fly ash. Through this critical synthesis, a framework is established for overcoming the soundness bottleneck, thereby repositioning magnesium slag as a viable and reliable constituent in next-generation sustainable building materials.