Abstract: Magnesium slag powder from Ningxia Huiye Magnesium Industry Co.，Ltd.was treated to improve the volume stability for its application in building materials.The slag was mixed with borates，pressed into briquettes， and then sintered at high temperature.SEM studies showed that under higher temperature Ca₂SiO₄ polymorphs were stabilized by Na and B ions in the added borates.The free MgO content in the slag was also decreased by the sintering treatment.The slag powder，after mixing with 0.4%~0.6%（mass fraction） of borates and sintering at 1 200 ℃ for 5~6 h，became volume stable aggregates.It is then possible to use the treated slag in building materials，saving valuable natural resources and decreasing the global warming impact in magnesium production via Pidgeon process.

Key words: magnesium, borates, pidgeon process, stability, building material