Effects and mechanisms of waste gypsum influencing the mechanical properties and durability of magnesium oxychloride cement

Abstract

Magnesium oxychloride cement (MOC) prepared by using magnesium chloride and light calcined magnesia has been widely used as decorative materials, refractory materials and adsorbing materials. But its poor water resistance restricts the application in construction and building. More importantly, the disposition of abundant waste gypsums is becoming a social issue, even a political issue in China. This study employs flue gas desulfurization gypsum (FGDG) and phosphogypsum (PG) to modify MOC, aiming to reduce the environmental hazards and production costs and improve the MOC properties. Effects of high content of FGDG and PG on the MOC properties and the mechanisms were systematically studied by a series of test and characterization experiments. The results demonstrated that the presence of FGDG and PG prolongs the setting time to some extent, and decreases the compressive strength to varying degrees. Compressive strength for FGDG-incorporated and PG-incorporated mixtures varies from 73 MPa to 62 MPa and from 73 MPa to 55 MPa, respectively, at curing age of 60 days. And the water resistance and volume stability of MOC are improved gradually with increasing the FGDG and PG contents. After adding FGDG and PG, no new hydration products were detected, while the formation of 5MgO ⋅ MgCl2⋅ 8H2O was reduced. However, the presence of FGDG and PG slightly increased the large harmful pores among the air curing groups and decreased the total porosity after water immersion. And, the microstructure analysis is in agreement with the results of comprehensive strength and the 5 phase products can bond gypsum particles together to form a dense structure. Moreover, the generation amount expansive brucite decreases, which is beneficial to the volume stability. The superior compatibility between waste gypsums and MOC not only provides an efficient method to utilize FGDG and PG and solidify heavy metals contained in them, but also improves the durability of MOC and decreases the production cost which will inevitably promote the large-scale application of MOC in constructional engineering.