Effects and mechanisms of waste gypsum influencing the mechanical properties and durability of magnesium oxychloride cement. (10th March 2022)

• Record Type: Journal Article
• Authors: Ma, Cong; Chen, Gege; Cao, Lei; Zhou, Haijun; Ren, Weixin
• Is Part Of: Journal of cleaner production. Volume 339(2022)
• Publication Date: 2022-03-10
• Subjects: Magnesium oxychloride cement -- Flue gas desulfurization gypsum -- Phosphogypsum -- Properties -- Microstructure -- Sustainability; Factory and trade waste -- Management -- Periodicals; Manufactures -- Environmental aspects -- Periodicals; Déchets industriels -- Gestion -- Périodiques; Usines -- Aspect de l'environnement -- Périodiques; 628.5
• DOI: 10.1016/j.jclepro.2022.130679
• Languages: English
• ISSNs: 0959-6526

Abstract

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 ⋅ 8H2 O 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. Graphical abstract: The flue gas desulfurization gypsum (FGDG) and phosphogypsum (PG) are employed to modified magnesium oxychloride cement (MOC). High FGDG addition (the mass ratio of FGDG and light burnt magnesia is 1:1 in MF100) decreases the compressive strength by only about 10%, and that in MOC with high content of PG is nearly 25%. XRD and TG-DTG analysis reveal that the main hydration products including 5MgO ⋅ MgCl2 ⋅ 8H2 O (phase 5) and magnesium hydrate decrease significantly in pastes MF100 and MP100. Accordingly, the porosities of MF100 and MP100 are also much higher than MOC without waste gypsums. It is found that the decrease in compressive strength does not match the change in the amount of hydration products and porosity. More importantly, the water resistance and volume stability of MF100 and MP100 are much better. This indicates that high content of FGDG or PG can improve the durability significantly with a slight decrease in compressive strength. Physical functions including strong bonding effect and filling or skeleton effect can be responsible for these phenomena. Image 1 Highlights: Flue gas desulfurization gypsum (FGDG) and phosphogypsum (PG) were introduced into magnesium oxychloride cement (MOC). Great improvement in durability of MOC with high content of FGDG or PG is confirmed with sacrificing very few strengths. Mechanisms of FGDG or PG are mainly physical functions including strong bonding effect and filling or skeleton effect.