# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 8 Application of MOC in Solid Waste Management  
Previous research has demonstrated that MOC exhibits strong bonding capabilities with various industrial byproducts, including sewage sludge ash (ISA), PFA, marble floor, phosphogypsum (PG), flue gas desulfurization gypsum (FDG), asbestos, expanded clays, and wood particles, and it exhibits rapid setting with a notable early strength gain [6; 82; 93; 99; 92]. The disposal of these industrial byproducts in landfills can have detrimental effects on the environment and human health. Therefore, MOC may serve as an environmentally friendly alternative, reducing waste accumulation and contributing to a more sustainable environment.  
[PERSON] et al. [92] incorporated different percentages of ISA, a byproduct of wastewater treatment plants, ranging from 10 to 30% as a partial replacement of MgO to improve the resistance performance of MOC. Their findings revealed that the inclusion of ISA resulted in enhanced water resistance and reduced expansion of MOC mortar resulting from the improved stability of phase 3 and phase 5 in water. ISA served as a source of soluble silicon (Si) and aluminum (Al), leading to the development of an amorphous gel of M-Cl-A-S-H type, which enhanced the stability of phase 3 and phase 5, consequently improving water resistance. [PERSON] et al. [94] further conducted a comparative analysis to investigate the impact of both ISA and PFA on the water resistance of MOC. They found that both ISA and PFA enhanced the water resistance of MOC, with the ISA-modified MOC paste demonstrating superior water resistance compared to the PFA-modified paste. The improvement was attributed to the development of an amorphous phase (Mg-A-S gel) resulting from the reaction between MgO and the active phases in ISA or PFA. This amorphous gel interspersed with Phase 5 and converted it to fibroid-like phases leading to a more stable and dense structure, thereby contributing to enhanced water resistance (Figure 14).  
[PERSON] et al. [77] conducted a study on the behavior of solidified waste sludge using GGBFS-incorporated MOC. They observed that the inclusion of GGBFS prevents the strength retraction of MOC-solidified sludge and promotes the formation of C-S-H gels through alkaline activation, resulting in improved long-term strength and durability of the solidified sludge. Similarly, [PERSON] et al. [112] studied the impact of MOC on the solidification and stabilization of sewage sludge and found that MOC effectively solidified the waste sludge, mitigating sludge handling and disposal challenges. [PERSON] et al. [99] incorporated recycled waste gypsum containing PG and FDG ranging from 10 to 30% (by weight) as a partial replacement of MgO. This was aimed at considering the huge annual production of PG and FDG in China amounting to approximately 30 million t  
Figure 14: Fibroid-like phases observed through (**a**) SEM and (**b**) transmission electron microscopy (TEM) of ISA-modified MOC [94].  
solid waste rather than being efficiently recycled. They reported that the incorporation of PG and FDG induced the formation of phase 3. Moreover, the presence of phosphate impurities in PG and FDG facilitated the nucleation of phase 5 and phase 3, thereby improving their water stability. Therefore, it was inferred that the utilization of waste gypsum in MOC not only improves the performance, but also supports the recycling of waste resources, promoting environmental sustainability.