# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 6 Recent Developments in Water-Resistant MOC  
The water resistance of MOC was one of the primary challenges limiting its application to mainly indoors. However, recent advancements in water-resistant MOC have widened the scope from interior to outdoor applications. The main reason for its poor water resistance was the conversion of compacted and dense hydrated phases into loose hydrated products such as brucite (Mg(OH)\({}_{2}\)), which results in significant strength reduction. Over the last decade, there has been a significant increase in research efforts to reduce or mitigate the water resistance issue of MOC, as illustrated in Figure 7. The figure depicts an annual summary of the number of articles published specifically on the water resistance of MOC as found on Google Scholar. It can be observed that in 2022, a maximum of 21 articles were published, followed by 18 articles each in 2021 and 2020, whereas in 2023, around 12 articles were published. This clearly shows the growing interest in this research topic, which is critical for construction applications. Figure 6: Multiple microcracking in FRMOC reinforced with 2% PE fiber [1]. The researchers have utilized a variety of SCMs to address this issue, including FA, silica fume, GGBFS, MK, alumina, nano silica, glass powder, sludge ash, and pulverized fuel ash, as well as soluble phosphates including PA, potassium phosphate, MFP, etc. [8, 23, 83]. The main aim of adding these admixtures and modifiers was to develop stable water-resistant hydrated products. [PERSON] et al. [84] doped 13 different additives individually in MOC to tackle the water resistance issue. Among them, seven were mineral additives (FA, GGBFS, baked clay, diatomic, silicate cement, coal gangue, and baked diatomic) and six were reagent modifiers (H\({}_{3}\)PO\({}_{4}\), Al(H\({}_{2}\)PO\({}_{4}\))\({}_{3}\), H\({}_{3}\)BO\({}_{3}\), Na\({}_{2}\)SO\({}_{4}\), Na\({}_{3}\)PO\({}_{4}\), and K\({}_{3}\)PO\({}_{4}\)). The specimens were immersed for three months in water, and the loss in compressive strength after water exposure was compared for different additives. It was reported that a suitable combination of FA with PA, Al(H\({}_{2}\)PO\({}_{4}\))\({}_{3}\), H\({}_{3}\)BO\({}_{3}\), or K\({}_{3}\)PO\({}_{4}\) can be effective for improving water resistance with only a 10-20% reduction. [PERSON] et al. [85] incorporated aluminate minerals in MOC and revealed that the addition of aluminate minerals improved MOC water resistance by transforming phase 3 and phase 5 into stable water-resistant hydrated phases. [PERSON] et al. [28] incorporated soluble phosphates (H\({}_{3}\)PO\({}_{4}\), NaH\({}_{2}\)PO\({}_{4}\):2H\({}_{2}\)O, and NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)) ranging from 0.5-1.0% by weight in MOC and studied its water resistance performance after 15, 30, 45, and 60 d water immersion. It was revealed that the strength retention coefficient was more than 0.8, which fulfills the minimal criteria for engineering applications. They attributed this to the presence of phosphate radical anions, which yielded a reduction in Mg\({}^{+2}\) ions and thereby the primary phases remained unchanged after water soaking. [PERSON] et al. [68] examined the water resistance characteristics of MOC with FA varying from 10-30% by weight of MgO. They reported that the strength retention coefficient was about 0.4 at 10% content, which increased to 0.65 and 0.85, respectively, at 20% and 30% FA content. From 2008 to 2023, a significant number of investigations were conducted, and different researchers incorporated different types of SCMs, industrial byproducts, and soluble phosphate to enhance the MOC's water resistance performance.