# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 6 Recent Developments in Water-Resistant MOC
The SCMs include FA [22, 86], silica fume [30], nano silica [67, 87], rice husk ash [88], MK [80, 89], and GGBFS [77]; the industrial byproducts include refuse floor tiles [88, 90, 91], incinerated sewage sludge [77, 92], granite wastes [93], pulverized fuel ash [94], waste wood [95, 96], plastic waste [97], agriculture and industrial waste [98], and waste gypsum [99, 100], and modifier reagents like H\({}_{3}\)PO\({}_{4}\)[39, 67], Ca(H\({}_{2}\)PO\({}_{4}\))\({}_{2}\)[101], MFP [48], tartaric acid [102] were added to MOC composites to enhance water resistivity of MOC. [PERSON] et al. [103] incorporated PA and phase-5 seed crystals in MOC cement to enhance strength and water resistance at the same time. They found that the addition of PA and phase-5 seed crystals reduced the porosity and made the MOC matrix more compact, leading to enhanced compressive  
Figure 7: Yearly data on published studies focused on the water resistance issue of MOC. strength and water resistance, as shown in Figure 8. It can be seen from Figure 8a that with the addition of PA, a large number of crystals grew in the pores; however, some interspace still remained in the middle of pores. This was overcome with the addition of phase-5 seed crystals which lead to a compact microstructure (Figure 8b). Similarly, [PERSON] et al. [104] found that phosphate may affect the hydration product composition during the hydration process and facilitate the development of stable water-resistant hydration products. [PERSON] et al. [23] also discovered that the MOC cement with KH\({}_{2}\)PO\({}_{4}\) generated gel-like phase 5. In general, the existence of phosphate ions in PA contributes to its ability to increase the water resistance of MOC cement. It is believed that insoluble phosphate is generated, which hinders phase 5 from decomposition. Ionization of PA occurs when PA is added to MgCl\({}_{2}\), as shown in Equations (6)-(8) [91]. \[\mathrm{H_{3}PO_{4}}\quad\rightarrow\quad\mathrm{H_{2}PO_{4}^{-}+H^{+}} \tag{6}\]  
\[\mathrm{H_{2}PO_{4}^{-}}\quad\rightarrow\quad\mathrm{HPO_{4}^{2-}+H^{+}} \tag{7}\]  
\[\mathrm{HPO_{4}^{2-}}\quad\rightarrow\quad\mathrm{PO_{4}^{-}+H^{+}} \tag{8}\]  
A stable phase, i.e., MgO\({}_{4}\).3H\({}_{2}\)O, with high water resistance is formed when HPO\({}_{4}^{2-}\) reacts with Mg\({}^{+2}\) in the solution. At the same time, MgO\({}_{4}\).3H\({}_{2}\)O can alter the interface regions to strengthen the configutinated force of the phases, improving water resistance and compactness. The process by which soluble phosphate improves the water resistance of MOC also resembles that of PA. Likewise, [PERSON] et al. [28] reported that phosphate ions might lower the Mg\({}^{+2}\) concentration in the solution, resulting in more stable crystalline phases in water. Improvement in water resistance has also been observed with the addition of some acids, including tartaric acid (TA) [102], hydroxyacetic acid (HA) [105], and citric acid [106]. Figure 9 further provides a summary of the observations where water resistance was found to be improved by the addition of organic additives. It can be observed that these  
Figure 8: Morphology of MOC specimens reinforced with (**a**) only 1% (by weight) PA (**b**) 1% (by weight) PA + 1% (by weight) phase-5 seed crystals [103]. chemical modifiers are effective in providing a higher softening coefficient. In particular, PA and HA have been found to have shown better effect in terms of strength retention. Nevertheless, the extent of improvement also depends on the raw MOC matrix composition. Also, though these modifiers improved the water resistance of MOC, there were some negative effects as well, such as the retarding impact of PA, which decreases the micro-level MOC compactness, leading to a decrease in strength [39]. [PERSON] et al. [26] investigated the water resistance of MOC composites enhanced with nano-dopants like carbon nanotubes, alumina nanosheets, graphene nanoplatelets, and graphene oxide. They observed water resistance coefficients ranging from 62.7% to 74.8% after 24 h of water immersion.