# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 3 Curing Mechanism of MOC
### CO\({}_{2}\) Curing and Its Effect on MOC-Based Cementitious Composites  
Cementitious composites are generally cured with CO\({}_{2}\) (accelerated curing) as opposed to natural carbonation since the carbonation reaction begins promptly and is utilized to quicken strength gains and lessen early drying shrinkage of cement products. Thus, the distinctive characteristics of rapid carbonation in hydration products, accelerated strength development, and reduced drying shrinkage in cementitious composites render CO\({}_{2}\) curing distinct from natural carbonation [49; 50]. Moreover, the high pressure and purity of CO\({}_{2}\) gas provide a quick interaction between CO\({}_{2}\) and cementitious materials [51]. Compared to OPC (calcination temperature 1450 \({}^{\circ}\)C), the reactive magnesia (MgO) is produced at a calcining temperature of 700-1000 \({}^{\circ}\)C [46]. The reactive MgO produces interfacial bonding by reacting with H\({}_{2}\)O and CO\({}_{2}\) to sequester CO\({}_{2}\) and acquire strength [52; 53; 54]. It has been suggested that CO\({}_{2}\) curing can lead to a variety of chemical modifications in the MgO cement system that result in the formation of hydrated magnesium carbonates like linsdoride (MgCO\({}_{3}\cdot\)5H\({}_{2}\)O), useabenonite (MgCO\({}_{3}\cdot\)3H\({}_{2}\)O), attribute (MgCO\({}_{3}\cdot\)Mg(OH)\({}_{2}\cdot\)3H\({}_{2}\)O), hydromagnesite (4 MgCO\({}_{3}\cdot\)Mg(OH)\({}_{2}\cdot\)4H\({}_{2}\)O, and dypinite(4 MgCO\({}_{3}\cdot\)Mg(OH)\({}_{2}\cdot\)5H\({}_{2}\)O) [55; 56; 57]. The strength and density of cement pastes can be increased by these hydrated magnesium carbonate compounds, which can also fill microscopic pores [58; 59]. Also, CO\({}_{2}\) curing might produce a weak acid source, which could be advantageous for increasing the strength of MOC cement [60; 61; 62]. According to the reported studies, phase 5 would be converted into phase 3 hydration products during the natural carbonation of MOC. The following new phases would form after phase 3 experienced further carbonation (Equations (4) and (5)) [32; 63]:  
\[5\text{Mg(OH)}_{2}\cdot\text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O}\to 3\text{Mg(OH)}_{2}\cdot \text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O}+2\text{Mg(OH)}_{2} \tag{4}\]  
\[3\text{Mg(OH)}_{2}\cdot\text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O}+2\text{CO}_{2} \to\text{Mg(OH)}_{2}\cdot\text{MgCl}_{2}\cdot 2\text{MgCO}_{3}\cdot 6\text{H}_{2} \text{O}+2\text{H}_{2}\text{O} \tag{5}\]  
Furthermore, this carbonation process has the potential to enhance the water resistance of MOC, attributed to the formation of a new phase (Mg(OH)\({}_{2}\cdot\)MgCl\({}_{2}\cdot\)2 MgCO\({}_{3}\cdot\)6H\({}_{2}\)O), which exhibits significantly lower solubility in water compared to phase 3 or phase 5. [PERSON] et al. [64] studied the influence of pulverized fuel ash (PFA) and CO\({}_{2}\) curing (25 \({}^{\circ}\)C and relative humidity 55%) on the water resistance and volume stability of MOC and found that both CO\({}_{2}\) curing and PFA addition significantly improved the water resistance of MOC. It was further reported that the strength retention coefficient of MOC increased from 7.9% to 72.2% when 30% of PFA was incorporated in the MOC mix as a partial replacement of MgO powder. After exposure to CO\({}_{2}\) curing, the strength retention coefficient for the control mix increased from 7.9% to 40%, whereas for the mix with 30% PFA, it increased from 72.2% to 81.4%. The authors attributed the improved water resistance (increased strength retention coefficient) to the development of insoluble amorphous gel (i.e., magnesium-chloride-silicate-hydrate (M-Cl-S-H) gel and magnesium-chloride-hydrate (M-Cl-H) gel) and densification of microstructure (Figure 1). Hence, CO\({}_{2}\) curing can enhance the water resistance of MOC blended with PFA but could not play any role in declining the expansion of MOC samples caused by water immersion [64]. Moreover, [PERSON] et al. [65] highlighted the potential of MOC to sequester CO\({}_{2}\) from the atmosphere, indicating its sustainable nature.