# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 2 Strength Development Mechanism of MOC  
MOC is a potential construction material with relatively high compressive strength, elastic modulus, and flexural strength [37; 38; 39]. The developed hydration products and microstructure of MOC have a significant impact on its strength development. The primary phases in MOC consist of MgO, Mg(OH)\({}_{2}\), and crystalline phases (phases 3 and 5). The phases are produced through the hydration process of MOC as represented in Equations (1)-(3) [40].  
\[3\text{MgO}+\text{MgCl}_{2}+11\text{H}_{2}\text{O}\to 3\text{Mg(OH)}_{2} \cdot\text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O}\text{ (Phase 3)} \tag{1}\]  
\[5\text{MgO}+\text{MgCl}_{2}+13\text{H}_{2}\text{O}\to 5\text{Mg(OH)}_{2} \cdot\text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O}\text{ (Phase 5)} \tag{2}\]  
\[\text{MgO}+\text{H}_{2}\text{O}\to\text{Mg(OH)}_{2} \tag{3}\]  
Particularly, the development of phase 5 crystals has generally been considered as the most desired reaction product, and it is largely recognized that the fast development of interlocked phase 5 crystals had a significant role in the mechanical characteristics of MOC [12; 41]. Needle-shaped phase 5 crystals formed during hydration extend into gaps, growing in a staggered and interlocking manner, leading to a dense microstructure and significant strength increase [42]. [PERSON] et al. [43] found that the hardening behavior and strength characteristics of MOC were primarily due to the development of needle-shaped phase 3 and phase 5 crystals within the matrix. Further, they mentioned that these crystals exhibit robust growth characteristics within the porous zones, resulting in a denser and more compact matrix.  
Moreover, [PERSON] et al. [44] suggested that the earlier hardening and commencement of stiffness were produced by the dense and compacted microstructure which was driven by needles (phase 5 crystals) produced in regions of porosity. The development of strength was due to the crystal phases' free growth into a denser structure. In addition to this, the fast chemical reaction of MgO with high reactivity may also result in an immediate increase in strength [44]. MgO is classified into three types based on the calcination temperature, resulting in different reactivity levels [45; 46].  
1. Dead-burned magnesia: High-temperature calcination (1500-2000 \({}^{\circ}\)C) reduces available surface area, resulting in unreactive MgO.  
2. Hard-burned magnesia: Calcination temperatures ranging from 1000-1500 \({}^{\circ}\)C yield MgO with limited reactivity. 3. Light-burned magnesia: Produced at 600-1000 \({}^{\circ}\)C, also known as caustic calcined magnesia, this form exhibits a reactive nature.  
[PERSON] et al. [47] investigated the impact of MgO calcined at temperatures between 600-1000 \({}^{\circ}\)C on the compressive strength of MOC. They observed a rise in compressive strength with increasing calcination temperature from 600 to 800 \({}^{\circ}\)C. However, no significant difference in compressive strength was noted between temperatures ranging from 800 to 1000 \({}^{\circ}\)C. [PERSON] et al. [45] also investigated the effect of MgO reactivity by analyzing the compressive strength of MOC with MgO calcined at varying temperatures. They observed that the 1-day (d) compressive strength of MgO ( calcined at 600 \({}^{\circ}\)C) modified MOC ranged from 5 to 7 MPa, significantly lower than the 1-day strength of 25-32 MPa for MgO ( calcined at 900 \({}^{\circ}\)C) modified MOC. Furthermore, MgO calcined at 1500 \({}^{\circ}\)C (dead-burned magnesia) failed to set even after 24 h. [PERSON] et al. [45] further recommended MgO calcined at 900 \({}^{\circ}\)C as optimal for MOC manufacturing. In summary, the discussion indicates that MgO calcined within the range of 800-900 \({}^{\circ}\)C may be best suitable for MOC production.