# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 1 Introduction  
Magnesium oxychloride (MOC) was discovered for the first time by [PERSON] in 1867, hence it is also referred to as Sorel cement [1]. It is an eco-friendly and nonhy-draulic cement developed by blending magnesia (light burnt MgO) with a concentrated magnesium chloride (MgCl\({}_{2}\)) solution. MOC has gained favorable attention due to its many characteristics that distinguish it from ordinary Portland cement (OPC). OPC production is an energy-intensive process and a leading source of CO\({}_{2}\) emissions, responsible for 5-10% of global greenhouse gas emissions. Studies have reported that approximately 0.85 tons of CO\({}_{2}\) are emitted for every ton of OPC produced [2, 3]. This substantial release of CO\({}_{2}\) poses significant environmental risks, including ozone depletion and exacerbation of global warming, underscoring the urgent need for more sustainable cement and production methods. On the other hand, the manufacturing of MgO, the main raw material of MOC, requires lower temperatures (<1000 \({}^{\circ}\)C) making it an energy-efficient alternative [4]. Moreover, the resulting cement using MgO also has high mechanical strength [5], air hardening ability, light weight, low thermal conductivity [6], good abrasion resistance [7], and resistance to oils, greases, and paints [8]. In addition, MOC possesses high early-age strength and has good bonding potential with various fillers, including gravel, sand, wood particles, expanded clays, asbestos, and marble dust [6, 9, 10]. MgO could also capture the greenhouse gas CO\({}_{2}\) to generate a variety of carbonates and hydroxy carbonates, making MOC a green and \"carbon-neutral\" cement [11]. Over the past few decades, extensive research has been conducted to investigate the strength development, reaction mechanism, and microstructure of the MgO-MgCl\({}_{2}\)-H\({}_{2}\)O ternary system. MOC production typically creates four primary reaction crystal phases, i.e., phase 2 (2 Mg(OH)\({}_{2}\cdot\)MgCl\({}_{2}\cdot\)4H\({}_{2}\)O), phase 3 (3 Mg(OH)\({}_{2}\cdot\)MgCl\({}_{2}\cdot\)8H\({}_{2}\)O), phase 5 (5 Mg(OH)\({}_{2}\cdot\)MgCl\({}_{2}\cdot\)8H\({}_{2}\)O), and phase 9 (9 Mg(OH)\({}_{2}\cdot\)MgCl\({}_{2}\cdot\)5H\({}_{2}\)O) [12]. The crystallization of such primary reaction phases is influenced by several significant factors, including the reactivity of MgO [13], MgO/MgCl\({}_{2}\) molar ratio, curing conditions [14], and chemical admixtures [10; 15]. Phases 2 and 9 stabilize when the curing temperature exceeds 100 \({}^{\circ}\)C, whereas phases 3 and 5 remain stable at temperatures below 100 \({}^{\circ}\)C. Phases 3 and 5 are described as scroll-tubular whiskers that resemble well-crystallized needles. The interlock and compact microstructure that occurs during the production of these phases make them the main reaction products that are responsible for the hardening and strength development of MOC. Phase 5 also undergoes rapid development, initiating approximately two hours after paste blending, and exhibits good void-filling characteristics, leading to a compact microstructure with minimal porosity. As a result, phase 5 is the preferred phase in the design of MOC. Despite the unique characteristics and advantages of MOC, it faces significant challenges in its use as a structural material. Firstly, MOC has very poor water resistance capability. It significantly degrades in a moist environment as a result of the leaching of MgCl\({}_{2}\), causing a loss in strength. Existing research has revealed that the compressive strength drastically decreases when MOC is soaked in water, owing to the transition of phases 3 and 5 [16; 17]. In consequence, the damaged matrix also considerably accelerates the process of water infiltration, increasing deterioration and creating a vicious circle of continuing damage [18]. In recent years, significant contributions have been made by researchers to enhance the water resistance of MOC [8; 19; 20].