# Magnesium Oxychloride Cement: Development, Opportunities and Challenges
## 1 Introduction
Various additives such as silica fume [19; 21], fly ash (FA) [22], sulphates [23; 24], nanomaterials [25; 26], silica glass [27], active silica, and soluble phosphates or phosphoric acid (PA) [28; 29] have been incorporated in MOC to obtain a dense matrix and water-stable hydration products [8; 19; 30], with some breakthrough in the development of water-resistant MOC. Past studies found that these additives facilitate the development of gel-like phase 5 and may induce the transformation of the micromorphology of phase 5, which may be the possible reason for its improved water-resistant behavior [8; 20]. [PERSON] et al. [30] extensively examined the effect of silica fume and FA on the water resistance of MOC, observing improved water resistance due to the formation of silicate 5.1.8 gel or alumina-silicate 5.1.8 gel in MOC modified with these additives. [PERSON] et al. [19] incorporated silica fume and a hybrid of FA and silica fume into MOC, achieving high compressive strength retention of around 100% and 95% with 28-day and 56-day water immersion, respectively, for a mix containing 15% FA and 15% silica fume. They attributed the improved water resistance to the formation of Mg-Cl-Si-H gel and the filler effect of FA and silica fume, resulting in a densified MOC matrix. These studies show that the incorporation of SCMs has the potential to produce water-resistant MOC. In addition to the issue related to water resistance, MOC is unsuitable for steel reinforcement. It is because of the chemical composition of the MOC which contains approximately 1.5-6% free chloride ions thus making MOC matrix more vulnerable to corrosion [31]. Chlorine ions are recognized to be a type of very potent depassivator. Its high concentration can break the passivating film of steel under low alkalinity conditions, which causes significant corrosion [12; 32]. Moreover, the susceptibility of MOC to failure under tension and its poor durability are other main flaws. Significant dimensional instability, poor weathering resistance, and the discharge of corrosive solutions upon steel reinforcements have all hindered MOC from being widely used in civil engineering infrastructures [32; 33]. Despite the above-mentioned challenges, MOC has been regarded as intrinsically fire-resistant [11; 34] attracting practical implementation in fire-resisting infrastructural applications like cladding and facade. However, the research efforts in this area are minimal, and to date, the systematic research work conducted to evaluate the mechanical performance of MOC-based composites at elevated temperatures is very scarce [35]. Nonetheless, some past studies have shown that MOC has the potential to be used as a fireproof coating material and can resist high temperatures without deteriorating the interior substrate [11; 34; 36]. Despite the extensive research and development, there remains a lack of studies which systematically summarize the findings focusing on the primary potential applications of MOC such as outdoor and fire-resistant applications. On account of the evident potential of MOC across various civil engineering applications, there is a need for a comprehensive review to aid ongoing researchers and engineering practitioners in understanding the developments, challenges, and future investigations. Therefore, this review attempts to bridge this gap by comprehensively summarizing the recent advancements in principal application domains of MOC. The paper is structured as follows. First, the strength development mechanism of MOC is introduced in Section 2. The curing method, including normal curing and CO\({}_{2}\) curing of MOC, and the effect of curing on the mechanical properties of MOC are explained in Section 3. Section 4 describes the effect of SCMs, including fly ash, ground granulated blast furnace slag (GGBFS), and metakaolin (MK) on the properties of MOC. The recent research and development of ductile FRMOC-based composites and water-resistant MOC are presented in Sections 5 and 6, respectively.