## 1 Introduction  
Magnesium oxychloride cement (MOC) is a gas-hardening gel material made of activated magnesium oxide powder and a certain concentration of magnesium chloride solution [1]. It has a short setting time, low alkalinity, fire resistance, heat resistance, and other properties [2]; after 28 days of curing, the compressive strength can reach 80-100 KPa and even 200 KPa after modification [3]. The above properties indicate that magnesium oxychloride cement is particularly suitable for foamed cement substrate and has application potential in light insulation board, composite wall panels, and other fields. The pore structure is an important property of foam materials. It also determines bulk density, heat preservation, heat insulation, and sound insulation performance [4]. On the other hand, its compressive strength is much lower than that of dense bulk materials [5]. In the actual use process, the continuous pore structure of the porous material leads to its extremely high water absorption, which further reduces the insulation effect, especially in moist and rainy areas. In addition, the water absorbed by the pore structures causes secondary damage such as a freezing fracture and mold propagation [6]. The setting time of ordinary Portland foam cement is long, the pore structure is unstable, and the volume water absorption rate increased rapidly after 72 h of blistering [7]. Magnesium oxychloride foam cement is made of magnesium oxychloride gel material as a matrix, thus it also inherits the disadvantage of poor water resistance of magnesium oxychloride cement. The main strength phase of magnesium oxychloride cement is the 5 Mg(OH)\({}_{2}\)MgCl\({}_{2}\)-8H\({}_{2}\)O (five phase), which easily changes from a needle-like network structure to a loosely packed layered structure after blistering [8; 9; 10]. Phosphoric acid and other additives can significantly improve the water-resistance of magnesium oxychloride cement [11]. However, the natural hydrophilicity of inorganic materials still cannot prevent water vapor from adhering to the inside and the outside of the porous material and gradually eroding its gel structure. It has been found that, by controlling the pore structure, minimizing the connection between pores can significantly reduce the capillary water absorption of foamed cement [12]. Due to the high dopability of magnesium oxychloride cement, the addition of graphene and graphene oxide [13], Magnesium Oxychloride 5 Mg(OH)\({}_{2}\)-MgCl\({}_{2}\)-8H\({}_{2}\)O [14], granulated Scrap tires andexpanded glass [15] can also increase its water resistance, but this is limited. The introduction of hydrophobic materials into foam materials can significantly improve the water absorption of foam cement. In the research of conventional silicate and sulfoaluminate foam materials, by dipping polystyrene into the material surface [16], silane-modified graphene oxide (GO) coating [17], organosilane coating [18], etc., can significantly reduce water absorption. The advantage of the surface dipping method is that the method is simple and does not affect the hydration reaction process of the cement material, but because the surface hydrophobic layer is only a few hundred nanometers, the hydrophobic layer wears easily. Therefore, improving the moisture resistance of foamed materials has become a key issue in the field of thermal insulation building materials. If the porous material is given overall hydrophobic properties, it can avoid the reduction of hydrophobicity due to surface wear and long-term use. Adding organosilane to the cement system and using the hydrophobicity of the organosilane hydrolysate can achieve the overall hydrophobicity of Portland cement materials [19], but the hydration reaction of MOC materials is quite different from that of Portland cement. According to our knowledge, there is no literature report on the hydrophobicity of MOC materials.