## 3 Results and Discussion  
### The Bulk Density and Pore Structure of Foam Material  
From the bulk density of F-MOC and N-MOC obtained with different H\({}_{2}\)O\({}_{2}\) content in Figure 1, it can be seen that the bulk density of F-MOC obtained with the same H\({}_{2}\)O\({}_{2}\) content was greater than that of N-MOC. The standard deviation of the bulk density of F-MOC and N-MOC shows that, although the conditions for preparing the samples were the same, the bulk density of the samples was different, and the stability was poor when only H\({}_{2}\)O\({}_{2}\) was used as the foaming agent. After adding FAS and TEOS, the standard deviation was reduced, and the stability was significantly improved. To a certain extent, the disadvantages of poor stability of the chemical foaming method can be improved. When preparing the hydrophobic foamed MOC material, FAS with MgO mass of 0.5%-2.5 wt % was added, and the content of TEOS was (0.5-2.5):6 based on the molar ratio of FAS to TEOS. The mass of H\({}_{2}\)O\({}_{2}\) was 10 wt % of the mass of MgO, and a series of foamed MOC materials were obtained. The bulk density of the material was maintained at 410-415 kg\(\cdot\) m\({}^{-3}\), indicating that the addition of FAS had little effect on the bulk density. Based on hydrophobicity and economy, in subsequent experiments, the FAS addition amount was selected to be 1% of the mass of MgO to explore the influence of H\({}_{2}\)O\({}_{2}\) and TEOS content on bulk density. From the three-dimensional graph of the influence of H\({}_{2}\)O\({}_{2}\) and TEOS content on bulk density in Figure 2, it can be seen that, when the TEOS content was fixed and the H\({}_{2}\)O\({}_{2}\) content increased from 4 wt % to 12 wt %, the bulk density decreased from 1200 kg\(\cdot\) m\({}^{-3}\) to 300 kg\(\cdot\) m\({}^{-3}\), thus the bulk density changed greatly. When the content of H\({}_{2}\)O\({}_{2}\) was fixed, n(FAS):n(TEOS) increased from 1:6 to 1:14, the relative content of TEOS increased, and the bulk density of the sample increased, with an average increase of 70 Kg\(\cdot\)m\({}^{-3}\). This indicates that TEOS may cause partial decomposition of H\({}_{2}\)O\({}_{2}\) during the pretreatment of the MgCl\({}_{2}\) solution. The results show that the content of H\({}_{2}\)O\({}_{2}\) had the greatest impact on the bulk density, followed by the content of TEOS, while the content of FAS had no effect. Figure 1: Comparison of foamed magnesium oxychloride cement (F-MOC) and magnesium oxychloride cement without 2H-tridecylfluoro-n-octylsilane (FAS) or tetraethylorthosilicate (TEOS) (N-MOC) bulk density under different H\({}_{2}\)O\({}_{2}\) content (FAS content was fixed at 1% of the mass of MgO; TEOS was added according to the molar ratio of FAS to TEOS of 1:6). Generally, the density grade of foam concrete with good thermal insulation properties is 300-1200 Kg\(\cdot\)m\({}^{-3}\)[24] Therefore, this study selected MOC samples within the above-mentioned density range for subsequent experiments and analysis. When adding the same content of H\({}_{2}\)O\({}_{2}\), the pore size distribution of F-MOC and N-MOC is shown in Figure 3 (F-MOC bulk density was 540 kg\(\cdot\)m\({}^{-3}\), N-MOC bulk density was 301 kg\(\cdot\)m\({}^{-3}\)). The pore size of F-MOC was small, and the pore size distribution was very uniform, mainly concentrated in \(210\pm 25\)um. On the contrary, N-MOC had a larger pore size and a wider distribution. The most easily achieved pore size distribution was \(490\pm 25\)um, which indicates that the addition of FAS-TEOS copolymer can make the pore size of the foamed MOC foam smaller. We chose F-MOC with a bulk density of 500-600 kg\(\cdot\)m\({}^{-3}\) to compare with N-MOC with similar bulk density (Figure 4). Although F-MOC and N-MOC had similar bulk densities, the pore size of N-MOC was significantly larger than that of F-MOC. The N-MOC sample had a large pore area and many solid parts, but the pores of the F-MOC foam material were very evenly distributed throughout the material.