## 3 Results and Discussion
As can be seen from Figure 3c, the addition of epoxy resin can enhance the compressive strength; the EPUP-WG-4 with 15 phr of WG and epoxy resin showed the highest compressive strength, which was 0.27 MPa. However, with the increase in WG, the compressive strength decreased dramatically in three types of grouting materials. When the content of WG reached 15 phr, the compressive strength of the system tended to be stable. The CO\({}_{2}\), which was produced from the reaction between isocyanates and water, served as the forming agent. Therefore, the amount of water played a crucial role in the expansion rates [19]. As can be seen from Figure 3d, the expansion rates increased with the addition of WG and finally stayed at a stable value. This was because the addition of WG led to an increase in water in the system, and then the content of CO\({}_{2}\) in the system increased, while the content of -NCO was fixed. Compared with the PU-WG system, the -OH of the epoxy resin could react with -NCO to reduce the expansion rate of the EPU-WG grouting material. The -NCO, which reacts with polyols to form CO\({}_{2}\), was further reduced during the process of prepolymer preparation. Therefore, the expansion rate of the EPUP-WG system was the lowest when the content of WG was less than 15 phr. Since  
Figure 2: (**a**) FTIR spectrum of epoxy\(-\)isocyanate prepolymer, (**b**) FTIR spectrum of EPUP-WG-1, EPU-WG-1 and PU-WG-1. the content of -NCO was fixed and the silicate could consume a part of the CO\({}_{2}\)[20], the production and consumption of CO\({}_{2}\) in the system reached an equilibrium value when the WG was 15 phr. At this time, the expansion rate of both the EPU-WG and EPUP-WG systems was almost 57 and did not increase with the increase in WG content. The morphologies of the fracture surfaces of the grouting materials are shown in Figure 4. Compared to the PU-WG system, the cell sizes of EPU-WG and EPUP-WG were found to be smaller and more regular. After the addition of WG, the cell size of these three systems increased, especially the cell size of EPU-WG and EPUP-WG systems, which doubled. Combined with the compressive strength of the corresponding system, it could be concluded that the increase in cell size had a negative impact on the material's properties. The thermal stabilities of the composite foams (PU-WG-3, EPU-WG-3, EPUP-WG-3) were investigated with TGA under a nitrogen atmosphere. The corresponding curves are presented in Figure 5. The derivative thermogravimetry (DTG) curve of PU-WG-3 displayed four maximum degradation temperature, suggesting a four-stage thermal degradation process. The first stage occurred between 100-200 \({}^{\circ}\)C, and was attributed to the evaporation of water and other substances with a low boiling point. The second stage of degradation occurred between 200-300 \({}^{\circ}\)C, due to the loss of bound water and reaction water from Si-OH dehydration. Degradation occurred between 300-400 \({}^{\circ}\)C and was caused by the decomposition of some organic components in the PU structure. The fourth stage occurred between 400-500 \({}^{\circ}\)C; this stage of degradation was attributed to the decomposition of isocyanate trimers [21]. Figure 3: (**a**) The reaction temperature curves of different PU-WG, (**b**) the maximum reaction temperature, (**c**) the compressive strength, and (**d**) the expansion rate of grouting materials. The derivative thermogravimetry (DTG) curve of EPU-WG-3 showed three obvious stages in the thermal degradation process. The first stage was between 100-300 \({}^{\circ}\)C, which showed the volatilization of water and other substances with a low boiling point. The second stage was between 300-400 \({}^{\circ}\)C, which was mainly caused by the elimination reaction of the terminal groups (hydroxyl groups, amino groups) of the modified epoxy resin cured structure. The third stage occurred between 450-500 \({}^{\circ}\)C, and was caused by the degradation of the main chain of the epoxy resin.