## 4 Conclusions  
Through the fracture grouting test of FCM grouting material and the unconfined compression test and electron microscope scanning of fractured sandstone grout-reinforced specimens, the following conclusions are obtained.  
(1) The flow ability and condensation ability of FCM under low water-material ratio are five times that of P.O 42.5 cement, which meets the relevant requirements of grouting materials, and FCM has certain expansibility, which is well cemented with red sandstone. The grout-reinforced specimens formed by the FCM and sandstone can reach the original rock or even exceed the original rock strength.  
(2) Grout-reinforced specimens and sandstone belong to brittle materials, and the degree of penetration and inclination angle of grout-reinforced specimens fracture determine the stress-strain relationship of the specimens. There is a certain similarity in the failure process of the specimens, which is composed of four stages: compaction stage, elastic deformation stage, crack propagation stage, and failure stage. For the full cracks, the correlation between the compressive strength of grout-reinforced specimens and the dip angle of cracks is high. When the dip angle increases from 0\({}^{\circ}\) to 60\({}^{\circ}\), the compressive strength of the reinforcement gradually decreases, and the compressive strength of grout-reinforced specimens with 90\({}^{\circ}\) cracks is between 45\({}^{\circ}\) and 60\({}^{\circ}\).  
(3) The acidic environment and freeze-thaw cycle will erode the grout-reinforced specimens, mainly the erosion of the grout-reinforced specimens surface, and as the fracture increases, the degree of erosion increases, and the strength of the grout-reinforced specimens decreases, and the degree of fracture penetration and inclination determine the degree of reduction of the strength of the grout-reinforced specimens.  
(4) FCM grouting material is well cemented with red sandstone. After soaking in acidic solution and freeze-thaw cycles in an acidic environment, the surface of grouting material appears to have particle loss, and the surface of the sandstone appears as a \"honeycomb\" area. With the increase of freeze-thaw cycles, the area of particle loss and honeycomb area increases.  
Figure 13: Different treatment methods of grout-reinforced specimens average aperture.  
**Author Contributions:** Methodology, D.Y. and J.Z.; Formal analysis, X.H.; Investigation, X.H.; Resources, D.Y.; Data curation, D.Y.; Writing--original draft, D.Y.; Writing--review & editing, X.H. and T.L.; Supervision, X.H. and T.L.; Project administration, J.Z.; Funding acquisition, J.Z. and T.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by National Natural Science Foundation of China: U2006213; National Natural Science Foundation of China: 42277139; National Natural Science Foundation of China: 42207172; China Postdoctoral Science Foundation: 2022M712989; Natural Science Foundation of Shandong Province: ZR2022 QD103.  
**Data Availability Statement:** No new data were created, or where data is unavailable due to privacy or ethical restrictions.  
**Conflicts of Interest:** The authors declare no conflict of interest.