# A State-of-the-Art Review on the Study of the Diffusion Mechanism of Fissure Grouting
## 2 Current Status of Research on Grouting Materials
### Inorganic Grouting Materials  
Inorganic grouting materials can be categorized into three types according to the slurry's state: particulate materials, adhesive materials, and composite materials. Particle materials are more common, such as ordinary cement slurry [16; 17], ultra-fine cement slurry [18], etc. Adhesive materials mainly include sodium silicate slurries [19] and silica sol [20]. Gelatinous materials mainly include sodium silicate and silica sol. Mixed materials mainly refer to a mixture of cement and sodium silicate slurry [21; 22]. Inorganic grouting materials can meet requirements for grouting under a large part of engineering geological conditions. The most commonly used slurries for underground water-sealing engineering include the common cement type, the cement-based composite type, the cement-water glass type slurry, etc. However, different ratios are allocated according to the actual engineering situation, taking into account the properties of different cement-based slurries. For example, common cement slurries are advantageous because they are low-cost, easy to obtain, and have properties similar to those of cemented rock masses, making them the main choice of grouting materials. However, they also have characteristics of high viscosity and strong granularity, so they have disadvantages such as easy sedimentation of particles, poor stability, and low viscosity, making it more difficult to inject them into small fissures and thus more difficult to meet construction requirements [23]. At this time, ultrafine cement  
Figure 1: Classification of grouting materials. slurry is considered for grouting. Compared with traditional cement slurry, ultrafine cement slurry has the advantages of smaller particles, less sedimentation, a larger surface area, and a smaller particle size [24; 25]. Nano-silica sol, created by uniformly dispersing nanoscale silica in water, undergoes gel formation upon mixing with cementitious materials [26]. This sol has the advantages of low initial viscosity, controllable gel time, and suitability for grouting engineering applications. In the field of grouting material selection and optimization, new materials have emerged in recent years. [PERSON] et al. [27] investigated the dynamic properties of cement paste specimens with varying dosages of carbon nanotubes (CNTs) under impact loads. The findings showed that cement paste's dynamic mechanical characteristics were somewhat improved with carbon nanotubes. [PERSON] et al. [28] developed anti-scouring grouts using a combination of bentonite, fly ash, potassium dihydrogen phosphate, sodium silicate, and light-burned magnesia. The grouts' compressive strength, viscosity, and setting time were influenced by the molar ratio of MgO to KH\({}_{2}\)PO\({}_{4}\) (M/P) and the contents of fly ash and sodium silicate. [PERSON] et al. [29] introduced a novel cement-based grouting material, SJP (S, Sichuan; JP, Jinping), which addressed diffusion and filling challenges in micro-fissured rock masses. [PERSON] et al. [30] investigated the diffusion and sealing of cement-water glass slurry in rough fissures using an orthogonal experiment, which offered guidance for practical applications in grouting and sealing engineering. [PERSON] et al. [31] prepared and tested four lime-based grout compositions, studied the prismatic behavior and effectiveness of fiber-reinforced lime-volcanic grout, and made macroscopic observations and measurements of pulse velocity and compressive strength before and after grouting. The results showed that the application of fiber-reinforced lime-based grout in the restoration of historical masonry is feasible. [PERSON] et al. [32] used a calcium sulfoaluminate cement (CSA)--silicate cement (PC)--gypsum ternary composite system to prepare a new type of grouting material. The factors affecting the performance of the grouting materials were analyzed by combining macro-experimentation and micro-analysis. [PERSON] et al. [33] enhanced the mechanical strength of a slurry by incorporating palm oil fuel ash into ordinary cement.