# Experimental Study of Imbibition Characteristics of Silica Sol in Coal-Measure Mudstone Matrix
## 1 Introduction  
Coal-measure mudstone has low bounding strength and developed micropores. When invaded by underground water through a dynamically developed fracture system, it showed nonlinear large deformation and significant creep characteristics. Mudstone is highly sensitive to mining and driving disturbances; these are the key weaknesses of the durability of the roadway surrounding rock-controlling structure, under the coupling effect of rock stress, water, and wind [1]. Many engineering practices (Figure 1) showed that grouting was an effective technology for fundamentally utilizing and developing the bearing capacity of surrounding rock, and was a key measure to prevent the degradation of mudstone by water.  
Coal-measure mudstone is a typical heterogeneous dual-porosity media. A cement slurry that is most commonly used in mining had a minimum groutability width of 0.1 mm, with almost no effect on the matrix system (nanoscale). Bleeding of the cement slurry would muddy and close the fracture surface, changing the seepage path and even blocking the grouting channels [2,3]. The groutability of sodium silicate materials and high-polymer materials was higher than the cement slurry, but also had little effect on the matrix system [4]. Hence, micropore sealing and reinforcement remained a difficulty that engineers committed to solve; this is an unavoidable fundamental question for serious deformation control of mudstone.  
Silica sol is an environment-friendly, nanoscale, unimolecular aerogel [5; 6], consisting of nanometer-sized particles of amorphous SiO\({}_{2}\) cores with hydroxylated surfaces. Therefore, it can be expressed as mSiO\({}_{2}\)-nH\({}_{2}\)O suspended in water [7]. The gelation does not take place in a stabilized silica sol before it is destabilized by the addition of appropriate electrolytes (such as NaCl and CaCl\({}_{2}\)). When silica sol starts to aggregate, a stable network comprising silica particles forms [8].  
[PERSON] built a shear gelation model and analyzed the experimental observations on silica sol grout injection into a sand column [9]. [PERSON] and [PERSON] modeled silica sol as fully soluble in UTCHEM (the University of Texas Chemical Flood Simulator) to simulate silica sol transport through sand columns [10; 11]. [PERSON] studied how the penetration length of silica sol in a fracture system can be computed analytically both as a 1-D channel flow and a 2-D radial flow [12]. In conclusion, studies to date have concentrated on the transport mechanism of silica sol in sand and in a fracture system; characteristics of silica sol flow in low-permeability porous media are insufficient.  
Using mudstone in the Huaibei mining area and silica sol as the grouting material as an example, this paper studies the imbibition effect of silica sol in coal-measure mudstone matrix. Groutability, driving force, and diffusion difficulty affecting the imbibition effect were tested by an AutoPore IV 9510 mercury porosimeter (Micromerities Instrument Corporation, Norcross, GA, USA), ZS90 nanoparticle size analyzer (Malvern Instruments, Worcestershire, UK), DSA100 optical contact-angle measuring device (KRUSS GmbH, Hamburg, Germany), K100 surface tension meter (KRUSS GmbH, Hamburg, Germany), NDJ-5S rotary viscosity meter (Shanghai Pingxuan Scientific Instrument Co., Ltd., Shanghai, China), and other devices. Finally, in order to study colloid spontaneous migration and phase characteristics, the pressureless imbibition process was conducted using nuclear magnetic resonance (NMR) equipment (Niumag Electronics and Technology Corporation Limited, Shanghai, China) for 216 h.