Interfacial porosity model and modification mechanism of broken coal grouting: A theoretical and experimental study

Grouting is the most effective approach for underground broken coal treatment. Traditional cement (TC) has difficulty achieving an ideal grouting anti-seepage and reinforcement effect due to the hydrophobic property of coal. This work developed an interfacial porosity model given Gibbs free energy changes during the droplet spreading on coal surface. And the key factors that affect the interface bonding behaviors were analyzed by means of model calculation. Accordingly, four types of wetting agents were selected to modify the coal hydro-phobic property via Fourier Transform Infrared Reflection, pulverized coal settling, and contact angle tests. Results showed that the anionic wetting agent WA-2 exhibited the greatest improvement on the coal wettability, and its addition resulted in a fine promotion effect at 0.5%. Furthermore, grouting simulation experiments on broken coal were conducted to compare the actual grouting effects of the modified and traditional cement. Nuclear Magnetic Resonance results reveal that the porosity of the grouted coal specimens by modified cement (MC) was less than that of TC. Moreover, the quantities of mesopore and macropore were remarkably decreased by 85.76% and 96.69%, respectively. Scanning Electron Microscopy results also presented a superior bonding performance between the MC and the injected coal, which indicates that the surface wettability of coal can be improved by the MC, thereby weakening the coal hydrophobic effect and promoting ion diffusion during the cement hydration process. In effect, resulting in the enhancement of the occlusion force and bonding strength of the grout-coal interface. The finding demonstrates that the wettability modification of cement grout contributes to improving the microstructure of the grout-coal interface, substantially ameliorating the grouting quality and prolonging the stability of broken coal on the macro-level.