Coal and gas outburst prevention using new high water content cement slurry for injection into the coal seam

As coal and gas outburst is one of the most serious mine disasters, it is very important to at least control it if not prevent it from occurring. Injecting cement slurry or grouting into the coal seam can strengthen the seam, increase its rigidity coefficient (f), and reduce the volumetric expansion due to gas energy release. This paper reports the results of laboratory experiments on cement-based high water content slurry having different water-cement ratios (W/C) to be used for coal injection. The results show that as the W/C increases, the mobility of the slurry and its setting time increase. The compressive strength and rupture strength, however, are reduced. Furthermore, high W/C grout shows early strength after 7 days, which can be 80% of its 14-day compressive strength. To achieve rapid setting and early strength, the addition of Na2SiO3 has proven to give the best result, when the concentration of the additive is 3%. The initial and final setting times are 13 and 21 min shorter than samples without Na2SiO3, while the compressive strength is more than double. As a retarder, the initial setting time can be extended to 83 min when tartaric acid of 0.4% concentration is added. Through the orthogonal experiment, the optimum recipe of the new high water content slurry has been determined to be: W/C = 2, tartaric acid = 0.2%, Na2SiO3 = 3%, and 12% bentonite. Reinforcement by injection simulation experiments show that the grouting radius of the new slurry mix is 250 mm when the applied grouting pressure is 60 kPa, 7-day rupture strength and compressive strength are 5.2 and 6.4 MPa, respectively, and are 37% and 88% higher than ordinary cement grout. It can be concluded that the newly developed slurry mix is more effective than the ordinary mix for reinforcing coal and controlling gas outburst.