Study on reasonable aggregate size ratio of coal as pumping cementitious materials

To advance the green development of coal mine support technology and reduce secondary pollution induced by pumped support materials, this paper proposes raw coal particles as aggregates for pumped cementitious materials. The optimal aggregate gradation ratio is determined via theoretical analysis, and the effects of coarse, medium and fine aggregates on the compressive strength, initial slump and 1 h slump of specimens are revealed through laboratory tests. The experimental results indicate that compressive strength is generally positively correlated with the increased mass proportion of 20~30 mm coarse aggregate and 5~20 mm medium aggregate, while it presents an overall negative correlation with the rising proportion of 0~5 mm fine aggregate. Both initial and 1-hour slumps generally show a trend of first increasing and then decreasing with the rising mass proportion of 20~30 mm coarse aggregates and 0~5 mm fine aggregates, while the 5~20 mm medium aggregates exert a weak influence on initial slump yet can significantly improve 1-hour slump retention performance. Combined with correlation analysis and weight scoring method, the optimal aggregate gradation is determined as follows: the mass ratio of 30~20 mm aggregate∶20~5 mm aggregate∶5~0 mm aggregate = 20%∶25%∶55%. In field application, coal aggregate pumped cementitious materials formulated with 30–20 mm coarse aggregates (20%~25% by mass), 20~5 mm medium aggregates (25%~35% by mass), and 5~0 mm fine aggregates (45%~55% by mass) can satisfy all performance requirements for underground mine roadway support.