Research on microstructure and physico-mechanical properties of grouting materials based on nano calcium carbonate-polyurethane composite modification

• Nano-calcium carbonate (NC) and polyurethane (PU) are used for composite modification of mining grouting materials, addressing issues like low strength and poor interfacial adhesion of broken coal grouting. • The optimal water-solid ratio of 0.5 and NC content of 1% are determined, with the composite-modified grouting material increasing the compressive strength of cemented broken coal by 43.6%. • NC promotes cement hydration to optimize pore structure, while PU enhances interfacial adhesion; their synergy effectively seals pores and microcracks, improving material density, durability, and damage resistance. • AE monitoring and creep tests confirm that the modified material reduces micro-damage during failure, maintains integrity, and exhibits superior long-term bearing strength and deformation capacity. Aiming at the problems of low grouting strength, high brittleness and insufficient interfacial adhesion of broken coal in coal mines, the optimum water-solid ratio of grouting materials was determined. Nano-calcium carbonate (NC) and polyurethane (PU) composite-modified grouting materials were used to conduct uniaxial compression, acoustic emission(AE) monitoring and electron microscope scanning tests. The strength and deformation characteristics of broken coal reinforced with NC and PU-modified mine grouting cement materials were studied. The macro and micro characteristics and long-term stability of the contact between broken coal and slurry were analyzed. The findings indicate that the altered slurry significantly enhances the compressive strength of cemented fractured coal, with an increase of 43.6% in strength. NC particles can enhance the hydration reaction of cement, expedite the development of hydrated calcium silicate (C-S-H) gel, and consequently improve the pore structure in the grouting material. PU enhances the adhesion between the slurry and the coal block interface. The synergistic interaction of the two can efficiently occlude pores and microcracks, enhancing the material’s density, durability and damage resistance. AE and electron microscopy characterization, along with mechanism analysis, demonstrated that incorporating NC and PU significantly diminishes the micro-damage extent of the sample during failure, preserves the sample’s integrity throughout the instability and failure phases post-cementation, and improves the material’s toughness and load-bearing capacity. The creep test further confirmed that the modified cement has higher long-term bearing strength and deformation ability. The grouting reinforcement mechanism is mainly due to the synergistic coupling of cement, NC and PU, which provides ideas for the research and development of high-performance grouting materials in deep coal mines.