Effect of CO2 foaming agent on the hydration and hardening properties of OPC-CSA-FA ternary composite filling materials

A safe, lightweight, inexpensive, quick-setting filling material is needed for high riser areas in coal mines. However, currently commonly used foaming agents, such as hydrogen peroxide, which produces the combustion gas O2, and aluminum powder, which is explosive gas H2, are not suitable for coal mine filling. Therefore, a new composite foaming agent that produces CO2, namely sodium bicarbonate and potassium aluminum sulfate, was proposed. The effects of CO2 foaming agent dosing on the fluidity, setting time, compressive strength, water absorption, and dry density of OPC-CSA-FA ternary composite paste filling material (CPFM) were systematically investigated. The hydration mechanism and pore structure evolution of the foamed filling materials were revealed by XRD, TG-DTA, SEM, etc. The results showed that CPFM blocks with a 28-day dry density of 790 Kg/m3, compressive strength of 2.73 MPa, and water absorption of 25.8% were prepared when the CO2 foaming agent was dosed at 10%. Compared with the control group, the setting time was shortened by 92.5%, the 28-day compressive strength was increased by 28.1%, and the 28-day water absorption was reduced by 48.3%. The addition of a small dose of CO2 foaming agent not only excites the fly ash activity and generates hydrocalumite and para-alumohydrocalcite, which are beneficial to the strength, but also a large amount of generated aragonite and nano calcite promotes the hydration and fills the pores to form a more dense microstructure. The above research provides new materials for foam filling management in high riser areas of coal mines.