Abstract：The cemented filling technology in coal mine can effectively solve the problems of solid waste surface storage and a series of geological disasters caused by coal mining. In view of the lack of systematic research on the fluidity and mechanical properties of slurry in the underground filling of fly ash based composite admixture, the water content, compressive strength, setting time, SEM and mercury injection experiments of paste with water reducing agent, cement and fly ash composite admixture were carried out when the slump was 220 mm. The strength evolution law and hydration mechanism of composite paste were analyzed and discussed from macro and micro perspectives.The results show that the addition of water reducing agent can guarantee the slump of filling slurry under low water cement ratio, and the strength of composite admixture paste can be increased by 33 % by selecting appropriate type and dosage of water reducing agent. The water reducing agent can reduce the water content of the filling slurry, and the water reducing rate is 3% ~ 7% compared with the filling slurry without water reducing agent. The setting time of fly ash composite admixture filling slurry can be reduced by adding water reducing agent and increasing the dosage of water reducing agent. Compared with A-type water reducing agent and non-doped water reducing agent, the short-term strength and long-term strength of B-type water reducing agent are higher than those of the latter. The main elements in the C-S-H gel of fly ash paste with water reducer are Si and Al, and the main elements in the paste gel without water reducer are Ca and O. The fully developed C-S-H gel isfilled in the pores and cracks of the filling body in the form of crystals, which is helpful to improve the strength and durability of the filling body. Adding water reducing agent and increasing the amount of water reducing agent can reduce the porosity of the filling body, making the filling body structure more compact and more conducive to the improvement of the strength of the filling body. The research results can enrich the theoretical research in the fields of fly ash based cemented filling technology and solid waste disposal in coal mines in China.
