Mechanical properties and microscopic characteristics of fly ash–slag composite backfill

Abstract The strength of the backfill is crucial for safe and efficient mining, and the cementing material is the key to obtaining a strong backfill. In this paper, industrial waste was used as a raw material, into which basis basalt fibres of different lengths and dosages were incorporated. First, the influencing factors were determined through orthogonal test designs. The optimum water–cement ratio and basalt fibre dosage and length were obtained, and thereby the complete proportion of the backfill material was obtained. The compressive strength and slump of the backfill material mixed with basalt fibres were investigated, and the interfacial mechanism of the backfill material was analysed at the microscopic level using scanning electron microscopy and X-ray diffraction. The results indicated that the degrees of influence of the different factors on the mechanical properties of the backfill material decreased in the following order: water–cement ratio > fibre dosage > fibre length. Furthermore, there were different degrees of interaction among them. Under the best test proportion, that is, when the water–cement ratio was 2, the fibre dosage was 2%, and the fibre length was 6 mm, the slump was 210 mm, and the compressive strength was 5.03 MPa. The mechanical properties were the best at this time. Through data regression, the relationship between the fibre dosage and the average peak stress was established, and the full curve expression for the uniaxial compressive stress–strain of the BFGF specimen was obtained. According to a microscopic characterisation analysis, the slag formed C-S-H gel, which contributed to the early-stage strength of the filling material; fly ash and coal gangue contained a large amount of aluminium, which formed hydrated aluminosilicate with a framework structure, and this contributed significantly to the later-stage strength of the backfill material. The addition of basalt fibres reduces the internal defects of the backfill, allowing the internal pore structure of the backfill to be optimised and the compressive strength to be increased. The results of this study have been successfully implemented in actual projects. Additionally, they provide a reference for similar mine management projects and have considerable promotion and application value.