Failure mechanism of a green substratum filling material based on digital scatter analysis

The utilization of solid waste products is of increasing importance. To improve the toughness of green geopolymers made from solid waste from the mining industry and prepare mine filling materials with excellent performance, this study proposes to prepare the filling material under the condition of tailing sand, slag, and fly ash (4:1:5) mixed with different amounts of polypropylene fibre (12 mm). Using the digital speckle technique, unconfined compressive strength tests were performed to study the internal energy evolution and failure mechanism of the samples. Micro-scanning techniques revealed the strength formation and fibre action mechanisms of the filling materials. The results obtained show that within the scope of the present study, the optimum unconfined compressive strength (UCS) of the specimen was 8.17 MPa when the NaOH doping was 3%, the water–solid ratio was 0.3, and the fibre doping was 6‰. The destabilization process of the specimen from loading to destabilization is a progressive destruction of the matrix material from void compaction to the same deformation of laminar displacement and then to local microcrack generation until the appearance of macroscopic vertical cracks. Fly ash and slag generate a large number of honeycomb topological network C-(A)-S-H polymerization products in alkaline environments and are filled between the tailing sand particles, between the tailings sand and the fibre interface, and inside the hollow polypropylene fibre; this promotes a continuous improvement in the compressive performance. In the matrix, to improve the UCS of the sample, the fibres mainly inhibit the initiation, development, and propagation of cracks by the three-dimensional network skeleton structure formed by 'bolt' mosaic and interlacing. The failure modes of fibre toughening and crack resistance mechanisms are mainly slip debonding and drawing fracture. The research results can provide a theoretical basis for the preparation of mine-filling materials from solid-waste materials.