Waste fines from rock breakage often negatively influence economics and environment. The Austrian Science Fund (FWF) sponsors a project to investigate the cause of the fines by studying blast fragmentation throughout small-scale blast tests and numerical simulations. The tests include blast-loading confined granite and mortar cylinders by detonating cord with 6, 12, and 20 g/m of PETN. The blast-driven dynamic cracking at the end face of the cylinder opposite to the initiation point is filmed with a high-speed camera. The filming is followed up by an analysis of surface and internal crack systems and sieving of the blasted cylinders to quantify the amount of fine material created. The numerical simulations cover the blast fragmentation of a mortar cylinder. These simulations use Finite and Discrete Element Methods (FEM, DEM) with explicit time integration. The model cylinders are loaded by a pressure evolution acting on the borehole wall. Both methods produce realistic crack patterns, consisting of through-going radial cracks with crack intersections around a crushed zone at the borehole. Furthermore, the DEM models have also yielded realistic fragment size distributions (FSD). The paper covers the present progress of the ongoing project and related future work.

Blast-induced fines in rock negatively influence multiple aspects of raw-mineral sustainability. The Austrian Science Fund (FWF) sponsored a project to investigate the cause of the fines by studying blast fragmentation through small-scale blast tests and numerical simulations. The paper covers the experimental part of the project focusing on internal blast-induced fracturing and related mechanisms. The blast tests were done by blast-loading confined granite and mortar cylinders. The blast-driven dynamic cracking at the end face of the cylinder opposite to the initiation point was filmed with a high-speed camera. Following analyses covered internal crack patterns, fracture surfaces, and sieving of the blasted cylinders to quantify the amount of fine material created. The internal crack patterns and fracture surfaces were analysed by means of computer tomography (CT) and scanning-electron microscopy (SEM). The CT scans show that the amount of explosive charge affects the changing of the topological features of the crack patterns along the cylinder. They also depict different deformation zones around the blast-hole wall with respect to the blasted material and the amount of charge. Although fracture surfaces of larger fragments do not clearly differ in measured roughness and curvature, the SEM scans of smaller fragments show clear difference in fracture surfaces with respect to the blasted material and the amount of charge. SEM scans of thin sections extracted from the blasted cylinders show different fracture features that could be related to the branching/merging mechanism.

High-resolution numerical simulations of cracks driven by an internal pressure in a heterogeneous and brittle granular medium produce fragment-size distributions with the same characteristics as experiments on blasted cylinders of mortar and rock in both the fine-and the intermediate-size-fragment regions. To mimic full-scale blasts used, e.g., within the mining industry, the cracks propagate in a medium that is under compression, neutral, or under tension. In a compressive environment, shear fracture produces a large volume of fines, whereas in a neutral or tensile environment, unstable crack branching is responsible for a much smaller volume of fines. The boundary between the fine-and the intermediate-size fragments scales as the average grain size of the material. The ultimate goal is to develop a blasting process that minimizes the fines, which, in mining, are both an environmental hazard and useless for further processing.

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