Macroscopic and Microscopic Investigation on Microfractures in Blast-Conditioned Rock and the Influence of Particle Size

In the mining industry, particle size reduction is the most energy-demanding activity. Blasting represents the first stage of comminution. Experimental and field observations have demonstrated that blasting produces two main effects on rock: (i) macroscopic fracturing and fragmentation, and (ii) microscopic fracturing, consisting of a network of microfractures that weaken the rock, reduce the specific Work Index, and make the material less resistant to crushing and milling. The present work represents an initial investigation into the relationship between blast-induced microfracturing, fragment size, and mechanical resistance. Blasted rock was analyzed using three approaches: macroscopic testing via point load tests, laboratory grinding tests using a Bond ball mill to determine the blasted Work Index, and microscopic optical observation of microfractures. The results show that macroscopic testing is unable to detect microscopic weakening, as no correlation was observed between point load strength and particle size. In contrast, laboratory ball mill tests and microscopic optical observations indicate a preliminary relationship between particle size and the internal weakening of particles. These results allow the formulation of a new hypothesis: that the Work Index may not be constant within a given volume of blasted rock and could depend on the particle size distribution.