This study investigates structural and morphological changes in limestone particles ground in a planetary ball mill. The grinding tests were carried out as a function of the revolution speed (100-300 rpm) and time (7-120 min). The feed size, the grinding media and the filling rate were kept constant. The size and shape of the particles were characterized by laser scattering and scanning electron microscopy (SEM), respectively. The uniformity index given by the Rosin-Rammler particle size distribution combined with SEM micrographs was useful to show the decrease of the grinding rate caused by the agglomeration of fines lying on partially broken particles. The effect of the mechanical action Of grinding on the crystalline structure of calcite grains was investigated by X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy was performed before and after the samples being irradiated with a dose of 5 kGy of gamma rays. XRD analysis only showed a net reduction in the intensity of the (10 (1) over bar4) peak and a slight increase in the line broadening of this peak. On the other hand, the energy input provided by the grinding action modified the population of paramagnetic defects existing in calcite lattice. In addition to the creation of electron traps responsible for a signal with a g-factor equal to 1.9999, the EPR measurements showed that the intensity of the hyperfine lines of Mn2+ substituting Ca2+ was affected by the grinding action. The morphological and particle size analysis together with the relationship found between the intensity of paramagnetic defects and specific surface area of limestone particles provided a picture of the onset of the agglomeration process. These results were discussed in order to explain the apparent grinding limit observed in ultrafine milling of limestone. (C) 2015 Elsevier B.V. All rights reserved.