The attrition of CaO-based solids operating in calcium looping (CaL) post-combustion CO2 capture systems is an important factor to consider when limestones are selected as sorbent precursors. In this work, four commercial natural limestones with similar chemical compositions but with very different mechanical properties were tested in a 30kWth calcium looping pilot plant. The main attrition mechanisms that act upon the limestones (i.e. fragmentation, decrepitation and abrasion) were identified on the basis of the evolution of their particle size distributions during the start-up and the first calcination of each batch. Great differences in the performance of the selected limestones were observed, which confirms the suitability of the experimental procedure employed for sorbent screening purposes in these high-velocity, high-temperature pilot plants. Several attrition indexes reported in the literature were tested to conclude that the Total Particle Generated Index (TPGI) and the Maximum Diameter of Particles Generated (MDPG) indexes are the most useful for quantifying attrition phenomena and rank limestones. (C) 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).