For the first time, a set of samples of European flue gas cleaning residues, mainly from the incineration of municipal solid waste (MSW), has undergone a mineralogical study. The residues are the result of the neutralization of acid flue gases by lime, the predominant method adopted in Europe, using dry and semi-dry washing processes. The study protocol combines physico-chemical analytical techniques (XRD, FTIR, DSC/TGA) and global chemical analysis enabling identification of the chemical composition of the main constituents, particularly chlorinated Ca-based phases, as well as establishment of modal distributions of the represented phases, both crystalline and amorphous. The samples are slightly hydrated and values vary for trapped Cl, S and even CO2. The main crystalline phases are NaCl, KCl, CaSO4, CaCO3, Ca(OH)2 and calcium hydroxychloride CaOHCl. CaOHCl is the main chlorine phase, regardless of the treatment process, filtration mode, and specific surface of the Ca-based sorbent. This phase develops during neutralization of HCl by excess lime present according to the reaction Ca(OH)2 + HCl → CaOHCl + H2O, to the detriment of a complete yield involving the two lime OH groups with formation of CaCl2 · 2H2O. In addition, it seems that gas temperatures above 150 °C increase competition between lime-based neutralization of HCl, SO2 acid flue gases and CO2 trapping, thus reducing washing efficiency.