The effect of the addition of three types of sludge (L, O and F) on both the Na, K and S volatilisation and the slagging behaviour of a bituminous coal (H) was studied. Na, K and S volatilise in the coal, the sludges and their blends partially, and there exist linear relationships between their contents in ashes and the ratio sludge ash/blend ashes for each coal-sludge blend ash series. However, in blends with F (treated with FeCl3 and lime), S relationship is not linear due mainly to the following effects: a) sulphur retention by CaO to form CaSO4, and b) decrease of free CaO due to the formation of ferrites, silicates and phosphates. To determine the slagging tendency, the ash fusion temperatures (AFTs) were related with ash chemical and mineral composition by using the slagging factor, the base/acid ratio, the Fe2O3/CaO ratio, the P2O5 content, and the ternary equilibrium phase diagram SiO2-Al2O3-CaO. Sludge addition to coal produces a decrease in AFTs that depends on the percentage of sludge added, the sludge phosphorus content and additives (FeCl3 and/or lime), the fact that ash composition is near to those of the eutectics, and the formation of Si and P amorphous compounds. (C) 2015 Elsevier B.V. All rights reserved.