Excessively hard filter cakes have occurred during filtration of fine particles from hot gases in pressurized fluidized bed combustion (PFBC) power plants. Hence, shear strengths were measured (at 22 degreesC) for cakes formed from powders taken from the hot-gas filtration vessel of a PFBC demonstration power plant. The measurements were made both before and after heating at 750 degreesC (1 h) or at 950 degreesC (1or 3 h). Sample strengths were significantly increased by the heat treatment at 950 degreesC, but not by heating to 750 degreesC. In filter vessels operating at 750 degreesC, formation of Mg2Ca(SO4)(3) from MgO, SOx, and CaSO4 is the main cause of the development of excessive cake strengths. However, these reactions and the concomitant strength increases cannot occur in the laboratory, if plant material is heated in the absence of one or more of the reactants (MgO and/or SOx). At 950 degreesC and in the absence of coal ash, the formation of Mg2Ca(SO4)(3) occurs even more rapidly. However, at 950 degreesC, Fe2O3 and other components of coal ash promote the decomposition of Mg2Ca(SO4)3. Hence, depending on the temperature, sorbent, amount of coal ash, and its composition, formation of Mg2Ca(SO4)(3) and/or sintering of CaSO4 may be the important cause(s) of cake hardening.