Samples of fly ashes generated from two different coals fired in a CO2-rich atmosphere by recycling part of flue gas have been characterized for chemical composition and particle size distribution. They have also been tested for reactivity against lime and gypsum in hydratory mixtures of interest in the held of pre-formed building materials. It has been found that firing coal in unusual conditions (mainly as far as flame temperature and flue gas composition are concerned) does not alter the reactivity of fly ashes in the systems referred to above. The main factor that affects fly ash reactivity is particle size distribution, as for the ashes produced in ordinary air firing operations.