For some years it has been possible to control the particle size of fly ashes, by-products of thermal power stations. Incorporating these very fine particles (obtained by grinding and/or pneumatic selection) improves the physical-mechanical characteristics of mortars and concretes. In this study, we measured the lime consumption of the various fractions (granulometric and densimetric) and identified by X-ray diffraction the neoformed phases by the pozzolanic reaction, to show that it is not sufficient to simply define the pozzolanicity of products based on lime consumption since it does not take into account the nature of the phases formed. The size of the particles used in the test samples also has a determining effect on the quantity of lime consumed. Before comparing results, it is necessary to ensure that the size of the particles (of the global ash and its constituents) be the same. Two distinct neoformed phases appear: CSH in the largest granular fractions (d>40 μm) and C<3AH6 in the smaller fractions.