Thermal power plants have a significant role in energy production. Within the combustion process, a by-product called fly ash is obtained and can be utilized by the cement industry as an additive for improving the overall economy of the manufacturing process. This study focuses on evaluating the performance of a fly ash classification plant with regard to quality development in the streams of material through the classifier. Within that scope, the cementitious properties of raw, fine and coarse ash were determined in dependence on changes in their degree of fineness. The values of fineness were adjusted both by changing the operating conditions of the classifier and by way of laboratory-scale grinding tests in which inter-grinding of cement and fly ash was taken into consideration. The study showed that the streams of material through the classifier had similar quality results, and that inter-grinding could increase the ash content by up to 45 % with 14 % improvement in strength. Moreover, it proved possible to nullify the negative effect of fly ash addition on early-age strength of fly ash cement mixture by means of inter-grinding.