Present paper presents the results of investigations of blast – furnace cement binders modified with mechanically activated brown coal ash formed in the fluidized bed combustion process. The properties of the fresh slurries were investigated with a special emphasis put on the rheological properties. Technological workability parameters were investigated, and also flow curves were determined with a rotational viscometer for all the slurries. In hardened state both compressive and flexural strength of the binder system were determined. Microstructure of hardened slurries was investigated with SEM observations. The addition of mechanically activated ash formed in the fluidized bed combustion of brown coal improves stability and workability of cementing slurries. In moderate amounts the addition of ash improves both flexural and compressive strength. 30% addition of the ash in respect to the total binder weight was found to be the optimal in investigated system. The best rheological models were found for all the slurries. SEM observations showed that the microstructure of pastes with fly ash is dense and consist mainly of C-S-H phase of type IV according to Diamond classification.