This paper presents the effect of fly ash grain-size fractions on Portland-fly ash cement hydration and its properties. Siliceous fly ashes of size fraction of 0 - 16 and 16 - 32 μm, separated from initial fly ash samples from 1^st, 2^nd and 3^rd hopper in ESP system, were analysed. Cement hydration was investigated by determination of hydration heat and content of Ca(OH)₂ and C₃S in cement samples. Water to cement ratio and initial setting time of cement pastes as well as compressive strength and microstructure of cement mortars were also analyzed. Results showed that the same amount of the same size ash fraction can give cement of lower or higher early strength and its lower or higher increase with time. Incorporation of 20 wt% of ash fraction of 0 - 16 μm can produce Portland-fly ash cement CEM II/A-V of strength class 42.5R (from 2^nd hopper) or 52.5N (from 3^rd hopper). Cement containing 40 wt% of ash fraction of 0 - 16 μm from 2^nd and 3^rd hopper can be classified as pozzolanic cements CEM IV/A-V of strength class 42.5 and normal or rapid early strength, respectively. Different development of strength of cement with addition of the same size ash fraction separated from the initial ash sample from the next hopper in ESP system is connected with higher depolymerization degree of SiO₄ units in ash glass, resulting from the greater amount of AlO₄ units replacing SiO₄ units. Ash fraction of 16 - 32 μm shows lower depolymerisation of glass network and as a consequence lower hydration degree of C₃S to portlandite and calcium silicate hydrates (C-S-H).