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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 C 3 S 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 SiO 4 units in ash glass, resulting from the greater amount of AlO 4 units replacing SiO 4 units. Ash fraction of 16 -32 µm shows lower depolymerisation of glass network and as a consequence lower hydration degree of C 3 S to portlandite and calcium silicate hydrates (C-S-H).ashes reveal greater amount of glassy component than their coarse fractions, which result from higher amount of grains below 45 μm, especially below 10 µm [1][2][3][4]. Ash fraction of 0 -45 μm is characterized by the greatest amount of active components, that is, SiO 2 and Al 2 O 3 , whoes total content is approximately 15% -20% according to ASTM C379-65T. The coarse ash fraction lowers content of active silica and aluminum oxide in it, which generally does not exceed 10% in case of fraction above 100 μm. Works [4,[8][9][10] presented that fly ashes separated from different hoppers, attached to an electrostatic precipitator system after coal combustion, have variable physical and chemical properties. Ashes collected from 3 rd hopper have improved fineness and particle size distribution in form of a normal distribution curve. Also, they show lower amount of SiO 2 , but greater amount of Al 2 O 3 and alkalies (Na 2 O and K 2 O) [2,6,7]. Consequence of that is increase in content of active chemical components in ash from 3 rd hopper, especially the largest increase is observed for active Al 2 O 3 [4]. According to E. TKACZEWSKA 55 Tkaczewska [2], summarized amount of active chemical components in ash of 0 -16 µm is higher than that in traditional siliceous fly ashes and difference could be as much as 1.5 times. Of course, this is due to greater fineness fly ash of 0 -16 µm, but also can be resulted from different chemical composition and structure of glassy compone...