Natural pozzolans are increasingly being used in blended cements due to their compositional uniformity and local abundance in some countries. They influence hydration, both at early and late ages. A control portland cement and three interground cements containing natural pozzolan were produced using a laboratory ball mill and sieved through 10-mu m, 35-mu m and 50-mu m sieves to yield different cement size subgroups. The heat evolution of each subgroup was investigated up to 2 days using isothermal calorimetry. The compositions of the size subgroups and their early hydration behaviors were found to differ from those of the unsieved cement subgroups. Subgroups containing finer pozzolan particles had higher heat of hydration due to the nucleation effect. In the coarse cement subgroups with relatively lower amounts of clinker, the dilution effect was dominant. It was shown that early heat evolution characteristics of cements can be controlled, among other ways, by adjusting the particle size distribution. (C) 2019 Elsevier Ltd. All rights reserved.