The chemical composition and heat evolution of <45 mu m (fine), and > 45 mu m ( coarse) portions of interground, natural pozzolan-blended cements were investigated. Cements with four different pozzolan contents were ground to 300 m(2)/kg, 500 m(2)/kg, and 600 m(2)/kg fineness and sieved through a 45 m sieve. Clinker, pozzolan, and gypsum contents in the fine and coarse portions after sieving were determined and compared with the unsieved cements. Heat of hydration evolution of the samples were determined up to 48 h using isothermal calorimetry. The fine portions of the cements always contained more gypsum, had higher pozzolan-to-clinker ratios, and slightly higher tricalcium silicate contents, which influenced the rate of heat development at early ages. The fine portions contribute more to the total early heat evolved than the coarse portions. A small amount of pozzolan enhances early hydration. The heat evolved up to 24 h was related linearly to the heat evolved up to 48 h. Such observations could be useful in the modeling of early hydration of blended cements.