The effect of three different natural pozzolans from Turkish deposits on the properties of blended cements produced by intergrinding cement clinker with a high volume of natural pozzolan (55 wt.% of the cementitious material) was investigated. The particle size distribution of blended cements, setting time, heat of hydration, and compressive strength of blended cement mortars were determined. Experimental results showed that the hardness of the pozzolanic material strongly influenced the particle size distribution and the related properties of the blended cements by affecting the fineness of the components of the blended product. The early strength of the mortars was strongly affected by the particle size distribution of blended cements, whereas the strength development performance of the mortars was more related to the pozzolanic activity of the natural pozzolan present in the blended cement. (C) 2004 Elsevier Ltd. All rights reserved.