This paper presents the results of preliminary studies on concrete mixtures containing high volumes of natural pozzolan (50% by mass of total cementitious materials) for structural applications. Three different kinds of natural pozzolans from Turkish deposits were used to prepare the high-volume natural pozzolan (HVNP) concrete mixtures. Two additional mixtures with a high-volume of low-calcium fly ash and granulated blast-furnace slag were also prepared for comparison purposes. In addition, a conventional Portland cement concrete mixture was prepared as a reference. The dosage required for a high-range water-reducing admixture for a given slump, air content, setting time, compressive strength, splitting-tensile strength, and resistance to chloride-ion penetration was determined for the mixtures. Preliminary results indicated that the high-volume natural pozzolan concrete mixtures studied are suitable for structural concrete applications with 1.74 to 2.03 ksi (12 to 14 MPa) and4.21 to 5.51 ksi (29 to 38 MPa) compressive strengths at 3 and 28 days, respectively. Copyright © 2007. American Concrete Institute. All rights reserved,.