Eight concrete mixtures were prepared using the same batch of ordinary portland cement (OPC) and ground low-lime fly ash. The aggregate grading used in the mixtures of concrete, water-binder ratio, and the maximum particle size of aggregate were kept constant in all concretes, but the partial replacement of cement by fly ash was varied from 0% (OPC concrete) to 70% in steps of 10%. The replacement was on a one-to-one weight basis. At 28 days, there was little reduction in compressive strength up to 40% cement replacement by ground fly ash; then a significant decrease was recorded for the further fly ash dosages. At 56 and 120 days, however, the compressive strength up to 40% cement replacement by fly ash is almost identical to that of the no fly ash concrete and for one year it was even higher. Beyond 40% replacement, the compressive strength decreased significantly. It was shown that the brittleness index increases substantially with increasing compressive strength of concrete. The results of the rapid chloride penetration tests indicated that high volume ground fly ash concrete had better resistance to the penetration of chloride ions. The mortar phases of these concretes were also prepared. As the dosage of fly ash increased, 2-, 7-, and 28-day compressive strengths of the mortars decreased. However, at later ages (that is, at 56, 120, and 365 days), up to 40% cement replacement by ground fly ash, the compressive strength of mortar with fly ash was nearly equal to that of mortar without fly ash. The pozzolanic effectiveness ratio, as a measure of pozzolanic reactivity and the contribution of fly ash to the strength development, increased with increasing curing time and fly ash content. Copyright © 2005, American Concrete Institute. All rights reserved.