The cementing properties of oil shale ash obtained by burning oil shales from Tzefa Efe, Israel in a fluidized bed, were modified by three means: Variation of the burning temperature in the range of 650/sup 0/ to 1015/sup 0/C; addition of gypsum and blending with portland cement. Pastes and mortars prepared from these modified systems were studied to characterize rates of hydration, nature of hydration products, and some mechanical properties. It was found that burning at about800/sup 0/C resulted in optimal properties: ash of highest reactivity with water and pastes of highest strength were obtained. Addition of gypsum was associated with an increase in the content of ettringite formed during hydration and resulted in higher expansion. Optimum strength was obtained in pastes containing 4 to 8% gypsum. Blending with portland cement resulted in pastes of water to cementing material ratios and strength values intermediate to those of pastes of portland cement and ash only. Highest shrinkage was obtained in the blended pastes, reflecting probably the more colloidal nature of their hydration products.