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Retort residue ash of El-Lajjun oil shale from central Jordan has been studied for its cementitious and pozzolanic properties and potential uses in construction. During the preliminary phase, properties of the ash were determined and some tests were performed on pastes of ash, cement and their mixtures. According to X-ray diffraction and thermogravimetric studies, the ash mainly consisted of calcite and silica and some lime silicates, providing cementitious character. Its hydration resulted in ettringite and lime silicate gel formation. The ash paste had relatively low setting times and hardened into a lightweight porous structure. Its 28 day strength was about half of the cement paste value for the same water content. The ash had weak pozzolanic activity. In blended pastes, addition of ash up to 30% caused some reductions in 28 day strengths. There seemed to be optimum ash contents which improved compactibility while causing minimum strength reductions. The results indicated that the ash could be used to produce lightweight masonry and building units having medium strengths. It showed potential for use as an admixture in cement and concrete.