The production of a new lightweight composite material based on polyurethane and Portland cement was investigated. The composite was obtained by the mixture of polyurethane foam precursors with different amounts of cement and water. To allow cement hydration, samples were aged in water and characterized through scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, and compressive testing. We studied the cement hydration reactions and the effect of the organic phase on hydration by determining the amount of chemically bonded water by calcination. The results showed that the amount of water affected the morphology and porosity of the foams and thereby affected the cement hydration reaction. Furthermore, the mechanical properties of the hybrid composite varied in a wide range, depending on the cement and water contents and on whether the hydrated cement particles behaved as fillers or were allowed to interact to form stronger inorganic networks within the polymeric matrix forming the bubble walls. The polyurethane-cement composite foams showed an increase in the stiffness and the yield strength. In addition, the ductile behavior of the polymeric foams was preserved, even at high filler loadings, due to the chemical compatibility between the hydroxyl groups of the polyol and the cement. (C) 2007 Wiley Periodicals, Inc.