Portland cement is by far the most important binding material used in both civil construction and oil well cementing. However, especially in the latter application, its brittle nature impairs its ability to withstand dimensional changes due to thermal gradients (RT to approximately 200°C), typical of heavy-oil recovery operations. This often results in extensive cracking of the cement sheath and debonding of the cement-casing interface, which leads to loss of zonal isolation, gas migration and production of water and oil through the well annular. Portland-Polyurethane composites have been formulated and tested in an attempt to improve the deformation ability of the cement during thermal cycles without affecting the pumping behavior or high-temperature resistance of the material. Preliminary results confirmed that the addition of small contents of polyurethane considerably improved the plasticity of the cement in approximately 50% as well as decreased porosity and permeability. No significant changes were observed in the rheological behavior of the composite slurries with respect to plain Portland cement.