The reactivity of hydrated portland cement pastes containing up to a 30% of a Spanish ground copper slag (CuGS) in an aggressive solution (Cl − + SO 4 =) has been studied in order to evaluate the changes in microstructural and mechanical properties of the composite material. Flexural strength data were related to microstructural parameters, studied by means X-ray diffraction as well as porosity and pore-size distribution analyses.