The aim of this work is to investigate the effect of the addition of Cu, Cd, Ni, Pb and Zn nitrate salts on the compressive strength of a CEM I Portland cement. Concentrations of 0.018 or 0.18mol/kg of cement of each trace element were tested. After 2days age, the compressive strength was reduced by various extents by addition of heavy metals, with the exception of Ni. This difference is due to a delay in tricalcium silicate hydration (C₃S) as shown by an isothermal calorimetry test. Trace elements also influence the 28-days compressive strength, whereas the measured degree of hydration of these cement pastes is the same. As shown by scanning electron microscopy and X-ray diffraction, Cu and Pb are predominantly absorbed in the calcium silicate hydrate gel (C–S–H) while Cd, Ni and Zn are mainly precipitated as hydroxides within the intergranular porosity. Thus, trace elements precipitated as hydroxides have only a slight effect on the compressive strength. In contrast, Cu and Pb cause an increase in mechanical resistance by changing the C–S–H nanometric assembly and its density.