Modelling heavy metal and anion effects on physical and mechanical properties of Portland cement by means of factorial experiments

The aim of the present work was to investigate the influence of a number of heavy metals (Cd, Cr(III), Cu, Pb and Zn), as well as anions (Cl-, SO42-) on the physical and mechanical properties of cementitious pastes, including setting time, bulk density and unconfined compressive strength. The above mentioned species and their concentrations were selected in order to simulate addition of 20% municipal solid waste incinerator ash to Portland cement. Spiking experiments were performed according to a 2(IV)(7-3) fractional factorial design by addition of pure compounds containing the contaminants of interest to Portland cement. The results from the experimental campaign, which were interpreted using statistical analysis techniques, showed that the investigated contaminants variously affected the hydration process of cement. The ranking for final setting time values was Zn(NO3)(2) > K2SO4 > NaCl > mean response > Cr(NO3)(3) > NaCl+Cr(NO3)(3), Cd(NO3)(2)+Pb(NO3), or CuCl2+Zn(NO3)(2) interaction, whereas the ranking for UCS values during hardening was Zn(NO3)(2) > CuCl2 > NaCl+K2SO4, CuCl2+Pb(NO3)(2) or Cd(NW,+Zn(NO3)(2) interaction > mean response. Conversely, none of the selected contaminants was found to significantly influence ultimate strength.