Role of scoria natural pozzolan in the passive film development for steel rebars in chloride-contaminated concrete environment

Abstract

This communication reports the effect of scoria natural pozzolan (NP) on rebar corrosion in a simulated pore solution (SPSL) and in mortars. The mortar mixes consist of 100% Portland cement (control) and Portland cement with 25% scoria NP. A third mix utilizing 25% fly ash (FA) was also cast for benchmarking. Specimens from the three mortar mixes were subjected to aggressive wet/dry treatment for long durations. Electrochemical impedance data were extracted after different durations of exposure of the rebars in SPSL and mortars. The addition of NP and FA to SPSL increased the corrosion potential of the rebars immersed in these solutions with time, indicating the strengthening of the passive film formed on the surface of the rebars. However, the addition of chloride to the pore solution considerably reduced the open-circuit potential in the active direction and reduced the polarization resistance, indicating the deterioration of the passive film. NP and FA enhanced the protective properties of the passive film on the surface of rebars embedded in mortars compared with the control one. The initially observed beneficial effect of NP and FA for improving rebar corrosion protection was greater in mortar than in the pore solution, which was attributed to the dual effects of the improvement in the passive layer in addition to the reduction in porosity. Electrochemical impedance spectroscopy (EIS) was used to assess the effect of NP and FA on the properties of the passive film. Cations leached out in the pore solution were analyzed using inductively coupled plasma-optical emission spectrometry. Raman spectroscopy and X-ray diffraction techniques were used to characterize corrosion products on rebars. The data generated using the above-mentioned techniques also helped to understand the mechanism of action of NP and FA observed on the passive film.