Chloride diffusion and chloride-induced corrosion of steel embedded in natural pozzolan-based alkali activated concrete

Abstract

Durability of emerging alkali-activated binder (AAB) widely viewed as alternative to ordinary Portland cement (OPC) is yet to be fully understood particularly when exposed to chloride-contaminated service environment. In this study, the resistance of natural pozzolan (NP) based alkali activated concrete (AAC) incorporating nanosilica (nSiO2) to chloride penetration and chloride-induced corrosion was evaluated. The performance of AAC upon exposure to chloride environment was assessed by conducting rapid chloride permeability, chloride migration and chloride-induced corrosion by measuring half-cell corrosion potentials, corrosion current density (Icorr) and gravimetric weight loss of embedded rebar. Additionally, accelerated corrosion by impressed anodic potential was conducted to validate the results. The chloride permeability was classified between moderate and high, while the chloride migration coefficient was in the range of 6.22 to 12.82 (×10−12) m2/s, lowest in the 7.5%-nSiO2 modified AAC followed by 5%-nSiO2. Though the corrosion potentials were more negative, the Icorr was in the range of 0.385 to 0.636 μA/cm2 in various AAC mixes as compared to 0.425 μA/cm2 recorded in the OPC concrete. The gravimetric weight loss of steel embedded in the AAC prepared with 0, 1, 2.5, 5 and 7.5% nSiO2 was 1.61, 0.99, 0.79, 0.67 and 0.68%, respectively. Therefore, it is postulated that the durability of AAC was comparable to that of OPC concrete exposed to chloride environment and the blend of 5–7.5% of nSiO2 in to NP outperformed other AAC mixes. However, it appears from the results that the electrochemical measurements do not reflect the accurate condition of the rebar embedded in AAC that could be attributed to lack of oxygen at the rebar level and lower pH in these binders. Hence, the specifications originally designed for OPC to assess the condition of the rebar may not be appropriate to be applicable for AAC.