Influence of chemistry of chloride ions in cement matrix on corrosion of steel

To assess the resistance of steel against corrosion, mortar specimens were manufactured with a centrally located steel reinforcing bar. The mortars were formulated by combining ordinary portland cement (OPC) with a single supplementary cementitious material. The proportions were 30% pulverized fuel ash (PFA), 60% groundgranulated blast-furnace slag (GGBS), or 10% silica fume (SF) by weight. In addition, a pure OPC mortar was prepared. The mortar specimens contained chlorides from 0.0 to 3.0% by weight of binder to accelerate the corrosion process. Then, the corrosion rate was measured by a polarization technique to determine the chloride threshold level for corrosion in concrete. The buffering capacity of the cement matrix to a pH reduction was quantified by measuring the acid neutralization capacity. The suspension consisting of distilled water and paste powder was exposed to nitric acid to characterize a decrease in the pH. As a result, the chloride threshold level for corrosion was dependent on binders, with values of 0.74 to 0.93%, 0.48 to 0.59%, 0.29 to 0.44%, and 0.63 to 0.78% by weight of binder for OPC, 30% PFA, 60% GGBS, and 10% SF, respectively. It was found that an increase in the buffering capacity resulted in an increased threshold value, of which characteristics were used for a new representation of the chloride threshold. The threshold ratio of[Cl ^-]:[H⁺] for acidification to a pH value of 10 at corrosion accounted for 0.0063 and 0.0082, irrespective of binder type, at 1.0 and 2.0 mA/m² (6.50 × 10^-4 and 1.29 × 10^-3 mA/in.²) of the corrosion rate, respectively.