Bound chloride and free Cl/OH thresholds for corrosion initiation in alkali-activated concrete

The paper presents results of hydroxyl ion (OH) and chloride (Cl) concentrations in the pore fluid of alkali-activated concrete (AAC) and their effect on steel reinforcement corrosion up to 1750 days of chloride exposure. A higher OH and corresponding lower Cl/OH was observed in AAC than the control Portland cement (PC) concrete at early ages of chloride exposure. A reversed trend was observed under long-term chloride exposure for AAC and PC concrete due to the superior chloride binding capacity of PC. The critical threshold Cl/OH ratio is between 2 and 3.28 for corrosion initiation in AAC, whereas it is <1 for PC concrete. Also, the critical bound chloride threshold for corrosion initiation in AAC is between 14.7 mg/g (1.47%) and 17.7 mg/g (1.77%) compared with 4 mg/g (0.4%) by binder weight for PC concrete. The steel reinforcement in AAC remains free from chloride-induced corrosion for a longer period and up to higher Cl⁻/OH⁻ ratios than PC concrete due to the presence of elemental sulfur (sulfides) on the steel surface in AAC.