Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

• Published in: Materials and corrosion 2020-05, Vol.71 (5), p.749-758
• Main Authors: Gluth, Gregor J. G., Ebell, Gino, Hlaváček, Petr, Mietz, Jürgen
• Format: Article
• Language: English
• Subjects: Activated carbon; Aging (natural); alkali‐activated materials; Carbon steels; Cement; chloride; Corrosion; Deionization; Exposure; Fly ash; Leaching; Mortars (material); Portland cements; Rebar; reinforcement corrosion; Reinforcing steels; Rheological properties; Sodium chloride; steel–concrete interface
• ISSN: 0947-5117; 1521-4176
• DOI: 10.1002/maco.202011541

Chloride contents at the steel–mortar interface that initiate steel corrosion were determined for carbon steel in alkali‐activated fly ash mortar for three different exposure conditions: exposure to 1 M NaCl solution; leaching in deionized water and then exposure to 1 M NaCl solution; and leaching in deionized water, aging in air at 20°C and natural CO2 concentration, and then exposure to 1 M NaCl solution. For comparison, a Portland cement mortar, exposed to 1 M NaCl solution, was studied. The median values of the corrosion‐initiating chloride contents (average over the full length of the rebar) in the alkali‐activated fly ash mortar varied between 0.35 and 1.05 wt% Cl with respect to binder, consistently lower than what was obtained for the Portland cement mortar, but with no clear trend regarding the exposure conditions. For most of the alkali‐activated fly ash mortar specimens, preferential corrosion at the connection between the working electrode and the external measurement setup was observed, while preferential corrosion did not occur for the Portland cement mortar. Scanning electron microscopy and auxiliary experiments in synthetic solutions indicated that this behavior was caused by inhomogeneities at the steel–mortar interface in the alkali‐activated mortar, likely due to its peculiar rheological properties in the fresh state. Steel corrosion‐initiating chloride contents were found to be lower in alkali‐activated fly ash mortar than in Portland cement mortar. For the former, preferential corrosion initiation at rebar ends with welding joints was observed. Increased formation of voids at the steel–mortar interface close to the rebar ends occurred in the alkali‐activated fly ash mortar and was interpreted as the cause for the observed behavior.