Characterizing Steel Corrosion in Different Alkali-Activated Mortars

Abstract

Alkali-activated materials (AAMs) present a promising potential alternative to ordinary Portland cement (OPC). The service life of reinforced concrete structures depends greatly on the corrosion resistance of the steel used for reinforcement. Due to the wide range and diverse properties of AAMs, the corrosion processes of steel in these materials is still relatively unknown. Three different alkali-activated mortar mixes, based on fly ash, slag, or metakaolin, were prepared for this research. An ordinary carbon-steel reinforcing bar was installed in each of the mortar mixes. In order to study the corrosion properties of steel in the selected mortars, the specimens were exposed to a saline solution in wet/dry cycles for 17 weeks, and periodic electrochemical impedance spectroscopy (EIS) measurements were performed. The propagation of corrosion damage on the embedded steel bars was followed using X-ray computed microtomography (μXCT). Periodic EIS measurements of the AAMs showed different impedance response in individual AAMs. Moreover, these impedance responses also changed over the time of exposure. Interpretation of the results was based on visual and numerical analysis of the corrosion damages obtained by μXCT, which confirmed corrosion damage of varying type and extent on steel bars embedded in the tested AAMs.

The corrosion properties of steel in pore solutions obtained from alkali‐activated mortars

“…[21] Comparisons of studies in mortars and in their pore solutions show that measured corrosion rate values in pore solution can go up to a couple of hundreds µm/year, [22] while the (local) corrosion rates in solid mortars can be 10 times higher. [23] According to one of our previous studies investigating solid AAM mortars, [16] a similar relationship was expected between the corrosion rates in the AAMs. In line with OPC, it was foreseen that more general corrosion damage, and somewhat lower corrosion rates, would be detected in the simulated AAM pore solutions compared to those in the AAM mortar specimens.…”

“…A graphite electrode served as the counter electrode, while a saturated calomel electrode (SCE) or Ag/AgCl electrode was used as the reference electrode. All potential measurements T A B L E 1 Chemical compositions of the simulated pore solutions, prepared on the basis of analysis of the pore solutions extracted from the three alkali-activated mortars (made from fly-ash [FA8], metakaolin [MK2], and steel slag [S3a-661]) [16] .…”

“…More information regarding the mix design of these mortars is available in our previous work. [ 16 ] The cast mortar mixes were cured in sealed plastic foil. After 28 days of curing, the mortar specimens, which were not sufficiently moist, were broken into smaller pieces, 3–4 cm in length, and slowly wetted with distilled water, such that the mortar absorbed all the excess water.…”

Application of electrochemical methods for studying steel corrosion in alkali‐activated materials

“…To accelerate Cl − ingress and thereby achieve meaningful results in a laboratory setup faster than with only diffusion of Cl − , the use of wetting/drying cycles has often been employed. [25,52,60,71] These cases are also nearly representative of concrete in a splash or tidal zone (exposure classes XD3 and XS3, respectively, according to EN 206 [75] ), which is known to be the most critical condition for steel-reinforced concrete.…”

“…In the case of AAMs, several different equivalent electrical circuits have been utilised, depending on the composition/precursors of the materials and the exposure conditions. [42,43,54,61,62,70,71,94] Because of this and because of the complexity of the method, it is difficult at present to draw general conclusions as regards the applicability of EIS to study steel corrosion in AAMs as F I G U R E 7 Anodic polarisation curves of mild steel immersed in 0.80 M NaOH solutions with different sulfide concentrations (as indicated in the legend). (Reproduced from Mundra and Provis [56] published under a Creative Commons license: https:// creativecommons.org/licenses/by/4.0/[CC BY 4.0]).…”