Corrosion behavior of bare and galvanized steel in alkali-activated and ordinary Portland cement-based mortars at the same strength class exposed to carbonation

This study compares the corrosion behavior of bare and galvanized steel reinforcements in cement and alkali-activated mortars based on fly ash and metakaolin after carbonation. Each mortar type belonged to three different strength classes: R1 (R c ≥ 10 MPa), R2 (R c ≥ 15 MPa), and R3 (R c ≥ 25 MPa), and has been tested during wet/dry cycles in tap water following exposure to a 3 vol% CO 2 environment. Results in terms of electrochemical tests and visual and metallographic analysis showed that steel reinforcements corroded less in fly ash-based mortars, thanks to their low porosity (spanning for 13–19 %) and high alkalinity, with pH values stable between 13 and 14 also after accelerated carbonation. For galvanized steel, the lowest consumption of the zinc coating (up to 40 µm in cement-based matrices) was observed in metakaolin-based mortars (less than 20 µm) since the combination of low alkalinity (with pH falling between 12.5 and 11.5) and high total porosity (around 35 %) of the matrix facilitate the penetration of oxygen and CO 2 which promote the formation of a passivating layer. The significant lower susceptibility to corrosion of galvanized steel compared to bare steel resulted in this work, regardless of the binder type, suggests that the use of galvanized steel is recommended in structures exposed to carbonation not only in cement but also in alkali-activated matrices.