Geopolymeric and cementitious mortars with the same strength class: comparison of the corrosion behaviour of embedded steel and galvanized steel reinforcements

Geopolymeric and cementitious mortars with the same strength class and workability were studied and compared in terms of mechanical and corrosion behaviour of possible reinforcements. To this aim, three different geopolymeric and cement mixtures were manufactured in order to obtain mortars belonging to R1 > 10MPa, R2 > 15MPa and R3 > 25 MPa strength class. In particular, mortars based on geopolymer binder were prepared with fly ash, calcium aluminate cement, a solution of sodium silicate/potassium hydroxide ratio of 0.85:1, with different concentration of KOH, and a sand/binder ratio of 2.7:1, by weight. The respective three cementitious mortar mixtures were prepared with different proportions of ordinary Portland cement and hydraulic lime as binders, different water/binder ratio by weight and the same sand/binder ratio (3:1 by weight). The corrosion resistance of steel and galvanized steel reinforcements was investigated by free corrosion potential and polarization resistance measurements. Finally, metallographic analysis on the cross sections of galvanized bars extracted from the specimens was also carried out. The results showed that, initially, the galvanized steel reinforcements are particularly susceptible to corrosion in geopolymeric mixtures due to their high alkalinity. However, after few days of curing, the corrosion rate decrease, even if this process is slower than that of same reinforcements in cementitious mixtures. On the other hand, in geopolymeric matrix, the steel reinforcements show a lower corrosion rate, if compared with galvanized steel reinforcements, and their corrosion behaviour do not change significantly with the type of binder.