Corrosion behaviour of galvanized steel in cement- and geopolymer-based concrete: a review on scientific work at the Polytechnic University of Marche

Hot-dip galvanized steel rebars are effective to increase the durability of reinforced construction materials, based on traditional cement and on recently studied alkali-activated binders, such as geopolymers. Instead of steel reinforcements, galvanized reinforcements in concrete are protected by calcium hydroxyzincate (CaHZn) layer, which, under carbonation, transforms into a less protective hydrozincite (HZ), but corrosion resistant layer. In the case of chlorides ingress, the threshold for localized corrosion initiation is at least three times higher for galvanized rebars than for steel rebars: 1.3 wt% vs 0.4 wt%, respectively, with respect to the weight of cement. Therefore, galvanized rebars increase considerably the durability of reinforced concrete structures, without significantly impairing construction costs. Concerning geopolymers, the matrix is more alkaline than the traditional cementitious ones and the scarce presence of calcium ions cannot guarantee the protection of galvanized steel by CaHZn. Actually, also in these conditions, galvanized steel passivates even if with a certain delay (4-7 days) from the cast, by the formation of ZnO passive layer. In particular, geopolymers obtained by fly ash slow the ingress of chlorides, protecting the reinforcements, if galvanized, from a penetrating localized corrosion of Zn-Fe alloys.