Inhibition of Steel Corrosion for Application in Carbonated Chloride-Polluted Ordinary Portland Cement and Fly Ash Mortar

The cement industry is regarded to be responsible for 6-7% of all the greenhouse gases emitted world-wide (0.85-1 ton of CO2 per ton of cement). Thus the construction sector is very interested in the development of new cement binder materials as an alternative to ordinary Portland cement (OPC). In this respect the most promising emerging approach is based on raw materials suitable for alkaline activation, essentially alkali-activated fly ash (AAFA), which originate new binding materials known generically as alkaline cements. Carbon steel reinforcements are compatible with AAFA mortars, where they show corrosion rates even lower than those recorded in sound OPC mortars. As in OPC-based, in fly ash-based structures, chloride pollution and carbon dioxide penetration may induce severe corrosion problems. The concomitant presence of both chlorides and carbonation produces an extremely severe environment, the effect of which has not yet been widely studied, particularly in fly ash systems. Under this condition, corrosion inhibition may help to withstand this problem in an easy-handling and cost-effective way. Therefore, the inhibiting behaviour of some organic compounds against steel corrosion was evaluated in a synthetic solution and preliminary results achieved in OPC and AAFA mortars are described. The polarization curves recorded over 3 days immersion tests show that in synthetic solution, the mixtures 0.05M sodium glycerophosphate (GPH) + 0.05M sodium 3-aminobenzoate (3AMB) and, particularly, 0.05M GPH + saturated sodium N-phenyl anthranilate (PhAMB) exhibit good inhibitive effects towards the rebar corrosion. The polarization resistance values collected during 150 days of partial immersion in chloride-polluted tap water of carbonated OPC mortars show Rp values of about 2-3  104 Ω cm2, indicating active corrosion on the reinforcements. AAFA mortars where the fly ash has been activated by either a sodium hydroxide solution (FAA mortar) or a sodium hydroxide + sodium silicate solution (FAB mortar) exhibit Rp values around 1-2  105 Ω cm2. In the presence of inhibitors at the following concentration ratios: [GPH] : [3AMB] : [Cl] = 1 : 1 : 4, and [GPH] : [PhAMB] : [Cl] = 1 : saturated : 4, high inhibiting efficiencies are obtained after 20 days of exposure (OPC : 50-70%; and AAFA: 98-100%), while at the end of the test (150 days) these efficiencies decrease (OPC: 30-60%; and AAFA: 50-70%).