EIS and Voltammetric study of passive film formation on steel bar embedded in Portland cement and innovative cementitious binders

Sustainable development is a key factor in the modern building industry, which is mainly characterized by a severe consumption of raw materials. More specifically, the cement production process is characterized by a high carbon footprint, which has to be reduced in order to comply with the most recent international agreements regarding Greenhouse gas emissions. Nowadays, innovative binders can be produced by low temperature processes, which lead to lower CO2 emissions. However, such binders show different properties compared with ordinary portland cement (OPC), usually considered for structural reinforced elements. Owing to the different hydration products of the constituents during hardening – leading to low alkalinity of pore solution – their use in reinforced concrete structures has to be further investigated in order to evaluate the effects on durability and corrosion behaviour of steel reinforcing bars. The paper is devoted to the study of passive film formation on steel by means of combined electrochemical measurements. Open circuit monitoring (OCV), Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were performed in reinforced concrete slabs manufactured with OPC and innovative binders. The experimental work moves from the study of the pH of the binder matrix to the study of the formation and characterization of passive film of rebars. The results show different passivation kinetics in innovative binders compared to traditional ones. Corrosion tests carried out by means of rotating cylinder electrode - under continuous scratching - outline that the rate determining step for the passive film formation is the oxygen and hydroxyl supply to the rebar surface. CV tests show a stable passivation of rebars embedded in OPC just after 28 days. Data collected by means of EIS tests showed a marked shift-to-the-left of the time constant over time since early exposures for rebars embedded in OPC – due to the higher pH values compared to innovative binders. More complex behaviour was noticed for rebars embedded in innovative binders depending on their formulations.