Samples of ASTM A-706 structural steel embedded in different concretes obtained from mixtures of alkali-activated steel slag and fly ash were exposed to accelerated carbonation under controlled conditions (4% CO2, 65% RH and 25°C). Concretes prepared with Ordinary Portland Cement (OPC) (Type I) and Granulated Blast Furnace Slag Cement (GBFS) were used for comparison. Analysis of the electrochemical phenomenon taking place on the surface of the steel due to CO2 transport through the concrete matrix was performed by measurements of open circuit potential (OCP) and linear polarization resistance (LPR). The steel-concrete interface was characterized using Mössbauer spectroscopy at room temperature and X-ray diffraction (XRD). The electrochemical measurements suggest passivation of the steel in the concretes prepared using mixtures of steel slag and fly ash without cement.