Characterisation of passive layer on steel immersed in MgO-SiO2 binder suspension solution

MgO-SiO₂ binders inherently have lower pH compared to Portland cement (PC), raising concerns about their suitability for structural applications involving steel reinforcement. To address this concern, this study investigated steel passivation in MgO-SiO₂ binders in the presence of sodium hexametaphosphate (SHMP), which improves workability and increases pH. The combined effect of SHMP and alkali addition in MgO-SiO₂ on steel passivation was also evaluated. Passive layer formation on steel was characterised with electrochemical impedance spectroscopy (EIS), Raman spectroscopy, low-kV backscattered electron (BSE) scanning and x-ray photoelectron spectroscopy (XPS). The obtained results demonstrated the formation of a protective passive layer on steel in the presence of SHMP, along with the absence of corrosion in mixes with SHMP or alkalis. Alkali addition also improved steel passivation in MgO-SiO₂ systems. While the passive layer in the MgO-SiO₂ system containing SHMP/alkalis was similar to those present in PC samples, differences in its composition and thickness were highlighted.