Carbonation of low-alkalinity mortars: Influence on corrosion of steel and on mortar microstructure

The ability of carbonated mortar to provide corrosion protection of embedded steel was studied on mortars prepared with low-alkalinity binders. Three mortars were investigated, mortar based on ordinary Portland cement, mortar based on high volume of fly ash and silica fume and mortar based on slag and nano-silica. Analysis of impedance spectra after carbonation revealed an increase in electrical resistivity of mortar based on OPC and significant decrease in electrical resistivity of low-alkalinity mortar. After exposure to cycles of wetting and drying steel embedded in carbonated low-alkalinity mortars has lower open circuit potential, lower charge transfer resistance and significantly higher corrosion current density compared to steel embedded in OPC mortar. To explain differences in corrosion protection properties, an extensive microstructural testing of carbonated mortar was performed. Difference in corrosion properties were explained by the changes in mortar microstructure, mainly changes in pore size distribution.