Corrosion of reinforcing steel in alkali-activated mortars caused by chlorides involves effects on binder chemistry, chloride penetration behavior, pore solution conductivity, steel surface state, and corrosion kinetics. It is not possible to apply criteria used for Ordinary Portland cement to alkali-activated materials since slag rich, slag/fly ash blended, and fly ash rich mortars generate dissimilar levels of alkalinity in steel environment. The aim of this research was to investigate the corrosion state of reinforcing steel embedded in three types of alkali-activated mortars, SN3 ($87.67\text{\,\%}$ mass blast furnace slag), BN3 ($41.12\text{\,\%}$ mass blast furnace slag, $46.61\text{\,\%}$ mass fly ash), and FN9 ($68.76\text{\,\%}$ mass fly ash). Reinforced mortars were tested either in tap water or in chloride environment with 8 h of spraying with $5\text{\,\% wt}$ NaCl solution, 24 h of storage under $100\text{\,\%}$ relative humidity, and drying for 24 h at $50\pm4~^\circ\mathrm{C}$. Corrosion potential, polarization resistance, electrochemical impedance spectroscopy, Tafel slopes, and gravimetric steel loss were considered as indicators of the corrosion status of steel bars. Mortar strength for SN3 was $47.6\text{\,MPa}$ at $28\text{\,days}$, its total porosity $4.59\text{\,\%}$, and chloride migration coefficient $1.64\times10^{-12}\text{\,m}^2\text{s}^{-1}$. In BN3 compressive strength after $28\text{\,days}$ was equal to $39.8\text{\,MPa}$, the chloride migration coefficient being $2.20\times10^{-12}\text{\,m}^2\text{s}^{-1}$. FN9 reached strength of $25.9\text{\,MPa}$ after $28\text{\,days}$ and exhibited very high chloride migration coefficient of $174\times10^{-12}\text{\,m}^2\text{s}^{-1}$. The electrochemical response corresponded to this material sequence. Mortars of types SN3 and BN3 remained in passive state with low corrosion current density throughout the time of chloride exposure. Mortar FN9 reached corrosion current density of approximately $6~\mu\mathrm{A cm^{-2}}$ after $40\text{\,days}$ and $30~\mu\mathrm{A cm^{-2}}$ near the end of the experiment. Fly ash rich mortar showed also very low mortar resistivity and early appearance of cracks. For the examined alkali-activated mortars, passive or negligible corrosion is indicated by $E_{\mathrm{corr}}>-400$ mV versus SCE, $R_{\mathrm{mortar}}>80~\mathrm{k\Omega\,cm^2}$, $n>0.6$, low $i_{\mathrm{corr}}$, and $B=15$--25 mV. Evidence of active corrosion was defined by $E_{\text {corr }} <-400 \, mV$ vs. SCE, $R_{mortar}<80~\mathrm{k\Omega cm}^{-2}, n<0.6$, increasing $i_{corr}$, concrete cracking or steel loss, and $B=55$ to 63 mV. These limits show that potential must be interpreted with polarization resistance, impedance parameters, Tafel slopes, and physical damage evidence when diagnosing chloride corrosion in reinforced alkali-activated mortars.

alkali-activated mortar,reinforcing steel,chloride corrosion,blast-furnace slag,fly ash,electrochemical impedance spectroscopy,Stern--Geary coefficient,polarization resistance