Influence of Nitrate Corrosion Inhibitors on Phase Stability of Alkali-Activated Slag against Chloride Binding and Natural Carbonation

Nitrate and nitrite-based corrosion inhibitors have been studied for decades for mitigating corrosion of steel embedded in ordinary portland cement (OPC) concrete; however, few studies are available regarding the effect of nitrate or nitrite on the performance of alkali-activated binders. This work, for the first time, investigates the influence of three nitrate-containing activators (i.e., sodium nitrate, magnesium nitrate, and aluminum nitrate, each mixed with sodium hydroxide solution) on the strength development, pore structure, phase assemblage, and phase stability of alkali-activated slag (AAS) binders upon exposure to chloride and natural carbonation. The results show that the addition of nitrate, regardless of its form, does not substantially alter the type of main reacted phases, i.e., calcium-aluminosilicate-hydrate (C-A-S-H) and Mg-Al layered double hydroxides (LDHs), in AAS; however, the addition of aluminum nitrate improves its compressive strength, boosts the formation of Ca-Al LDHs (AFm-type phase), and likely inserts nitrate anions in the interlayer of formed Ca-Al or Mg-Al LDHs. The findings suggest that the nitrate-intercalated Ca-Al LDHs and/or Mg-Al LDHs, if formed to some extent, can potentially release nitrate ions during chloride binding and carbonation, working as a smart corrosion inhibitor.