Mitigation of Carbonation-Induced Corrosion in Alkali-Activated Slag Concrete Using Calcined Mg–Al Hydrotalcite: Electrochemical and Microstructural Evaluation

This study investigates the effectiveness of calcined magnesium–aluminium layered dou-ble hydroxide (CLDH) as a functional additive for mitigating carbonation-induced corro-sion in alkali-activated slag concrete (AASC). Mixtures incorporating different CLDH con-tents (0%, 2%, 4%, 6%, and 8%) were evaluated under accelerated CO₂ exposure (3%, 65% RH, 25 °C) for 90 days. Performance characterisation included electrochemical impedance spectroscopy (EIS) to assess corrosion of embedded steel, phenolphthalein spraying to de-termine carbonation depth, and complementary techniques such as X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM/EDX) for microstructural evolution. The results demonstrate that CLDH significantly enhances resistance to CO₂ ingress, increasing the polarisation resistance (Rp) to over 55 kΩ·cm² (at 6% CLDH) and reducing carbonation depth by more than 50% compared to the reference mix. These improvements are attributed to the memory effect-induced regeneration of LDH-type lamellar phases, controlled release of OH⁻ and CO₃²⁻ anions, and progressive densification of the microstructure, thereby limiting the ingress of aggressive agents. The optimal dosage was identified as 6%, as higher contents offered no further improvement and evidenced the formation of residual phases such as MgO. This work highlights the potential of CLDH as an effective and sustainable strategy to enhance the durability of al-kali-activated cementitious materials against degradation processes driven by carbona-tion and corrosion.