4D quantification of C-(A)-S-H and Mg-Al LDH phase alterations and microstructural evolution during accelerated carbonation of alkali-activated slag pastes

In situ synchrotron X-ray diffraction computed tomography (XRD-CT) and micro-tomography (μCT) are used to determine the effects of accelerated carbonation on sodium silicate- and carbonate-activated slag cement pastes, focusing on changes in crystalline and semi-crystalline phases, and pore structures. Accelerated carbonation leads to decalcification of the interlayer of aluminium-substituted calcium silicate hydrate (C-(A)-S-H), resulting in reduced interlayer distance, volume shrinkage, and increased porosity with larger pore volumes. The hydrotalcite-like Mg-Al LDH phase acts as a CO₂ sink, mitigating the increased concentration of CO₃²⁻ in pore solution via interlayer anion exchange of OH⁻ for CO₃²⁻, playing a more significant role in sodium silicate slag cement paste. Additionally, sodium silicate-activated slag cement is found to have a finer, more tortuous pore distribution and higher carbonation resistance than sodium carbonate-activated slag cement, as evidenced by a smaller degree of carbonation-induced C-(A)-S-H shrinkage, and a smaller increase in porosity volume during carbonation.