4D quantification of C-(A)-S-H and Mg-Al LDH phase alterations and microstructural evolution during accelerated carbonation of alkali-activated slag pastes
Zixian Su, Zengliang Yue, Alastair Marsh, Marco Di Michiel, Timothy L. Burnett, John L. Provis, Partha P. Paul, Susan A. Bernal, Philip J. Withers
Abstract
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 CO 2 sink, mitigating the increased concentration of CO 3 2− in pore solution via interlayer anion exchange of OH − for CO 3 2− , 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.