Mechanistic insights into carbonation shrinkage of hardened cement paste via RH-dependent pathways of decalcification and decomposition of C-A-S-H

This study examined the effect of water on carbonation of hardened cement paste with three water-to-binder ratios, considering four types of relative humidity (RH) during carbonation. The role of the water content in calcium aluminosilicate hydrate (C-A-S-H) on decalcification and decomposition under RHs was quantitatively assessed using total carbon analysis, XRD/Rietveld-PONKCS, 1 H NMR relaxometry, SEM-EDS, and carbonation shrinkage measurements. At different RHs, carbonation shrinkage increased with reduced Ca/(Si + Al) ratio and increased aluminosilicate gel content. Regardless of RHs, the H 2 O and Ca released from C-A-S-H during decalcification were equimolar. Minor variations in the released H 2 O/Ca ratio during decomposition were observed, which could be attributed to the effect of RH on Ca-modified silica gel formation. Carbonation shrinkage was assessed based on the Ca and Si released from C-A-S-H, demonstrating that decomposition from aluminosilicate gel formation significantly affects carbonation shrinkage compared with decalcification and water loss.