Multi-scale investigation on composition-structure of C-(A)-S-H with different Al/Si ratios under attack of decalcification action

This paper examines the deteriorating behavior of C-(A)-S-H samples with three Al/Si ratios from atomic to mesopore scales under the decalcification action. The composition, chemical structure, pore structure, micro/ nanomorphology of samples were studied using XRD, FTIR, 29Si and 27Al NMR, TEM, SEM and N2 adsorption. Results indicate bridging Al(IV) in alumino-silicate chain and most of Al-incorporated phases (e.g., Doyleite and Zeolite) dissolve in NH4Cl solution. Furthermore, the decalcification action improves polymerization and elongates chain length with simultaneous transformation of C-(A)-S-H into amorphous Al-Si gel. Therefore, the crystallinity of C-(A)-S-H is damaged and the nanomorphology becomes flocculent from foil-like, with large reduction on Ca/Si ratio but slight reduction on Al/Si ratio. The dissolution of C-(A)-S-H coarsens but the generation of amorphous Al-Si gel refines pore structure. Samples with moderate Al/Si ratio exhibit stronger resistance to structural evolutions than samples with other Al/Si ratios. Our work paves the way for understanding Ca-leaching induced multiscale deteriorating mechanism of blended cement-based/alkali-activated materials and the improvement on structural stability by moderate Al addition.