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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 N 2 adsorption. Results indicate bridging Al(IV) in alumino-silicate chain and most of Al-incorporated phases (e.g., Doyleite and Zeolite) dissolve in NH 4 Cl 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. [ABSTRACT FROM AUTHOR]

The formation of C-S-H in cementitious materials is critical to their gain in strength. As one of the key parameters, pH is reported to affect the nucleation and growth of C-S-H; however, the underlying mechanism remains unclear. In this study, C-S-H was synthesized under different pH conditions, to elucidate the underlying mechanism. Changes in the chemical composition, morphology and structure depending on the reaction time and interactions between pH and C-S-H were tracked using a series of characterizations. Three types of C-S-H (amorphous calcium silicate, C-S-H precursor and semi-crystalline C-S-H) formed at different pH and time points. pH affected the formation of C-S-H mainly through the protonation of silicates, which determined the surface charge of silicate tetrahedra and consequent chemical composition of C-S-H. At high pH, the formation of CaOH+ as the first step before the formation of C-S-H was responsible for the high Ca/Si ratio under such conditions. [ABSTRACT FROM AUTHOR]

Calcium silicate aluminate hydrate (C(-A)-S-H) is the main hydration product used in cement-based materials. It plays a specific role in both the mechanical properties and volumetric deformation of the cement matrix. This study aims to clarify the effect of porosity and chemical composition of C(-A)-S-H on its micromechanical properties using nanoindentation. The results showed that porosity significantly affected the indentation, elastic, and creep moduli of the porous C(-A)-S-H compacts. Micromechanical properties of the "pure" C(-A)-S-H were improved by decreasing the Ca/Si ratio. The incorporation of Al slightly altered the creep modulus by affecting the volume of the large gel pores, basal spacing, and mean chain length (if no cross-linking occurred in C(-A)-S-H). Finally, the nanoindentation results of the mixed C(-A)-S-H and Al(OH) 3 confirmed the distinct contribution of the individual component characteristics to the overall properties of porous mixtures. [ABSTRACT FROM AUTHOR]