Influences of leaching on the composition, structure and morphology of calcium silicate hydrate (C-S-H) with different Ca/ Si ratios

Leaching of hydration products in cement-based materials may reduce the service life of concrete constructions, such as radioactive waste disposal containers and hydro structures. C-S-H is the most important cement hydration product, and systematic research on its nanostructures change under leaching is necessary to clarify the leaching mechanism in cement-based materials. This work aims to disclose the leaching mechanism of C-S-H with different Ca/Si ratios by immersing them in NH4Cl solution. The chemical structure, composition and morphology of samples were investigated by XRD, TG, ICP-OES, FTIR, 29Si NMR and TEM. Results show that decalcification occurs both in interlayer and intralayer of C-S-H, reducing the Ca/Si ratio to 0.67-0.70. Although initial Ca/Si ratios are different, the eventual decalcification behavior of C-S-H tends to be congruent dissolution. Additionally, Ca(OH)2 buffers the decalcification of C-S-H. Ca leaching breaks the charge balance of C-S-H and therefore changes the chemical state of silicate tetrahedra, which increases the length of silicate chains. Meanwhile, the silicate chain in adjacent layers was linked and the amorphous silica gel was formed. Leaching coarsens the pore structure of C-S-H and transforms partial foil-like C-S-H into silica gels with flocculent morphology.

Automated summary

Leaching of hydration products in cement-based materials can lead to a reduced service life of concrete structures. Understanding the leaching mechanism is crucial for preventing damage to concrete structures. This study investigated the leaching mechanism of C-S-H with different Ca/Si ratios by immersing them in NH4Cl solution. The results revealed that decalcification occurred in both interlayer and intralayer of C-S-H, leading to a reduced Ca/Si ratio and the formation of silica gels.