Influences of leaching on the composition, structure and morphology of calcium silicate hydrate (C–S–H) with different Ca/Si ratios. (15th October 2022) - Ma, Yuefeng
Abstract: 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 NH4 Cl solution. The chemical structure, composition and morphology of samples were investigated by XRD, TG, ICP-OES, FTIR, 29 Si 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. Highlights: Congruent dissolution of calcium silicate hydrate in accelerated leaching experiment are found. The decalcification occurs both in interlayer and intralayer of leached calcium silicate hydrate. The transformation of foil-like calcium silicate hydrate into silica gels with flocculent morphology was observed firstly.