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Xonotlite and Hillebrandite as Model Compounds for Calcium Silicate Hydrate Seeding in Cementitious Materials

The demand for more environmentally friendly cement with no disadvantages in relation to hydration activity has led to the development of various additives to accelerate cement hydration. As calcium silicate hydrate (C-S-H) is the major hydration product of cement and is responsible for its mechanical properties, it plays an outstanding role in the discussion of nanoparticle additives. Nevertheless, the investigation of its mechanism of action is complicated by the similarity of its properties to those of the C-S-H that forms as an initial hydration product. Crystalline C-S-H phases, on the other hand, can be easily distinguished from the original hydration products, which makes them a valuable model compound for studying the mechanisms of nucleation seeding in cementitious materials. In this paper, the effect of crystalline types of C-S-H as nucleation seeds are presented. Xonotlite and hillebrandite were thoroughly characterized using nuclear magnetic resonance, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IR) and were then used as an admixture for alite pastes. Low-vacuum SEM images of the hydrated pastes revealed that xonotlite can significantly promote the visible etch pit formation on C3S clinker particles, which was not found to be true for hillebrandite. Whether the phases act as a nucleation site is assumed to be strongly dependent on the mineralogy: hillebrandite appeared to be heavily overgrown, but xonotlite did not show any hydration products on its surfaces after the same hydration time of up to 24 h. The diverse effect of the minerals was confirmed by the accelerating behavior in isothermal heat flow calorimetry and by XRD.