Concrete is the single most widely used material in the world. Still, hydration of common (Portland) cement is not well understood, predominantly due to the complex defect chemistry of its main binding phase: calcium-silicate-hydrate (C-S-H). The exact microstructure of the defect-rich metastable C-S-H phase is driven by kinetics, which can be tuned by using organic chelating additives, providing a huge economic potential for synthesizing high performance cements. Moreover, recent electron probe and cryo-transmission electron microscopy studies revealed that C-S-H crystallization involved two distinct nucleation processes: 1) Nucleation of a highly disordered proto-CSH that is composed of globular particles followed by 2) a slower crystallization of nano-crystalline CSH of sheetlike-particles.Chelating additives increase the lifetime of disordered proto-CSH. To elucidate the mechanistic role of this proto-CSH phase, we have performed in-situ High-Energy X-ray Scattering (HEXS) experiments of CSH-crystallization in the presence and absence of sodium gluconate, followed by pair distribution function (PDF) analysis. HEXS-patterns confirm that the formation of nanocrystalline CSH is preceded by a highly disordered phase. Refinements of randomly generated clusters led to multiple solutions. Neural network analysis on this collection of clusters suggests that the metastable proto-CSH contains predominantly 6-fold coordinated calcium polyhedra CSH. This structural study will be useful to ascertain the role and mechanism by which organic molecules interact with CSH during cement setting.

High energy X-ray Scattering revealing the short-range order of a metastable proto-CSH phase and its implications on cement nucleation

ACS Spring Meeting 2021

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