Abstract
Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.
Highlights
Le Châtelier[1] already established that Portland cement hydration starts by the dissolution of calcium silicate species in water from the most soluble silicate phase
We have employed synchrotron X-ray total scattering to study the short- and medium- range atomic arrangement in the C-S-H gel nanoparticles. With this knowledge and observations from electron microscopy and previous reports, we propose a model for this complex heterogeneous system, developing a multiscale picture for the hydration of alite in order to explain the observed mass densities and Ca/Si atomic ratios at the different scales
Below 10 nm, C-S-H gel are composed of a fine intermixing of defective clinotobermorite, particle sizes ranging 3–5 nm with Ca/Si ratio close 1.2, and monolayers of Ca(OH)[2]
Summary
Le Châtelier[1] already established that Portland cement hydration starts by the dissolution of calcium silicate species in water from the most soluble silicate phase This process is followed by the precipitation of complex poorly-crystalline calcium-silicate-hydrates (generically named C-S-H gel) and the crystallization of Ca(OH)[2], portlandite, see overall reaction (1)[2]. We have employed synchrotron X-ray total scattering (and 29Si MAS-NMR) to study the short- and medium- range atomic arrangement in the C-S-H gel nanoparticles With this knowledge and observations from electron microscopy and previous reports, we propose a model for this complex heterogeneous system, developing a multiscale picture (see Fig. 1) for the hydration of alite in order to explain the observed mass densities and Ca/Si atomic ratios at the different scales. Above 100 nm, the hydration reaction of alite is well known resulting in crystalline Ca(OH)[2], named portlandite, C-S-H gel and capillary water, see Fig. 1
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