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.
