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.

1Ye'elimite is the main phase in calcium sulfoaluminate cements and also a key phase in sulfobelite 2 cements. However, its hydration mechanism is not well understood. Here we reported new data on 3 the hydration behaviour of ye'elimite using synchrotron and laboratory powder diffraction coupled 4 to the Rietveld methodology. Both internal and external standard methodologies have been used to 5 determine the overall amorphous contents. We have addressed the standard variables: water-to-6 ye'elimite ratio and additional sulfate sources of different solubility. Moreover, we report a deep 7 study of the role of the polymorphism of pure ye'elimites. The hydration behaviour of orthorhombic 8 stoichiometric and pseudo-cubic solid-solution ye'elimites is discussed. In the absence of additional 9 sulfate sources, stoichiometric-ye'elimite reacts slower than solid-solution-ye'elimite, and AFm-10 type phases are the main hydrated crystalline phases, as expected. Moreover, solid-solution-11 ye'elimite produces higher amounts of ettringite than stoichiometric-ye'elimite. However, in the 12 presence of additional sulfates, stoichiometric-ye'elimite reacts faster than solid-solution-ye'elimite. 13 14 15

Eco-friendly belite calcium sulfoaluminate (BCSA) cement hydration behavior is not yet well understood. Here, we report an in-situ synchrotron X-ray powder diffraction study for the first hours of hydration of BCSA cements. Rietveld quantitative phase analysis has been used to establish the degree of reaction (α). The hydration of a mixture of ye'elimite and gypsum revealed that ettringite formation (α~70% at 50 h) is limited by ye'elimite dissolution. Two laboratory-prepared BCSA cements were also studied: non-active-BCSA and active-BCSA cements, with β-and α′ H -belite as main phases, respectively. Ye'elimite, in the non-active-BCSA system, dissolves at higher pace (α~25% at 1 h) than in the active-BCSA one (α~10% at 1 h), with differences in the crystallization of ettringite (α~30% and α~5%, respectively). This behavior has strongly affected subsequent belite and ferrite reactivities, yielding stratlingite and other layered phases in non-active-BCSA. The dissolution and crystallization processes are reported and discussed in detail.

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