Multiscale understanding of tricalcium silicate hydration reactions

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.

Quantitative disentanglement of nanocrystalline phases in cement pastes by synchrotron ptychographic X-ray tomography

“…For estimating the water content, the Ca/Si ratio within the C-S-H gel must be known. We have assumed an overall Ca/Si ratio of 1.80 (Cuesta et al, 2018) which is in agreement with previous studies (Richardson, 2008;Papatzani et al, 2015). Following this methodology, the water stoichiometry determined for the gel was (CaO) 1.80 SiO 2 (H 2 O) 3.96 and then a density of 2.11 g cm À3 is obtained.…”

“…The interested reader is directed towards recent reviews on C-S-H for detailed information (Richardson, 2008;Jennings, 2008;Papatzani et al, 2015;Palkovic et al, 2016). At the nanometre scale, it has been shown recently that C-S-H from hydrating alite had a defective clinotobermorite structure, with an approximate composition of Ca 11 Si 9 O 28 (OH) 2 Á8.5H 2 O, and monolayers of Ca(OH) 2 and gel water (Cuesta et al, 2018). Nanocrystalline C-S-H, particle size ' 5 nm, has a Ca/Si molar ratio of $1.3; which has been previously reported (Cong & Kirkpatrick, 1996;Skinner et al, 2010;Chen et al, 2010;Richardson, 2014;Grangeon et al, 2017;Cuesta, Zea-Garcia et al, 2017;Andalibi et al, 2018) However, it is very well established that the neat PC pastes produce C-S-H gels with an average Ca/Si molar ratio of 1.7-1.8 at the micrometre scale (Bensted & Barnes, 2002;Taylor, 1997) with an average stoichiometry close to (CaO) 1.8 SiO 2 Á4H 2 O.…”

“…Nanocrystalline C-S-H, particle size ' 5 nm, has a Ca/Si molar ratio of $1.3; which has been previously reported (Cong & Kirkpatrick, 1996;Skinner et al, 2010;Chen et al, 2010;Richardson, 2014;Grangeon et al, 2017;Cuesta, Zea-Garcia et al, 2017;Andalibi et al, 2018) However, it is very well established that the neat PC pastes produce C-S-H gels with an average Ca/Si molar ratio of 1.7-1.8 at the micrometre scale (Bensted & Barnes, 2002;Taylor, 1997) with an average stoichiometry close to (CaO) 1.8 SiO 2 Á4H 2 O. There is still some debate if the excess of calcium with respect to the defective clinotobermorite structure, Ca/Si ratio ' 1.3, is caused by the presence of monolayers of Ca(OH) 2 (Grangeon et al, 2017;Cuesta et al, 2018) or by the layers of calcium hydroxide intergrown within the clinotobermorite nanoparticles (Kumar et al, 2017). In each case, the component phase in the cement pastes with a Ca/Si molar ratio ' 1.8 at the micrometre scale originates from the fine intermixing at the nanometre scale of defective tobermorite and calcium hydroxide.…”

“…It is widely accepted that the strength and other mechanical and chemical properties of mortars and concretes mainly rely upon the formation of C-S-H gel during cement hydration which forms about half the volume of all cement hydrates (Taylor, 1997;Bensted & Barnes, 2002). Because of the complex hierarchical arrangement of cement pastes in general (Mehta et al, 2014), and C-S-H gel in particular (Papatzani et al, 2015;Cuesta et al, 2018;Andalibi et al, 2018;Kumar et al, 2017), techniques yielding spatially resolved information are key to their characterization. There are different approaches for obtaining spatially resolved information including scanning electron microscopy, transmission electron microscopy, nanoindentation and X-ray imaging.…”

A Microstructure Insight of MTA Repair HP of Rapid Setting Capacity and Bioactive Response

“…Typically, no peaks corresponding to calcium silicate hydrated products (CSH) were observed by XRD analysis due to poorly crystalline nanoparticles generation, usually described as needle-like, foil or globule formations [25,28]. CSH phase has been mainly based on tobermorite-like structure [28,29], and the proposed nanostructure as a gelled colloid, which enclosed water-filled of internal and interlayer spaces [30]. TEM images displayed in Figure 4 are very illustrative of the complex nano-porosity exhibited by the MTA Repair HP hydrated material.…”