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.

Figure 1

Schematic understanding of the alite hydration reaction at different length scales. (Top) Hydration reaction of tricalcium silicate at the microscale. (a1) SEM microphotograph for C₃S_21 µm. (a2) SEM microphotograph for C₃S_3 µm. (b) SEM microphotograph for C₃S_21 µm_080 paste showing a homogeneous portlandite plate microparticle, voids arising from capillary water, and three agglomerates of heterogeneous C-S-H gel. (c) Enlarged view of one C-S-H gel region in (b). (d) TEM microphotograph of C₃S_3 µm_080_arrested:16d showing interspersed foil-like C-S-H nanoparticles at the mesoscale. (e) Schematic representation of the C-S-H colloidal nanoparticles of clinotobermorite (blue) and monolayer Ca(OH)₂ (orange) generating the small gel pores (SGP) and large gel pores (LGP) of Jennings’s model (25). (f) Schematic representation of a single C-S-H nanoglobule composed by defective clinotobermorite and two monolayers of Ca(OH)₂ at the nanoscale. (Bottom) Hydration reaction of tricalcium silicate at the nanoscale highlighting the three main components of colloidal C-S-H nanocomposite: nanocrystalline clinotobermorite, amorphous (monolayer) calcium hydroxide and gel pore water. The (approximate) densities, mass and volume percentages of the different components are also given for an overall water content of four water molecules per silicate.

Figure 2

Calorimetric data. (a) Heat flow calorimetry curves and (b) Cumulative heat released for C3S_21 µm_045, C3S_21 µm_080, C3S_7 µm_080, C3S_3 µm_080, C3S_21 µm_080_qz, and C3S_7 µm_080_qz. Data collected in the same run for a Portland cement type-I (OPC_045 and OPC_080) and OPC_080_qz are given as reference.

Figure 3

Quantitative phase analysis results from SXRPD. (a) C₃S_21 µm_080 and (b) C₃S_7 µm_080. The lines for C-S-H gel (blue) and portlandite (green) show the theoretical amounts expected from the measured dissolution of alite according to reaction (1).

Figure 4

²⁹Si MAS-NMR spectra for C3S_3 µm_080_arrested:16d measured. Spinning rate of 15 kHz and a magnetic field of 14.1 T.

Figure 5

PDF refinements. Experimental (blue circles), fitted (red lines) and difference (grey lines) PDF patterns for C₃S_3 µm_080_arrested:16d (a) from 40 to 70 Å; (b) from 10 to 25 Å; and (c) from 2 to 10 Å. For details of the fits, the readers are referred to the text.