Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus

Abstract

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C₃S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C₃S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C₃S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C₃S paste. We further prepared thin films of C₃S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C₃S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C₃S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C₃S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

Figure 1

Amplitude sweep (AS) measurements on C₃S pastes (dark and light red) and C–S–H gels (dark and light blue). The evolution of the elastic G′ (○) and viscous G″ (□) moduli is plotted as a function of the imposed amplitude strain γ (f = 1 Hz). The vertical lines indicate the elastic limit of the samples. The two pictures on the left correspond to the C₃S paste (light-red frame) and the C–S–H gel (dark-blue frame) and were taken immediately after mixing.

Figure 2

(Long) time structuration (TS) measurements on the C₃S paste (red) and the C–S–H gel (blue). The evolution of the elastic modulus G′ is followed in time. The pictures correspond to the loading (left) and unloading (right) of the samples and have the same color code as the graphics.

Figure 3

XRD diffraction patterns of the (bottom) C₃S PLD sintered target and the (top) calcium silicate film PLD-deposited on a mica substrate. The sharp peaks in the film XRD correspond to mica substrate Bragg reflections stemming from Cu Kα,β and W-Lα₁ wavelengths simultaneously.

Figure 4

X-ray photoelectron spectroscopy (XPS) spectra of the PLD-deposited calcium silicate films. (A) Wide-scan XPS survey spectra with the main photoemission peaks used for the calcium silicate film composition semiquantification. (B) High-resolution photoemission peaks corresponding to O, Ca, and Si.

Figure 5

SEM-EDS semiquantitative Ca mapping on the ∼200 nm-thick calcium silicate film sample deposited on a mica substrate. (A) SEM SE image of the region chosen for the mapping, showing a flat topography with a few scattered μm-sized particles; the scale bar corresponds to 10 μm. (B) Ca EDS map of the region in panel (A) showing a largely uniform distribution of Ca as indicated by the orange color. A higher relative amount of Ca is measured for the μm-sized particles on the surface; the scale bar is 10 μm. (C) Fragment of a point EDS spectra showing Ca and K signals from a region on a calcium silicate film and on a bare mica substrate. (D) High-resolution secondary electron (SE) image of the calcium silicate film deposited on mica.

Figure 6

Atomic force microscopy topography maps of calcium silicate films in air (A) and in water ((B) sample immersed in H₂O for 30 min). The panels below AFM maps show height profiles along the center of each AFM image as marked with a dashed magenta line. Note that the y axis is the same in both panels. (C) Comparison of the root-mean-square (rms) roughness measured in air and in water (over 1.5 h in the same position) for a 1 × 1 μm² scan size. Each point corresponds to one AFM scan, including the measurement in air.

Figure 7

SFA measurements of forces between two opposing calcium silicate surfaces (C₃S). (A) Schematic representation of SFA experiments, with C₃S surfaces reprecipitating in water into calcium silicate hydrate (C–S–H). (B) Representative force–distance (D) SFA curves measured between reacting C₃S surfaces. The hard wall position (i.e., separation distance at 0 nm) corresponds to the initial thickness of dry calcium silicate surfaces (T_DRY). The separation distance is expressed as D – T_DRY to highlight the shift of the hard wall contact position with time that corresponds to the growth of C–S–H. The layer growth is correlated with the appearance of attractive force (force < 0) on separation. The inset shows the thickness increase and the change of the contact shape (extracted from the interferometric fringes shown in panel (C)). The approach and detachment rate was 100 nm/s for all force curves. (C) Shift of the interferometric FECO fringes at the hard wall contact position, corresponding to the growth of the C–S–H layer. The dashed yellow line marks the position of the FECO fringe of the same chromatic order. The white dotted line outlines the size of the contact region. (D) Adhesive pressure between calcium silicate surfaces and calcium silicate film thickness (on a single surface) as a function of elapsed time after the injection of water. Adhesive pressure is calculated as the pull-off force (see panel (B)) normalized by the nominal contact areas (see panel (C)). Further calculation details are included in the Materials and Methods section.

Figure 8

Adhesion (pull-off force) between two opposing calcium silicate surfaces measured in water in the surface force apparatus after a 3 h-long immersion in water. All data were collected in the same contact position (also the same as in Figure 1) after the layer growth ceased and surface thickness stabilized. (A) Adhesion as a function of applied load at constant approach and detachment velocity (100 nm/s), measured from high loads to low loads. Here, each data point corresponds to at least three loading–unloading cycles. (B) Adhesion as a function of time in contact at a constant load of 6000 mN/m and a constant approach and detachment velocity (100 nm/s). Dwell time was applied in a random order. Each data point corresponds to a single loading–unloading cycle. (C) Adhesion as a function of detachment velocity at a constant load of 6000 mN/m and no dwell time in contact. Approach velocity was varied in a random order. Each data point corresponds to a single loading–unloading cycle.