Ca2+ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy

Hygroscopic Growth of Adsorbed Water Films on Smectite Clay Particles

Hygroscopic growth of adsorbed water films on clay particles underlies a number of environmental science questions, from the air quality and climate impacts of mineral dust aerosols to the hydrology and mechanics of unsaturated soils and sedimentary rocks. Here, we use molecular dynamics (MD) simulations to establish the relation between adsorbed water film thickness (h) and relative humidity (RH) or disjoining pressure (Π), which has long been uncertain due to factors including sensitivity to particle shape, surface roughness, and aqueous chemistry. We present a new MD simulation approach that enables precise quantification of Π in films up to six water monolayers thick. We find that the hygroscopicity of phyllosilicate mineral surfaces increases in the order mica < K-smectite < Na-smectite. The relationship between Π and h on clay surfaces follows a double exponential decay with e-folding lengths of 2.3 and 7.5 Å. The two decay length scales are attributed to hydration repulsion and osmotic phenomena in the electrical double layer (EDL) at the clay-water interface.

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

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.