Calcium-silicate-hydrate
(C–S–H) is the major binding
phase responsible for strength and durability of cementitious materials.
The cohesive properties of C–S–H are directly related
to the intermolecular forces between its layers at the nanoscale.
Here, we employ free energy perturbation theory (FEP) to calculate
intermolecular forces between crystalline C–S–H layers
solvated in aqueous medium along face-to-face (FTF) and sliding reaction
coordinates. Contrary to mean-field theories, we find that our counterion-only
system exhibits an oscillatory behavior in FTF interaction. We correlate
these oscillations with the characteristic length scale comparable
to the distance between interfacial water layers at the hydrophilic
surface of crystalline C–S–H. We attribute the sliding
intermolecular forces to the atomic level roughness of crystalline
C–S–H layers stemming from the local arrangement of
nanoscale structural motifs. These intermolecular forces provide a
direct access to the key mechanical properties, such as surface energy,
cohesive pressure and elastic properties. The simulation results are
in close agreement with the available experimental measurements. Furthermore,
we present these intermolecular forces in a mathematical framework
to facilitate coarse-grain modeling of crystalline C–S–H
layers. These results provide a novel route that paves the way for
developing realistic mesoscale models to explore the origins of chemophysical
properties of crystalline C–S–H.
Calcium-silicate-hydrate (C-S-H), the main binding phase in cementitious materials, possesses a complex multiscale porous texture where nanosized particles interact effectively and contribute to the macroscopic properties of concrete. Engineering the morphology and properties of cementitious materials can thus be obtained by, first, studying the impact of the variable chemical composition on the cohesion and properties of nanolayers of C-S-H at the nanoscale and, then, translating these information to the mesoscale so that a textural analysis can be accomplished. Here, we aim to establish a foundation for such a comprehensive study. First, we construct variable atomic structures of C-S-H nanolayers and validate them against experimental measurements. Then, we conduct free energy perturbation analysis to measure the potential-of-mean-force (PMF) between C-S-H nanolayers with varying chemical compositions. We find a strong correlation between the chemical composition as well as polymorphic structure of C-S-H and characteristics of measured PMFs. In particular, we observe a transition in PMF shape from a single minimum to multiple minima, indicating the emergence of metastable states in the interparticle interactions. We show that key mechanical properties of C-S-H calculated via the PMF approach are in a reasonable agreement with the available experimental data. The proposed PMFs can be directly used to investigate the textural attributes as well as the study of the hydration process in cementitious materials.
Line shift coefficients, line broadening coefficients, and line narrowing coefficients have been measured in the ν1+3ν3 band of acetylene using a diode laser system operating at 788 nm and a multipass Herriot absorption cell. Experimental data have been obtained for 20 lines of the P and R branches broadened by N2, O2, air, and the rare gases He, Ne, Ar, Kr, and Xe. The observed line shapes could successfully be reproduced by employing Galatry and Rautian functions which include the phenomenon of Dicke narrowing. Our results for the line broadening coefficients are in good agreement with the values reported previously for other vibrational bands. Thus, the present work confirms the vibrational independence of the broadening coefficients. On the other side, we observed a clear dependence of the narrowing coefficients on rotation in a vibrational transition of acetylene for the first time. In addition, the line center frequencies have been determined with improved accuracy.
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