Insights into the interfacial strengthening mechanisms of calcium-silicate-hydrate/polymer nanocomposites

Abstract

The mechanical properties of organic/inorganic composites can be highly dependent on the interfacial interactions. In this work, with organic polymers intercalated into the interlayer of inorganic calcium silicate hydrate (C-S-H), the primary binding phase of Portland cement, great ductility improvement is obtained for the nanocomposites. Employing reactive molecular dynamics, the simulation results indicate that strong interfacial interactions between the polymers and the substrate contribute greatly to strengthening the materials, when C-S-H/poly ethylene glycol (PEG), C-S-H/poly acrylic acid (PAA), and C-S-H/poly vinyl alcohol (PVA) were subject to uniaxial tension along different lattice directions. In the x and z direction tensile processes, the Si-OCa bonds of the C-S-H gel, which were elongated and broken to form Si-OH and Ca-OH, play a critical role in loading resistance, while the incorporation of polymers bridged the neighboring silicate sheets, and activated more the hydrolytic reactions at the interfaces to avoid strain localization, thus increasing the tensile strength and postponing the fracture. On the other hand, Si-O-Si bonds of C-S-H mainly take the load when tension was applied along the y direction. During the post-yield stage, rearrangements of silicate tetrahedra occurred to prevent rapid damage. The polymer intercalation further elongates this post-yield period by forming interfacial Si-O-C bonds, which promote rearrangements and improve the connectivity of the defective silicate morphology, significantly improving the ductility. Among the polymers, PEG exhibits the strongest interaction with C-S-H, and thus C-S-H/PEG possesses the highest ductility. We expect that the molecular-scale mechanisms interpreted here will shed new light on the stress-activated chemical interactions at the organic/inorganic interfaces, and help eliminate the brittleness of cement-based materials on a genetic level.

Effect of polymer-coated silica particles in a Portland cement matrix via in-situ infrared spectroscopy

“…From the thermodynamic viewpoint, as the pure C-S-H phase undergoes the tensile process, cause of the activation energy reductions, two hydrolytic reactions are taken place (equations (5) and (6)). 34–36 One is that, considering formula No. 5, the ionic bonds between the Ca and Si−O groups are stretched in the presence of water until they break which leads to formation of Ca−OH and Si−OH.…”

“…Presumably, the coordination of carbon from the functional group of the polymers makes Si susceptible to separating from the Si−O group and forming 2 Si−OH. 34 Further point is that in the PEG-600-C, the intensity of the C−O bond related to calcium silicate carbonate phase is also enhanced in comparison to the cement without particles (Figure 10). It seems that the carbonate silicate phase in this sample is a weakest phase comparing to the presented phase in pure hardened cement paste.…”

Perspective/Discussion on “Quantum Mechanical Metric for Internal Cohesion in Cement Crystals” by C. C. Dharmawardhana, A. Misra and Wai-Yim Ching

“…On the contrary, it is broadly thought that the backbone or templates of the C-S-H disordered arrangements are designed by the mixture of Jennites (d.8) and Tobermorites (d.2-d.4) [45]. Nevertheless, the TBOD and the TO computation indicate that C-S-H arrangements in hydrated cement cannot just be surmised as C-S-H crystal mixtures or even some disordered forms of such mixtures.…”

Bioinspired Cementitious Materials: Main Strategies, Progress, and Applications

“…Zhou et al employed molecular dynamics to study the nature of interactions between C-S-H and three different types of polymer, poly(acrylic acid) (PAA), poly (vinyl alcohol) and poly (ethylene glycol), all of which have been investigated experimentally as an organic component for C-S-H/polymer composites (Zhou et al, 2018). The authors found that PAA exhibits the strongest interaction among the three types of polymer, primarily via bonding between calcium from C-S-H and double bonded oxygen of carboxylic acid.…”