Viscous Behavior of Exfoliated Graphite and its Cement-Matrix Composites
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This dissertation addresses the viscoelastic behavior of graphite and graphite cement-matrix composites. The graphite is exfoliated graphite, with the cell wall in its cellular structure consisting of ∼60 graphite layers and exhibiting extraordinarily viscous behavior, in addition to elastomeric behavior, due to interfacial sliding in the nanoscale multilayer. The viscous character of the cell wall decreases with increasing solid (graphite) content, due to the increasing difficulty of shear, which becomes limited at solid contents above 4 vol.%. At the lowest solid content of 1.0 vol.%, the loss-tangent/solid-content is 35 and 25 under flexure and compression respectively, the storage-modulus/solid-content is 125 MPa and 46 kPa under flexure and compression respectively, and the loss-modulus/solid-content is 45 MPa and 13 kPa under flexure and compression respectively. The loss-tangent/solid-content decreases with increasing solid content, leveling off at 0.9 at 15 vol.% solid. Exfoliated graphite compacts exhibit elastomeric behavior, as enabled by the high-amplitude reversible and easy sliding of the graphite layers relative to one another in the cell wall of exfoliated graphite. The total shear strain and reversible shear strain of the cell wall are up to 40 and 35 respectively during nanoindentation (in the compaction direction) without fracture of the graphite layers, compared to corresponding values of 12 and 8 for flexible graphite with 38 vol.% solid. The fraction of displacement that is irreversible is as low as 12%, compared to 29% for flexible graphite. The modulus is as low as 83 kPa, compared to 790 kPa for flexible graphite. The load per unit displacement is as low as 0.32 μN/nm, compared to 1.1 μN/nm for flexible graphite. Microscale constrained-layer damping has been achieved by incorporating the cell wall of exfoliated graphite in a cement matrix. The incorporation involves compaction of exfoliated graphite prior to the cement curing. Without the compression, the exfoliated graphite remains fluffy and the cell walls are not adequately constrained by the cement matrix. Without the compression, the cement-matrix composite is isotropic; with the compression, the composite is anisotropic. With the compression, the composite exhibits high values of both the loss tangent (up to 0.8) and the loss modulus (up to 7 GPa), as previously reported, in contrast to conventional damping materials which do not have high values for both of these quantities. Silica fume and the abovementioned composite made with the compression (used prior to curing) are effective as admixtures for enhancing the mechanical energy dissipation of cement-based materials, as shown under small-strain dynamic flexure at 0.2 Hz. The fraction of energy dissipated reaches 0.26, 0.58 and 0.22 for cement paste, mortar and concrete respectively, as provided by silane-treated silica fume and the cementitious admixture, which cause concrete to increase the dissipation, loss modulus, loss tangent and storage modulus by 11000%, 260000%, 9000% and 190% respectively. The highest loss tangent and loss modulus obtained are 0.14 and 3.5 GPa respectively. Silane-treated silica fume alone causes concrete to increase the dissipation by 11000%; untreated silica fume alone gives a 9100% increase. In exfoliated graphite, the mechanism of energy dissipation involves interfacial friction. This notion is supported by an analytical model.