Dongshuai Hou

Mechanism of cement paste reinforced by graphene oxide/carbon nanotubes composites with enhanced mechanical properties

This study presents the enhanced mechanical properties of cement paste reinforced by graphene oxide (GO)/carbon nanotubes (CNTs) composites. The UV-vis spectroscopy and optical microscopy results show that the dispersion of CNTs in the GO solution is much better than in an aqueous solution due to the higher electrostatic repulsion, which allows a completely new approach of dispersing CNTs rather than by incorporating a dispersant. More importantly, the GO/CNTs composite plays an important role in improving the compressive and flexural strength of cement paste by 21.13% and 24.21%, which is much higher than cement paste reinforced by CNTs (6.40% and 10.14%) or GO (11.05% and 16.20%). The improved mechanical properties of cement paste are attributed to better dispersed CNTs and enhanced interactions among CNTs by the GO incorporation. Finally, the space interlocking mechanism of the GO/CNTs/cement paste composite with enhanced mechanical properties is proposed.

Molecular Dynamics Study of Water and Ions Transported during the Nanopore Calcium Silicate Phase: Case Study of Jennite

AbstractDurability is an important property that determines the long-term behavior of cement-based materials. Water and ions are transported in nanopores of calcium-silicate-hydrate (C-S-H) gels, the main element in cement-based material, which significantly influences the durability of cement. Because of its structural similarity, jennite, an important mineral analog of C-S-H gel, is first taken to investigate the transport behavior at a molecular level. In this paper, structural and dynamical properties of the water/ions and the jennite interface are studied by the molecular dynamics (MD) simulation method. On the (001) surface of jennite, water molecules diffusing in the channel between silicate chains demonstrate the following structural water features: large density, good orientation preference, ordered interfacial organization, and low diffusion rate. The channel water molecules have more H-bonds connected with the neighboring water molecules and solid surface. As the distance from the channel incre...

Reactive force field simulation on polymerization and hydrolytic reactions in calcium aluminate silicate hydrate (C–A–S–H) gel: structure, dynamics and mechanical properties

The reactive force field method was first utilized to characterize the structure, dynamics and mechanical properties of calcium–aluminate–silicate–hydrate, which is essential in the chemistry of high alumina layered gel in Portland cement. In order to study the role of Al atoms, the properties of Al atoms located in the calcium silicate sheet and the interlayer region have been investigated. The Si–Al substitution in the calcium silicate sheet has not changed the layered structure of the C–A–S–H gel. On the other hand, the presence of Al atoms in the interlayer region improves the structure and mechanical performance significantly. The connectivity factor, Q, species evolution indicates that the aluminate species in the interlayer region play an essential role in bridging the defective silicate chains and transforming the layered C–A–S–H gel at low Al/Ca levels to the branch network structure at high Al/Ca levels. The structural transition is partly attributed to the aluminate–silicate connection by the NBO sites and is partly caused by the polymerization reaction between the aluminate species, both of which can be described by the reactive force field. Additionally, the polymerization reaction by the aluminate species also leads to a hydrolytic reaction. In this way, a lot of water molecules are transformed to hydroxyls, even bridging oxygen atoms. Dynamically, due to the high strength of the Al–O bond, the aluminate–silicate network in the C–A–S–H gel has a better stability at higher Al/Ca ratios. Furthermore, uniaxial tension tests on the C–A–S–H gels demonstrate the mechanical behavior and large structural deformation of the gel. Both the Young’s modulus and tensile strength are improved significantly with increasing aluminum content, indicating a good loading resistance ability in the aluminate–silicate network. The tensile deformation, simulated by the reactive force field, is also coupled with de-polymerization of the aluminate species and the water dissociation reaction, which shows good plasticity due to the Al atom addition.

Preparation and characterization of an expanded perlite/paraffin/graphene oxide composite with enhanced thermal conductivity and leakage-bearing properties

A novel phase change material (PCMs) of expanded perlite/paraffin/graphene oxide (EP/PA/GO) with enhanced thermal conductivity and leakage-bearing properties was fabricated by depositing GO films on the surface of the EP/PA composite. The as-prepared EP/PA/GO composite was characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The experimental results indicated that due to the small loading of the GO incorporated, the EP/PA/GO composite showed a small latent heat capacity and weight loss, and the thermal conductivity significantly increased with increasing the content of GO up to 0.5 wt%. The heat storage/release performance test results demonstrated that the EP/PA composite with 0.5 wt% GO had 2 times faster heat storage/release rate compared to the EP/PA composite because of the enhanced thermal conductivity. In addition, the FTIR and TGA results indicated that the EP/PA/GO composite had good chemical compatibility and thermal stability. More importantly, the GO films covering the surface of the EP/PA composite can greatly prevent the leakage problems of molten paraffin. No leakage of paraffin occurred even after thermal cycling 3000 times. Therefore, the EP/PA/GO composite has a great potential for thermal energy storage applications due to its enhanced thermal properties, good leakage-bearing properties and excellent chemical compatibility.

Confined Water Dissociation in Disordered Silicate Nanometer-Channels at Elevated Temperatures: Mechanism, Dynamics and Impact on Substrates

The effects of elevated temperature on the physical and chemical properties of water molecules filled in the nanometer-channels of calcium silicate hydrate have been investigated by performing reactive molecular dynamics simulation on C-S-H gel subjected to high temperature from 500 to 1500 K. The mobility of interlayer water molecules is temperature-dependent: with the elevation of temperature, the self-diffusivity of water molecules increases, and the glassy dynamic nature of interlayer water at low temperature transforms to bulk water characteristic at high temperature. In addition, the high temperature contributes to the water dissociation and hydroxyl group formation, and proton exchange between neighboring water molecules and calcium silicate substrate frequently happens. The hydrolytic reaction of water molecules results in breakage of the silicate chains and weakens the connectivity of the ionic-covalent bonds in the C-S-H skeleton. However, the broken silicate chains can repolymerize together to form branch structures to resist thermal attacking.

Interfacial Connection Mechanisms in Calcium–Silicate–Hydrates/Polymer Nanocomposites: A Molecular Dynamics Study

Properties of organic/inorganic composites can be highly dependent on the interfacial connections. In this work, molecular dynamics, using pair-potential-based force fields, was employed to investigate the structure, dynamics, and stability of interfacial connections between calcium-silicate-hydrates (C-S-H) and organic functional groups of three different polymer species. The calculation results suggest that the affinity between C-S-H and polymers is influenced by the polarity of the functional groups and the diffusivity and aggregation tendency of the polymers. In the interfaces, the calcium counterions from C-S-H act as the coordination atoms in bridging the double-bonded oxygen atoms in the carboxyl groups (-COOH), and the Ca-O connection plays a dominant role in binding poly(acrylic acid) (PAA) due to the high bond strength defined by time-correlated function. The defective calcium-silicate chains provide significant numbers of nonbridging oxygen sites to accept H-bonds from -COOH groups. As compared with PAA, the interfacial interactions are much weaker between C-S-H and poly(vinyl alcohol) (PVA) or poly(ethylene glycol) (PEG). Predominate percentage of the -OH groups in the PVA form H-bonds with inter- and intramolecule, which results in the polymer intertwining and reduces the probability of H-bond connections between PVA and C-S-H. On the other hand, the inert functional groups (C-O-C) in poly(ethylene glycol) (PEG) make this polymer exhibit unfolded configurations and move freely with little restrictions. The interaction mechanisms interpreted in this organic-inorganic interface can give fundamental insights into the polymer modification of C-S-H and further implications to improving cement-based materials from the genetic level.

Molecular Simulation of the Ions Ultraconfined in the Nanometer-Channel of Calcium Silicate Hydrate: Hydration Mechanism, Dynamic Properties, and Influence on the Cohesive Strength

Reactive force field molecular dynamics was utilized to investigate the structure, dynamics, and mechanical nature of different cations solvated in the nanometer-channel of highly disordered calcium silicate hydrate. The local structures of different cations bonded with hydroxyl groups are characterized by the long spatial correlation, bond angel distribution preference, and featured coordinated number, resembling those of the tetra-/penta-/octahedron for cation-oxygen structure in the defective region of the silicate glass. Al atoms in the interlayer region play a role in bridging the defective silicate chains and enhance the connectivity of the silicate skeleton. Dynamically, the mobility of ultraconfined water molecules and cations is significantly influenced by the ionic chemistry: the residence time for water molecules in the hydration shell of Al and Mg ions is longer than that in the environment of Na and Ca ions. Furthermore, uniaxial tension simulation provides insight that while both the stiffness and cohesive strength of the C-S-H gels are significantly improved due to the silicate-aluminate branch structure formation, sodium ions with unstable Na-O connection weaken the loading resistance of the C-S-H gels. During the tensile process, the hydrolytic reaction is also affected by the cationic type: water molecules coordinated with Al and Mg cations at high stress state are likely to decompose, but those aggregated with sodium ions are hard to be stretched broken due to the low failure stress.

The Effect of Water Repellent Surface Impregnation on Durability of Cement-Based Materials

In many cases, service life of reinforced concrete structures is severely limited by chloride penetration until the steel reinforcement or by carbonation of the covercrete. Water repellent treatment on the surfaces of cement-based materials has often been considered to protect concrete from these deteriorations. In this paper, three types of water repellent agents have been applied on the surface of concrete specimens. Penetration profiles of silicon resin in treated concrete have been determined by FT-IR spectroscopy. Water capillary suction, chloride penetration, carbonation, and reinforcement corrosion in both surface impregnated and untreated specimens have been measured. Results indicate that surface impregnation reduced the coefficient of capillary suction of concrete substantially. An efficient chloride barrier can be established by deep impregnation. Water repellent surface impregnation by silanes also can make the process of carbonation action slow. In addition, it also has been concluded that surface impregnation can provide effective corrosion protection to reinforcing steel in concrete with migrating chloride. The improvement of durability and extension of service life for reinforced concrete structures, therefore, can be expected through the applications of appropriate water repellent surface impregnation.

A novel Zn(II) dithiocarbamate/ZnS nanocomposite for highly efficient Cr6+ removal from aqueous solutions

A simple one-step method was designed for the first time to produce a novel Zn(II) dithiocarbamate/ZnS nanocomposite. The developed Zn(II) dithiocarbamate/ZnS nanocomposite was found to exhibit outstanding performance on Cr6+ removal from aqueous solutions and the removal rate could reach more than 98% in just a few seconds. Various influencing factors such as the addition quantity, pH value and reaction temperature were investigated in order to determine the optimal removal conditions. It was shown that the Cr6+ removal ability of the Zn(II) dithiocarbamate/ZnS nanocomposite remained high at almost all the pH values and the degradation rate increased gradually with the reaction temperature up to 323 K. Compared to the traditional catalysts, which usually involve a complex production process with a high cost and a low degradation rate, the novel Zn(II) dithiocarbamate/ZnS nanocomposite has great potential in applications for wastewater treatment.