Tiejun Zhao

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.

Damping Performances of Carbon Nanotube Reinforced Cement Composite

Multi-walled carbon nanotubes (MWCNTs) were dispersed in an aqueous cement matrix using surfactant decoration, ultrasonic treatment, and, subsequently, intensive mixing to fabricate MWCNT/cement composites with six different MWCNT concentrations. Damping performances of these cured nanocomposites were studied with forced vibration testing, half-power bandwidth, and Morlet wavelet transform identification methods. The micro-crack bridging and interfacial “stick-slip” capacity of nanotubes among cement matrix contributes to balanced enhancement on structural damping capacity and flexural strength of the nanocomposite. With the addition of 2.0% nanotubes, the fundamental frequency, damping ratio, and flexural strength of the nanocomposite increases around 13 Hz, 60, and 32%, as compared to the reference, respectively.

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.

Orthogonal experimentation for optimization of TiO2 nanoparticles hydrothermal synthesis and photocatalytic property of a TiO2/concrete composite

An orthogonal experimental design was applied to optimize the hydrothermal preparation parameters of TiO2 nanoparticles by the analysis of means (ANOM) and variances (ANOVA). The ANOM & ANOVA results on crystalline and size show that the optimal process of synthesized TiO2 nanoparticles is 5.0% TBT, an alkalescent environment (pH = 9), 160 °C reaction temperature, and 3 h reaction time. The main factors affecting the photocatalytic properties of TiO2 nanoparticles were explored by measuring the UV light absorbance index, and the results show that their photocatalytic performance are excellent when methyl orange (MeO) concentration is 2.5 mg L−1, the pH value is 6, and the Ag-doped amount is 5.0%. Then optimal TiO2 nanoparticles aqueous suspensions with varied concentration (cTiO2) were sprayed on permeable concrete to fabricate TiO2/concrete composites, and their photocatalytic activity was evaluated by measuring the degradation rate of soaked MeO along the UV radiation time. It is concluded that when cTiO2 is 2.0 g L−1 with above fixed conditions, the produced TiO2/concrete composite has an optimal capacity (35.7%) in the photo-degradation of azo pollutants.

A novel microporous amorphous-ZnO@TiO2/graphene ternary nanocomposite with enhanced photocatalytic activity

Rational design and synthesis of graphene-based photoactive heterostructures is in great demand for various applications. Herein, a novel microporous amorphous-ZnO@TiO2/graphene heterostructure was developed via a facile approach for the first time. This heterostructure possesses excellent characteristics such as high surface area (336 m2 g−1), excellent mobility of charge carriers, and enhanced photocatalytic activity. The higher photocatalytic activity of the developed novel microporous amorphous-ZnO@TiO2/graphene hybrid was demonstrated through the degradation of water pollutants, MB and RhB. The mechanistic analysis result shows that the numerous unsaturated sites on the surface of amorphous-ZnO@TiO2 facilitate the separation of photogenerated electrons and holes, and graphene mainly acts as an electron transfer bridge. The combination of amorphous-ZnO@TiO2 and graphene constructs a new class of photocatalysts and also has a synergistic effect on improving the photocatalytic activity. The resultant amorphous-ZnO@TiO2/graphene ternary nanocomposite as a novel high performance photocatalyst is of a great potential for water pollution treatment due to its high catalytic activity, low cost, long-term stability, and easy recovery.

Aqueous processing and effects of V2O5 on microwave dielectric properties of multilayer Li1.075Nb0.625Ti0.45O3 ceramics

In the present work, we report the development of an aqueous tape casting method and a low temperature co-firing process for fabrication of multilayer Li1.075Nb0.625Ti0.45O3 microwave dielectric ceramics. A co-binder, consisting of polyvinyl acetate latex (PVAc) and polyvinyl alcohol (PVA), was used to prepare aqueous Li1.075Nb0.625Ti0.45O3 tapes. PVA addition increased the tape flexibility and adhesiveness but resulted in decreased tensile strength. Rheological tests indicated that the aqueous ceramic slurry exhibited a typical shear thinning behavior without thixotropy, suitable for tape casting. Scanning electron microscopy (SEM) studies revealed that the green tapes have a defect-free surface and that the multilayer ceramics sintered at 900°C have a fine plate like, grainy microstructure of uniform size. At lower temperatures, increased densification rates were achieved by addition of V2O5 to Li1.075Nb0.625Ti0.45O3 ceramics. The saturated bulk densities and dielectric constants (ɛr) of Li1.075Nb0.625Ti0.45O3 multilayer ceramics affected by lower sintering temperatures with an increase in V2O5 doping, and then an improvement in the quality factor (Q × f value) of the samples was achieved at the lower sintering temperatures. As a result, the ɛr of 64.9 and the Q × f value of 8800 GHz were obtained in the sample with an addition of 3 wt.% V2O5, sintered at a temperature of 900°C. No reaction was observed between the ceramic and silver layers when sliver inner-electrode, was sintered with ceramic tapes at 900°C.

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.

Long-term behavior of fiber reinforced concrete exposed to sulfate solution cycling in drying-immersion

The damage process and corrosion ion distribution in concrete, which was exposed to 60 and 170 drying-immersion cycles of sulfate solution, were systematically investigated. The effects of plain concrete, plain concrete mixed with 4 and 8 kg/m3 modified PP fiber and high-performance concrete (HPC) mixed with 0.8 kg/m3 fine PP fiber on the damage process were also studied. The experimental results showed that thenardite-induced surface scaling, as well as gypsum- and ettringite-induced cracks, were the main degradation forms of concrete under attack of sulfate solution and drying–immersion cycles. The relative dynamic modulus of elasticity of concrete initially increased, then reached stability and finally decreased to failure. The sulfate diffusion coefficients of plain and HPC were 10-12 and 10-13 m2/s, respectively. The concentration of sodium ion increased with depth, then maintained stability and finally decreased rapidly with concrete depth. The content of calcium ion on the concrete surface was 110%-150% of that in the interior of specimens. Although fiber worsened the surface scaling of concrete, better resistance capacity of sulfate ion penetration into concrete was observed in plain concrete with 4 kg/m3 modified PP fiber and HPC.