Shu Jian Chen

Carbon nanotube-cement composites: A retrospect

Although ordinary Portland cement (OPC) is widely used in the construction industry, its weak tensile strength, to some extent, limits its application. A carbon nanotube (CNT), on the other hand, has outstanding mechanical properties with a tensile strength of 63 GPa and Youngs modulus of 1 TPa, making it a candidate as nano-scale reinforcements in OPC. Past research studies have reported improved mechanical and electrical properties of carbonnanotube–ordinary Portland cement (CNT–OPC) composites, which show future promise in practical civilengineering applications. In this study, recent research studies in developing CNT–OPC composites are comprehensively reviewed. Highlighted herein are the considerable efforts been made in the study of fabrication, hydration, porosity and transport properties of the CNT–OPC composites. There are, however, future investigations needed to provide a better understanding in the areas of uniform dispersion of CNTs within the OPC paste, durability, impact, fatigue properties and the theoretical modelling of CNT–OPC interaction.

Distribution of carbon nanotubes in fresh ordinary Portland cement pastes: understanding from a two-phase perspective

Significant research advances have been made in the field of carbon nanotube (CNT) reinforced ordinary Portland cement (OPC) paste composites in recent years. However, the distribution of CNTs in fresh OPC paste is yet to be fully researched and quantified, thereby creating a technical barrier to CNT utilization in concrete construction. In this study, fresh OPC paste was treated as a two-phase material containing solid particles (cement grains) and liquid solutions (pore solutions). A centrifugation-based technique was proposed to separate these two phases and the presence of CNTs in each phase was quantified. UV-Vis spectrometry showed that the degree of dispersion can achieve above 90 wt% using polycarboxylate superplasticizer. The results suggested an upper limit of 0.26 wt% for CNT addition into water before mixing with OPC, and the dispersion was found to be stable for at least 4 hours. Based on scanning electron imaging, the adsorption phenomenon of CNTs on OPC grains with size less than 4 μm was discovered. Energy-dispersive X-ray spectroscopy indicated these adsorptive particles have lower Ca to Si ratio. It was observed that about 0.5 mg of CNTs per gram of OPC grains was adsorbed in solid OPC grains in typical fresh OPC pastes. On the basis of these results, a conceptual model was proposed for the distribution of CNTs in fresh OPC paste where about 33 wt% of the CNTs stay in pore solution and 65 wt% of CNTs are adsorbed on OPC grains.

Effects of nanoalumina and graphene oxide on early-age hydration and mechanical properties of cement paste

The effects of nanoalumina (NA) and graphene oxide (GO) on the early-age hydration and mechanical properties of portland cement pastes were investigated in this study. The hydration heat release rate and cumulative heat of cement pastes incorporating different dosages of NA and GO were evaluated using an isothermal calorimeter measurement method. Early-age electrical resistivity development was investigated by a noncontact electrical resistivity technique. The results show that both NA and GO could efficiently accelerate cement hydration. As a physical filler, NA significantly accelerates the hydration of tricalcium aluminate (CA) in cement. On the other hand, GO is able to obviously reduce the dormant period of cement hydration and shift the heat flow peaks to the left by accelerating the hydration of tricalcium silicate (CS) in cement. Compared to plain cement pastes, both the compressive and flexural strengths of cement pastes incorporating NA or GO are significantly increased. However, when NA and GO contents exceed the optimal amounts, improvements in flexural strength tend to decline, which is probably due to particle agglomeration. NA-cement paste exhibited slightly higher electrical resistivity than plain cement paste during hydration acceleration and deceleration stages. But GO-cement paste clearly showed lower electrical resistivity, which might be attributed to iron diffusion caused by GO with large surface areas.

Improvement of mechanical properties by incorporating graphene oxide into cement mortar

ABSTRACT As a two-dimensional nanomaterial, graphene oxide has attracted much attention for its use in reinforcing cementitious materials. However, the dispersion of graphene oxide in cementitious materials has been found unsatisfactory due to crosslinking of divalent calcium ions. In this study, we propose a modified mixing procedure to improve graphene oxide dispersion in cement mortar by utilizing silica sand to mechanically separate graphene oxide nanosheets. Apart from the improved graphene oxide dispersion, adhesion between sand and cement matrix is also believed to be enhanced due to the improved roughness of the sand surface. According to our mechanical properties study, with the introduction of 0.02% by weight graphene oxide in cement mortar, compressive strength was significantly improved by more than 25% and tensile splitting and flexural strength were improved by around 15%. In a microstructural investigation, the interfacial transition zone in cement mortar was found to be denser due to the addition of graphene oxide. Moreover, graphene oxide incorporated cement mortar also showed pore structure refinement and porosity reduction. Therefore, improvement in mechanical properties may result from an improved interfacial transition zone and a more refined pore structure with the introduction of a small quantity of well-dispersed graphene oxide nanosheets.

Quantification of evaporation induced error in atom probe tomography using molecular dynamics simulation

Non-equilibrium molecular dynamics was used to simulate the dynamics of atoms at the atom probe surface and five objective functions were used to quantify errors. The results suggested that before ionization, thermal vibration and collision caused the atoms to displace up to 1Å and 25Å respectively. The average atom displacements were found to vary between 0.2 and 0.5Å. About 9 to 17% of the atoms were affected by collision. Due to the effects of collision and ion-ion repulsion, the back-calculated positions were on average 0.3-0.5Å different from the pre-ionized positions of the atoms when the number of ions generated per pulse was minimal. This difference could increase up to 8-10Å when 1.5ion/nm2 were evaporated per pulse. On the basis of the results, surface ion density was considered an important factor that needed to be controlled to minimize error in the evaporation process.

Nano-Impact Tests with Ultra-High Strain Rate Loading Using Graphene and Ion Impact

Strain rate is essential in study of the physics of fluids and solids undergoing deformation. State-of-art high strain rate tests have mainly been in macro-scale with an upper limit of 108s−1. A graphene-based layered system is proposed to conduct nanometer-scale high strain rate testing. The process is investigated by molecular dynamics simulations. Accelerated single ion or group of ions are used to impact on the proposed system to generate ballistic or plate-like impact scenarios. The effects of impact energy, shape of impact absorber and the impaction location are investigated. The graphene layer is the key of the proposed system to spread the load and protect the sample. The results indicate that ion group and single ion impacts produce strain rates of 5.0–5.2 × 1011s−1 and 0.9–1.2 × 1011s−1, respectively. Ion group impact produces a more significant signal than single ion impact on the sensing nano-layer in the system. The ultimate strength of an Al-Cu alloy sample during ion impact is estimated to be 215MPa to 251MPa, significantly lower than predicted by the Johnson–Cook model because of the rapidly increased temperature and melting in the sample. The results demonstrate new possibilities for understanding high strain rate effects at nano-scale.

Water Sorption Hysteresis in Cement Nano Slits

An important feature of the water sorption in cement has been the pronounced hysteresis. The hysteresis in capillary pores has been explained by so-called «ink-bottles» theory and/or the meniscus theory. In contrast, the hysteresis in gel pores (nanopores) received little attention. This paper investigated the sorption hysteresis in cement slits via atomistic simulations. Tobermorite 11 was adopted to represent the cement atomistic structures. In order to create the surface for adsorption, the Wollastonite chains in Tobermorite 11 were removed and the dangling bonds were saturated with hydrogen atoms. Charges were balanced to be neutral for the whole simulation cell. The sorption isotherms were calculated using the Sorption module in Material Studio (a commercial software for MD simulations), which calculates adsorption isotherms for water in the pores from a series of fixed pressure simulations. Three Tobermorite11 slits of widths H = 10 A, 20 A and 30 A were studied. The temperature was kept at 298 K. The results for all three slits showed significant and slit-size-dependent hysteresis sorption-desorption loops. The hysteresis shrinks when the width of the slits decreases. The role of Coulomb interaction in the hysteresis, as well as the water molecular density within the slits, were discussed. The results on sorption hysteresis in nano slits can be used to revise/refine the existing water hysteresis models for cement and concrete. This paper provides the pathway towards a better understanding of creep and drying shrinkage of ordinary Portland cement on the nanoscale.