In situ observing the hydration process of K-PSS geopolymeric cement with environment scanning electron microscopy

The hydration process of K-PSS geopolymeric cement was in situ quantitatively investigated by environment scanning electron microscope (ESEM) under 80% relative humidity. An energy dispersion X-ray analysis (EDXA) was also employed to determine the chemical composition of the hydration product. The ESEM micrographs showed that metakaolin particles pack loosely at 10 min after an initial mixing, resulting in an existence of many large voids. As the hydration proceeded, some gels were produced and gradually precipitated on the surfaces of these particles. At a later stage, these particles were covered by thick gel layers, and their interspaces among the metakaolin particles were also completely filled up. The corresponding EDXA results illustrated that the molar ratios of K/Al and Si/Al decreased with the development of hydration. The molar ratios of K/Al and Si/Al of the hydration products at an age of 13 h amounted to 1.06 and 2.14 respectively, which were very close to the theoretical values (K/Al = 1.0, Si/Al = 2.0) for K-PSS geopolymeric cement hardened paste. In addition, well-developed crystals could not be found at any ages, instead sponge-like amorphous gels have always been observed during the whole hydration process.     

Effect of addition time of a superplasticizer on cement adsorption and on concrete workability

The effect of addition time of a naphthalene-based superplasticizer (SNF) on the adsorption behavior on type I Portland cement slurries and on the concrete workability was studied. Test results indicate that the adsorption behavior of SNF on cement particles follows a Langmuir isothermal adsorption model. As the addition time increases, the saturated adsorption amount of SNF decreases sharply at the beginning and then more slowly. In comparison, the concrete workability decreases slightly in the early phase and then falls off abruptly. Most importantly, the transition points in both cases appear to be the same, at about 10–15 min. This strongly suggests that a close relationship exists between the SNF adsorption behavior on cement particles and the workability of concrete. In addition, the optimum addition time of SNF to concrete should be in this period, which corresponds to the beginning of the dormant period of the cement hydration process.     

The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt

Two ethylene–vinyl acetate (EVA) copolymers containing 10 and 25 wt.% vinyl acetate (EVA10 and EVA25) were utilized to explore the effect of molecular polarity on the formation of conductive carbon nanotube (CNT) network in EVA melt under an electric field. Because of the different interfacial energy, it was supposed to be stronger molecular chain-CNT interaction in CNT/EVA25 than that in CNT/EVA10. The critical time for conductive CNT network formation decreased with annealing temperature, filler loading and EVA polarity. The activation energy of conductive CNT network formation (93.9 kJ/mol) in CNT/EVA10 is lower than that (104.7 kJ/mol) in CNT/EVA25. By a thermodynamic percolation model, the percolation threshold at the equilibrium state was about 0.19 vol.% for CNT/EVA10, while it rose to 0.27 vol.% for CNT/EVA25. Morphological observation showed a high degree of CNT alignment in CNT/EVA10 compared to CNT/EVA25 after application of an electric field. The results suggested the strong CNT–EVA chain interaction and higher viscosity of polymer matrix limited the CNT alignment and the conductive network tended to form easily in EVA melt with a low chain polarity.     

An investigation on diffusion bonding of aluminum to copper using equal channel angular extrusion process

A new method for production of bimetallic rods, utilizing the equal channel angular extrusion (ECAE) process has been introduced before by previous researchers, but no attempt has been made to assess the effect of different temperatures and holding times in order to achieve a diffusional bond between the mating surfaces. In present research copper sheathed aluminum rods have been ECAEed at room temperature and subsequently held at a constant ECAE pressure, at different temperatures and holding times to produce a diffusional bond between the copper sheath and the aluminum core. The bonding quality of the joints was examined by shear strength test and a sound bonding interface was achieved. Based on the results, a bonding temperature of 200 °C and holding time of 60-80 min yielded the highest shear strength value.     

Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)₂ content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)₂ in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.     

An investigation into the effects of graphite particles on the damping behavior of ZA-27 alloy composite material

The objective of present work is to investigate the effect of macroscopic graphite particles on damping behavior of ZA-27 alloy composites. Compo casting technique was used to prepare the composites, in which the graphite particles were used to reinforce ZA-27 alloy, in varying fractions in order to arrive at optimum graphite reinforcement for bearing applications and establish a correlation of experimental readings. The experimental method used was the cantilever technique with Dynamic Mechanical Analyzer to evaluate damping properties. Standard strips of size 60 mm × 10 mm × 1 mm thick were tested by cantilever method of vibration tests at various temperatures from ambient to 300 °C. It was observed that the damping capacity of the material increased with increasing temperature and fractions of graphite particles.     

Identification of hydration stage of calcium sulfoaluminate cement at an early age with helium pycnometry

This work is to study the mechanism of the hydration of calcium sulfoaluminate (CSA) cement at an early age by identifying the hydration stage with helium pycnometry, heat evolution test, X-ray diffraction, thermo-gravimetric/differential scanning calorimetry and scanning electron microscope. The results show that the hydration process of CSA cement has four stages: rapid dissolution stage, dissolution-crystallization stage, crystal growth stage and stable stage. In the rapid dissolution stage, exothermic wetting and fast hydration result in the shrinkage occurring from 0 to 15 min. A significant volume decrease happens in the dissolution-crystallization stage. In the crystal growth stage, ettringite prefers to crystalize at the neighboring places of pores at the nano scale due to the combined action of crystallization pressure and size effect. The volume increase considerably slows down throughout the stable stage indicating the deceleration of CSA cement hydration. In addition, the influence of the proportions of clinker and gypsum in the CSA cement on its hydration is studied as well.     

Essential work of fracture and failure mechanisms of polypropylene-clay nanocomposites

The fracture behavior of polymer nanocomposites (PNCs) based on a polypropylene with organo-modified clays (2 wt.%) and different coupling agents was studied by means of essential work of fracture (EWF). The PNC microstructure was characterized by clay particle dispersion at the micron scale (>1 μm) and sub-micron scale (200 nm to 1 μm), with good intercalation and partial exfoliation (<100 nm). Tensile testing showed significant improvements (+25-50%) corresponding to nanoparticle reinforcement effects. Fracture surfaces revealed that fracture occurred by void initiation at larger clay particles, followed by void growth and coalescence as the surrounding matrix stretched into ligaments. EWF improvements (+20%) were noted for PNCs that had fewer micron scale particles and showed higher tensile improvements. Toughness improvements were attributed to higher voiding stresses and improved matrix resistance attributed to finer, more oriented clay nanoparticles.     

A comparative analysis of tensile and impact-toughness behavior of cold-worked and annealed 7075 aluminum alloy

The present research reports comparative analysis of effects of cold working (CW) and annealing on tensile and impact-toughness behavior of 7075 Al alloy. Cold-rolled samples were annealed at various temperatures in the range of 225–345 °C for 5 min. A remarkable increase in ductility and impact toughness was observed when specimens were annealed at temperatures above 265 °C for 5 min. It was also found that cold rolling has a profound effect on strength anisotropy that enhances with amount of % CW. The maximum strengths were observed in the transverse direction in the investigated alloy. Cold rolling has been found to impart a significant effect on decreasing the impact toughness of alloy that enhance with amount of % CW; this loss in impact energy could not be compensated by recrystallization process. It has also been shown that impact test can be considered as a simple method for measurement of toughness and plastic anisotropy in sheet and plate. The analysis of the fracture surfaces with the scanning electron microscope presented dimpled morphology for the failure ductile mechanism in starting material and fibrous structure with some quasi-cleavage regions in cold-rolled samples, corresponding to the ductile to brittle fracture mechanism.     

In-plane shear strength of a carbon/carbon composite at different loading rates and temperatures

The in-plane shear strength (IPSS) of a carbon/carbon composite (C/C) was measured at different loading rates and temperatures by compressing the double-notched specimen (DNS). The fracture surfaces were examined by scanning electron microscopy. The results indicate that IPSS measured by loading in compressing DNS is very close to that determined by the Iosipescu method at room temperature. There is a linear relationship between IPSS and the loading rate on the log-log coordinate, as the loading rate increases from 0.005 to 2 mm/min. IPSS at 1873 K is about two times of that at room temperature. The results were caused by the degassing effect of the absorbed water, release of the thermal stress and enhancement of the fiber strength.     

Vacuum brazing of aluminium metal matrix composite (55 vol.% SiCp/A356) using aluminium-based filler alloy

Aluminium matrix composites with high volume fractions of SiC particles, as the reinforcements, are potentially suitable materials for electronic packaging. These composites, due to their poor weldability, however, have very limited applications. The microstructure and shear strengths of the bonds made in 55 vol.% SiC_p/A356 composite, using an aluminium based filler alloy containing Cu, Si, Mg and Ni, were investigated in this paper. The brazing temperature had a clear effect on the bond integrity, and the samples brazed at 560 °C demonstrated good bonding between the filler alloy and the SiC particles. The maximum shear strength achieved in this work was 102 MPa.     

Release of internal curing water from lightweight aggregates in cement paste investigated by neutron and X-ray tomography

A sealed sample of cement paste containing a pre-wetted and a dry lightweight aggregate (LWA) particle was investigated in the period between 0.5 and 20.3 h after mixing. Changes in the local water distribution in the sample during hydration were evaluated using the subtraction of 3D images obtained by subsequent neutron tomographies (NT). As both water retention in the LWA and its release to the cement paste are influenced by the pore structure of the aggregate, a high-resolution image of the sample was subsequently captured by X-ray tomography. The internal curing water released from the LWA traveled at least 3 mm from the LWA into the cement paste in the first day. Hardly any gradient in the water content of the cement paste against the distance from the LWA was observed. This suggests that the release of water for internal curing (IC) is relatively fast and the water is distributed fairly homogeneously from the LWA for at least 3 mm within the hydrating cement paste.     

A comparative study of EVA with and without thermal history for different lamination process parameters

In more than 80% of the worldwide photovoltaic (PV) modules, mostly very fragile and 200 μm thick, crystalline silicon solar cells are encapsulated into ethylene-vinyl acetate (EVA) foils, which bond the module components together, provide physical protection, electrical insulation and a barrier for moisture ingress. The understanding of what can happen with EVA during its transport, storage and lamination process is necessary to optimize the quality of the PV module for its long exposure to outdoor weather conditions. Achieving a proper cross-link density of over 70%, it is essential to overcome the cold flow of EVA and to make the module durable. In this work, the feasibility of the use of differential scanning calorimetry (DSC) compared with the solvent extraction (SE) method by toluene were evaluated in order to provide structural information on the EVA curing kinetics and the cross-link density. DSC tests were performed on a DTA DuPont1600 tester. The temperature range for the test was from −50 °C to 200 °C, with the heating rate of 10 °C/min, and the endothermic and exothermic peaks were evaluated. Toluene solvent extractions were performed on the same set of samples that were analyzed by DSC. The measured cross-link density shows a direct dependence on the pre-lamination conditions of EVA, which is in good agreement with the data obtained with the DSC method.     

An experimental study on the ballistic impact behavior of some metallic armour materials against 7.62 mm deformable projectile

The investigation describes and analyses the ballistic impact behavior of a high strength armour steel and Al-7017 alloy under 7.62 mm deformable projectiles at a velocity of 830 ± 10 m/s at normal angle of attack. The high strength armour steel is subjected to two different heat treatments to see the effect of different mechanical properties on the ballistic behavior. The ballistic result of the Al-7017 alloy is compared with that of the steel. Some observations relating to the adiabatic shear bands formation have also been presented. Experimental results showed that among the investigated materials, the best ballistic performance was attained with the armour steel at 910 °C austenitisation followed by 200 °C tempering condition.     

Mixtures of silicon and aluminum oxides to optimize the performance of nanoporous thin films in concrete

This study explored the effect of two combinations of silicon and aluminum oxides, nanosilica–nanoboehmite and nanosilica–gibbsite, on the hydration reaction of cement and the porosity of the interfacial transition zone (ITZ). The influence of sols on the cement hydration reaction was investigated using isothermal calorimetry while their effect on the porosity of the aggregate–paste interface was validated using scanning electron microscopy. The nanosilica–nanoboehmite mixtures were found to accelerate the hydration reaction to a higher degree than the individual components, nanosilica and nanoboehmite. Further, the effect was also found to be dependent on the stoichiometry of the mixture of nanoparticles. The nanosilica–gibbsite combinations not only accelerated the reaction but also increased the cumulative heat of hydration. In this case, the enhancement is attributed to the seeding effect of the gibbsite particles, being more prominent at the smaller particle sizes. Lastly, when these materials were applied as nanoporous thin films on the aggregates, all sol mixtures not only helped to decrease the overall porosity but also contributed to refinement of the porosity in the cement paste adjacent to the aggregate. These effects were observed up to 250 μm away from the surface of the aggregate thus not restricted to the typical length of the interfacial transition zone in concrete (40–50 μm).     

Controlling cement hydration with nanoparticles

Nanoparticles have recently become a focus in the development of new accelerators for cement hydration. We have produced nanoparticles of different materials like Al₂O₃, C–S–H-phase, and quartz by different top-down and bottom-up production methods. The effect of these particles on the cement hydration was followed by heat flow calorimetry to evaluate their acceleration potential as a function of the particles material, particles size and their percentage in the cement paste. In this work we will demonstrate which particles have the best potential as accelerators for cement hydration and therefore are most suitable for further investigations. Interestingly, nanoparticles which have a retarding effect on cement hydration were also found.     

From microstructure to macrostructure: an integrated model of structure formation in polymer-modified concrete

A model is proposed for the formation of the microstructure in polymer-modified cementitious materials. Cement hydration and polymer film formation were studied, with specific emphasis on the synergetic effect between cement particles and polymer particles. Alterations at the microstructure level result in macroscopic changes in the properties of the modified material. In this paper, the influence of the polymer addition on the appearance of the cement hydrates and the presence of the polymer film through the cement hydrates are presented in relation to the minimum film forming temperature. Owing to the presence of the cement particles and to cement hydration, film formation can take place at lower temperatures, so that a polymer dispersion with a slightly higher MFT (minimum film forming temperature) can be used. This is important for the physical and mechanical properties of the polymer-modified materials. The findings have been included in an integrated model based on the three-step model of Ohama, in which the polymer film formation and the cement hydration processes are integrated in relation to each other. A time-dependent evaluation of both processes was incorporated. The research presented in this paper was part of a PhD research at the Civil Engineering Department, University of Leuven, Belgium [1].     

High-temperature shear strength of lead-free Sn-Sb-Ag/Al2O3 composite solder

The lead-free Sn-1.7Sb-1.5Ag solder alloy and the same material reinforced with 5 vol.% of 0.3-μm Al₂O₃ particles were synthesized using the powder-metallurgy route of blending, compaction, sintering, and extrusion. The mechanical properties of both monolithic and composite solders were studied by shear punch testing (SPT) at temperatures in the range of 25-130 °C. Depending on the test temperature, the shear yield stress (SYS) increased by 4.8-8.8 MPa, and ultimate shear strength (USS) increased by 6.2-8.8 MPa in the composite material. The strength improvement was mostly due to the CTE mismatch between the matrix and the particles, and to a lesser extent to the Orowan strengthening mechanism of the submicro-sized Al₂O₃ particles in the composite solder. The contribution of each of these mechanisms was used in a modified shear lag model to predict the total composite-strengthening achieved.     

The influence of nano-silica on the hydration of ordinary Portland cement

Nucleation seeding is a new approach to control the kinetics of cement hydration. It is known that nano-silica has an accelerating effect on cement hydration. It is assumed that the surface of these particles act as a nucleation site for C–S–H-seeds which then accelerate the cement hydration. In this case the acceleration should depend on the particles surface area. To verify this, nano-silica particles of different sizes and specific surface areas were synthesised. The acceleration of cement hydration clearly correlates with the total surface size of the added particles, which was varied by either using smaller particles or higher concentration of particles in the cement lime. Additional in situ-XRD experiments show that the consumption of C₃S and the formation of portlandite are accelerated by the addition of nano-silica. In both cases the surface size is the major factor for the hydration kinetics.     

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.