Location and Visualization of Working p-n and/or n-p Junctions by XPS

X-ray photoelectron spectroscopy (XPS) is used to follow some of the electrical properties of a segmented silicon photodetector, fabricated in a p-n-p configuration, during operation under various biasing configurations. Mapping of the binding energy position of Si2p reveals the shift in the position of the junctions with respect to the polarity of the DC bias applied. Use of squared and triangular shaped wave excitations, while recording XPS data, allows tapping different electrical properties of the device under normal operational conditions, as well as after exposing parts of it to harsh physical and chemical treatments. Unique and chemically specific electrical information can be gained with this noninvasive approach which can be useful especially for localized device characterization and failure analyses

Effect of slags of different origins and the role of sulfur in slag on the hydration characteristics of cement-slag systems

The effect of slag of different origins (synthetic slag produced in the laboratory and commercial slag collected from different steel factories) with comparable chemical composition, amorphous content and particle size distribution, on the hydration characteristics of slag cement was investigated. In order to study the effect of sulfur in slag, a model cement paste of a C3S-slag blend was also produced. It was found that origin of slag has very little impact on the hydration process of cement-slag system. Synthetic slag shows a comparable compressive strength gain to commercial slag at 1 day, and from then on, a lower compressive strength is reached by it compared with that of commercial slag until 28 days. The sulfur in slag starts to participate in reaction after 1 day and it dominates the rate of heat release in calorimetric measurement, consistent with the result of compressive strength test. It significantly affects the elemental composition of the cementitious matrix at 7 days, and higher Al/Si and S/Ca ratios can be detected in cement-commercial slag blend at 7 days. The sulfur is involved in the formation of AFm-phase, such as calcium monosulfoaluminate, and the thermodynamic modelling shows that upon the gradual incorporation of sulfur in slag, calcium monosulfoaluminate precipitates continuously with the consumption of strätlingite and portlandite. The conclusion obtained in the paper provides a basis to understand the role of sulfur in slag on the hydration process of slag cementMaterials and Environmen

Experimental and numerical study on mechanical properties of cement paste pipes subjected to uniaxial tensile loading

The aim of this paper is to investigate the mechanical properties of cement paste specimens by both experimental and numerical methods. Firstly, the specimens subjected to uniaxial tensile loading were studied experimentally. Afterwards, numerical investigation was carried out based on the experimental observations. Two types of specimens were used, which were unnotched and single notched specimens. The uniaxial tensile experiments of the unnotched specimens provided the Young's modulus and tensile strength of the specimens. The complete stress-strain responses of the specimens were derived from the uniaxial tensile experiments on the single notched specimens. The crack initiation and propagation were discussed. The uniaxial tensile loading experiments were simulated by a 3D lattice model. The local mechanical properties of lattice elements were determined through simulations. The tensile simulations of the unnotched specimen provided the Young's modulus and tensile strength for the local lattice elements. Then, the softening behavior of lattice elements was obtained from tensile simulations of the single notched specimen. The experimental and simulated stress-strain responses and cracking process were compared with each other. It was found that the simulated results matched quite well with the experiments with the set of local mechanical properties that was determined. This set was used in a further study for the simulations on external sulfate attack.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen

Effect of viscosity modifier admixture on Portland cement paste hydration and microstructure

Significant attention has been given to the development of new materials and techniques to be employed in the construction market. One of the techniques which has drawn noticeable attention is the additive manufacturing process (a.k.a. 3-dimensional printing (3D printing)). One of the approaches of this construction technique is the extrusion of cementitious composites to form contour of a desired geometry. To achieve high viscosity in cementitious materials, usually viscosity modifying admixtures (VMA) are employed. However, the consequences of using these admixtures at high dosages is still not fully understood. This study characterized the influence of different VMA dosages on Portland cement paste, through a microstructure analysis. Hydration development was assessed, and effect of the admixture was quantified at different curing ages. Techniques such as thermogravimetric analysis, optical and electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, micro computed tomography scan and nanoindentation were employed. Important negative side effects were found such as: VMA increasing the cement setting time, anomalous dispersion of hydration products in the bulk and increasing the void content. On the other hand, positive effects were also found such as: evidence of internal curing, higher degree of hydration and lack of undesired hydration products.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen

The Effect Of Viscosity Modifier Agent On The Early Age Strength Of The Limestone And Calcined Clay-Based Sustainable And 3D Printable Cementitious Material

Recently, our group attempted to develop the ternary blended (Portland cement, calcined clay and limestone) cementitious material for 3D concrete printing (3DCP). Due to the elimination of formwork during the layer-by-layer casting process, the printed material should have favorable elastic properties and green strength at the fresh state. A small amount of Hydroxypropyl methylcellulose (HPMC) based viscosity modifier agent (VMA) is used in the printable mixture to enhance the printing shape stability during the printing process. However, adding VMA may delay the hydration of cement-based materials and affect the strength development at an early age. It is necessary to determine how the VMA additions affect the early age strength development of a 3D printable cementitious material. In this paper, three mix designs with different amounts of VMA were selected to perform the uniaxial compression test at different early ages (30 min, 1, 2, 3, 4, 6h). The setting time and compressive strength tests at 1, 7 and 28 days of those mix designs were also measured. Besides, the heat flow of different mix designs was recorded by using the isothermal calorimeter. Finally, it has been found that: (1) adding VMA could contribute to increase the green strength within the first 2h after mixing water and weaken the strength development from 2h to 6h; (2) the VMA additions mainly delayed the initial set and small effects on the final set; (3) about 50% of compressive strength at 1, 7 and 28 days were reduced for the specimens with VMA. (4) the more amounts of VMA was used in the mixture, the more retarding effects on cement hydration.Materials and Environmen

Deliberate Deformation of Concrete in the Fresh State: Crack Risk and Efficient Production of Curved Precast Elements

The production of double-curved precast concrete elements for cladding or shell structures requires expensive CNC (computer numerical control)-milled formwork. As an alternative method, the innovative flexible mould for economically efficient and sustainable production of such elements is discussed in this paper. This method comprises the use of a flexible, CNC-controlled formwork, which is filled with self-compacting concrete. After a short period of thixotropic stabilization in the fresh state, the flexible mould is then deformed into its desired geometry, typically having a strong curvature radius of only a few metres in one or two direction(s). After hardening and de-moulding, the flexible mould can be reused for elements with the same or different curved geometry. The present paper describes the outcomes of a study focussing on two aspects relevant for the abovementioned production method: effect of change of rheological properties in the first 90 min after casting and assessment of the risk of cracking and development of cracks during the deformation process. In an experimental study the following parameters were modified: radius of deformation, moment of deformation in time, panel thickness and water-cement ratio. The presence of cracks after deformation was investigated quantitatively, using a petrographic technology. The results show that for the application of the flexible mould method the plastic stage of concrete is important to be considered

Freeze-thaw resistance and air-void analysis of concrete with recycled glass-pozzolan using X-ray micro-tomography

Recent studies have shown promising potential for using Glass Pozzolan (GP) as an alternative supplementary cementitious material (SCM) due to the scarcity of fly ash and slag in the United States. However, comprehensive studies on the freeze-thaw (FT) resistance and air void system of mixtures containing GP are lacking. Therefore, this study aimed to evaluate GP’s effect on FT resistance and characterize mixtures with different GP contents, both macro- and microscopically. In this study, six concrete mixes were considered: Three mixes with 20%, 30% and 40% GP as cement replacements and two other comparable mixes with 30% fly ash and 40% slag, as well as a mix with 100% Ordinary Portland cement (OPC) as a reference. Concrete samples were prepared, cured and tested according to the ASTM standards for accelerated FT resistance for 1000 cycles and corresponding dynamic modulus of elasticity (Ed). All the samples showed minimal deterioration and scaling and high F/T resistance with a durability factor of over 90%. The relationships among FT resistance parameters, air-pressured method measurements of fresh concretes and air void analysis parameters of hardened concretes were examined in this study. X-ray micro-tomography (micro-CT scan) was used to evaluate micro-cracks development after 1000 freeze-thaw cycles and to determine spatial parameters of air voids in the concretes. Pore structure properties obtained from mercury intrusion porosimetry (MIP) and N2 adsorption method showed refined pore structure for higher cement replacement with GP, indicating more gel formation (C-S-H) which was verified by thermogravimetric analysis (TGA).</p