The effect of zeolite on the properties and hydration of blended cements

In this paper the properties and the hydration of cements containing natural zeolite, coming from the Metaxades area, Thrace, Greece, are studied. The experimental part comprises three stages. In the first stage a complete mineralogical characterization of the zeolite was performed. In addition, the pozzolanic reactivity of the zeolite was evaluated on the basis of the Chapelle test. In the second stage, the mechanical and physical properties of blended cements, incorporating 0%, 10% and 20% per weight of fine zeolite were determined. Finally, the hydration rate and products were studied by means of X-ray diffraction and FTIR spectroscopy, in combination with thermoanalytical methods (TG/DTG and DTA). As it is concluded, the examined zeolite consists mainly of heulandite type-II and is a pozzolanic material that contributes to the strength development of zeolite-cement mixtures, the consumption of Ca(OH)₂ formed during the hydration of Portland cement and the formation of cement-like hydrated products. Finally, the addition of zeolite up to 20% w/w does not significantly affect the physical and mechanical properties of the blended cements.     

Microstructural analysis of hardened alite paste

New information about the pore structure of hydrating alite (impure tricalcium silicate) of 0.59 water∶alite ratio has been obtained by examining polished cross-sectional surfaces with a scanning electron microscope. Micrographs were used to analyse quantitatively the volume of porosity which was impregnated with epoxy and these results were compared with independent measurements of porosity using volumetric technqiues. The results suggest that pores less than 50 nm diameter are not impregnated by epoxy and that they may be incorporated as part of the pore structure of the calcium silicate hydrate; however, only the finest of these pores may remain in pastes formed at low water ∶ solid ratios. The calcium silicate hydrate formed in the late stages of hydration may not incorporate as much fine porosity as that formed during the early stages. The structure of the large “capillary pores” (i.e., pores greater than 50 nm diameter) changes with time from an open network to an array of disconnected pores. This study reveals that mature specimens are very heterogeneous and contain regions, hundreds of micrometres across, having few, if any, capillary pores. These regions are not found in young specimens.     

Microstructural analysis of hydrated alite paste

The distribution and composition of the products of hydrating alite have been studied using a backscattered electron detector in a scanning electron microscope. Polished surfaces of specimens were examined and analysed quantitatively. Calcium hydroxide forms in the available water filled space. Calcium silicate hydrate appears to have several distinct morphologies, and their fomation can be associated with the different stages of the reaction. These observations are compared with classification schemes derived from fracture surfaces which heretofore have been the usual kind of specimen examined.     

Lattice orientation and crack size effect on the mechanical properties of Graphene

The effect of lattice orientation and crack length on the mechanical properties of Graphene are studied based on molecular dynamics simulations. Bond breaking and crack initiation in an initial edge crack model with 13 different crack lengths, in 10 different lattice orientations of Graphene are examined. In all the lattice orientations, three recurrent fracture patterns are reported. The influence of the lattice orientation and crack length on yield stress and yield strain of Graphene is also investigated. The arm-chair fracture pattern is observed to possess the lowest yield properties. A sudden decrease in yield stress and yield strain can be noticed for crack sizes <10 nm. However, for larger crack sizes, a linear decrease in yield stress is observed, whereas a constant yield strain of \(\approx \)0.05 is noticed. Therefore, the yield strain of \(\approx \)0.05 can be considered as a critical strain value below which Graphene does not show failure. This information can be utilized as a lower bound for the design of nano-devices for various strain sensor applications. Furthermore, the yield data will be useful while developing the Graphene coating on Silicon surface in order to enhance the mechanical and electrical characteristics of solar cells and to arrest the growth of micro-cracks in Silicon cells.     

Flash-calcined dredging sediment blended cements: effect on cement hydration and properties

Dredging of docks and waterways generates a large and continuous supply of sediments currently destined for disposal. Transforming this currently wasted materials into new resources still requires meeting technical challenges. One of the options is to process the sediments into a supplementary cementitious material by flash-calcination. This paper describes the effect of cement replacement by flash-calcined dredged sediments on cement hydration and key properties. The hydration kinetics, products and microstructure are studied to explain changes in cement properties such as compressive strength development and workability. The flash-calcined dredging sediments show clear pozzolanic activity which surpasses that of typical coal combustion siliceous fly ash (V, EN 197-1). This is manifested in (1) the rate of compressive strength development, (2) reduced portlandite and (3) increased ettringite and bound water contents. The results show that calcination can transform wasted dredging sediments into a new supplementary cementitious resource for producing large volumes of low-CO₂ blended cements.     

The effect of a retarder on the early stages of the hydration of tricalcium silicate

Cement is used in the oil industry to line oil wells. The major component of oilwell cement is tricalcium silicate (C₃S), which is responsible for the initial thickening of the cement slurry. It is important to control the time that it takes for this slurry to thicken, and this is achieved in practice by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose main effect is to form a complex with calcium ions is investigated by use of a model for the hydration of C₃S previously investigated by Preece, Billingham and King (2001). It is found that such a retarder can significantly increase the thickening time of pure tricalcium silicate.     

The effect of temperature on the hydration of composite cements containing limestone powder and fly ash

The effect of the curing temperature (5, 20 and 40°C) on the degree of hydration, amount of bound water and calcium hydroxide, porosity and the development of mechanical properties was investigated on pastes and mortars prepared with fly ash (FA)?Climestone (L) Portland composite cements. Increasing the curing temperature for ordinary Portland cement (OPC) leads to a more inhomogeneous distribution of hydration products, resulting in an increased coarse porosity and therefore a lower compressive strength after 7?days and longer. In contrast, the FA containing mortars showed higher compressive strength with increasing curing temperature up to 90?days. The reaction of the FA is increased at 40°C and strongly retarded at 5°C. At 20 and 40°C, FA reduces the porosity at later ages. The replacement of 5% of the OPC or FA by L powder did not impair the strength at 5 and 20°C, but lowered strength slightly at 40°C for the FA blended cements. The porosity appears to be the dominating factor regarding the compressive strength, independent of whether part of the OPC is replaced by FA and L powder or not.     

Combined effect of chemical nature and fineness of mineral powders on Portland cement hydration

This paper focuses on the influence of the chemical nature and the fineness of the fillers on the hydration process and on the compressive strength development. Four different types of fillers are considered in combination with Portland cement: quartzite filler, alumina filler, limestone filler, and silica fume. The study deals with blended mortars having a 0.45 water to powder (cement and filler) ratio with a 10% substitution of cement by filler. Quartzite fillers do not seem to accelerate the hydration process in a significant way. No positive effect is noticed on the strength development either. The presence of a fine inert alumina powder increases the rate of early hydration of Portland cement. The greater the fineness, the faster the rate of hydration heat development. This reactivity leads to an increase in the compressive strength at early age for mortar containing the finest alumina powders. In case of coarse alumina powder, no acceleration effect is obtained. Finely ground limestone (calcite) fillers promote heterogeneous nucleation of hydrates which significantly accelerates hydration. At early age, this also results in an increased mortar compressive strength in comparison with the control mortar. From the obtained results, it is clear that both chemical natures as well as fineness are important with regard to the accelerating effect of the hydration process. With increasing fineness, the accelerating effect increases. For powders with comparable fineness, it is clear that limestone powder has a more significant accelerating effect than silica fume and alumina filler. Quartzite filler seems to have no significant effect.     

Lattice modelling of size effect in concrete strength

This paper uses a recently improved lattice network model to study the size effect in the strength of plain concrete structures. The several improvements made to the lattice network model are: (i) tension softening of the matrix phase is included in the material modelling; (ii) the structural response is modelled by incrementing the deformation rather than the load. This eliminates the need for introducing arbitrary scaling parameters in the beam element failure criteria and; (iii) a square rather than a triangular lattice beam network is found to be adequate for modelling concrete, thus greatly reducing the computational time.The improved square lattice network has been used to simulate the complete load-deformation response of notched three-point bend beams of different sizes with a view to checking the validity of several size effect models available in the literature. Lattice simulation was found to identify microcracking, crack branching, crack tortuosity and bridging, thus allowing the fracture process to be followed until complete failure. The improved lattice model predicted smooth structural response curves in excellent agreement with test results.The simulated nominal strengths also correlated very well with the test results, apart from that for the smallest beams (depth 38.1 mm). However, even in the relatively broad range of sizes (1:8) of the test beams, there was no clear evidence that one size effect model is superior to the other. In fact, rather surprisingly the test data would appear to be equally well described by all the available size effect models. The lattice simulations however indicated a trend which is better predicted by the multifractal scaling model.     

The effect of synthesis conditions on the efficiency of C-S-H seeds to accelerate cement hydration

C-S-H seeding is a promising approach to accelerate cement hydration in a very effective way. Until now little is known about the way the C-S-H seeds work and how they affect the properties of the hydrated cement paste. To be able to analyse this in detail it is necessary to have a preparation method for the C-S-H seeds, which is reproducible and able to prepare seeds with defined properties. The present work describes two methods for the synthesis of C-S-H seeds, a mechanochemical approach and a sol-gel synthesis. A factorial design of experiments setup was used to find those parameters, which affect the efficiency of the C-S-H seeds to accelerate cement hydration and to adjust these parameters in order to achieve the optimized seeds, which are very effective accelerators. Heat flow calorimetry shows that the dormant period of cement hydration can be eliminated by the use of the optimized C-S-H seeds and compressive strength tests show a multiplication of early compressive strength even at very low concentrations of seeds, indicating that an optimization of the synthesis by factorial design of experiments is a promising way for further studies on the mode of action of C-S-H seeds.     

Antagonistic effect of superplasticizer and colloidal nano-silica in the hydration of Alite and Belite pastes

The dependence on the hydration rate for Alite and Belite clinker phases in the presence of a polycarboxylate superplasticizer PC SP upon addition of colloidal nano-silica CNS were monitored by means of Diffuse Reflectance Infrared spectroscopy DR-FTIR. Spectral signatures of clinker dissolution and product formation were acquired for both materials. The rates for the build-up of product vibrational band intensities were found to depend sensitively on addition of CNS. The hydration product was proposed to be calcium-silicate-hydrate C–S–H. Details in the spectral signatures were found to differ. Quantum chemical calculations were employed and found to be consistent with the interpretation that small clusters dominate the Alite C–S–H spectrum, whereas the Belite C–S–H spectrum results from extended polymers.     

The effect of material defects on the fatigue behaviour and the fracture strain of ABS

The structural integrity and durability of a construction are highly dependent on the material quality. Poly(Acrylonitrile-Butadiene-Styrene) Copolymer (ABS) is a material which is preferably chosen for high performance products, because of its superior toughness. The toughness of ABS is revealed by its high fracture strain in a tensile test, and high notched Izod impact fracture energy. However, the fatigue resistance of ABS is less favourable. This investigation is mainly devoted to the fatigue behaviour of ABS and to the fracture strain and the notched Izod fracture energy. Various mechanical tests, performed in conjunction with scanning electron microscope investigations of the fracture surfaces, demonstrate that fracture initiates from small defects which are abundantly present in the material. Especially the fatigue fracture surfaces show numerous cracks which had initiated from the defects. The fracture strain in tensile tests is high, but shows a large scatter. It is demonstrated that the fracture strain is also related to the presence of defects. A pre-fatigue load up to 40% of the anticipated fatigue life, followed by a tension test shows a significant reduction of the fracture strain as compared with a tension test on non damaged as-moulded material. Microscopic investigations show that this fracture strain reduction is caused by the presence of small cracks which initiated from the defects, during the preceding fatigue load. A similar but much smaller effect of pre-fatigue was observed for notched Izod tests. Finally it is concluded that the fatigue behaviour of ABS is dominated by the growth live of microscopic small cracks from material defects.     

The effect of drying method on ordinary Portland cement surfaces during the early stages of hydration

During the early stages of ordinary Portland cement (OPC) hydration, ettringite, calcium hydroxide and C–S–H form on the OPC surface while nanoscale pits are formed in the exposed surface areas. In order to study these features using high resolution electron microscopy, samples must be appropriately dried to remove unbound water. The work reported here compares the effects of freeze-drying to drying by solvent exchange using four different solvents on the surface features of OPC after four hours of hydration in order to determine whether the solvent exchange process produces nanoscale changes in the surface features that can be detected using high resolution scanning electron microscopy (SEM). A quantitative image analysis of secondary electron SEM images was performed and the results analyzed using small sample statistics. They suggest that all of the solvent exchange methods damage the OPC surface and produce surface structures not seen in the freeze-dried samples. Caution is therefore warranted in the use of solvent exchange in the preparation of samples for the study of the dissolution of OPC surfaces.     

Lattice imaging of structural defects in a chain silicate: The pyroxenoid mineral rhodonite

The pyroxenoid mineral rhodonite has been examined by electron diffraction and high-resolution lattice imaging. Planar defects parallel to (110) and (001) have been observed. The former are comparable to the (100) stacking faults frequently observed in the related mineral wollastonite, whereas the latter involve unit cell strips of the more complex pyroxmangite structure occasionally inserted into the parent rhodonite matrix.     

The effect of extrinsic defects in pyrolytic graphite on the a-axis resistivity

A contactless r.f. resistivity technique is shown to be well suited to the measurement of room temperature a-axis resistivities in lamellar compounds, particularly those of graphite. The method is used to investigate the relative damage induced by different techniques employed in cutting of samples.Nato exchange visitor, University of Nancy, France.     

Calcium sulphate anhydrite based composite binders; effect of Portland cement and four pozzolans on the hydration and strength

The strength, hydration products, microstructure and heat of early hydration were investigated on alternative hydraulic green cements based on calcium sulphate anhydrite partially blended with Portland cement and pozzolans. Four pozzolans of different physical and chemical nature, namely a geothermal waste, silica fume, metakaolin and pulverized fuel ash were characterized. The composite binders showed hydraulic behavior. The use of Portland cement favoured the strength, which was also higher with the incorporation of siliceous nanometric pozzolans compared to the micrometric silicoaluminate pozzolans. The nanoparticles enhanced the early hydration and changed the gypsum morphology promoting denser matrices of hydration products. The geothermal waste pozzolan was the most effective, while one of the composites with metakaolin showed formation of ettringite and strength losses. The heat of hydration of the composites was considerably lower than that of the neat Portland cement.     

Competing Interactions of Spin and Lattice in the Kondo Lattice Model

The magnetic properties of a system of coexisting localized spins and conduction electrons are investigated within an extended version of the one-dimensional Kondo lattice model in which effects stemming from the electron–lattice and on-site Coulomb interactions are explicitly included. After bosonizing the conduction electrons, is it observed that intrinsic inhomogeneities with the statistical scaling properties of a Griffiths phase appear, and determine the spin structure of the localized impurities. The appearance of the inhomogeneities is enhanced by appropriate phonons and acts destructively on the spin ordering. The inhomogeneities appear on well-defined length scales and can be compared to the formation of intrinsic mesoscopic metastable patterns which are found in two-fluid systems.     

The role of defects in the quantum size effect

Many macroscopic properties of films exhibit an oscillatory dependence on thickness when the thickness is comparable with the de Broglie wavelength of an electron at the Fermi surface. This behavior is called the quantum size effect. In this paper the influence of defects in thin semimetal films on the electrical conductivity is explored by extending the work of Sandomirskiǐ to include a scattering potential of finite range. An isotropic sample bounded by infinitely high potential walls is assumed, and the Greenwood-Peierls formula is the starting point for this calculation. The potential selected and the approach taken allow the conductivity to be calculated with correction terms which have not previously been given. Within this model the calculations show that for δ-function scattering, Sandomirski?'s results are exact. Compared with Sandomirski?'s work, the calculations in this paper predict a reduction in the amplitude of the oscillations in addition to an overall decrease in the conductivity. A physical explanation of the results is given.     

The effect of mechanical defects on the strength distribution of elementary flax fibres

Flax fibres are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibres are affected by the natural variability in plant as well as the damage accumulated during processing, and thus have considerable variability that necessitates statistical treatment of fibre characteristics. The strength distribution of elementary flax fibres has been determined at several fibre lengths by standard tensile tests, and the amount of kink bands in the fibres evaluated by optical microscopy. Strength distribution function, based on the assumption that the presence of kink bands limits fibre strength, is derived and found to provide reasonable agreement with test results.     

The effect of two neighboring defects on the mechanical properties of carbon nanotubes

The tensile behavior of single-walled nanotubes (SWNTs) having two defects (vacancy or Stone-Wales) positioned next to each other was simulated in this study to investigate the influence of the spatial arrangement of defects on the mechanical properties. The simulations were performed using classical molecular dynamics (MD) at the atomic scale. Two neighboring vacancy defects reduced the failure strength as much as 46% and the failure strain as much as 80% in comparison with those of pristine SWNTs, while two neighboring Stone-Wales defects reduced them as much as 34% and 70% respectively. SWNTs having two defects in the loading (axial) direction showed higher failure strength than SWNTs with defects perpendicular to the loading direction. For both types of defect, the closer the defects, the weaker the SWNTs. As result, the defect arrangement in the SWNT structure must be one of the key factors in determining its mechanical properties, as well as the population of defects.