Simulation of the flow of fresh cement suspensions by a Lagrangian finite element approach

In this paper a Lagrangian formulation of the Navier–Stokes equations, based on a Particle Finite Element Approach, is applied to the simulation of rheological tests for homogenized cement pastes and mortars, i.e. the L-box, slump and mini-slump and Marsh cone tests. Comparisons with both experiments and analytical predictions are addressed by assuming a simple Bingham-like constitutive model. The analyses explicitly and automatically account for the presence of evolving free surfaces thanks to a continuous re-triangulation of the domain. These feature and the excellent accuracy obtained in the benchmarks addressed make the proposed approach an ideal tool for more advanced applications.     

The phenomenological description of the thixotropic behaviour of fresh cement pastes

The present work reports an investigation on the rheological behaviour of fresh cement pastes. Three types of Portland cement were used. The water/cement ratio was varied in the range 0.35 ÷ 0.40. The rheological tests were performed using the coaxial-cylinder viscometer Rotovisko-Haake RV 11. The material to be tested was subjected immediately after mixing to a constant shear rate until a steady value of shear stress (equilibrium value) was attained. All the pastes tested exhibited a flow behaviour of the partially thixotropic type. A rheological model consisting of a sole constitutive equation \(\tau = \tau (\dot \gamma ,t)\) was defined according to the Cheng and Evans approach. The parameters of the constitutive equation were correlated with the cement specific surface and the water/cement ratio.     

Rheological properties of fresh cement pastes and clinker reactivity

This paper presents research results concerning the influence of clinker reactivity on rheological properties of cement pastes at early stages of hydration (lasting less than one hour). The research was carried out on clinkers synthesized in a laboratory, on non-alkaline cements and cements containing in-built alkalis (sodium, potassium) in the clinker phase.The clinker reactivity was estimated from the flow curves and stress changes in time occurring in clinker pastes while maintaining constant measurement parameters. It was found that the effectiveness of gypsum as a setting controller depends on clinker reactivity. Increased clinker reactivity is accompanied by a decreased consistency of a cement paste.The research results indicate that decreased reactivity of clinkers containing inbuilt alkalis is connected with decreased setting controlling effectiveness of added gypsum. This is manifested by a consistency increase of such system at early stages of hydration.     

Relationship Between the Microstructure/Pore Structure of Oil-Well Cement and Hydrostatic Pressure

In cementing operations, hydrostatic pressure reduction in cement slurries is a serious threat to operation safety and cementing quality. This study combines X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, low-field nuclear magnetic resonance, and nano-computed tomography to investigate the mechanism of hydrostatic pressure reduction in cement slurries. The experimental results reveal that hydrostatic pressure transmission in a fresh cement slurry follows Pascal’s law. However, in the slurry, some cement particles undergo sedimentation with an increase in the hydration time, which reduces a part of the hydrostatic pressure. Further, another part of the hydrostatic pressure, which is maintained by the free water in the slurry pores, is reduced during the hardening stage. When the hydration reaction of the slurry is accelerated, the pore water in the slurry is supersaturated and hydration products start to rapidly nucleate and grow between the cement particles. These hydration products are porous gel structures and can change the pore structure of the cement slurry; a macro-pore is divided into many micropores, such as capillary pores and gel pores. Because these pores are filled by water in the slurry, during this process, free water in the macro-pores is changed to capillary water and gel water. However, gel water and capillary water cannot transmit hydrostatic pressure in the cement slurry. Meanwhile, in the fresh cement slurry, many pores containing free water are connected and some hydration products rapidly grow in the macro-pores and fill them, which may reduce the column height of the free water in the pores and lead to hydrostatic pressure reduction in the slurry.     

Rheology and microstructure of pastes with crystal network

This article reviews scientific studies which have been focused on the structure–property relationships of pastes with crystal network by considering fat crystal network as the key example. The review illustrates how rheology can be a valuable tool to characterize the mechanical behavior in these pastes and also provide information about the underlying microstructure in conjunction with various complimentary techniques. Rheological tests which can be adopted to determine the yielding behavior of the crystal network containing pastes are also presented. We have used these methods and concepts to characterize fatty acid crystal network to study a system emulating a skin cream formulation. It is hoped that the rheological fundamentals and the microstructural characterization techniques discussed in context of fat crystal network can be translated to other experimental systems where crystal-forming components are added to impart pasty material characteristics.     

Alkali activated fly ash: effect of admixtures on paste rheology

In this paper, an investigation related to the rheological behaviour of alkali-activated fly-ash pastes (AAFA) is described. Those pastes were prepared by mixing the fly ash with an alkaline dissolution containing 85% of a 12.5 M NaOH solution and 15% of waterglass and adding some commercial chemical admixtures usually used in the Portland cement concrete fabrication, like lignosulphonates, melamines (first and second generation products) and polycarboxylates (latest generation). The fly ash rheological data were determined by rotational viscometry measurements as well as by the use of the flow table test. Results indicate that chemicals admixtures used do not work the same in the Portland cement systems than in alkali-activated fly ash systems. As a general rule, it seems that the most efficient admixtures for these new cementitious pastes (AAFA) are those based in polycarboxylates.     

INFLUENCE OF GLASS CULLET IN CEMENT PASTES

1. Introduction The necessity for glass cullet recycling is becoming more and more imperative not only due to the general need of solid waste volume reduction, but also due to the strict European legislation, which defines the levels of re- cycling packaging materials including glass cullet. Accord- ing to valid estimates, the total consumption of packaging materials was 58 million tones for 1997 in Europe, 25% of which comprised glass. The present target for recycling is a minimum of 25% for…     

Rheology of milled black cumin (Nigella sativa L.)

Black cumin (Nigella sativa L.) paste is a local food in the Middle East, which is produced from dehulled black cumin seeds. The rheological properties of a commercial black cumin paste in the temperature range of 5-40 °C have been studied. Shear-thinning flow behavior with a yield stress was observed for the samples tested. The Herschel-Bulkley model described well the flow curves of the black cumin pastes at different temperatures. It was found that the yield stress, the consistency coefficient, and the flow behavior index decrease with temperature. As far as the effect of steady shearing on the rheological properties of black cumin paste is concerned, the structural kinetic approach was used to predict the thixotropic behavior. It was found that the thixotropic behavior of black cumin paste increases with increasing shear rate and is mitigated by increasing temperature. The completely destructed black cumin paste flow curves were also measured after subjecting the samples to a high shear rate for 2 h. The flow curves of the completely destructed samples were modeled well by the Herschel-Bulkley model. A comparison between the fresh and the completely destructed black cumin paste results at different temperatures revealed that there was a systematic difference, such that the completely destructed paste showed lower yield stress and flow behavior index, and showed a greater consistency coefficient.     

Evolution of properties in hydration of cements—A numerical study

The present work develops a numerical method for analysis of the microstructure and property evolution in the hydration of the cement. A time-dependent micro-mechanical model is established to investigate the microstructure development and the effective property evolution of the cement paste, while the input parameters of the model are based on experimental data. It is assumed that the cement paste composite consists of the anhydrous cement particles, cement gel and pores. The cement particles have a periodically spatial array and are wrapped by the cement gel. The Young’s modulus and Poisson’s ratio of the cement paste are calculated by direct average method and two-scale expansion method. The comparisons between the numerical results and experimental data show that this model can simulate the evolution of the microstructure and properties during the hydration of the cements quite satisfactorily.     

The rheological properties of solder and solar pastes and the effect on stencil printing

Solar and solder pastes are widely used in the electronics industry. Solder paste is the principal joining medium in the assembly of surface mount components, whilst solar paste is used in the manufacture of semiconductor solar cells in the photo-voltaic industry. The stencil printing of both solder and solar pastes is a very important and critical stage in the assembly process. With miniaturisation of components, this is likely to continue. The challenge in stencil printing at such dimensions is in achieving repeatable deposition of both solar and solder pastes from print to print. To meet this challenge requires an understanding of the flow behaviour of both solar and solder pastes. The rheological properties of solar and solder pastes have been evaluated through three different types of experiments. Existing models were applied to compare their rheological behaviour under these schemes. One striking difference was that solar paste showed a higher viscosity than solder paste. Both solar and solder pastes were found to be non-Newtonian materials, showing a decrease in viscosity with increasing shear rates. In this paper we investigate the rheological properties of both solder and solar paste under steady shear and creep-recovery tests. Received: 12 October 1999 Accepted: 11 July 2000     

Phenomenological description of the thixotropic behaviour of gypsum plaster pastes

Gypsum plaster pastes are concentrated suspensions of calcium sulfate hemihydrate in water. The knowledge of their rheological behaviour plays an important role in the manufacture of gypsum products. In the present work, the approach suggested by Cheng and Evans has been taken into consideration in order to derive a constitutive equation suitable for characterizing completely the flow behaviour of these materials in the first stages after preparation. To this end, gypsum plaster pastes were produced with solid volume fractions in a range of practical interest. The rheological tests were performed using a coaxial-cylinder viscometer.     

Beneficial use of a cell coupling rheometry, conductimetry, and calorimetry to investigate the early age hydration of calcium sulfoaluminate cement

A specific cell was designed to monitor simultaneously the evolution of the viscoelastic properties, electrical conductivity, and temperature of a cement paste with ongoing hydration. Hydration of calcium sulfoaluminate cement by demineralised water or by a borated solution was then investigated as an example. Borate anions acted as set retarders but to a smaller extent than with ordinary Portland cement. The delay in cement hydration resulted from the precipitation of an amorphous or poorly crystallized calcium borate, which also caused a rapid stiffening (and thus a loss of workability) of the paste after mixing. The gypsum content of the CSA cement was shown to play a key role in the control of the cement reactivity.     

Characterization of physical aging by time-resolved rheometry: fundamentals and application to bituminous binders

Physical aging is a ubiquitous phenomenon in glassy materials and it is reflected, for example, in the time evolution of rheological properties under isothermal conditions. In this paper, time-resolved rheometry (TRR) is used to characterize this time-dependent rheological behavior. The fundamentals of TRR are briefly reviewed, and its advantages over the traditional Struik’s physical aging test protocol are discussed. In the experimental section, the TRR technique is applied to study physical aging in bituminous binders. Small-diameter parallel plate (SDPP) rheometry is employed to perform cyclic frequency sweep (CFS) experiments over extended periods of time (from one to 8.6 days). The results verify that the mutation of rheological properties is relatively slow during physical aging (mutation number N′_mu?<<?1), thus allowing rheological measurements on a quasi-stable sample. The effects of temperature, crystallinity, and styrene-butadiene-styrene (SBS) polymer modification on the physical aging of bitumen are evaluated. The time-aging time superposition is found to be valid both for unmodified and for polymer-modified bitumen. Vertical shifts are necessary, in addition to horizontal time-aging time shifts, to generate smooth master curves for highly SBS-modified bitumen.     

Dense packing properties of mineral admixtures in cementitious material

The effect of ultra-fine fly ash （UFFA）, steel slag （SS） and silica fume （SF） on packing density of binary, ternary and quaternary cementitious materials was studied in this paper in terms of minimum water requirement of cement. The influence of mineral admixtures on the relative density of pastes with low water/hinder ratios was analyzed and the relationship between paste density and compressive strength of the corresponding hardened mortars was discussed. The results indicate that the incorporation of mineral admixtures can effectively improve the packing density ofcementitious materials; the increase in packing density of a composite with incorporation of two or three kinds of mineral admixtures is even more obvious than that with only one mineral admixture. Moreover, an optimal amount of mineral admixture imparts to the mixture maximum packing density. The dense packing effect of a mineral admixture can increase the packing density of the resulting cementitious material and also the density of paste with low water/binder ratio, which evidently enhances the compressive strength of the hardened mortar.     

Extraction of Effective Cement Paste Diffusivities from X-ray Microtomography Scans

The problem of extracting effective diffusivities of cement pastes on the basis of X-ray microtomography images is considered. A general computational homogenization framework is developed and applied to a variety of cement pastes whose microstructure has been digitized to a resolution of 1 μm. At this resolution, important submicron features are not resolved. Consequently, we propose a methodology whereby the pore space is ascribed a diffusivity less than the free diffusivity. For this purpose, a simple rule that incorporates microtomography data is proposed and shown to yield satisfactory results.     

Rheology of crosslinking poly vinyl alcohol systems during film formation and gelation

Gelation of crosslinking polymers, with ionic groups, is of interest in membrane processing of these materials. The gelation of polyvinyl alcohol in the presence of an ionic crosslinker, sulfosuccinic acid, was monitored through rheological measurements. The evolution of rheological material functions during film formation, which involves solvent evaporation and crosslinking, was observed at different polymer and crosslinker concentrations. Relative effect of water evaporation leading to physical gelation and the chemical crosslinking was examined. To understand the effect of crosslinker type, the rheology of nonionic crosslinker, glutaraldehyde was also examined. The gel points as well as network parameters for these covalent networking systems were different and depended on crosslinker type and polymer/crosslinker concentrations. However, qualitative evolution of rheological behavior during film formation was largely similar, demonstrating the dominating effect of solvent evaporation. Crosslinking in the absence of solvent evaporation was examined at different temperatures. It can be concluded that similar mechanism was involved, independent of temperature, in the temperature range of interest. The effect of crosslinker concentration on evolution of rheological properties near gelation was insignificant for ionic crosslinker though marginally significant for nonionic crosslinker.     

EQUIVALENT MODEL OF EXPANSION OF CEMENT MORTAR UNDER SULPHATE EROSION

The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.     

Influence of thermal history on rheological properties of various bitumen

This paper focuses on the influence of thermal history on the rheological properties of unmodified and polymer modified bitumen (PMB), measured at elevated service temperatures, and contributes to the development of test methods for measuring binder properties, which can be used as indicators for asphalt rutting. It was found that the storing and preparation conditions prior to the rheological measurement can have a large influence, especially in the range of long loading times or low frequencies. For elastomer modification, the homogenization and sample pouring temperature and the corresponding change in microstructure, as revealed by fluorescence microscopy, have a large impact on the rheological measurements. For binders with semi-crystalline modifiers, the storage conditions between sample preparation and testing have the largest impact on the rheological behaviour. This can be related to variations in crystallinity, as shown by calorimetry. The main conclusion from this study is that sample preparation and handling is extremely important for the rheological properties of PMBs. Reproducibility can only be achieved when these conditions are controlled more accurately, especially in binder specification tests for rutting susceptibility.     

Viscoelastic behaviour of wheat starch pastes

Summary The dynamic mechanical behaviour of starch pastes has been investigated over a frequency range of 0.02 to 5.0 radian sec^–1. The influence of concentration of starch, source of starch and method of paste preparation on rheological properties have been studied. The results suggest that under the conditions examined differences in the dynamic rigidity and viscosity of pastes may be attributed to differences in the number fraction of large granules and the volume fraction of swollen starch particles they contain.With 4 figures and 4 tables