Behavior Of Fresh Cement Pastes Research Articles

Modeling thixotropic behavior of fresh cement pastes

It is crucial to describe the thixotropic behavior of cement-based materials, which plays an important role during the production process such as mold casting or 3D printing. This study establishes and validates a thixotropic model of fresh cement pastes considering de-flocculation using experimental data with different shear rates and water-to-cement ratios.

On the “Thixotropic” Behavior of Fresh Cement Pastes

Thixotropic behavior describes a time-dependent rheological behavior characterized by reversible changes. Fresh cementitious materials often require thixotropic behavior to ensure sufficient workability and proper casting without vibration. Non-thixotropic behavior induces a workability loss. Cementitious materials cannot be considered as an ideal thixotropic material due to cement hydration, which leads to irreversible changes. However, in some cases, cement paste may demonstrate thixotropic behavior during the dormant period of cement hydration. The aim of this work is to propose an approach able to quantify the contribution of cement hydration during the dormant period and to examine the conditions under which the cement paste may display thixotropic behavior. The proposed approach consists of a succession of stress growth procedures that allow the static yield stress to be measured. For an inert material, such as a calcite suspension, the structural build-up is due to the flocculation induced by attractive Van der Waals forces. This structural build-up is reversible. For cement paste, there is a significant increase in the static yield stress due to cement hydration. The addition of superplasticizer allows the thixotropic behavior to be maintained during the first hours due to its retarding effect. However, an increase in the superplasticizer dosage leads to a decrease in the magnitude of the Van der Waals forces, which can erase the thixotropic behavior.

Role of shear stress at rest on the viscoelastic response of fresh cement pastes

Fresh concrete is composed of aggregates and cement paste, which itself is a dense suspension that exhibits a yield stress and serves as a binder that solidifies through cement hydration. Although there is relatively little reaction during the fresh state (i.e., before setting), there is still the formation of early hydration products that will influence the paste rheology. Nonzero stress at rest widely exists in concrete practice, e.g., interruption during pumping, static formwork casting, and 3D concrete printing, but its impact on eventual viscoelastic properties is not well understood. In this study, we investigated the influence of different levels of applied shear stress at rest on fresh cement pastes and show that it dramatically alters the eventual solid performance (i.e., static yield stress and stiffness). With increasing applied shear stress at rest, the cement paste exhibited increasing static yield stress and increasingly elastic behavior, where the origin was tied to the progression of hydration. Cement pastes were compared against magnesium oxide suspensions, which served as an inert model system, and an accelerated rate of static yield stress increase over time was observed in the former but not in the latter. This highlights the role of early cement hydration on the solidification behavior of fresh cement pastes in the presence of nonzero shear stress at rest. We further support this by showing that the structural evolution of cement paste under applied stress can primarily be attributed to an enhanced elastic limit, which is associated with early hydration. The influence of two common viscosity modifying admixtures, nanoclay and diutan gum, on these properties was also examined. This work demonstrates the importance of taking the shear stress at rest into consideration for laboratory rheological experiments and industrial concrete applications, as it can substantially impact the eventual solidification behavior.

Rheological properties for fresh cement paste from colloidal suspension to the three-element Kelvin–Voigt model

In the present work, rheological behaviors of fresh cement paste are studied based on multi-disciplinary approaches, i.e., colloidal suspension using attractive van der Waals force, rheology using the Bingham model and Bingham–Papanastasiou model, which are able to describe the behavior of cement pastes before and after yield stress, and finally the continuum mechanics based on Maxwell and Kelvin–Voigt models. To achieve this, the fresh cement paste with different water-to-cement ratios of 0.3 up to 0.6 is prepared. The attractive van der Waals forces are estimated based on the distances between solid cement particles, which vary at every single water-to-cement ratio. The rheology experiments of all water-to-cement ratios are performed using a rheometer. According to our experimental outcomes, the Bingham and Bingham–Papanastasiou models are applied in the modeling of the experimental curves and determination of yield stress and viscosity. Maxwell and the Kelvin–Voigt models are utilized in describing solid-like behavior before yield stress and fluid-like behavior beyond yield stress. It is observed that the increase of water generates a decrease in the viscosity, yield stress, and packing concentration of solids. It also increases the distances between two cement particles in the cement pastes. According to the modeling results, the Bingham–Papanastasiou model is well adapted for the cement paste flow due to its additional modeling parameter, which is known as m. The role of m is understood and described by linking the van der Waals interaction, rheology, and three-element Kelvin–Voigt model as a whole in function of water-to-cement ratio. m is understood as a key parameter in which the distance between particles affects the rheological behavior of fresh cement pastes. Lastly, the two-phase flow simulations have been successfully achieved and compared with the experiments. The conclusion and outlooks are summarized and discussed at the end of the paper.

Rheological characterization of the viscoelastic solid-like properties of fresh cement pastes with nanoclay addition

At sufficiently low applied shear deformation or shear stress, fresh cement paste exhibits viscoelastic solid-like behavior. This solid-like state is particularly important for some applications, such as self-consolidating concrete formwork pressure and 3D concrete printing. In this study, the viscoelastic solid-like behavior of fresh cement pastes is probed using a rheometer. Static yield stress, storage modulus, and cohesion are measured by creep recovery and constant shear rate test, small amplitude oscillatory sweep (SAOS) test and tack test, respectively. The effect of nanoclay addition is studied in various aspects of viscoelastic performance of fresh cement pastes. It is found that nanoclay addition not only enhances yield stress and cohesion, but also stiffness in terms of storage and tangent modulus.

Rheological Behavior of Fresh Cement Pastes

Rheology of a concrete is mainly controlled by the rheological behavior of its cement paste. This is the main practical reason for the extensive research activity observed during 70 years in this research subfield. In this brief review, some areas of the research on the rheological behavior of fresh cement pastes (mixture method influence, microstructure analysis, mineral additions influence, chemical additives influence, blended cements behavior, viscoelastic behavior, flow models, and flow behavior analysis with alternative methods) are examined.

Role of the filler on Portland cement hydration at very early ages: Rheological behaviour of their fresh cement pastes

The rheological behaviour of fresh Portland cement (PC) pastes with different chemical and potential mineralogical compositions was analysed when were blended with crystalline mineral additions (fillers). These two fillers were very different: a siliceous, Q, α-quartz type, and another limestone, C. Trials were conducted during latent hydration at 25°C. The findings showed that apparent viscosity was higher in the PC with low C3A and high C3S content, consequently, the Na2Oeq.(%) content of everyone also contributed very significantly. Moreover, it was also observed that the incorporation of each filler affects differently the rheological behaviour of fresh cement pastes. The different influence is due to the different physical and chemical properties of each filler (nature, chemical composition and character and texture intimate of the particles), and also of the type of PC to interact. Finally and at these very early ages, both fillers stimulated the hydration of the PC which they were mixed by direct and non-direct way, and in the case of C filler by indirect way as well, but without any pozzolanic activity. Despite the fact that blended cement P1/Q 60/40 has proved to be a “false positive” in Frattini test.

New Carbon Materials 2016, 31(6): 574–584]. Rheological behavior of fresh cement pastes with a graphene oxide additive

New Carbon Materials 2016, 31(6): 574–584]. Rheological behavior of fresh cement pastes with a graphene oxide additive

Rheological behavior of fresh cement pastes with a graphene oxide additive

The rheological properties and morphology of fresh cement pastes with different contents of graphene oxide (GO) were investigated by a rheometer and a laser confocal scanning microscope, respectively. The rheological data were fitted by the Modified-Bingham (M-B) model and Herschel-Bulkley (H-B) model. A mechanism for the effect of GO on the rheological properties is proposed. Results show that the cement particles are re-agglomerated and new flocculation structures are generated by the addition of GO. The new flocculation structures significantly alter the rheological properties of the pastes. The degree of re-agglomeration and the number of new flocculation structures increase with increasing GO content, leading to a sharp increase in the yield stress, plastic viscosity and the area of the hysteresis loop. GO can effectively reduce the degree of shear-thickening, and increase the critical shear rate and the stability of the pastes.

Rheological behaviors of fresh cement pastes with polycarboxylate superplasticizer

The rheological behaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated. Influential factors including superplasticizer to cement ratio (Sp/C), water to cement ratio (w/c), temperature, and time were discussed. Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0, 20 and 40 °C, respectively. Flowability and rheological tests on cement pastes were conducted to characterize the development of the rheological behavior of fresh cement pastes over time. The exprimental results indicate that the initial flowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer. Higher temperature usually leads to a sharper drop in initial flowability and flowability retention. However, for the cement paste with high Sp/C or w/c, the flowability is slightly affected by temperature. Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influential factors at low Sp/C. In the case of high Sp/C, yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature. Moreover, two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C, w/c, temperature and time are developed on the basis of the existing models, in which experimental constants can be extracted from a database created by the rheological test results.

Influence of nano-sized highly purified magnesium alumino silicate clay on thixotropic behavior of fresh cement pastes

Thixotropic behavior of cementitious materials is attracting much attention due to its important applications in concrete practice, such as formwork pressure, multi-lift casting, slip form paving, pumping and segregation. This study investigates the influences of nano-sized highly purified magnesium alumino silicate clay (3nm in diameter, called nanoclay) on thixotropic behavior, particularly, the structural rebuilding of fresh cement pastes. In this study, rheological properties (yield stress, viscosity and thixotropy) of cementitious pastes, with and without nanoclay, were measured at every 15min after the pastes were mixed. Flow table tests were conducted, and the results were compared with those measured from the rheology tests. Statistical methods were utilized to determine the rates of changes in thixotropy with time. The results indicate that a small amount of nanoclay addition (0.5–1% by mass of cement) significantly facilitates particle re-flocculation or structural rebuilding, and effectively enhances the thixotropy of cement pastes.

Rheological behaviour of fresh cement pastes: Influence of synthetic zeolites, limestone and silica fume

New generation concretes have specific properties, often obtained by using additives, and these properties are strongly affected by rheological characteristics of the fresh cement paste. In this work small amplitude oscillations were adopted to investigate the effects of synthetic zeolite, limestone and silica fume on the rheological properties of fresh pastes. Time sweep tests were used to describe paste evolution with time evidencing that zeolite is more able to increase the rate of structure development with respect to other additives. Frequency sweep tests evidenced that zeolite, at least at low concentration, is able to improve the mechanical strength and to increase the liquid-like behaviour of the paste. Adopted rheological tests and data analysis are suitable to describe paste structure and evolution with time and synthetic zeolite seems able to give better properties to cement paste than other additives can.

Modelling thixotropic behavior of fresh cement pastes from MRI measurements

From MRI measurements we show that in a flowing cement paste thixotropic effects dominate over short time scales while irreversible effects become significant over larger timescales. The steady and transient flows exhibit a yielding behavior which differs from usual yield stress model: the transition from the solid to the liquid regime is abrupt. We propose a simple thixotropic model based on these observations. The validation is done on the steady and transient state on local experimental tests. We build the local rheogram which is representative of the intrinsic rheological properties. Comparisons with apparent rheograms demonstrate that it is possible to use correction techniques from the literature to have access to the real behavior law of the material from standard measurements but that, in the case of the material studied in this paper, this would nevertheless lead to an underestimation of the yield stress.

Modelling thixotropic behavior of fresh cement pastes from MRI measurements

From MRI measurements we show that in a flowing cement paste thixotropic effects dominate over short time scales while irreversible effects become significant over larger timescales. The steady and transient flows exhibit a yielding behavior which differs from usual yield stress model: the transition from the solid to the liquid regime is abrupt. We propose a simple thixotropic model based on these observations. The validation is done on the steady and transient state on local experimental tests. We build the “local” rheogram which is representative of the intrinsic rheological properties. Comparisons with “apparent” rheograms demonstrate that it is possible to use correction techniques from the literature to have access to the real behavior law of the material from standard measurements but that, in the case of the material studied in this paper, this would nevertheless lead to an underestimation of the yield stress.

Thixotropic Behavior of Fresh Cement Pastes from Inclined Plane Flow Measurements

Abstract We show that the rheological characteristics of a fresh cement paste can be determined from inclined plane tests. The apparent flow curve measured from inclined plane flows coincides with the apparent rheogram from classical rheometer tests and the flow curve obtained from local Couette flow measurements with magnetic resonance imaging (MRI). In order to describe the thixotropic properties of these fluids we suggest to use a simple model, the four parameters of which may be determined from specific, practical, inclined plane experiments.

Rheological behaviour of fresh cement pastes formulated from a Self Compacting Concrete (SCC)

Self Compacting Concrete (SCC) has a high flowability and can be placed without vibration. It is defined as a concrete that exhibits a high deformability and a good resistance to segregation. This kind of concrete is of great interest and has gained wide use especially in the case of difficult casting conditions such as heavily reinforced sections. From a rheological point of view, the use of a Viscosity Enhancing Admixture (VEA) along with an adequate superplasticizer content enables to ensure high deformability and stability. However, little is known about the interactions between superplasticizer and viscosity agent. Hence, we propose to study several cement pastes formulated from the original paste of a typical SCC mix. Depending on their rheological behaviour, these pastes will be used later to study the stability of coarse aggregates. The major aim of this paper is to show that empirical tests such as spread and flow time are suitable to characterise the rheological behaviour of cement pastes instead of more complex ones. Rheological properties, i.e. viscosity and shear yield stress, are well correlated with empirical test results in the range of flowable mixes. Moreover, the experimental program leads to emphasize the effects of the mixing procedure on the rhelogical properties of cement pastes. Finally, test results enable to underline the interactions between superplasticizer and Viscosity Enhancing Admixture used in designing Self Compacting Concrete.

Steady and transient flow behaviour of fresh cement pastes

Fresh cement pastes behave as non-Newtonian viscous fluids. During steady flow, their apparent viscosity depends on the applied strain rate. During transient flow, the apparent viscosity is a function of time. In this work, a thixotropic model is presented. Its four parameters are identified experimentally for a tested cement paste using coaxial viscometer test. Viscometer flow simulations are then carried out. The model proves to be able to predict the trends of the fresh behaviour of cement pastes in various flow situations.

A model for the flow of cement pastes

This paper presents a simple mathematical model of the flow behaviour of fresh cement pastes and mortars based on a soil mechanics approach. A soil mechanics model can mimic many of the features of the pastes rheological behaviour indicating that physical, as well as chemical, factors might control many of these. Specifically, we have developed a simple mathematical model of the evolution of the packing of the clinker or sand grains and its effect on the flow of the paste or mortar. This not only incorporates the role of the pore water but also copes with the large deformations typical of the mixing and placement of mortars. This model is used here to simulate small cyclic deformations with a large drift accumulating in one direction, and is able to capture both the liquefaction and locking phenomena seen in practice when mortars are vibrated.

The rheological behavior of fresh cement pastes

The rheology of fresh cement pastes is difficult to obtain and data is often difficult to reproduce. Factors involving mixing, testing techniques, test geometries, etc contribute to the measurement problems. This review addresses these problems and comments on the analysis of experimental data.

The effect of fly ashes on the rheological behaviour of cement pastes

A study of the behaviour of fresh cement pastes under shear in a rotational viscometer of the Rheotest-2 type, especially designed for this purpose, is described. It is shown that an addition of fly ash to Portland clinker influences the change of flow behaviour of cement pastes from thixotropic flow to thixotropic-dilatant flow.