Rheological Behavior Of Cement Pastes Research Articles

Mathematical models of apparent viscosity as a function of water–cement/binder ratio and superplasticizer in cement pastes

The water–cement/binder ratio and the admixture of water-reducing agents strongly affect the rheological properties of cement pastes. This study develops mathematical models to predict the apparent viscosity of cement pastes with varying water-cement/binder ratios and polycarboxylate-based superplasticizer content by introducing the power law shear stress-shear strain relation of non-Newtonian fluids into the Navier–Stokes motion equations. The developed models are compared with the results of rheological experiments and verified for their accuracy in simulating the apparent viscosity of cement pastes. These models provide insight into the rheological behaviour of cement pastes and could have practical applications in the construction industry.

The Role of MWCNTs in Enhancing the Foam Stability and Rheological Behavior of Cement Pastes That Contain Air-Entraining and Superplasticizer Admixtures.

This research delves into the intricate dynamics between multi-walled carbon nanotubes (MWCNTs), air-entraining admixtures (AEAs), and a range of superplasticizers (SPs) in cementitious systems, shedding light on key aspects of construction material innovation. The focus is on how MWCNTs, AEAs, and specific SPs-namely, lignosulfonate (LS), polycarboxylate (PCE), and polyacrylate (PA)-influence the stability of foams and the viscosity and setting times of cement pastes. To assess the impacts of these components, we employed foam stability assessments, viscosity measurement techniques, electrical conductivity analysis, and evaluations of dispersion and setting times. Results indicate that MWCNTs enhance foam stability and viscosity, with the degree of improvement contingent on the type and concentration of SPs and the presence of AEAs. Notably, SPs, particularly PCE and PA, markedly influence the properties of cement paste, including increasing dispersion values and modulating setting times, especially when combined with MWCNTs and AEAs. The study concludes that strategically combining MWCNTs with specific SPs and AEAs alters the physical properties of cementitious materials significantly, underscoring the potential for customizing material design in the construction sector.

Rheological characterization of cement pastes with microcrystalline and nano-fibrillated cellulose under static and dynamic regimes

The study of the rheological properties of cement-based materials is essential to the processes directly linked to the flow performance of the fresh mixture. As well, the use of micro- and nano-cellulose-based materials is gaining visibility in its application within cementitious matrices. Despite the great potential represented by those materials, there are still informational gaps to be filled. This work aims at exploring the rheological behavior of cement pastes with micro- and nano-materials beyond the single-point testing customarily presented. An experimental program was developed to characterize and discuss the effects of microcrystalline cellulose (MCC) and nanofibrillated cellulose (NFC) on the rheological behavior of cement pastes. The investigation included both dynamic and static regimes, with a specific range of percentages of MCC and NFC. During the dynamic evaluation both MCC and NFC presence resulted in stiffer cement pastes, with increases in yield stress of up to 136% with 0.040% of NFC and 94% of MCC. MCC-specimens showed non-monotonic behavior regarding the static yield stress values. The rheological characteristics of the NFC gel itself affected the general behavior of the cement paste. The water present in the gel was found to be only partially available as mixing water.

Rheological Behavior of Cement Paste: A Phenomenological State of the Art

With the interest aroused by the development of modern concretes such as printable or self-compacting concretes, a better understanding of the rheological behavior, directly linked to fresh state properties, seems essential. This paper aims to provide a phenomenological description of the rheological behavior of cement paste. The first part is devoted to the most common testing procedures that can be performed to characterize the rheological properties of cement suspensions. The second one deals with the complexities of the rheological behavior of cement paste including the non-linearity of flow behavior, the viscoelasticity and yielding, and the structural build-up over time.

Nonequilibrium thermodynamics modeling of the rheological response of cement pastes

Undoubtedly, cement is one of the most important materials in the construction industry. For its effective use, it is particularly important to fully comprehend the rheological behavior of cement paste. When cement is mixed with water, a suspension is initially formed and the rate of hydrolysis reactions accelerates leading to the formation of a new irreversible structure, i.e., the cement paste gradually solidifies. At the same time, the viscosity of the paste initially decreases with time, while at long times it gradually increases due to the formation of the irreversible structure. We herein introduce a continuum model for predicting the rheological behavior of cement pastes. The model is developed using nonequilibrium thermodynamics, in particular, the Generalized Brackets formalism, to guarantee model admissibility with thermodynamic laws. To this end, we consider two scalar structural variables: a reversible, λrev, characterizing the reversible structure, and an irreversible one, λirr, characterizing the irreversible structure resulting from the hydrolysis reactions. Also, we consider a tensorial structural variable, the conformation tensor c, to characterize the deformation of the cement particles. The predictions of the new model compare reasonably well with available experimental data, especially at large times, and further highlight the capacity of the new model to address the thixotropic behavior of cement pastes. It is expected that the use of this model in concrete rheology simulators will allow for the in silico testing and tailor-designing of concrete to meet specific processing needs.

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Circulating fluidized bed fly ash (CFA), by-product of coal gangue power generation industry based on circulating fluidized bed combustion technology, differs from ordinary fly ash (OFA) in its physical and chemical properties. This paper aims to investigate the influences of CFA with three particle sizes on the workability and rheological properties of cement paste. The results show that untreated raw CFA (RCFA) can significantly degrade many properties of fresh cement paste. Directly using CFA as a mineral admixture in cement-based materials is not suitable in terms of workability. However, CFA through grinding methods can significantly mitigate the detrimental effect of CFA on the flowability of cement paste. When 15% ultrafine CFA (UCFA) was added, the cement paste exhibited relatively favorable workability and rheological properties. The rheological behavior of cement paste containing CFA with three particle sizes follow the shear thinning, which is in line with the modified Bingham model. Furthermore, the reasons for the impact of CFA on the microscopic level of cement workability are analyzed and discussed. The addition of CFA reduces the amount of free solution in the cement paste, leading to the formation of a flocculated structure and an increase in the thixotropic loop area. Based on the analysis of TOC, BET and TEM, it was determined that numerous polycarboxylate superplasticizer (PCE) molecules are adsorbed onto the rough and porous surface structure of CFA particles, hence the destructive ability of PCE to the flocculation structure caused by cement hydration is weakened. Therefore, the mechanism that CFA affecting the working performance of cement paste can be determined as: the adsorption of free water and PCE molecules.

Portland/Sulfoaluminate Cement Blends for the Control of Early Age Hydration and Yield Stress

Early age hydration and the rheological behavior of cement pastes are related to their reactivity. The reactivity of Ordinary Portland Cement (OPC) and calcium sulfoaluminate cement (CSA) blends, with a CSA percentage of less than 10%, are investigated in this paper. Different percentages of CSA replacement are studied. First, an isothermal calorimetry study of the different cement pastes is performed to understand the effect of CSA on the evolution of the heat of hydration. Then, in order to understand the reactional mechanism of these cement pastes, X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) studies were conducted on one blended mix made out of 7% CSA and compared to the pure cement pastes of OPC and CSA. Later, the evolution of the yield stress as a function of the CSA percentage was studied. A synergy is shown between both OPC and CSA once mixed together, leading to a 57% and 45% higher ettringite formation for the blend compared to 100% OPC and 100% CSA cement pastes, respectively. This implies that the CSA amount affects the reactivity of OPC/CSA blends. This was confirmed by the variation in both the heat of hydration and yield stress evolution as a function of the percentage of CSA.

Effect of Plasticizer on Hydration and Rheological Behavior of Cement Pastes

Lignosulfonate-based admixtures (LS) obtained from sulfite pulping processes were the first dispersants added as a water-reducer to concrete. Less effective than the latest superplasticizers, the low cost and low environmental footprint of LS continue to allow these materials to compete in the building and construction market. In addition to the dispersion effect, LS is known to retard cement hydration. The aim of this paper is to characterize the effect of LS on the hydration and rheology of cement paste. The effect of the time at which the LS is added, either by immediate addition or after ten minutes of hydration, has been investigated. It appears that the immediate addition of LS is less effective than the delayed addition when it comes to dispersion of cement, likely due to the initial ettringite formation.

Rheological Behavior and Strength Characteristics of Cement Paste and Mortar with Fly Ash and GGBS Admixtures

A cement paste or mortar is composed of a mineral skeleton with micron to millimeter-sized grains, surrounded by water filaments. The cohesion or shear resistance in the cement paste and mortar is caused by capillary forces of action. In the case of mortar mixes, there is friction between the particles. Therefore, the mortar mixture shows both friction between particles and cohesion, while the paste shows only cohesion, and the friction between particles is negligible. The property of the cement paste is greatly influenced by the rheological characteristics like cohesion and internal angle friction. It is also interesting that when studying the rheology of fresh concrete, the rheological behavior of cement paste and mortar has direct applicability. In this paper, the rheological characteristics of cement paste and mortar with and without mineral admixtures, that is, fly ash and ground granulated blast-furnace slag (GGBS), were studied. A cement mortar mix with a cement-to-sand ratio of 1:3 was investigated, including fly ash replacement from 10% to 40%, and GGBS from 10% to 70% of the weight of the cement. A suitable blend of fly ash, GGBS, and ordinary Portland cement (OPC) was also selected to determine rheological parameters. For mortar mixtures, the flow table was conducted for workability studies. The flexural and split tensile strength tests were conducted on various mortar mixtures for different curing times. The results indicate that in the presence of a mineral mixture of fly ash and GGBS, the rheological behavior of paste and mortar is similar. Compared with OPC-GGBS-based mixtures, both cement with fly ash and ternary mixtures show less shear resistance or impact resistance. The rheological behavior of the mortar also matches the rheological behavior in the flow table test. Therefore, it is easy to use the vane shear test equipment to conduct cohesion studies to understand the properties of cement paste and mortar using mineral admixtures. The strength results show that the long-term strength of GGBS-based mixtures and ternary mixed mixtures is better than that of fly-ash-based mixtures. For all mixtures, the strength characteristics are greatest at a w/b ratio of 0.6.

Rheological behavior of cement paste with nano-Fe3O4 under magnetic field: Magneto-rheological responses and conceptual calculations

The magneto-rheological responses of cement paste with nano-Fe3O4 particles are experimentally investigated. The estimated magneto-dynamic force between two neighboring nanoparticles and equilibrium movement velocity of the nanoparticles in cement-based suspensions are calculated. Results show that the nanoparticles have a potential to move to form magnetic clusters when a magnetic field is applied, which creates a sort of agitation effect breaking down the early C-S-H links between cement particles, and thus the corresponding suspensions exhibit liquid-like behavior immediately after applying the magnetic field. The solid-like property of the studied suspensions becomes more dominant with magnetizing time due to the formation of magnetic clusters and the reconstruction of C-S-H bridges. The rheological properties of paste medium exert significant influences on the magneto-rheological responses of cement paste containing nano-Fe3O4 particles. It is revealed that the calculated magnetic yield parameter and nanoparticle movement velocity are useful relevant indicators to evaluate the magneto-rheological effect of cementitious paste.

What is the mechanism of the fiber effect on the rheological behavior of cement paste with polycarboxylate superplasticizer?

Compared with most common construction materials, fiber reinforced cementitious materials are well known to exhibit better physical, working and mechanical properties. In this study, three fibers were selected: polypropylene fiber (PPF) and polyvinyl alcohol fiber (PVAF), which represented synthetic fibers, and sisal fiber (SF), which represented natural fibers. Effects of these fibers on the flowability, rheological properties, and adsorption behavior of the cement paste with polycarboxylate superplasticizer (PCE) were investigated. Furthermore, the above experimental results were verified by measuring the contact angle of the fiber with water and PCE solution and the apparent morphology of the fiber. Results revealed that the addition of fibers significantly reduces the fluidity of the cement paste, while the yield stress and plastic viscosity of the cement paste increase with the addition of fibers. From the contact angle and scanning electron microscope, the surface of SF was relatively rough, and the contact angle of SF with water or PCE solution was the smallest. It can be concluded that the plant fiber has a significant influence on the fluidity and rheology of cement paste.

Resting time effect on the rheological behavior of cement paste in presence of superplasticizer

Rheology has been widely used to describe the flow properties of fresh cement-based suspensions. Nevertheless, the effect of early hydration product formation on rheology, and the effect of shear on breaking of hydration bonds is not fully understood. Therefore, the characterization of the time-dependency of rheology has become necessary to predict precisely the behavior of concrete during placement. In this paper, variations in the resting time in between rheological tests were imposed to evaluate the effects on the development of internal structure of the mixture using different dispersing agents. Increasing resting time in between measurements mainly enhances viscosity, but the magnitude seems strongly dependent on the dispersant used. However, the effect of the employed procedure, as two procedures were evaluated, seems to have a stronger influence on the time-evolution of the rheological properties than changing the resting time in between measurements.

Effect of nano-silica on Portland cement matrices containing supplementary cementitious materials

abstract: For a controlled granulometry, this study evaluates the effect of nano-silica on mechanical and rheological properties, as well in the microstructure of Portland cement matrices containing a fixed amount of supplementary cementitious materials and three different types of cements. The rheological behavior of cement pastes was evaluated by rotational rheometry and mechanical performance was measured througth the compressive strength. The microstructure was analyzed by intrusion mercury porosimetry and scanning electron microscopy. There was an increasing on the viscosity of the cementitious matrices, as a consequence of the reduction in the inter particle separation of these suspensions. The optimum content of nano-silica varied according to Ca/Si ratio of Portland cement matrices containing supplementary cementitious materials. The use of nano-silica allowed to modify the pore size distribution of cementitious matrices. And the structure of nano-silica in cementitious matrices has occurred in layers or agglomerates of nano-particles covered by hydration products.

Effect of Limestone Filler Dosage and Granulometry on the 3D printable Mixture Rheology

The article presents the experimental results concerning the influence of the concentration and a particle size distribution of limestone filler on the rheological behaviour of cement pastes as matrixes of 3D printable concrete. The squeezing test is used in this paper as a rheological behaviour identification tool of cement-based materials in order to evaluate the extrudability and buildability. It has been established that introduction of filler as a factor changing the properties of solid phase in the system “cement + water” is an effective method for plasticity regulation and increasing the 3D printable cement-based materials resistance to load during the printing. Structural stability of cement paste can be improved by 2 times by introduction of filler with efficient ranging of a particle size in dosage of 20 – 40 % by weight of cement. Cement paste with limestone filler having a particle size distribution ranging from’ to 75 μm is characterized by the optimal plasticity, structural strength and deformability under the load. Using limestone filler with equal particle size impairs these characteristics of cement paste.

Modeling on Rheological Behavior of Cement Paste under Squeeze Flow

Modeling on Rheological Behavior of Cement Paste under Squeeze Flow

Investigation of the Incompatibilities of Cement and Superplasticizers and Their Influence on the Rheological Behavior.

The rheological behavior of cement paste and the improvement of its flowability takes center stage in many research projects. An improved flowability can be achieved by the addition of superplasticizers (SP), such as polycarboxylate ethers (PCE). In order to be able to use these PCEs effectively and in a variety of ways and to make them resistant to changes in the environment, it is crucial to understand the influence of SPs on cement hydration. For that reason, the topic of this paper was the incompatibility of a specific SP and an ordinary Portland cement (OPC). The incompatible behavior was analyzed using rheological tests, such as the spread flow test and penetration test, and the behavior was compared by means of an ultrasound technique and explained by the phase content measured by in-situ X-ray diffraction (XRD) the heat evolution measured by calorimetry, and scanning electron microscope (SEM) images. We showed that the addition of the SP in a high dosage led to a prevention of the passivation of the most reactive and aluminum-containing clinker phases, aluminate and brownmillerite. This induced the aluminate reaction to take place in the initial period and led to an immediate stiffening of the cement paste and, therefore, to the complete loss of workability. The results showed that in addition to the ettringite, which began to form directly after water addition, hemicarbonate precipitated. The fast stiffening of the paste could be prevented by delayed addition of the SP or by additional gypsum. This fast stiffening was not desirable for SPs, but in other fields, for example, 3D printing, this undesirable interaction could be used to improve the properties of printable mortar.

Synthesis of dispersing agents from starch – Influence on rheological properties and early age hydration of OPC

This study deals with the influence and effectivity of synthesized starch-based superplasticizers on rheological behaviour of cement pastes and on cement hydration. By varying molecular parameters, like molecular weight and the amount of introduced anionic charges, key parameters which induce thickening or liquefying behaviour in cement pastes were identified. Cement hydration was monitored by isothermal calorimetry, ultrasonic measurements and conductivity experiments. The investigations were supplemented by adsorption measurements by the phenol-sulphuric acid method. Results clearly show that low molecular weights and anionic charges in the molecular structure are necessary to create superplasticizers from starch. Also, the amount of anionic charges implemented in the molecular structure by chemical modification is a key parameter for a high dispersing performance. Furthermore, the starch-based superplasticizers delay the hydration of cement pastes, because of adsorption on cement surface, especially on the first hydration products.

Rheological impact of using cellulose nanocrystals (CNC) in cement pastes

The rheology of cement pastes containing cellulose nanocrystals (CNC) was assessed. Nine CNC obtained from various sources were added to cement pastes. Cement pastes were made using Type I/II and Type V cement with CNC addition rates from 0.023 vol% –3.311 vol%, relative to dry cement. Pastes were made using water to cement ratios of 0.30, 0.35, and 0.40. A Bingham model was used to evaluate changes in yield stress. The goal of these experiments was to identify the characteristics of the CNC which impact rheological behavior. The results showed the rheological behavior of cement pastes with CNC was not strongly related to Zeta potential, average CNC particle length, nor the average CNC particle respect ratio. However, it was found that maximum yield stress reduction occurred at higher CNC dosages as water to cement ratio increased. In addition, the rheological measurements suggested that at lower dosages (<0.2%), CNC behaved more like a water reducing admixture, reducing the yield stress up to 54%, while at higher dosage (more than 0.5%), CNC increased the paste yield stress, similar to certain viscosity modifying admixtures.

Rheological model selection and a general model for evaluating the viscosity and microstructure of a highly-concentrated cement suspension

A better fit to rheological measurement of cement pastes is only part of model selection. The selected models are likely to describe the rheological behavior of cement paste, but they need to be generalized. This paper provides a generalization on the various models to describe the rheological behavior of cement pastes. The viscosity curve following Sisko model characterizes the steady-state response of cement paste. A power-law exponent called the flow behavior index is constant for all cement paste samples produced from various mix proportions. The flow consistency factor and infinite viscosity decrease with a higher water-to-cement ratio and a higher dosage of polycarboxylate. In the relaxation process, the viscosity shows exponential decay at a constant rate of shear strain. The generalized model is finally compared with Quemada's microstructural model, Roussel's thixotropy model, and the Bingham model.

Influences of shear-mixing rate and fly ash on rheological behavior of cement pastes under continuous mixing

The shear rate experienced by the cement pastes inside fresh concrete depends on the placing operations and the nature of concrete, and it plays an important role in the evolution of rheological properties and stability of fresh concrete over time. In this study, the effects of continuous shear mixing and fly ash on thixotropy, apparent viscosity, and shear-thickening behavior of cement pastes were experimentally investigated. Various shear rates of 1 s−1, 15 s−1 and 30 s−1 were selected to continuously shear the cement pastes. Results showed that the action of high-rate shear mixing slowed down the evolution of thixotropy, but the efficiency of reduction was gradually limited. Under the state of continuous shear mixing, the apparent viscosity of reference paste was significantly increased and the shear-thickening intensity at higher shear rate range was decreased. However, for the cement pastes containing fly ash, only slight increase in apparent viscosity was observed and the shear-thickening intensity also exhibited a slight increase behavior. It would be an effective way to hinder the evolution of flocculation structure by exerting a continuous shear mixing at an appropriate shearing rate and adding fly ash.