Cement-based Suspensions Research Articles

Assessing the stress-transfer capability of mineral impregnated PBO yarns in a limestone calcined clay cement-based (LC3) matrix

Technical textiles made of poly(p-phenylene-2,6-benzobisoxazole) (PBO) represent attractive candidates for strengthening and repairing damaged concrete and masonry structures, due to the outstanding durability and mechanical performance of PBO fibres. Similarly to their aramid counterparts, PBO fibres have proved very effective against dynamic and impact loading. In this contribution, the pull-out behaviour of PBO multifilament yarns embedded into a blended cement-based matrix is investigated, with particular reference to its stress-transfer capacity. In addition to the as-received PBO yarns, impregnation with a cement-based suspension, which can fully preserve the inorganic nature of the composite system, is also evaluated. Experimental results are presented and interpreted using a one-dimensional mechanical model. The findings indicate that mineral impregnation of the yarns provides a 40% increase in the stress-transfer capacity with the matrix, corresponding to a halving of the anchoring length. These performance gains are also supported by a transition in the failure mechanism which shifts from friction-based pull-out to fibre rupture.

Thixotropy of superplasticized cement pastes – Underlying mechanisms considering the polycarboxylate molecular structure, interparticle interactions and hydration kinetics

This paper deals with the decisive mechanisms of thixotropic structural build-up in superplasticized cement-based suspensions related to the PCE molecular structure. We investigate the effect of the side chain length and anionic charge ratio of tailored PCE superplasticizers on thixotropic structural build-up under quantitative consideration of colloidal and contact interactions as well as very early hydration kinetics. The thixotropic structural build-up of superplasticized cement pastes with systematically varying the PCE molecular structure was investigated at constant workability, i.e. dynamic yield stress, by adjusting the individual PCE dosage. Thixotropy was quantified using plate-plate rheometry, while at the same time superplasticizer adsorption, specific surface area, granulometric measurements and quantitative XRD analysis provided an experimental basis for the identification of the respective contribution to the structural build-up.Our results show that the thixotropic structural build-up increases with decreasing PCE side chain length and increasing anionic charge ratio. This is due to accelerated very early hydration kinetics, causing an increase in the specific surface area. The latter is primarily attributed to ettringite, modifying the particle size distribution of the hydrated cement paste. Our approach of separating the colloidal and contact interactions indicates that the PCE-induced change in particle size distribution due to accelerated ettringite formation as well as the change in the average effective layer thickness are the dominant mechanisms for the thixotropic structural build-up in this context.

Effect of Mixing Procedure and Injection Pressure on Injectability of Cement-Based Suspension Grouts

Effect of Mixing Procedure and Injection Pressure on Injectability of Cement-Based Suspension Grouts

Development of viscosity models of concentrated suspensions: Contribution of particle-size and shape indices

Blending cementitious powders marks multiple morphological features which dictate the rheology of cement-based suspensions. Models, whether theoretical or empirical, unveiled the convolution in describing the consequence of particles’ morphology on the rheology of cement-based suspensions. The viscosity of various cement suspensions designed with different liquid-to-binder ratios was examined. Models, for instance, Krieger–Dougherty and Mooney, were implemented to foresee the relative viscosity of the employed suspensions in terms of the coupled physical properties and dry relative solid fraction. The coefficient kE and intrinsic viscosity (η) are in good agreement with particle-size indices, including fineness index (SSA × Gs) and particle-size ratio (d502/SSA). In addition, the current models were adjusted with a focus on new coupled-physical properties, including static (αR × αS), dynamic (αF × cohesion), and total flowability (ftotal) indices. This resulted in significant improvement in the precision of determining the viscosity of concentrated cement-based suspensions.

Ultrafast stiffening of concentrated thermoresponsive mineral suspensions

Extrusion-based 3D printing with rapidly hardening polymeric materials is capable of building almost any conceivable structure. However, concrete, one of the most widely used materials for large-scale structural components, is generally based on inorganic binder materials like Portland cement. Unlike polymeric materials, a lack of precise control of the extent and rate of solidification of cement-based suspensions is a major issue that affects the ability to 3D-print geometrically complex structures. Here, we demonstrate a novel method for controllable-rapid solidification of concentrated mineral suspensions that contain a polymer binder system based on epoxy and thiol precursors as well as one or more mineral fillers like quartz and calcite. The thermally triggered epoxy-thiol condensation polymerization induces rapid stiffening of the hybrid suspensions (0.30 ≤ ϕ ≤ 0.60), at trigger temperatures ranging between 50 °C and 90 °C achieving average stiffening rates up to 400 Pa/s. The use of nucleophilic initiators such as 1-methylimidazole provides control over the activation temperature and curing rate, thereby helping to achieve an adjustable induction period and excellent thermal latency. By using multiple techniques, we provide guidelines to create designer compositions of mineral suspensions that utilize thermal triggers to achieve thermal latency and ultrafast stiffening – prerequisite attributes for 3D-manufacturing of topologically-optimized structural components.

A new mix design method for low-environmental-impact blended cementitious materials: Optimization of the physical characteristics of powders for better rheological and mechanical properties

The use of blended cements has attracted much interest in the quest to overcome the environmental challenges facing the concrete industry. However, partially replacing cement with supplementary cementitious materials (SCMs) can result in complex rheological behavior that is influenced by the type of SCM and the replacement percentage, as well as the physical characteristics of the particles. Adequate physical control of the blended cements is essential for improving the rheology of the suspensions. A new mix design methodology based on the physical optimization of SCMs is proposed and validated to ensure adequate rheology of the suspensions. The physical characteristics that influence the viscosity of the cement-based suspensions investigated in this study include particle-size distribution, surface roughness, maximum packing density of the powder, and interparticle distance. Based on the experimental results presented in this paper, ternary binders with optimized physical characteristics enhanced the 28-day compressive strength of mortar by 20% while decreasing the clinker content by 15% and maintaining constant fluidity.

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.

Effect of sedimentation on the rheological properties of cement pastes

Rheology has been widely used to describe the flow properties of fresh cement-based suspensions. Nevertheless, the characterization and understanding of the rheological behavior of cement-based materials have become essential with the recent introduction of novel applications like digital construction in the field of civil engineering, among others. It has been well documented that concrete and mortar may suffer stability problems (i.e. the ability of the mixture to maintain homogeneous). At the scale of cement paste, this phenomenon can still arise and affect the rheological properties of the measured paste. Therefore, inaccuracies during the measurements and incorrect interpretation may occur, resulting in erroneous conclusions. This study explores the impact of sedimentation of cement particles when characterizing the rheological properties of cement paste. The w/c employed in the pastes were 0.45 and 0.35 with the addition of superplasticizer to obtain a dispersed system. The rheological properties were measured using a coaxial cylinder configuration and followed up to 180 min after the contact between cement and water. Repeated measurements were executed with and without a homogenization procedure after each application of a measuring procedure. Comparing the results shows a clear influence of sedimentation on the evolution of rheological properties with time, and the effect is more pronounced for the mixtures with high w/c and for the procedure with the larger shear rate. Therefore, it is strongly recommended to keep a measuring procedure sufficiently short in duration.

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.

Influence of flexible fibers on the yield stress of fresh cement pastes and mortars

In the present work, we aim at improving our understanding of the effect of synthetic flexible fibers on the rheology of fresh cement-based suspensions. We first measure the way that synthetic fibers do increase the yield stress of various fresh cement pastes and mortars. We then measure, using micro CT, the way these fibers are deformed in a fresh paste. Finally, we propose a simple physical model allowing for the prediction of the fibers conformation as a function of their geometry, their elastic properties and the rheology of the cement-based material, into which they are dispersed. We validate this model on our results and discuss its application at the scale of mortars and concretes.

Flowability characteristics of dry supplementary cementitious materials using Carr measurements and their effect on the rheology of suspensions

Particle-size distribution, fineness, and morphology of powders used in flowable concrete greatly affect their rheological properties. An experimental investigation was undertaken to determine the physical characteristics of dry cementitious binders using Carr measurements and evaluate their coupled effects on rheology of cement-based suspensions. Cement, glass pozzolans, limestone filler, fly ashF, and ground granulated blast-furnace slag were investigated. The angles of repose, fall, and spatula, compressibility, and cohesion were measured. The experimental results showed that dry flowability of blended systems are correlated to their particle-size distribution, Blaine specific surface area, morphological characteristics, packing density, specific gravity, and cohesion. The mean average diameter, fineness, and morphological properties showed the most significant effects on dry flowability of the investigated dry powders. On the other hand, the yield stress and plastic viscosity values of the suspensions were found in good agreements with the Carr's angles and dispersion indicators of the investigated systems.

On the Use of the Coaxial Cylinders Equivalence for the Measurement of Viscosity in Complex Non-Viscometric, Rotational Geometries

The rheology of macroscopic particle suspensions is relevant in many industrial applications, such as cement-based suspensions, synthetic and natural drilling fluids. Rheological measurements for these complex, heterogeneous systems are complicated by a double effect of particle size. On the one hand, the smallest characteristic length of the measuring geometry must be larger than the particle size. On the other hand, large particles are prone to sediment, thus calling for the use of rotational tools that are able to keep the suspension as homogeneous as possible. As a consequence, standard viscometric rotational rheometry cannot be used and complex flow geometries are to be implemented. In this way, however, the flow becomes non-viscometric, thus requiring the development of approximate methods to translate the torque vs. rotation speed raw data, which constitute the rheometer output, into viscosity vs. shear rate curves. In this work the Couette analogy methodology is used to establish the above equivalence in the case of two complex, commercial geometries, namely, a double helical ribbon tool and a square-shaped stirrer, which are recommended for the study of relatively large size suspensions. The methodology is based on the concept of the reduction of the complex geometry to an equivalent coaxial cylinder geometry, thus determining a quantitative correspondence between the non-standard situation and the well-known Couette-like conditions. The Couette analogy has been used first to determine the calibration constants of the non-standard geometry by using a Newtonian oil of known viscosity. The constants have been subsequently used to determine the viscosity curves of two non-Newtonian, shear thinning fluids, namely a homogeneous polymer solution and two heterogeneous concentrated suspensions. The results show that the procedure yields a good agreement between the viscosity curves obtained by the reduction method and those measured by a standard viscometric Couette geometry. The calibration constants obtained in this work from the coaxial cylinder analogy are also compared with those provided by the manufacturer, indicating that the calibration can improve the accuracy of the rheometer output.

Modelling of particles retention in a porous soil

Construction of foundations and underground structures requires grouting the soil for groundwater protection. To select the technology of soil grouting and the type of grout, it is necessary to calculate the suspended particles retention for the filtration of bentonite or cement-based suspension in a porous soil in dependence of sizes of the soil pores and the grains of the injected mortar. The study of transport and retention of solid particles in the complex structure of a porous medium makes it possible to calculate the strength and impermeability of a grouted foundation at the stage of pre-construction preparation.The solid particles retention in the nonlinear process of deep bed filtration of a suspension in a porous medium is considered. It is assumed that the retained particles can not be detached from the frame of porous medium by a fluid flow or suspended particles. The purpose of the study is to calculate the dynamics of deposit formation for different types of variable filtration coefficients with the predominance of the size-exclusion mechanism of particles capture.The mathematical model of filtration is the system of mass transfer equation and the kinetic equation of deposit rate with the conditions of injection of a constant concentration suspension into an empty porous medium. In this paper an exact solution of the problem at the inlet of a porous medium is constructed. The dynamics of retained particles growth is studied by numerical modeling method, depending on the type of blocking filtration coefficient.The dependence of retained particles concentration on time at the inlet of a porous medium for various blocking filtration coefficients was obtained. Plots of the dependence of the deposit concentration on time for smooth and non-smooth filtration coefficients are constructed.When reducing the number of free pores of small sizes, the filtration rate slows down. It is shown that slowing down the filtration process and the growth rate of the deposit are determined by the multiplicity of the root of the blocking filtration coefficient. Depending on the type of filtration coefficient, the process of retention of solid particles in a porous medium can last indefinitely, or it may cease after some time with complete blocking of small pores by retained particles. The limiting time for the end of the filtration is given by a compact analytical formula.

Experimental study on performance of cement-based grouts admixed with fly ash, bentonite, superplasticizer and water glass

To ensure desirable flowability, enough penetrability and less leaching of grouting materials in geotechnical engineering, the ASTM Type I Portland cement (PC), class F fly ash (FA), bentonite (B), superplasticizer (SP) and water glass were designed to improve properties of grouting materials in different aspects. The fresh-state properties i.e. flowability, spreading ability, bleeding and setting time; mechanical properties i.e. flexural strength and compressive strength; and microstructures of the cement-based grouts were investigated. The water/solid (W/S) ratio is selected as 1.0, the amounts of FA, bentonite and SP by mass of PC were 0–40%, 0–15% and 0.7%, respectively. The volume ratios of cement-based suspensions and water glass were selected as 1:1, 2:1 and 3:1. The results show that the FA and 0.7% SP enhance the flowability and spreading ability of fresh grouts, and the bentonite can enhance the stability. The effects of various FA, B, SP and water glass contents on flexural strengths of grouts are different. The class F FA, B and 0.7% SP reduce the compressive strength of hardened grouts. The performance of cement-based grouts with 20–30% class F FA, 0–5% bentonite, 0.7% SP and volume ratios of 2:1–3:1 can satisfy the requirements of high performance, environmental friendliness and low costs in geotechnical and underground engineering.

Physico-chemical kinetics of structural build-up of neat cement-based suspensions

The rate of structural build-up of cement-based suspensions at rest is a result of combined physical and chemical structuration processes. The non-destructive dynamic rheometry was used to investigate the influence of cement fineness and content, alkali sulfate content, and temperature of the suspension on the percolation time and rigidification rate of neat cement suspensions. Zeta potential, calorimetric, and spectrometric measurements were performed to explore the corresponding microstructural changes in cement suspension. Test results showed that the percolation time is controlled by the frequency of Brownian collisions, the distance between dispersed particles, and intensity of cohesion between cement particles. On the other hand, a high rigidification rate can be secured by increasing the number of contact points per unit volume of paste, nucleation rate of cement hydrates, and intensity of inter-particle cohesion. A semi-empirical model correlating the build-up indices and the microstructural characteristics was developed and validated.

Constitutive parameter identification: An application of inverse analysis to the flow of cement-based suspensions in the fresh state from synthetic data

Rheometers with specific impellers, developed to characterize the behavior of cement-based suspensions in the fresh state, are used to limit the heterogeneities induced during shearing but they make the identification of the rheological parameters less straightforward compared to conventional rheometers. This paper presents the inverse analysis method and discusses the quality of this identification procedure when applied to such materials. The procedure includes a CFD simulation based on the finite element method using a Herschel–Bulkley model. Two kinds of optimization algorithms are used: a deterministic simplex method and a stochastic genetic method. As a first step of a larger study, the procedure reported in this paper used 2D synthetic data, i.e. 2D numerically generated experimental data. Two numerical viscoplastic materials, characterized by shear-thinning and shear-thickening, were selected and studied. The results obtained with the two algorithms are systematically compared to the known parameter solution. Three approaches corresponding to three levels of user’s knowledge about the material under study are considered successively: (1) the user has no a priori knowledge about the material, (2) the user knows whether the material is shear-thinning or shear-thickening and (3) the user is able to estimate the behavior index. The time-consuming genetic method appears to be suitable when the a priori knowledge of the material is slight, whereas the simplex method gives a reliable solution in a few iterations when the level of knowledge is higher. Both algorithms encounter more difficulties with the shear-thinning material than with the shear-thickening one. In this paper, the advantage provided by the genetic method regarding the non-uniqueness of the identification procedure for real experimental data is also highlighted: this method provides a collection of satisfactory solutions among which the user can select the optimal one based on his scientific background and/or on further experimental test results.

New approach to assess build-up of cement-based suspensions

The structural build-up of cement suspensions at rest is a complex phenomenon affected by both physical and chemical structurations. Better understanding of this phenomenon relies on the efficiency of measurement approach. Dynamic rheometry was used to investigate the evolution of both storage modulus and phase angle of cement and inert calcium carbonate suspensions. Two independent build-up indices were proposed to identify the rest time needed to build a percolated elastic network and its rigidification rate with time. The structural build-up of various cement suspensions was evaluated using the proposed indices. These indices were then compared to the well-known thixotropic index (Athix.). It was found that combining the percolation time and the rigidification rate can provide deeper insight into the structuration process of cement suspensions. Furthermore, these indices were found to be well-correlated to the decay in lateral pressure of cement suspensions.

Effect of solid concentration and shear rate on shear-thickening response of high-performance cement suspensions

Flow performance, placement, and consolidation of concrete are mainly related to its rheology. Rheology of paste plays a key role on rheology of concrete. Cement pastes exhibit complex rheological behavior affected by several physical and chemical factors, including the solid concentration, type and dosage of high-range water-reducer (HRWR), cement characteristics, and shear history. An experimental investigation was carried out to investigate the effect of solid concentration, HRWR–cement combinations, and shear rate regime on pseudoplastic behavior of high-performance cement grouts. Grout mixtures proportioned with w/c of 0.30, various cement–HRWR combinations, 8% silica fume, and different limestone powder additions were investigated.Solid concentration and shear rate regime are shown to be key factors affecting shear-thickening response of concentrated cement-based suspension (i.e. low w/c). Concentrated high-performance grout mixtures are shown to exhibit shear-thickening behavior. The use of polycarboxylate HRWR acting by hindrance effect exhibited greater shear-thickening behavior compared to polynaphtalene type acting by electrostatic effect. The use of finer particles enhances the powder skeleton and ensures polydisperse systems, hence resulting in lower shear-thickening response. For concentrated cement-based suspensions, shear-thickening is due to disorder state, and at higher shear rate, hydrocluster formation is prevailing.

The nature of limestone filler and self-consolidating feasibility—Relationships between physical, chemical and mineralogical properties of fillers and the flow at different states, from powder to cement-based suspension

This paper is a part of a large study aimed at identifying the physical and chemical properties of limestone fillers (LF) that govern their behaviour towards self-consolidating flow. Five LF were studied, complying with the standards and selected for their significant differences in properties on the basis of the supplier's database. Despite their specific manufacturing, a thorough characterization showed that the selected LFs had very different properties in terms of surface charges, morphology, wettability and size distribution. Then, relationships were sought between these properties and the flow of LF in powder form and suspended in water, or water + polycarboxylate type High Range Water Reducer Admixture (HRWRA), or water + HRWRA + cement (OPC or slag blended cement). The flow measurements concerned flowability, floodability and shear under consolidation in the dry state, and static yield stress and apparent viscosity in the suspension state. The main results show that the LFs act in the same way on the flow as long as cement is not incorporated into the suspension. From the dry state to the water + HRWRA suspensions, the flow is dependent on the fineness of the LF. The significant relationships between the surface charges, wettability and fineness of LFs show that impurities like clays are key factors in the flow of LF suspensions. When cement was incorporated into the suspension, the flow was dependent on the interactions existing among all the constituents. Then, with a view to self-consolidating applications, it becomes possible to identify how best to incorporate LF in a cement-based matrix through the measurement of the arrangement of cement and filler particles in suspension.

Reducing shear thickening of cement-based suspensions

Rheological measurements were made on concrete and mortars to characterize the shear thickening behavior of certain concrete mix designs. Shear thickening reduction levers were found by selecting and designing admixtures. Since the shear-thickening phenomena occur at the scale of the finest particles, industrial limestone fillers were studied that behave like cementitious materials. Theories based on previous academic works were relevant. The shear stress-dependent effects of shear thickening and size scaling were very helpful to distinguish between surface interactions, such as lubrication and volumetric contributions and also including the packing effects. The suspension viscosity curves vary accordingly to the Newtonian viscosity of the solvent medium. In both the shear thinning and shear thickening regimes, viscosity is controlled by adjusting the amount of two specific admixtures. The reduction of friction between polymer-coated materials appears to be a key phenomenon to delay onset shear thickening in industrial processes.