Study of the shear thickening effect of superplasticizers on the rheological behaviour of cement pastes containing or not mineral additives

doi:10.1016/S0008-8846(00)00330-6

Cement and Concrete Research

Volume 30, Issue 9, September 2000, Pages 1477-1483

https://doi.org/10.1016/S0008-8846(00)00330-6 Get rights and content

Abstract

Fresh cement pastes, when dispersed by a superplasticizer acting mainly by electrostatic effect, present a viscosity that increases with the shear rate (shear thickening). When mineral additives are used, the intensity of the phenomenon depends on their nature: it can be amplified (metakaolin), unchanged (quartz, fly ashes) or reduced (silica fumes). According to the literature, shear thickening occurs when suspensions have a high solid concentration and when repulsive interactions are predominant. These conditions can be achieved in cement pastes owing to the dispersing action of superplasticizer. In that case, the shear thickening behaviour could be due to local mechanical actions created by the increased shearing forces.

Introduction

Cement pastes, without the use of any mechanical action or superplasticizer, present a structure called “flocculent,” a word coined by Powers [1], who states that “under the effects of combined interparticle forces of attraction and repulsion the grains make up floccules so concentrated that they merge into a continuous mass, thus giving the whole system the aspect of a single large floc.”

The consequence of such a structure is a particular rheological behaviour that has already been described [2]. To sum up, without any vibration, cement pastes are viscoplastic. To make them flow, sufficiently strong shearing forces are necessary to break the bonds between grains that are the cause of an initial yield value. Because of the structural breakdown that occurs when this point is exceeded, the shearing forces necessary to maintain the flow at a constant rate decrease strongly during time. A good vibration periodically breaks the bonds and tends to decrease the initial yield value. It can even suppress it and, at the same time, the time-dependent behaviour.

Whether the flow is facilitated by local shear or by vibration, the apparent viscosity decreases nearly always with the increase in the strain rate (shear thinning). This property can give some interesting effects on the workability of concretes.

Legrand [3] pointed out possible causes to explain the shear thinning, especially the fact that all the bonds are not broken and that flocs can remain and be dragged by the flow. The number, size, and shape of such flocs will change as a function of the strain rate.

For some years, new products have appeared, like superplasticizers, which cause an important decrease in the initial yield value without any external mechanical action. The effect is mainly physicochemical and consists in interparticle repulsive forces that tend to change a flocculent state into a dispersed one. This results in concentrated and dispersed suspensions that can exhibit an increase in their apparent viscosity with an increase in the strain rate (shear thickening). This behaviour can induce unexpected consequences on concrete workability.

Already, a few authors have accidentally found the shear thickening behaviour when using superplasticizers and mineral additions [4], [5], [6], [7] but the knowledge remains very partial.

The objective of this paper is to experimentally point out the conditions of occurrence of the shear thickening behaviour in cement pastes. Rheological tests have been carried out varying the nature and dosage of superplasticizers and mineral additives. Some microstructural interpretations of such behaviour have then been attempted with the help of recent theories about colloidal and monodispersed suspensions.

Section snippets

Viscosimetric equipment and pastes composition

The equipment used is a modified Rotovisco RV2 (Haake). The inner rotating bob is replaced by a six-blade vane, which drags a cylindrical block of paste (to avoid slip). The gap is wide enough to allow the medium to be considered as infinite. Because of the time-dependent behaviour when the paste is at rest, a single flow curve cannot represent the complete rheological behaviour; thus, the paste is sheared at a high shear rate to get, in the sheared zone, the best structural breakdown. A pseudo

Results

All the flow curves have been modelled by Herschel–Bulkley's equation (τ=τ₀+kγⁿ). The exponent n characterises the behaviour of the pastes: shear thinning for n<1 and shear thickening for n>1.

Present knowledge about shear thickening behaviour

The shear thickening behaviour was first associated, and even confused, with the phenomenon of volumetric dilatancy of coarse aggregate, originally described by Reynolds [10]. This is not the case here, where the particles are small and far from being overlapped.

All kinds of suspensions of solid particles in a fluid can show a shear thickening behaviour [11], [12], if they present two particularities: the volume fraction of solids in the suspension must be very high and the suspension must be

Conclusion and prospects

Some products like superplasticizers and mineral additives are now inevitable when designing a concrete. These products can give unexpected rheological behaviour of pastes as the main phase affecting the rheology of concrete: shear thickening is often observed.

Rheological tests have been carried out to point out the conditions of occurrence of the shear thickening behaviour in cement pastes, varying nature and dosage of superplasticizer and mineral additives. The following conclusions were

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PaperStudy of the shear thickening effect of superplasticizers on the rheological behaviour of cement pastes containing or not mineral additives

Abstract

Introduction

Section snippets

Viscosimetric equipment and pastes composition

Results

Present knowledge about shear thickening behaviour

Conclusion and prospects

Cem Concr Res

Cem Concr Res

J Colloid Interface Sci

The Properties of Fresh Concrete

L'état floculent des pâtes de ciment avant prise et ses conséquences sur le comportement rhéologique

Contribution à l'étude de la rhéologie du béton frais

Mater Constr

A mathematical model for the flow curve of cement paste

Mag Concr Res

On the combined effect of water solubles lignosulfonates and carbonates on Portland cement and clinker pastes: 1. Physical properties

Cem Concr Res

Détermination, par diffraction des rayons X, de la teneur en phase amorphe de certains matériaux minéraux

J Phys IV

On the dilatancy and media composed of rigid particles in contact

Philos Mag

Paper
Study of the shear thickening effect of superplasticizers on the rheological behaviour of cement pastes containing or not mineral additives