Adsorbing polymers and viscosity of cement pastes
Introduction
Most academic researchers or practitioners quantify the consequences of a change in mix design on workability of fresh cementitious materials in terms of yield stress variations (i.e. in terms of variations in the critical value of the stress to be applied to initiate flow of the material). They sometimes do so without even knowing it by measuring, for instance, a change in slump or slump flow, this industrial test being, in most circumstances, directly correlated to yield stress [1], [2], [3], [4]. Although cementitious materials are not only yield stress fluids (i.e. they are also thixotropic non-Newtonian viscous fluids [5], [6]), this pragmatic approach is justified by the fact that yield stress is often the most relevant parameter to describe the ability of a material to fill, under its own weight (i.e. under the sole effect of gravity), a formwork or more generally a mold [7]. It is now accepted that this yield stress finds its origin in the colloidal interaction network between cement particles [8], [9], [10], [11] more or less amplified by the presence of the rigid aggregates [12], [13], [14], [15] and more or less modified by adsorbed polymers such as plasticizers [9].
Recent trends in mix design of cementitious products show however that water to cement mass ratio of modern cementitious materials is progressively decreasing [16]. This decrease in liquid content (or increase in solid volume fraction) leads to a decrease in the porosity of the hardened material and allows for increased mechanical performances or increased durability [17]. More specifically, within an environmental perspective, this decrease in porosity often allows for the substitution of pure Portland clinker by less efficient alternative binders such as blended cements while maintaining the performances of the resulting hardened product [18]. This increase in solid volume fraction has however dramatic consequences on the workability of the material and workers at the building site often complain about these “sticky” concretes that they are unable to vibrate and surface finish. This “stickiness” and, more specifically, the additional stress needed to place the material have nothing to do with yield stress but seem to increase with the rate of shear applied to the material and therefore seem to relate to the energy dissipation occurring in the flowing concrete, which is at the origin of its viscosity.
From a practical point of view, this raises therefore the question of how to decrease this energy dissipation in the case of these “sticky” mixes with high solid volume fractions. A decrease in apparent viscosity (i.e. the ratio between shear stress and shear rate) when plasticizers are added to the paste is sometimes measured in literature [19], [20], [21], [22], [23]. It is moreover accepted, in the field of attractive suspensions [24], that particle flocculation, which finds its origin in the attractive colloidal interactions between particles, may have a very strong influence on viscosity by modifying the micro-structure of the flowing suspension and the way shear concentrates between flocs or particles. This is often described in the cement community as cement flocs releasing some water as de-flocculation occurs [25]. As a consequence, by changing the flocculation state of a cement paste, adsorbed polymers seem to be able to reduce both yield stress and viscosity. It could then be expected that, at a given water to cement ratio for a given cement, a given flocculation state would correspond to a given yield stress and a given viscosity no matter the polymer. However, during the recent Tenth International Conference of Super-plasticizers and Other Chemical Admixtures in Concrete held in Prague, Czech Republic, in October 2012, the existence of various super-plasticizer molecules allowing for various reductions in stickiness (i.e. reduction in apparent viscosity) at constant slump values (i.e. constant yield stress) was reported and discussed [26].
In this paper, we aim at improving, mostly from rheological measurements, the understanding of the physical mechanisms at the origin of the above effects. We suggest, from our results, similar to other authors, that, for a given mixing and polymer introduction protocol, plasticizers are able to decrease apparent viscosity by modifying the flocculation state of the system, which, in turn, impacts the way shear localizes and concentrates in fluid layers. Our measurements moreover suggest that the thickness of the fluid layers scales with the surface-to-surface separating distance imposed by the adsorbing polymers. These effects being identical for all polymers at a given flocculation state of the cement particles, we suggest that the residual difference between polymers on the final apparent viscosity of cementitious products comes from the more or less pronounced increase in the local viscosity of the interstitial fluid between neighboring cement particles.
Section snippets
Cement
The cement used in this study is a Portland cement equivalent to ASTM Type I cement. Its chemical composition obtained through ICP-AES and ATD-ATG is given in Table 1. Its maximum packing fraction was estimated to be around 60% in [27] and its Blaine specific surface is 3650 ± 100 cm2/g. All cement pastes studied here are concentrated pastes and are prepared with a water to cement mass ratio (W/C) between 0.275 and 0.475. The total amount of water added to the system takes into account the water
Apparent shear viscosity measurements
We plot in Fig. 3 the measured apparent viscosity as a function of shear rate for cement pastes prepared with polymer A for three dosages along with the reference paste without polymer. The water to cement ratio is 0.35.
As expected, the reference paste is obviously a shear thinning fluid (i.e. apparent viscosity decreasing with shear rate). When polymer is added to the system, this shear thinning behavior becomes less pronounced. This can be obviously attributed to the deflocculating action of
Conclusions
In this paper, we studied the potential physical mechanisms at the origin of a decrease in viscosity of concentrated cement pastes. We suggested from our results, similar to other authors, that plasticizers are able to decrease viscous dissipation by modifying the flocculation state of the system, which, in turn, impacts the way shear localizes and concentrates in fluid layers. Our results suggest that shear concentrates in the gap between cement grains and that the typical size of this gap
Acknowledgment
The authors wish to thank Dow Construction Chemicals® for providing some of the polymers and Prof. R. Flatt for his very useful comments.
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