Cement paste rheology

These early views are, perhaps, too simplistic to explain in full the rheological changes that occur in polyelectrolyte cement pastes before and at gelation. There are several physicochemical processes that underlie... 

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete. Thus a high water-cement ratio concrete will contain a paste content which is more fluid than that of a low water-cement ratio concrete. 

The fluidity of the cement paste can be measured in rheological terms by the torque transmitted to a stationary bob inside a revolving outer cylinder placed in a water-cement system as shown in Fig. 1.10. The shear stress measured at the stationary bob is plotted against the rate of applied shear when, for pastes of varying water-cement ratios, the results shown in Fig. 1.11 are obtained for readings taken of the shear stress as the shearing rate is increased (the up curve). 

The rheological characteristics of cement pastes are related to the nature of the attractive and repulsive forces which exist between cement and cement hydration product particles and can be categorized as follows ... 

The cement composition affects the rheological behavior of the system cement pastes having low C3S/ C2S and C3A/C4AF ratios have a higher viscosity when the superplasticizer addition is delayed [27],... 

The effect that air-entraining agents have on the rheology of fresh cement pastes can be considered from the point of view of changes due to the admixture itself, and those due to the presence of entrained air. 

Most admixtures of this type do not significantly alter the rheology of cement pastes at early ages. The quicker stiffening of accelerated pastes will, of course, result in higher viscosities at a later age. More complex formulations occasionally include water-reducing admixtures to reduce the water-cement ratio, and their effect will be a function of the water-reducing admixture type and content (see Section 1.3.1). 

The properties of an overall concrete system in the plastic state will be a function of many parameters such as aggregate types and shapes, cement contents and characteristics, etc., but it is useful to isolate the effect of the paste rheology where it can be stated that ... 

The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint, it includes those of mixing, placing and compaction. 

Rheological studies on cement pastes and concrete have been reviewed in a book (T42) and several shorter articles (H40,L40,S76). The results in the literature show wide variations, many of which reflect the large effects of seemingly minor differences in experimental technique. Since cement pastes do not show Newtonian behaviour, methods giving only a single parameter are inadequate. Viscometers in which the material is studied in shear between a shallow cone and a plate, or between concentric cylinders, have generally been used. 

The tricalcium aluminate reacts with water at a highest rate among the clinker phases and affects substantially the rheological properties of cement paste. The gel products are formed very rapidh and subsequently crystalhze as a mixture of the two phases C2AHg and C4AHJ3 respectively. These products cover the C3A grains and further... 

There are several hypotheses as the rheological properties of cement pastes are concerned. As it is commonly known the rheology deals with the flowing and deformation of materials imder stress. The Newtonian fluids show a simple relationship between the shear stress and shear rate. When a thin layer of fluid is placed between the two parallel plates, of which one is fixed and the second will be subjected to the shearing force F, then the shearing of this layer will occur. The dynamic equilibrium will be attained when the force F, in the condition of stationary flow, will be balanced by the viscosity of Newtonian fluid and the relation between the shear stress and shear rate gradient will be linear (Fig. 5.1). 

As it was aforementioned, the rheological properties of cement pastes are dependent on many factors. In Fig. 5.7 the evolution of the properties of pastes from the pseudoplastic fluids through the Bingham fluid to the showing dilatancy, at higher... 

The water to cement ratio and the fineness of cement are the two important factors determining the properties of cement paste. The thickness of water layer on cement grains and the distance between the grains are dependent upon these parameters, which have the significant effect on the rheological properties of paste, first of all on the yield stress value. 

The analysis of the rheological properties of cement pastes is complicated additionally by the interaction of solid and liquid phase. In Table 5.1 the approxiniate size of hydrating cement grains and the distances attributed to the particular types of interactions are shown according to Massazza [21],... 

Nawa, T., and Eguchi, H., "Effect of Alkali Sulfate on Rheological Behavior of Cement Paste," Reviews, 42nd General Meeting of the Cement Association of Japan, 1988, pp. 50-53. 

Non-reactive constituents of inorganic binders do not react chemically to any significant extent in the course of hydration. They may, however, alter the rheology of the fresh cement paste and/or some properties of the hardened material by their physico-mechanical action. 

---