LEPTICO épticas del
1: Índice de Marcas
4.11 COSTO DE PRODUCCIÓN PARA CADA TRATAMIENTO
An illustration of typical experimental flow curves obtained as close to steady state as possible is provided in Fig. 1. On Fig. 1(a) was plotted the measured stress vs the applied shear rate; the shape of the curve is counter-intuitive in the sense that the general consensus stands upon an ever-increasing curve when shear rate increases. In numerous cases during this study, such U-shaped curves are obtained, the minimum of which defines a critical stress σc and critical shear rate γc. The origin of such a shape will be discussed below but
has also been reported in the past8,15 and is discussed more precisely in the paper SP-089 of
the present conference.
Fig. 1- Example of the general shape of flow curves
Influence of Superplasticizers on the Flocculation Degree of Cement Suspensions 81
Fig. 1(b) shows the same data plotted in terms of apparent viscosity vs shear rate. After
the expected shear-thinning behaviour (decreasing viscosity branch), a viscosity minimum
ηmin is also observed, beyond which the suspension enters a shear-thickening regime. The
critical shear rate defining the onset of this behaviour was noted γst, the ‘st’ index standing for ‘shear thickening’.
As already stated, these features are quite ubiquitous among the cement suspensions studied by the authors of this paper, the only differences from one system to another being the values obtained for the four critical parameters described above.
Low shear limit: the influence of suspension ageing
As previously stated, Fig. 1 shows an unexpected non-monotonous trend in shear stress vs shear rate. Some previous work seems to point at the influence of ageing or thixotropy; an analysis by Roussel et al8 shows that the introduction of a time-dependence component
in a flow curve equation leads to U-shaped flow curves (see also paper SP-089). A more general theoretical approach developed by Picard et al15 also concludes that non monoto-
nous curves are expected. Both approaches rely upon the coupling of a general flow equa- tion with a differential equation providing a time-dependence of an underlying parameter (structure level for Roussel et al., fluidity for Picard et al.) showing that structure evolution through time is involved.
A closer look to the raw data recorded here allows observing such an influence. As a matter of fact, all data points below γc appear to be flagged with a ‘non-steady state alarm’ by the rheometer software. This means that the data points were recorded despite the fact that no steady torque value was achieved for the requested shear rate. Fig. 2 shows the transient data in terms of steady-state index, a value supposed to reach one when the steady state is achieved. Fig. 2(b) shows that when the applied shear rate is higher than the critical shear rate, steady flow is achieved in mere seconds whereas an applied rate lower than the critical value induces an oscillating behaviour with an increasing amplitude as seen on Fig.
2(a), preventing the rheometer to establish a steady flow. Roussel et al8 explain that below
critical stress or rate no steady flow may indeed be achieved anymore and the suspension ages (i.e. its structure level increases), though slower than at complete rest. It may be argued that since no homogeneous or steady flow is possible anymore, the critical stress acts as an apparent yield stress.
Some further insight about this interesting flow feature may be brought up by a stress step protocol applied to a freshly mixed batch of the same suspension. Fig. 3 shows the flow curves obtained by applying a decreasing stress step program, then an increasing stress step program with a steady-state condition. The obvious observation is that the flow curves do not have the same shape as the applied shear rate curve.
At relatively high applied stress where flow is established all data sets are consistent. The decreasing stress stage shows a critical stress below which shear rate suddenly decreases by almost five decades to reach very low values in the range 10-5-10-4 s-1. The subsequent
increasing stress protocol features the same rate jump, but for a higher stress.
There appears a hysteresis loop around a low value of stress that is a result of the compe- tition between shear and ageing, as supported by the work by Picard et al,15 quoted below:
‘When [stress] is imposed, the flow curve jumps in a hysteretic fashion between two branches: one that corresponds to no flow but for a wall layer in the vicinity of the walls; the other corresponds to a fully fluidized situation. When the [global shear rate] is imposed, shear banding can occur, as well as sometimes a stick-slip like oscillating behaviour at small shear rate that corresponds to a localized oscillation of the fluidity.’
This description based upon the assumption of a time-depending evolution is then totally consistent with the observations presented in this paper. It was shown previously9 and is
shown in paper SP-089 in the same book that superplasticizer nature and dosage both influ- ence the values of critical stresses and shear rate, with a noticeable effect on concrete flow during casting.
Fig. 2- Steady state index during the transient periods right after the setting of a new shear rate value. (a) below critical shear rate. (b) above critical shear rate.
Fig. 3- Decreasing then increasing stress steps applied to the suspension near the critical stress. Shear rate jumps over several decades are observed for different applied stresses depending on the shear history.
Influence of Superplasticizers on the Flocculation Degree of Cement Suspensions 83
Here the influence of suspension solid volume fraction φ was studied by gradually decreasing the amount of water in the mix. Fig. 4 shows that an increase of solid volume fraction at a constant PCE superplasticizer dosage of 0.65% by weight of binder yields both an increase of apparent viscosity throughout the observed shear rate range and an increase in critical shear rate and stress, a sign of a faster ageing.
Ageing itself was studied as described above under a constant stress of 6.5 Pa. The results are plotted in Fig. 5, which shows for φ = 0.555 a rather slow viscosity increase through time, with a sharper slope towards 30 min corresponding to gelling. For φ = 0.580, the flow period before gelling is shorter while the upper two φ values yield gelling in less than a minute.
There seems to be a connection between the values of σc and γcand the ageing kinetics of
the suspension since they feature the same trend. It may then be stated that suspension ageing is responsible for the flow behaviour at low shear rates, the above parameters defining a boundary between homogeneous flow and heterogeneous or impossible flow. This contributes to showing how suspension ageing through hydration is closely connected to the low-shear-rate flow characteristics.
Steady flow regime: effective volume fraction flow curves, admixture robustness
As stated above, the flow regime is composed of a shear-thinning domain followed by a shear-thickening regime beyond a shear rate γst.
It seemed interesting to compare flow curves of both superplasticizers at equal work- ability, which is the purpose of Fig. 6. In terms of viscosity, there appears a crossover between the diphosphonate and the PCE technology at around 4 s-1, beyond which shear
thickening is more intense for the PCE. Fig. 7 corresponds to the same data transformed into the ratio of effective volume fraction to maximum packing fraction through the use of
Fig. 4- Influence of solid volume fraction on the shape of flow curves. Grout mix #1. PCE superplasticizer with a 0.65% dosage.
Eq. 6. It may be observed that the phosphonate technology indeed allows a lower structure
level of the suspension, thus a higher amount of deflocculation, beyond 10 s-1, i.e. for shear
rates corresponding to pumping or mixing. Interestingly, shear thickening is here inter- preted as an increase of structure with an increase of shear rate, i.e. shear is here considered as a flocculation mechanism for example through the formation of hydroclusters.12,13 Shear
thickening may interfere with operations at high shear rate, i.e. pumping, and these results show the advantage of a phosphonate technology over a PCE technology on this property. The tradeoff is a much higher dosage, thus a higher cost and a possible influence on the early strength.
Fig. 5- Ageing curves in rotational protocol as a function of solid volume fraction. PCE superplasticizer. 0.65% by weight of total binder.
Fig. 6- Comparison of flow curves at 5 min obtained with both technologies at equivalent workability
Influence of Superplasticizers on the Flocculation Degree of Cement Suspensions 85
Another important admixture property is its robustness to the water amount. Any inac- curacy in the weighing of water or the moisture measurement of aggregates may lead to a variation in initial workability. Water reduction at constant dosage was investigated, leading to an increase of the solid volume fraction φ. The data recorded at 5 min, expressed in terms of relative structure through Eq. 6, are shown in Fig. 8.
There is an obvious difference between the two technologies; while the phosphonate is able to keep a consistent structure level in the 0.1-10 s-1 range despite the increase in φ, the
PCE is unable to do the same and the structure degree increases with φ at all shear rates, leading to a loss of workability. It may then be concluded that the phosphonate admixture is more robust to water variations, the tradeoff being once again a much higher dosage.
Another interesting feature of the phosphonate experiments is that the shear thickening effect seems to intensify when φ increases, showing that this phenomenon depends on the solid volume fraction. This feature will be further investigated in the next section.
High shear limit: possible mechanisms at the origin of shear thickening
The first mechanism that may explain shear thickening is the rise of inertial forces16,17
and this was investigated here by following the approach used by Brown and Jaeger16 who
defined a suspension Reynolds number under the form:
Re min =ρ γ ηl d 2 (7)
ρl: liquid specific gravityd: particle diameterηmin: minimum viscosity defined on Fig. 1.
Even by taking the diameter of the largest particle (315 µm), at the highest shear rate investigated (200 s-1), the Reynolds number of the suspension is no higher than 5.10-3. This
shows that inertial effects may be neglected in the system.
The high shear rate limit was investigated on Grout #2 the advantage of which is to require almost the same dosage for both technologies (2.0% for the PCE, 2.5% for the
Fig. 7- Flow curves of Fig. 6 transformed according to Eq. 6. Grout mix #1. φ = 0.555.
phosphonate) at equal workability. This difference from Grout #1 comes primarily from a much lower water amount and the presence of fly ash in Grout #2. The phosphonate robustness may be stressed out again with a small dosage difference between grouts at the expense of a high overall dosage with respect to the PCE.
Grout #2 features a much more intense shear thickening regime than Grout #1 as shown in Fig. 9, though the phosphonate superplasticizer still seems to mitigate shear thickening when compared to the PCE. Indeed the apparent structure level computed from Eq. 6 on
Fig.9(b) shows obviously that the phosphonate induces a much lower degree of structure
in the range 10 – 200 s-1, though shear thickening still appears towards 100 s-1.
A final insight may be given about the possible origin of shear thickening. Though the vane geometry is not properly designed for such a purpose, normal force was measured during the experiments. Fig. 10 represents the evolution of the normal force exerted by the suspension onto the vane versus the recorded shear stress. A very sharp increase of normal force (up to 0.14 N) may be observed at high stress in the case of the PCE superplasticizer while for the phosphonate the normal force remains lower than 0.025 N in the same stress range.
It was previously shown16-19 that the rise of contact forces between particles, especially
of frictional nature, are often involved whenever such a coupling between shear and normal stresses is observed. This means that the occurrence of frictional regimes may be a cause of shear thickening in the observed suspensions.
If such is the case, it would mean that the phosphonate adsorbed layer is able to mitigate friction between particles while the PCE adsorbed layer is less able to do so, leading to a sharper rise of friction, normal stresses and shear viscosity. This would be obviously linked to the structure of the adsorbed layers, which would be dependent upon the polymer structure.
FURTHER RESEARCH
If the low shear rate regime may be well understood in the framework of suspension ageing, there still remains some work to better understand the underlying mechanisms of
Fig. 8- Relative structure levels of suspensions (Eq. 6) with a variable volume fraction
Influence of Superplasticizers on the Flocculation Degree of Cement Suspensions 87
the shear-thickening regime. If inertia may be ruled out, hydrocluster formation or the rise of contact or frictional forces at high shear rates may still be involved. Superplasticizer technology seems to have an influence on the phenomenon, but the exact molecular param- eters controlling the performance are still to be determined.
CONCLUSIONS
This paper is dedicated to seldom-studied features of the flow of cementitious mate- rials, beyond the often described shear thinning or Bingham behaviours. It was first shown that low-shear-rate regimes may lead to a discontinuous behaviour with a critical value of applied rate or stress below which no steady flow may occur. Previous work points at the influence of the ageing of the suspensions, through cement hydration, that interferes with
Fig. 9- Comparison between apparent viscosity and apparent suspension structure on both technologies. Grout mix #2.
Fig. 10- Normal force applied on the geometry as a function of stress. Grout mix #2.
shear. The diphosphonate and the PCE technologies both behave in a similar manner in this regime.
Beyond the main shear thinning regime, there appears a high shear rate regime of viscosity increase similar to what some authors define as continuous shear thickening.17,25
This is where the studied technologies differ, the phosphonate superplasticizer producing much lower viscosities in the 10-200 s-1 shear rate range. Elements from the literature and
some experimental facts allow arguing that the underlying mechanism is related to the way the adsorbed phosphonate polymers mitigate contact forces between particles in a more efficient manner than the PCE, at the expense of its water reducing ability, which seems noticeably lower.
AUTHOR BIOS
Dr Lucia Ferrari is the Physical Chemistry manager in the main research and devel- opment laboratory of CHRYSO in France. She received her PhD from the Technische Universität München (Germany) after she completed her PhD work with the EMPA in Dübendorf (Switzerland) under the supervision of Dr Frank Winnefeld and Pr. Dr. Johann Plank.
Dr Pascal Boustingorry is the Head of the Interface Physical Chemistry Team in the main research and development laboratory of CHRYSO in France. He received his PhD from the INP Grenoble along with the Ecole des Mines in Saint Etienne (France). Their main research interests are the interaction of organic molecules with cement suspensions and the links between superplasticizer chemical architecture and flow properties of building materials.
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Influence of Superplasticizers on the Flocculation Degree of Cement Suspensions 91
Superplasticizers are often used in conjunction with other additives and this can produce either an adverse or synergistic effect on rheology and setting properties of cementitious systems. These effects can be enhanced when temperatures are increased due to environ- mental changes or induced temperature as in hydrothermal curing. This research focuses on the compatibilities of different types of superplasticizer either sulfonated naphthalene or polycarboxylate based in combination with a lignosulphonate or hydroxycarboxylic acid type retarder.
Rheological measurements were made using a rotational viscometer at temperatures from 25°C (77°F) to 120°C (248°F) under pressure, and plastic viscosity and yield point determined based on the Bingham Plastic model though in almost all cases it was noted that the Power Law or more so the Herschel-Buckley model gives a better fit. Zeta poten-