HIGH MOLECULAR WEIGHT CELLULOSE ETHERS INFLUENCE
ON THE RHEOLOGICAL PROPERTIES OF FRESH MORTARS
Van Tien Phan1,* Huu Cuong Nguyen1, Dinh Quoc Phan1,
Ngoc Hung Le2
1Department of Civil Engineering - Vinh University, 182 Le Duan Str., Vinh, Nghe An
2 Université de Bordeaux, I2M, Bordeaux, France
This paper presents an experimental
study on the effect of three types of water-soluble high molecular weight cellulose
ethers on the
rheological properties of cement mortars in fresh state. Vane-cylinder test was used to determine the rheological
properties of fresh mortars. The flow curves, obtained in the tests, were
exploited to determine the rheological parameters, including the yield stress,
the consistency coefficient and the fluidity index. The results
indicate a difference between the mixes
crorresponding to the different cellulose ethers at high shear rate, while at
low shear rate, all the mortar mixes behaved as a shear thinning fluid. The investigation of the
influence of molecular weight on the properties of fresh mortars has shown a
similar observation to the reported research in literature. The yield stress of
the mortar decreases with the increase of molecular weight. This decrease is
not significant at low molecular weights, and becomes much more significant at
high molecular weights. Inversely, the mortar consistency is found to increase
with the increase of molecular weight.
Keywords: Rheological properties; Mortar; High molecular weight, Rheology test,
The term 'cellulose ether" refers to a wide range of commercial
products and differs in term of substituent, substitution level, molecular
weight (viscosity), and particle size. The most widespread cellulose ethers
used in dry mortars as admixtures are methyl cellulose (MC), methyl-hydroxyethyl
cellulose (MHEC) and methyl-hydroxypropyl cellulose (MHPC) .
According to their properties, cellulose ethers are used in various
industrial fields, including food industry, pharmaceutical industry, in paints
and adhesives, etc. They significantly modify the properties of materials even
if they are introduced in small amounts (0.02-0.7 % ). They are used to
control the viscosity of a medium, as thickeners or gelling agents. In mortar,
cellulose can be added before or during the mixing as thickening and water
retaining agents. However, the effect of cellulose ethers on the mortar in
fresh state was not fully studied . For example, there are few studies on
the effect of methyl-hydroxyethyl cellulose (MHEC) on the rheological behavior
of fresh mortar.
Cellulose ethers such as hydroxyethyl methyl cellulose (HEMC) is a common
admixture in factory made mortars for various applications including cement
spray plasters, tile adhesives, etc. The influence of HEMCs have been published
by many researchers in the case of various application fields, such as
biological macromolecules [3,4,5], carbohydrate polymers [6,7,8], etc. However,
there are few published studies concerning the influence of HEMCs on the fresh
state properties of cementitious materials including cement grouts [9,10],
cement-based mortars .
Patural et al.  had investigated the influence of cellulose ether on the
properties of mortars in fresh state, in which the molecular weight of polymers
is rather low (90-410 kDa). The effect of high molecular weight cellulose ether
hasn't been studied. Thus, it is interesting to deal with high molecular weight
cellulose ether in order to complement the effect of molecular weight of
cellulose ether on the properties of fresh mortars.
The influence of high molecular weight cellulose ether on the adhesive
properties of fresh mortars had been investigated, which indicated an important
role of molecular weight of cellulose ether on controlling the adhesion force,
the cohesion force and the interface adherence . In this paper, the
rheological properties of fresh mortar under the variation dosage of three type
of HEMCs will be examined.
2. MATERIALS AND
comprises a Portland cement (CEM I 52.5 N CE CP2 NF from Teil-France) and a
hydraulic lime (NHL 3.5Z). In order to minimize phase
separation, the standard sand CEN EN 196-1 ISO 679 has been used. In this
study, the effect of three types of high molecular weight cellulose
ethers have been
investigated. Typical characteristics of HEMCs are introduced in Table 1.
Table 1. Typical physical characteristic of three types of HEMCs
pH (2% solution)
(1) solution in water, Haake Rotovisko RV 100, shear rate
2.55 , 20°C
A certain dosage rate of a commercial air-entraining
agent, NANSA LSS, is used to guarantee moderate rheological properties within
the resolution range of the rheometer.
proportion of each constituent of the mortar is represented in Table 2.
Table 2. Mortar formulation
% wt. of dry mixture
content in the mortar formulation is varied according to the following
proportions: Ce =[0.19; 0.21; 0.23; 0.25; 0.27; 0.29; 0.31] % by weight. The
water dosage rate is fixed to 19% by weight for all the investigated samples. The
mortar composition corresponds actually to a basic version of commercially-available
render mortar .
characterizing the rheological properties of the fresh mortars, the rheometer
AR2000ex is equipped with 4-blade vane geometry. Vane geometry is appropriate
for high yield stress fluids such as dense granular suspensions, including
mortars , as
slippage can be avoided and the material can be sheared in volume.
The yield stress
is measured with the vane-cylinder geometry in stress controlled mode in which
a "ramp" of steps of increasing stress levels is applied to the vane
immersed in the material, and the shear rate is measured as a function of
applied stress. The yield stress is determined from the critical stress at
which the material starts to flow.
Depending on each
specific experiment, test will be performed at least three times to determine
the best possible experimental procedure. In the first run, the interval between two
successive steps must be chosen large enough to reduce the duration of the
test. The yield stress is determined, but with a low precision. For latter
runs, the measuring points must be increased around the determined yield point.
That would help to determine a high accuracy yield stress of the materials.
Figure 1. Typical
flow curves of mortar with the addition of 0.29% of polymer
Figure 2. Perform
the best fit of flow curves to Herschel-Bulkley models
A typical curve
obtained in the rheology test is presented in Figure 1. The yield stress is determined by
the critical stress at which we observe the transition from solid state to
liquid state of the material. However, in actual experiments, almost all cases,
the transition from solid to liquid state is occurred gradually and is hard to
detect. Therefore, it is difficult to determine the exact value of the yield
stress. So, different models have been developed in order to determine the
value of the yield stress as well as other rheological parameters by fitting
the flow curves’ data with the model’s equation. In this study, the most
general models for concentrated suspensions, Herschel-Bulkley’s, which is characterized
by the following equation, has been used:
coefficient K, the fluidity index n, and the yield stress τ0 are
three parameters characterize Herschel-Buckley fluids. The consistency K is a
simple constant of proportionality, while the flow index n measures the degree
to which the fluid is shear-thinning or shear-thickening.
In some cases the
use of Herschel-Bulkley model leads to non-physical values of the yield stress
(negative), this parameter is then determined by the applied stress at which we
obtained a finite shear rate (0.01 s–1). Figure 3 shows the best
fits of flow curves to Herschel-Bulkley models in the variation content of A,
in which m1=yield stress, m2= consistency coefficient, m3=fluidity index.
3.1. Evolution of the shear rate versus applied stresss
A comparison of
the loading curves corresponding to different polymer contents is presented in Figure 1, both in linear and in logarithmic
scale. A qualitative similarity of the rheological behaviour with the
increasing of polymer content has been observed. The flow curves indicate that
the mortar pastes behave as a shear thinning fluid with a yield stress.
evolution of the applied stresses as a function of recorded shear rate at some
given stresses and for different polymer contents of B, we can see that: At
certain stress, for instance 600 Pa, the
recorded shear rates are about 60 s-1 for 0.21%, and 500 s-1 for 0.25% and
0.29%. This indicates that for certain given applied stresses, the recorded
shear rates increases with the increase of polymer content. This observation is
inverse to that in case for mortars with polymer A. The crossover of the flow
curves indicates that the evolution of the apparent viscosity (stress divided
by shear rate) versus polymer content is dependent of the shear-rate interval
considered. This may be attributed to the different antagonistic effects of the
In case of C, the
mortar rheological behavior is close to that of a Bingham fluid. The mortars
are shear thinning at lower polymer content. However if we zoom in the flow
curves around low shear rates (see figure 3b) we can observe that the mortar
behave rather as Herschel-Bulkley shear-thinning fluids for all the dosages
rates. This change in the rheological behavior of mortar pastes at low and high
shear rates is represented by the evolution of the rheological parameters,
including yield stress, consistency and fluidity index, which will be discussed
in the following.
Figure 3. Flow curves comparisons with the variation of
of the rheological parameters
parameters are determined by performing the best fits of the loading curves
with the Herschel-Bulkley model. Figure 4-6 represents the
evolutions of the rheological parameters, including the yield stress, the
consistency coefficient and the fluidity index as a function of polymer
content, in case of different polymer type.
can be seen that an optimum is observed in the evolution of the yield stress
with the variation of polymer content. The yield stress reaches a minimum for a
content of 0.25%. The observation of such a minimum has already reported by
several authors concerning other types of mortars [9, 10]. This has been
attributed to the air-entraining effects of cellulosic ether polymers . In
fresh state, the air bubbles in the mortar may lead to an increase of the
resistance to flow initiation due to capillary forces. However, these bubbles
along with the lubrication effects of the polymer would decrease the resistance
to flow initiation due to decrease of granular contacts. These effects have
opposing impacts. The interplay between them would lead to the appearance of
minimum value in the resistance to flow initiation.
Figure 4. Evolutions of
the yield stress as a function of polymer content, in case of different polymer
In case of A, the consistency reaches a
maximum for a concentration of 0.25 %. In contrast of the yield stress, the
consistency increases slightly, reaching a maximum at 0.25%, followed by a
decrease of the consistency when increasing the polymer content. As discussed
above, the interplay between increasing of pore solution viscosity, lubricating
and air-entraining effects would lead to the decreasing of the viscous effects.
The presence of a maximum in the evolution of the consistency can be attributed
to the competition of the three effects, which lead to the increase or decrease
of the viscous effects.
Figure 5. Evolutions of
the consistency coefficient as a function of polymer content, in case of
different polymer types
two other cases of B and C, which have higher molecular weight, we observe a
monotonous increase of the consistency when increasing the polymer
concentration, reflecting the increase of the viscous drag effects with polymer
content. A similar observation concern the effect of another type of cellulose
ether polymer on mortar joints has been reported . B has much higher effect
on consistency than A. This is rather expected since the molecular weight of B
is higher, so its effect on the viscosity of the pore solution is higher due
more entanglement. A huge increase of the consistency can be observed when the
polymer content is above 0.23 %. They may correspond to a transition from
dilute/semi-dilute to concentrated regimes in the polymer pore solution.
evolution of the fluidity index is less significant. We observe a slightly
increase of the fluidity index when increasing the content of A. This is
followed by an approximate plateau and for a dosage rate of 0.27%, the fluidity
index decreases. This evolution of the fluidity index is similar to the
observation of A.Kaci et al.  in case of mortar joints with the variation
of another type of cellulose ether polymer. The evolution of fluidity index in
case of B indicates that the fluidity of the mortar is high at low content, and
significantly decreases to a small value at high polymer contents. We can
recognize two areas of the fluidity index of mortar as circled in Figure 6. At
low polymer contents, including 0.21 and 0.23 %, the mean value of the fluidity
index of is around 0.34, while it is around 0.21 at high polymer contents. It
means that the mortar becomes more shear-thinning with increasing polymer
content. The transition from high to low fluidity indexes coincides with that
of low to high consistency.
presence of a minimum value of fluidity index in case of C may result from the
competition between the shear-thinning character of the addition polymer and
the shear-thickening contribution of the granular phase in the suspension. In
addition some associative polymers are known to present shear-thickening at low
shear-rates, and this is probably the case here.
Figure 6. Evolutions of
the fluidity index as a function of polymer content, in case of different
of the molecular weight Mw
3.3.1. Influence of Mw on
the yield stress
effect of molecular weight on the yield stress of the mortar is highlighted in Figure
7. It can be seen that we observe an evolution with an optimum for a
concentration of 0.25 % independently of the molecular weight. As discussed in
the previous sections, several authors have reported the presence of such a
minimum and this has often been attributed to the air-entraining effects of
cellulose ether polymers. There is no
direct correlation between the depth of the minimum and the molecular weight.
Evolution of yield stress in shear for the variation of molecular weight
figure 7 shows the dependency of the yield stress on the molecular weight.
Increasing the molecular weight first leads to a slightly increase of the yield
stress to reach a maximum value, followed by decrease of the yield stress. These trends are observed for all the polymer
concentrations expect the highest one (0.29%). For this dosage, the maximum
transforms into a minimum.
Influence of Mw
on the consistency of the mortar
evolution of the consistency of the mortar pastes as a function of molecular
weight is represented in figure 8. We
can observe a significant increase of the consistency of mortar pastes when the
molecular weight increases. This dependence of the consistency of mortar on the
molecular weight is also in agreement with the results reported by L.Patural (2011)  on the effect of other types of
cellulose ethers on cement-based mortars. The increase of consistency with
molecular weight is not surprising since the viscosity of polymer solution make
up by the cellulose ether dissolved in the pore solution should increase with
Evolution of the consistency of mortar pastes as a function of molecular weight
The rheological properties of mortars in
the fresh state have been investigated by varying the content of three types of
hydroxyethyl methyl cellulose denominated A, B and C. These polymers differ
from each other mainly in their molecular weights.
At low shear rates, all the mortar
mixes behaved as a shear-thinning fluid. However at high shear rates, we
observed a difference between the mixes corresponding to the different
cellulose ethers. In case of A, the mortar pastes behave as shear-thinning
fluids for all investigated concentrations. In case of B, the rheological
behavior of mortar is shear thinning at low concentrations, while it behaves as
Bingham fluids at high contents. In case of C, the mortars behaved much like
Bingham fluids through the entire shear-rate interval investigated.
The investigation of the influence of
molecular weight on the properties of fresh mortars has shown a similar
observation to the reported research in literature. The yield stress of the
mortar decreases with the increase of molecular weight. This decrease is not
significant at low molecular weights, and becomes much more significant at high
molecular weights. Inversely, the mortar consistency is found to increase with
the increase of molecular weight.
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