Characterisation of rendering mortars by squeeze-flow and rotational rheometry
F.A. Cardoso a, V.M. John a, R.G. Pileggi a , P.F.G. Banfill b
a
Department of Construction Engineering, Escola Politécnica, University of São Paulo
05508 900, São Paulo, Brazil
b
School of the Built Environment, Heriot-Watt University
Edinburgh, EH14 4AS, United Kingdom
This paper reports the first experimental comparison between squeeze-flow at controlled
displacement rate and rotational shear at controlled speed for evaluating the rheological
behaviour of mortars. Several Brazilian and European rendering products showed a wide
range of workability behaviour in both testing modes. The flow curves and hysteresis
effects during the shear cycle varied significantly, since the mortar’s composition affects
structural breakdown, interfacial slip and phase segregation. The latter plays a critical
role during squeeze-flow and is the main reason for the higher loads required to deform the
samples at lower rates. A gravimetric-based methodology was developed to assess phase
segregation induced by rotational tests. Encouraging agreement between the methods was
observed, with yield stress in the structured (unsheared) state showing a good linear
correlation. For some mortars showing low segregation it was possible to compare shear
viscosity and extensional viscosity and Trouton ratios between 20 and 40 were obtained.
Keywords: Rheology (A), Mortar (E), Squeeze flow.

Corresponding author
e-mail address: rafael.pileggi@poli.usp.br (Rafael G. Pileggi)
phone number: +55 1130915442
1. INTRODUCTION
Inorganic mortars, besides being a fundamental portion of concretes, are also widely used
for bonding ceramic tiles, masonry and many rendering functions. Nowadays, rendering
products vary according to their use and/or placement method, with speciality mortars often
designed to meet more specific demands. The rheological requirements associated with the
various uses and processing combinations are, therefore, expected to be diverse as well.
Therefore proper rheological evaluation is crucial to providing useful information for the
optimisation of fresh mortar performance.
Rotational rheometry is capable of overcoming the limitations of the traditional single-point
methods, by evaluating concretes and mortars under a considerable range of shear rates
generated through the varying the rotation speed of the impeller or sample container [1-7].
The rheometer Viskomat (and its predecessor ViscoCorder), based on the latter setup with a
concentric stationary impeller on which torque is measured [1,2,7], has been extensively
applied to evaluate the flow behaviour of mortars and to determine the influence of mix
proportions [1,2,8-10], binder type [11], mineral and slag additions [2,10,12], sand
characteristics [13,14], admixtures [1,8,9,12,14-17] and fibres [18] on the Bingham
rheological parameters g and h (equation 1). In equation 1 T is torque, N is speed of
rotation and g and h are respectively proportional to yield stress and plastic viscosity [1-3].
T = g + hN
2
Equation 1
However, problems were reported regarding wall slip during evaluation of low workability
(stiff) mortars in the equipment [1,8] and, moreover, phase segregation can be an issue
when testing highly fluid mortars such as self-compacting compositions, which has led to
the use of a basket probe to avoid both slippage and segregation [7,14]. The development /
evolution of methods for concretes and mortars generally seeks more adequate
experimental conditions through the use of specific devices / geometries and testing
routines [3-6,19] aiming to fulfil the basic physical requirements of no slip or segregation
for the proper application of rheological models and/or to simulate flow situations of
scientific or technological interest.
In this sense, squeeze-flow testing – i.e. compression of a cylindrical sample between
parallel plates – which is widely used in food, pharmaceuticals and suspensions [20-23],
has been applied as an alternative / complementary technique to assess the flow behaviour
of building materials such as cement paste [24-27], lime paste [28], gypsum plaster [29],
and mortars for adhesives [30-32], rendering [33-36], masonry [37], extrusion [38] and
fibre-reinforcement [39,40]. Its geometry change during gap reduction makes the method
particularly interesting, as it creates flow conditions similar to those involved in processing
and application of pastes and mortars (for example, flow through a narrow nozzle during
pumping or spraying; spread over a surface and then finishing; squeezing between bricks;
extrusion of cement-based materials). Furthermore, properties of the plate(s) such as
roughness [24,36,37] and absorption [27,37], can be altered in order to better simulate
application of the material to different substrates. The method may also be considered for
3
the mortar fraction of concretes, which is squeezed locally between coarse aggregates
during fresh concrete flow.
The radial flow caused by a squeezing force or displacement occurs by shear and/or biaxial
extension (or elongation) of the material, depending on the geometric setup (the likelihood
of shear increases with the sample’s diameter / height ratio) and on the boundary conditions
at the material-plate interfaces: for a no-slip condition, pure shear occurs but, conversely,
for perfect slip (no friction), only the extensional flow occurs [20,22,36]. In practice, both
types generally happen, but depending on the experimental setup (d/h and plate roughness)
and on the lubrication characteristics of the material, one or the other can be predominant
[20-22,36].
For building materials, previous publications [24,26] have employed both squeeze-flow and
rotation to investigate pure and modified cement pastes, and the comparison of methods
indicated significant differences between the behaviours measured. However, no systematic
comparison between the methods on the mortar scale has been reported. Therefore, the
main goal of this work is to compare the rheological behaviour of rendering mortars
evaluated by squeeze-flow and rotational rheometry.
2. EXPERIMENTAL
2.1 Materials
Fourteen proprietary factory-produced mortars (Table 1) were investigated, including
Brazilian (G, H, K, P, S, Z and Alfa) and European products from Scandinavia, Portugal
4
and UK (Eur1, Eur3, Eur4, Eur5, Eur6, Eur7 and Eur16) [36] designed for different
rendering purposes or application methods such as: (i) general render with multi-coat
application (some also for masonry), (ii) one-coat render for external and/or internal areas,
(iii) final coat with smooth or scratched finish, (iv) renovation render of brick walls and (v)
final coat to receive dry dashing. Most products have both cement and lime as binders and
are recommended for manual application (manufacturers’ information) with exceptions
noted in Table 1. The products were delivered in bags and kept in airtight containers until
used.
Table 1 also shows aggregate (>75m) and matrix (<75m) contents (determined by
sieving, ASTM #200) and specific gravity (Helium pycnometry, Multi Pycnometer Quantachrome Instruments) of the dry mortar mixes, maximum aggregate size D99 (99% of
aggregates below this diameter as determined by laser diffraction using free fall dispersion
method, Malvern MSS Mastersizer) as well as specific surface area (N2 BET,
Micromeritics ASAP 2010) of the fines.
Table 1 - Mortar characteristics: type of rendering use, specific gravity (), aggregate and
matrix volumetric contents, matrix specific surface area (SSA), maximum aggregate size
(D99), water to solids weight percentage (w/s), fresh density at 15 minutes (Fresh), air
content at 15 (Air15) and 55 minutes (Air55) after mixing of the batches used for rotational
and squeeze tests.
2.2. Mixing procedure
Two-kilogram batches of the dry mortars were prepared in a five-litre capacity planetary
mixer (Hobart - N50) with the water content specified by the manufacturer’s information
5
on the bag (Table 1). The mixing procedure used consists of two steps, starting with the dry
mortar in the bowl and mixer at speed I: (1) continuous addition of half of the water content
in a constant rate (5g/s, during approximately 40s) followed by mixing until 60s; (2)
repetition of step 1, with the rest of the water. Total mixing time was 120s [36]. Distinct
batches were prepared for squeeze-flow and rotational experiments, and the times discussed
below are reckoned from the start of mixing.
2.3. Fresh density and air content
Apparent density was determined by gravimetric method with a 225ml container (Brazilian
standard ABNT NBR 13278) immediately after and 50min after mixing, respectively
ascribed as 15 and 55min values to facilitate association with the rheological results . In the
meantime the mortars remained at rest in the covered bowl. Air content was calculated
from the apparent density, water content and specific gravity.
2.4. Squeeze-flow
Squeeze-flow tests were conducted on a two-column universal testing machine (JJ Lloyd
Instruments - M5K) with a 6kN load cell. The bottom plate (200x150mm) was mounted
over the fixed compression base of the equipment and the top plate (diameter = 101mm)
fixed to the load cell at the crosshead. Plates were made of steel with smooth surfaces.
Immediately before testing, a cylindrical sample of fresh mortar (diameter = 101mm, height
= 10mm) [36] was cast over the clean and dry bottom plate using a plastic ring mould. The
squeeze-flow tests consisted of compressing the samples, at speeds of 0.1, 1 and 3mm/s, to
6
a maximum displacement of 9mm or maximum load of 1kN (whichever was reached first).
Squeeze-flow tests at the different speeds were performed consecutively on different
samples over a 10min period in the order 3, 1 and 0.1mm/s. The 15min tests were thus
complete by 25min after mixing and the 55min tests by 65min.
2.5. Rotational rheometry
The rheometer for pastes and mortars used (Viskomat NT - Schleibinger Testing Systems)
is a rotating cylindrical sample container plus a measuring head that monitors the torque
generated on the concentrically-fixed impeller due to the shear resistance of the material
flowing around its blades [1,2,7]. The details of the measuring system (Figure 1) are as
follows: sample volume ≈ 400ml; container diameter = 83mm; distance impeller - container
bottom ≈ 2mm; mortar impeller 3 (diameter 60mm) [11], distance blades 3 - container wall
= 13mm; mortar impeller 4 (diameter 49.6mm) [11], distance blades 4 - container wall =
18.2mm; the scraper was not used.
Fig. 1 Viskomat NT testing geometry (schematic).
Mortar impeller 4 (shorter blades) had to be utilized for the evaluation of stiff mortars K
and S due to torque overload with impeller 3, which was employed for all other products .
The correlation procedure is mentioned when appropriate.
7
The testing routine comprised a shear cycle (from 0 to 200rpm) applied through 15s steps at
50, 100, 150rpm and a 30s step at 200rpm, followed by a decrease to 150, 100, 50, 10, 0
rpm taking 180s in all. Samples were placed in the container up to the marked level a few
minutes before the tests, which were started 15, 35 and 55min after mixing started. There
was neither remixing nor reuse of specimens. Additional 35min-samples from the squeezeflow batches were also tested for control purposes.
2.6. Phase segregation tests
10 of the 14 products were subjected to the segregation tests based on gravimetric methods.
New batches of the mortars were prepared following the mixing procedure described in 2.2.
Segregation experiments consisted of three steps:
Step 1 – Rotation: Fresh mortar samples (400ml) were subjected to the same rotational
program used on Viskomat tests (2.5). The experiments were performed in the Poli-USP
rheometer [36] using a rotating container with the same dimensions as the Viskomat’s
container, but with no impeller shearing the sample.
Step 2 – Determination of water content: Immediately after the device stopped rotating, the
sample was divided in two parts (Border and Centre) using a metallic tool (internal
diameter = 56mm). Both portions were weighed before and after microwave drying
(Electrolux MEX55 - 1500W) for 20min.
Step 3 – Determination of particle size: Both dried mortar portions, Border and Centre,
were gently crushed by hand in a plastic bag and sieved. The material was sieved /
8
dispersed using a soft brush to break the agglomerates and to “clean" the aggregates from
the fine particles and then the samples were actually sieved with vibration for 1 hour. The
sieves used were: 2.8mm, 1.7mm, 1.0mm, 0.8mm, 0.6mm and 0.5mm.
3. RESULTS AND DISCUSSION
3.1. Squeeze-flow
Fig. 2 shows the squeeze-flow results of the mortars tested 15 min after mixing. The loaddisplacement curves show two distinct regions of behaviour: (i) viscous flow or plastic
deformation at low loads, with a roughly linear increase in deformation with small
increasing load, which undergoes a transition to (ii) strain hardening with a small increase
in deformation achieved in spite of a large increase in load [24,35,36]. The materials
presented a wide range of behaviour. High workability mortars flowed easily (G, Eur3,
Eur4 and Eur16), with a viscous flow stage at very low loads and extending for most of the
curves. The transition to the strain hardening stage occurred only at large displacements (>
6.5mm). The results obtained at different displacement rates were similar, with the curves
at 0.1mm/s in all cases shifted to higher loads than those at 3mm/s.
Fig. 2 - Squeeze-flow results of the mortars tested at 0.1 and 3mm/s, 15min after mixing.
9
Low workability or stiff mortars (K, P and S), on the other hand, were very difficult to
deform and the viscous flow stage was absent, with instead an intense strain hardening
stage, even at low strains. Curves at different displacement rates could be easily
differentiated. The behaviour of the mortars in the middle range of workability (H, Z, Alfa,
Eur1, Eur5, Eur6 and Eur7) varied in terms of the load level and extent to which the plastic
deformation / viscous flow stage occurred. Additionally, these mortars were influenced
more significantly by the displacement rate, with the loads at 3mm/s considerably lower
than those at 0.1mm/s, which resulted in an extended plastic deformation stage as well as
greater final displacement.
The strain hardening behaviour is associated with the friction between particles due to
geometry restriction or increase of solids concentration in the central region between the
plates [25,35,36,38]. The first occurs when the gap reaches a critical value near the
maximum particle size present in the material being tested, while the latter is related to
liquid-solid segregation. For granular suspensions and especially those like mortars that are
highly concentrated with macroscopic particles [25,35,36,38], radial migration of liquid
plays an important part in the rheological behaviour under squeeze-flow, as it causes an
increase of solids concentration in the central region of the sample and, consequently, of the
required loads. When the sample is squeezed at slow speeds the liquid has a long time to
flow outwards throughout the porous structure of packed particles (fines and aggregates),
and thus the likelihood of phase segregation is high. Conversely, when the sample is tested
at high displacement rates, the liquid may not have enough time to flow separately and the
chance of segregation is lower. The likelihood of segregation also increases as the
10
displacement increases and is inevitable at some stage if the gap is continuously reduced
[25,35,36,38] .
For mortars, it has not yet been established experimentally whether the fluid-solid
segregation is due to water bleeding through fine particles or to paste separating from
aggregate, because the experiments reported so far were based on visual observations of
drainage [38] and water content measurements [36]. Nevertheless, phase segregation
depends on mortar characteristics as well [25,35,36,38]. The ease of flow of liquid through
the sample depends on its viscosity and the permeability of the packed particles: low
viscosity and high permeability result in compositions prone to segregation. This was
probably the case of Brazilian products K, P and S, since their results indicated that even at
3mm/s phase segregation was occurring and flow was still difficult. In contrast, the high
workability mortars (G, Eur3, Eur4 and Eur16) showed a lower likelihood of segregation
(probably due to their high air content  20-30%, which enhances the paste’s cohesion) and
could be deformed up to high strains even at 0.1mm/s.
The products with intermediate behaviour (H, Z, Alfa, Eur1, Eur5, Eur6 and Eur7) flowed
much more easily at 3mm/s than at 0.1mm/s and, except for Eur1, exhibited substantially
increased final displacement. The intense dependence of the rheological behaviour of these
mortars on the displacement rate suggests that the critical rate (minimum speed to obtain
homogeneous flow for most of the test [25,38] for each material lay within of near the 0.13mm/s range.
11
It is also important to point out that, for each mortar, the plastic deformation (or viscous
flow) stage occurred at basically the same load level regardless of the displacement rate
applied, and the point where the curves started to differ from each other is probably at the
displacement where fluid migration begins to affect the flow at 0.1mm/s more significantly.
Fig. 3 illustrates the relationship between the maximum displacement achieved during
squeeze-flow tests and the maximum particle size of the aggregates. Fig. 3(a) shows a
reasonable correlation at 3mm/s if mortars K, P, S and Eur1 are excluded, suggesting that,
when segregation is absent or minimal the material behaves like a homogeneous fluid, and
the maximum displacement is inversely proportional to D99 (99% by volume of aggregates
smaller than this value) and the minimum gap tends to this value. At 0.1mm/s (Fig. 3(b)),
the only data points that remained in the same region of the linear behaviour at 3mm/s
(dashed line) were those of mortars G, Eur3, Eur4 and Eur16 (which had a low segregation
tendency), while the others were scattered.
Fig. 3 - Maximum displacement under squeeze-flow vs. maximum particle size (D99) of the
aggregates portion of the mortars tested 15min after mixing at (a) 3mm/s and (b) 0.1 mm/s.
Linear regression of data at 3mm/s excluding K, P, S and Eur1 values is shown in (a) and
displayed also in (b) as a dashed line just to guide the eyes.
3.2. Rotational rheometry
12
Raw rotational rheometry results (torque) for the mortars submitted to the speed profile
shown by the continuous black line at 15 and 55min after mixing are shown in Fig. 4 and
are presented as flow curves (torque vs speed) in Fig. 5. They group themselves into three
types of behaviour. In the first group, the highly flowable mortars G, Eur4 and Eur16 (air
content  20-30%) the torque remained below 50N.mm and the flow curve showed a
Bingham plastic response (Fig. 5). The behaviour did not change significantly from 15 to
55min, and little or no structural breakdown was observed, as shown by the close
coincidence of the up and down curves and nearly zero hysteresis area (Fig. 5(a)). The
Bingham model was applied to the down curves and the parameters g and h and the
correlation coefficient (R2) are shown in Table 2. h was around 4N.mm.s for G and Eur16
and 2N.mm.s for Eur4, while g remained in the 15-30N.mm range. Table 2 also presents
the torque at 10rpm (T10rpm) during the upcurve, which can be taken as a proxy for the
initial yield stress in an unsheared (fully structured) condition. For these mortars, g and
T10rpm were very similar since there was little or no structural breakdown.
Fig. 4 - Torque vs. time results of the mortars tested in the Viskomat 15 and 55min after
mixing.
In the second group, products Eur5, Eur6 and Eur7 were stiffer and displayed higher torque
levels (40-90N.mm). Eur7 showed little structural breakdown even after 55min because it is
a lime-based mortar. Neither did Eur6 at 15min (Fig. 5(a)) but, after 55min, the torque
levels of Eur6 increased significantly, and there was significant structural breakdown. Eur5
had the highest torque values and the greatest hysteresis area of this group. It is also
13
important to point out that the Eur5 curves (torque vs. time, Fig. 4) were the most scattered
of all results, probably due to its large maximum aggregate size (4mm) interfering with the
flow between the impeller and the container wall. The common feature of all Eur5, Eur6
and Eur7 curves is an increase of torque at 10rpm at the end of the tests, which can be
observed in both Figures 4 and 5, and seems to be a designed thixotropic behaviour, since
these three mortars are from the same producer. Because of this behaviour, the linear fitting
of the downcurves was difficult (resulting in some negative h values), and, for Eur6 and
Eur7 the linear fitting excluded the torque values at 10rpm (Table 2). Table 2 shows that for
these three mortars T10rpm is much higher than g, confirming the structural breakdown. g
was practically the same ( 40-50N.mm) for these materials (and about double that of G,
Eur4 and Eur16), while h was 4-5N.mm.s for Eur5 and 1-3N.mm.s for Eur6 and Eur7.
Fig. 5 - Flow curves showing torque vs. rotation speed for the mortars tested 15 min after
mixing. Torque values taken at the end of each speed step.
In Fig.4b and Fig.5b at 15min mortars Z and Eur3 behaved similarly to G, Eur4 and Eur16
with little structural breakdown and nearly zero hysteresis but at higher torque levels. At
55min Z showed some breakdown during the upcurve, but the down curves practically
coincide and consequently g and h values are very similar at 15 and 55min (Table 2). On
the other hand, at 55min Eur3 showed some signs of structural breakdown and, most
importantly, underwent the highest shift of torque over time. This behaviour can be
14
ascribed mainly to its substantial loss of entrained air, from 23.5 to 13% (Table 1). At
55min, Eur3 also showed an increase of torque at 10rpm (like Eur5-7) and this value had to
be eliminated from the fitting.
Mortars H and Eur1 (Fig. 4b) showed the typical breakdown pattern of an increase of
torque with rotation speed followed by its decrease during the dwell time at each speed [13] at both tested times. For Eur1 this pattern was more intense and occurred at higher
torques, especially at 55min. Hysteresis area (even at 15min) was large for both mortars
(Fig. 5b) and, although T10rpm was considerably higher (especially for H) than for the other
mortars, and g of both H and Eur1 was similar (40N.mm, Table 2), h of Eur1 (20N.mm.s)
was twice that of H, and both were considerably higher than h of the other materials (Table
2). Besides lying at the same torque levels of Eur1, the behaviour of mortar Alfa was the
reverse, as it presented only a little structural breakdown at the beginning of the test (small
hysteresis area), the highest g and T10rpm (Table 2) and low h (4N.mm.s at 15min).
Table 2 - Rotational parameters of the mortars tested 15 and 55min after mixing: T10rpm =
torque at 10rpm during accelerating curve; g = torque proportional to Bingham yield stress;
h = parameter proportional to Bingham plastic viscosity; R2 = coefficient of determination
of the linear regression (downcurves).
The final group were the stiff mortars K, P and S and their results are shown in Figures 4c
and 5c. Due to their stiffness, products K and S could not be tested using the mortar
impeller 3 because the torque overloaded the transducer and the instrument cut out. Hence,
15
these mortars were tested with impeller 4 (with shorter blades) and a calibration curve was
constructed from the ratio of the results of mortar P tested with both impellers at 35min. In
Figure 4c, an intense reduction of torque during the upcurve can be seen for all three
mortars (K, P and S) and, then, the torque remained almost constant for mortar P, while for
K and S it continued reducing slightly. As a consequence, flow curves (Fig. 5c) show a
significant drop during the upcurves and an almost horizontal line for the downcurves.
Such unusual behaviour may be a result of slippage at the mortar-container interface due to
their very low workability [1] and the smooth container surface. A second possibility is the
loss of contact between the blades and the material during the test. After the first few
revolutions of the container, a hole in the centre of the sample may be created if the
material is insufficiently fluid to flow back after being deformed. In these plastic mortars
with high yield stress the impeller could be underestimating the resistance to flow. Visual
confirmation of this possibility is hindered by the design of the instrument.
Thirdly the behaviour may be due to phase segregation, with the aggregates moving
radially out of the measuring zone. The fact that mortars K, P and S presented very low
displacements during squeeze-flow tests (Fig.2), which were characterized by a strainhardening behaviour caused by phase segregation (liquid phase migrates radially and
increases solids concentration in the central region) is consistent with this suggestion.
Furthermore, previous work on fibre cement composites reported that stiff compositions
with strain-hardening behaviour in a cone consistency test resulted in heterogeneous
products when cast by spinning [41].
16
Further investigation of the segregation in these stiff mortars under rotational testing is
described in the next section but it is clear that the results of K, P and S are compromised.
3.3 Phase segregation
The occurrence of phase segregation during rotational testing was assessed by simulating
the rotation program by gravimetric methods. Table 3 confirms that all mortars contained
more water in the centre than in the border of the sample, although the relative difference
was small (around 1%) for most of the products. Mortars P and Eur5 presented higher
values of approximately 3%, while mortar S, with almost 10%, showed the most fluid-solid
segregation.
Table 3 – Water content and difference in water content between the Centre and Border of
the samples subjected to rotational segregation tests.
As the container rotates a centripetal acceleration is generated and, for the configuration
with paddle 3, at 200rpm (maximum rotation speed) this acceleration varies from 7m/s2 at
the middle of the blades to 18m/s2 near the container wall. Depending on the viscosity of
the paste and the size of the aggregates, the latter moves radially outwards of the measuring
zone, hence increasing the water content in the centre.
17
Segregation as a function of particle size was small, but also took place as can be seen in
Table 4. Most of the mortars presented a slightly higher content of bigger particles in the
border (positive values), while the centre had more particles smaller than 0.6mm (negative
values). Mortars Eur5 and Eur6 displayed more significant differences of particles greater
than 1mm.
Table 4 – Particle content as a function of size and difference in particle content between
the Centre and Border of the samples subjected to rotational segregation tests.
Among the stiff mortars, S showed the most liquid-solid segregation, while P showed a
moderate difference in water content and K’s water content remained practically
homogeneous. The differences in particle size distribution between centre and border were
very low for all three products. Therefore, it seems that phase segregation cannot explain
the unusual rheological results for P, K and S, since a more significant segregation was
measured only for S. While segregation is certainly occurring for these mortars, it is not
clear whether these levels of segregation have any measurable influence on the rheological
behavior.
3.4. Comparison of rheological parameters
18
In order to compare the results of rotational and squeeze-flow testing it is necessary to use
fundamental units of stress and viscosity.
3.4.1. Yield stress
As already noted, the initial yield stress was taken from the torque at 10 rpm (T10rpm ) in the
upcurve. There are two reasons for this: (1) in the very beginning of the rotational test there
is no influence of phase segregation or slippage and (2) the squeeze-flow tests were
performed on unsheared specimens. Prior calibration of impeller 3 [11] gives the initial
yield stress 10rpm in Pa as
10rpm = 6.417 x 10rpm
Equation 2
The squeeze-flow yield stress assumed for the comparison was the normal stress (load /
area of the top plate) at 0.5mm of displacement 0.5mm) (5% strain) taken from the low
speed (0.1mm/s) tests.
The comparison of yield stress results in Fig. 6 indicates a coherent behaviour between the
values obtained by the different techniques. If the stiff mortars (K, P and S) are excluded
there is a reasonable linear correlation, which is an encouraging result from this, the first
comparative investigation of rotational and squeeze-flow rheometry. Further work to
resolve the approximately five-fold difference in yield stresses is justified. Factors that
could contribute to this difference are (i) the fact that the calibration of the Viskomat is
based on the behaviour of the flowing material, leading to a yield stress obtained by
extrapolation to zero shear rate [1], whereas initial yield stress is a static measurement, and
19
(ii) the state of the structure existing in the mortar at 10rpm in the Viskomat and at 0.5mm
squeeze-flow displacement may not be identical, particularly since structure is very
sensitive to shear history in the early stages of shearing [3].
Fig. 6 - Shear stress at 10rpm during accelerating curve (10rpmvs. squeeze stress at 0.5mm
and 0.1mm/s 0.5mm). Linear regression: K, P and S (green); other mortars tested at 15
(blue), 55 (red) and 15 + 55min (black).
3.4.2. Viscosity
Comparison of viscosity results is a more complex issue because rotational rheometry
estimates shear viscosity but during squeeze-flow both extensional and shear strains occur
and which is dominant depends on the interfacial slip conditions. In the present work, the
boundary condition was defined as a perfect-slip interface owing to the use of smooth
metallic plates, so a biaxial extensional model was assumed [21,35]. However, as the
rheological models require that the fluid remains homogeneous during the tests, extensional
viscosity curves were calculated as follows, for only those mortars that displayed low
segregation – G, Eur3, Eur4 and Eur16. The biaxial extensional strain rate  B  is equal to
one-half the vertical Hencky strain rate H  [21,35]:
 B 
20
 H
v

2 2h 
Equation (3)
where: h is the instantaneous height of the sample, v is the displacement velocity of the
top plate. The extensional viscosity  B  is defined as the ratio between the biaxial
extensional stress  B  , which is the squeezing load divided by the top plate area, and the
extensional strain rate:
B 
B
 h  vt
 2L 0 2 
 B
 vR 
Equation (4)
where: L is the load, h0 is the initial height and R is the radius of the sample [21,35].
Fig. 7 shows biaxial extensional viscosity curves as a function of the extensional strain rate,
for the mortars tested both at 0.1 and 3mm/s. Viscosity values increased when the strain
rate increased as a result of gap reduction for all the tested samples, however after a
compaction phase that occurs at the beginning of each curve a plateau value is reached after
which the viscosity starts to increase significantly as strain-hardening starts The plateau
value is taken as the biaxial extensional viscosity and varies by almost two orders of
magnitude, indicating that extensional viscosity of mortar is very sensitive to the
displacement rate used in the test.
Fig. 7 - Biaxial extensional viscosity B) vs. biaxial extensional strain rate °B) curves of
mortars G, Eur3, Eur4 and Eur16 tested 15min after mixing. Curves showing the variation
of biaxial extensional strain rate °B) with displacement at each speed are also shown.
21
Because mortar is non-Newtonian comparisons of viscosity between different techniques
must be done at the same strain rate. From equation 3 and Fig. 7 the extensional strain rate
at 5mm of displacement is 0.01 s-1 at 0.1mm/s and 0.3 s-1 at 3mm/s. The apparent viscosity
app of a Bingham material under rotational shear at a shear rate  is given by:
 app 
  0  



Equation 4
where  0 and  are the yield stress and plastic visosity, respectively. To ensure a similar
degree of structural breakdown in each technique, the torque value at 10rpm during the
upcurve (T10rpm) is used. The corresponding yield stress is given by [1]:
 0  K G  T10rpm
where K
Equation 5
 6.42 by calibration of the Viskomat [11]. The plastic viscosity, obtained from
G
h, the slope of the downcurve, is given by [1]:
  1G  h
where 1
G
Equation 6
 0.709 by calibration [11]. Using equations 4, 5 and 6, the apparent viscosity
under rotational shear can be calculated at the two shear strain rates of 0.01 and 0.3 s-1.
Table 5 summarises the resulting shear and extensional viscosities.
22
Table 5 – Comparison of viscosities at strain rates of 0.01 and 0.3s-1. Shear: apparent
viscosity at 0.066rpm (app0.066) and 1.98 rpm (app1.98). Biaxial extension: viscosity at
displacement of 5mm and velocities of 0.1mm/s B0.1) and 3mm/s B3).
The clear points to emerge from Table 5 are that the two measurement techniques place the
viscosity of the four mortars in the same rank order, that viscosity decreases with increasing
shear rate and that extensional viscosity is 20-40 times the shear viscosity. These are further
encouraging results from this comparison of rotational and squeeze-flow rheometry. The
ratio between extensional and shear viscosity, the Trouton ratio, for a Newtonian liquid in
biaxial extension is 6 [42], while for non-Newtonian liquids, such as polymer solutions,
which are prone to entanglement of chains and show significant elasticity, values up to
3000 have been reported. The results in Table 5 are consistent with mortar being a granular
but relatively inelastic material.
4. CONCLUSIONS
The first systematic comparison of squeeze-flow and rotational rheometry (Viskomat) for
evaluation of rendering mortars with a wide range of workability levels has been
performed. The products presented three main groups of behaviour when tested by squeezeflow. The first group included mortars that could be easily deformed up to large
displacements with low loads both at slow and fast displacement rates, without phase
segregation. On the other hand, a group of stiff mortars were very difficult to squeeze (even
at faster rates) and required high loads to achieve only small deformations, mainly as a
result of phase segregation in the test. The intermediate group was mainly composed of
materials that were sensitive to the rate of squeeze-flow displacement: at higher rates the
loads were lower and consequently larger maximum displacements were achieved, while at
23
lower rates phase segregation had time to occur and mortar flow was more difficult.
Besides the occurrence of segregation, which also depends on mortar characteristics such as
paste viscosity and aggregate packing permeability, the results showed that the maximum
particle size in the material limits the maximum displacement (or minimum gap) that can be
achieved.
Rotational test results not only confirmed the wide range of torque levels of the materials
tested, but also pointed out differences in measured rheological parameters (plastic
viscosity and yield stress) and structural breakdown. In general, the results agreed between
the two test methods, which were able to arrange the materials in the same rank order.
Yield stresses in squeeze-flow were about five times those in rotation, while extensional
viscosities were 20-40 times the shear viscosities.
The stiff mortars were difficult to evaluate properly by rotational rheometry, resulting in
unusual flow curves, due either to wall slippage (on the mortar-container interfaces) or to
loss of contact between mortar and impeller blades. While low to moderate liquid-solid and
particle size segregation took place during rotational testing, this does not seem to be the
main cause of the behaviour of the stiff mortars (K, P and S). Nevertheless, an experimental
methodology for the evaluation (both identification of type and quantification) of the
segregation phenomenon was successfully developed and can be used as complementary
technique to provide a better understanding of the rheological results of concentrated
suspensions containing macroscopic particles.
ACKNOWLEDGEMENTS
The authors would like to thank: the Brazilian agencies FAPESP and CNPq for funding;
Fábio Campora (ABAI – Brazilian Association of Mortar Industries), CONSITRA
(Brazilian Consortium for Innovation of Rendering Mortars Technology), Votomassa and
Cimpor Brasil for support; Dr. Paul Houang and Weber Saint-Gobain (Brazil and Europe)
24
for major assistance with European products; Andy Cowland (CPI Euromix) for mortar
samples; and at Heriot-Watt University, Dr. Gerry Starrs for rheometer instructions and
James Maguire for laboratory assistance.
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strength of rendering mortars. Doctoral thesis, Department of Construction Engineering,
University of São Paulo, São Paulo, 2005 (in Portuguese).
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different squeezing rates, Cem. Concr. Res., 39, 748-753, 2009.
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distribution and rheological behaviour, Doctoral thesis, Department of Construction
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Mech. 137 (2006) 15-23.
28
Figures
Fig. 1 Viskomat NT testing geometry (schematic)
29
1000
(a)
K_3mm/s
800
Load (N)
K_0.1mm/s
H_0.1
H_3
600
Z_0.1
400
200
Z_3
G_0.1
G_3
0
1000 0
1
2
(b)
800
Load (N)
P_0.1mm/s
600
3
4
5
6
7
8
9
Displacement
Eur1_0.1 (mm)
Alfa_0.1
Eur1_3
P_3mm/s
Eur3_3
400
Alfa_3
200
Eur16_0.1
Eur3_0.1
Eur16_3
0
1000 0
1
2
3
4
5
6
7
8
Eur6_3
Displacement (mm)
S_3mm/s
(c)
9
800
Eur4_0.1
Load (N)
S_0.1mm/s
Eur6_0.1
600
400
Eur7_0.1 Eur5_3
Eur5_0.1
200
Eur7_3
Eur4_3
0
0
1
2
3
4
5
6
7
8
9
Displacement (mm)
Fig. 2 - Squeeze-flow results of the mortars tested at 0.1 and 3mm/s, 15min after mixing.
30
Maximum displacement (mm)
10
Eur16
Eur7
8
Alfa
G
H
Eur4
Z
Eur3
Eur6
Eur5
6
Eur1
4
y = -0.6802x + 9.7358
R² = 0.605
P
K
S
2
(a) 3mm/s
Maximum displacement (mm)
0
10 0
1
2
4
G
Eur16
8
3
D90 (mm)
Eur3
Eur4
Z
Eur7
6
H
4
Alfa
Eur6
Eur1
Eur5
P
K
2
S
(b) 0.1mm/s
0
0
1
2
3
4
D99 (mm)
Fig. 3 - Maximum displacement under squeeze-flow vs. maximum particle size (D99) of the
aggregates portion of the mortars tested 15min after mixing at (a) 3mm/s and (b) 0.1 mm/s.
Linear regression of data at 3mm/s excluding K, P, S and Eur1 values is shown in (a) and
displayed also in (b) as a dashed line just to guide the eyes.
31
G_55min
Eur16_55
Eur6_55
90
G_15min
Eur16_15
Eur6_15
Eur4_55
Eur5_55
Eur7_55
Eur4_15
Eur5_15
Eur7_15
(a)
150
50
100
Speed (rev/min)
Torque (N.mm)
200
50
10
0
H_55min
Alfa_55
Eur3_55
1H_15min
Z_552
Time (min)
Alfa_15
Eur1_55
Eur3_15
Z_15
3
Eur1_15
(b)
200
150
90
100
50
50
10
0
290 0
K_55min1
K_15min
Time
(min)
S_55
S_15
2
P_15
Speed
3
200
250
Torque (N.mm)
(c)
210
150
170
100
130
90
Speed (rev/min)
Torque (N.mm)
130
Speed (rev/min)
0
170
50
50
10
0
0
1
Time (min)
2
3
Fig. 4 - Torque vs. time results of the mortars tested in the Viskomat 15 and 55min after
mixing.
32
Torque (N.mm)
90
G
Eur5
50
H
Torque (N.mm)
Eur16
Eur7
(a)
50
10
170 0
100
Z
150
Speed
Eur1(rev/min)
Eur3
200
Alfa
(b)
130
90
50
10
290 0
50
100
KSpeed (rev/min)
P
250
Torque (N.mm)
Eur4
Eur6
150
S
200
(c)
210
170
130
90
50
10
0
50
100
150
200
Speed (rev/min)
Fig. 5 – Flow curves showing torque vs. rotation speed for the mortars tested 15 min after
mixing. Torque values taken at the end of each speed step.
33
2.8
S
10rpm (kPa)
2.4
y = 0.12x + 1.19
R² = 0.96
2.0
K
1.6
S
P
1.2
y = 0.20x
R² = 0.80
y = 0.21x
R² = 0.92
0.8
KP S
15min
55min
y = 0.19x
R² = 0.63
0.4
K
Linear (15+55min)
0.0
0
2
4
6
 0.5mm (kPa)
8
10
12
Fig. 6 - Shear stress at 10rpm during accelerating curve (10rpmvs. slow speed (0.1mm/s)
squeeze stress at 0.5mm 0.5mm). Linear regression: K, P and S at 15 and 55min (green);
other mortars tested at 15 (blue), 55 (red) and 15 + 55min (black).
34
1.E+07
9
7
1.E+06
6
 B (Pa.s)
G
Eur3
Eur4
Eur16
1.E+05
5
4
3
1.E+04
2
0.1mm/s
3mm/s
1.E+03
1.E-03
Displacement (mm)
8
1
0
1.E-02
.
 B (s-1)
1.E-01
1.E+00
Fig. 7 - Biaxial extensional viscosity B) vs. biaxial extensional strain rate °B) curves of
mortars G, Eur3, Eur4 and Eur16 tested 15min after mixing. Curves showing the variation
of biaxial extensional strain rate °B) with displacement at each speed are also shown.
35
Tables
Table 1 - Mortar characteristics: type of rendering use, specific gravity (), aggregate and
matrix volumetric contents, matrix specific surface area (SSA), maximum aggregate size
(D99), water to solids weight percentage (w/s), fresh density at 15 minutes (Fresh), air
content at 15 (Air15) and 55 minutes (Air55) after mixing of the batches used for rotational
and squeeze tests.
Rendering
Mortar
use
G
generalc
H
one coat d
general
K
P
generalc
S
generalc
one coat
Z
one coat
Alfa
internal
Eur1
Eur3
one coat e
Eur4
generald
Eur5 scratched
Eur6 renovationf
Eur7
smoothg
Eur16 dash receiv. e
c
Anhydrous characteristics

AgregMatrix SSA D99
3
(g/cm ) (%v)
2.68
2.85
2.47
2.69
2.72
2.79
2.65
2.67
2.66
2.68
2.74
2.60
2.61
2.62
80.2
60.2
71.6
66.9
83.9
76.0
73.3
73.1
64.2
71.1
76.0
78.6
68.5
85.7
Rotational rheometry Squeeze-flow
w/s   Fresh Air15 Air55 Air15 Air55
(%v) (m2/g) (mm) (%wt) (g/cm3)
(%)
(%)
(%)
(%)
19.8
39.8
28.4
33.1
16.1
24.0
26.7
26.9
35.8
28.9
24.0
21.4
31.5
14.3
23.6
12.2
1.4
8.2
8.6
22.7
12.5
11.8
23.5
21.6
6.7
14.0
11.0
31.6
21.5
11.6
0.1
7.5
20.7
10.5
8.9
13.0
-
22.8
12.8
1.4
9.7
8.2
24.4
13.6
19.1
18.1
5.5
11.2
7.6
30.9
18.9
0.5
9.0
12.7
16.8
4.3
8.9
5.3
27.4
2.5
1.6
5.1
4.1
4.0
1.1
1.8
2.0
1.1
1.7
3.8
2.7
4.6
-
2.2
1.4
0.6
0.8
2.0
1.9
2.7
1.8
3.0
2.7
4.0
2.7
1.2
0.6
15.0
16.5
18.0
16.0
17.0
17.2
15.0
16.0
21.0
18.0
16.0
18.0
24.0
20.0
also used for masonry; d also for spray application;
lime-based mortar; g lime-based mortar.
36
e
1.68
1.99
1.99
2.00
1.99
1.71
1.91
1.91
1.58
1.67
2.06
1.80
1.77
1.43
for spray application; f hydraulic
Table 2 - Rotational parameters of the mortars tested 15 and 55min after mixing: T10rpm =
torque at 10rpm during upcurve; g = torque proportional to Bingham yield stress; h =
parameter proportional to Bingham plastic viscosity; R2 = coefficient of determination of
the linear regression (downcurves).
55min
15min
Parameter
T10rpm (N.mm)
g (N.mm)
h (N.mm.s)
R2
T10rpm (N.mm)
g (N.mm)
h (N.mm.s)
R2
G
Eur4 Eur16 Eur5 Eur6 Eur7
Eur1 Eur3 Alfa
K
P
S
28
27
26
20
5.3 1.8
0.98 0.99
17
16
4.6
0.98
83
63
67 123 67
85
54 130 271 214 282
37
39
43
41
39
42
42
80
31
70
49
4.0 1.8 1.3 10.3 5.9 20.1 8.1 3.4 2.5 2.8 1.3
0.82 0.83 0.97 0.99 0.97 0.99 0.98 0.88 0.45 0.88 0.42
33
33
29
22
3.8 1.6
0.99 0.95
20
19
4.3
0.99
109 129 84 137 90 136 124 172 357
49
48
49
48
44
48
94
78
36
5.0 3.6 1.5 9.4 4.4 23.7 5.7 7.3 2.4
0.70 0.92 0.90 0.95 0.93 0.99 0.86 0.98 0.51
Obtained by linear fitting excluding torque at 10rpm
37
Mortar
H
Z
-
405
78
0.1
0.01
Table 3 – Water content and difference in water content between the Centre and Border of
the samples subjected to rotational segregation tests.
Water content (%wt)
Mortar
h
38
Difference (%)
Nominal Border Centre Total Absoluteh Relativei
G
13.04
12.70 12.85 12.75
0.15
1.17
K
P
S
Z
Eur3
Eur4
Eur5
Eur6
Eur7
15.25
13.79
14.53
14.68
17.36
15.25
13.79
15.25
19.35
14.86
13.51
13.65
14.36
17.01
14.98
13.06
14.89
19.03
0.12
0.43
1.41
0.28
0.11
0.18
0.36
0.22
0.21
0.81
3.16
9.83
1.94
0.67
1.18
2.72
1.45
1.07
14.98
13.94
15.06
14.64
17.13
15.16
13.42
15.11
19.23
14.91
13.71
14.35
14.50
17.07
15.06
13.23
15.00
19.12
Centre-Border; i 100*(Centre-Border)/Total
Table 4 – Particle content as a function of size and difference in particle content between
the Centre and Border of the samples subjected to rotational segregation tests.
Particle content (%wt)
Total
Absolute difference j
Mortar
Size (mm)
Size (mm)
<0.6 0.6-1.0 >1.0 <0.6 0.6-1.0 >1.0
G
73.4
22.6
3.9
0.12 -0.14
0.02
k
l
m
l
98.5
1.4
0.1 -0.045k 0.04
0.005m
K
P
97.9
2.1
0.0
0.10 -0.10
0.00
S
92.3
4.9
2.8
-0.20
0.07
0.13
Z
92.5
6.0
1.5
-0.12
0.14
-0.02
Eur3 78.8
9.1
12.1 -0.58
0.47
0.10
Eur4 87.8
8.1
4.1
-0.57
0.13
0.44
Eur5 55.5
11.1
33.4 -1.22
0.00
1.22
Eur6 69.6
15.1
15.2 -0.93 -0.70
1.63
Eur7 93.8
6.0
0.2
-0.30
0.32
-0.02
39
j
Border particle content-Centre particle content
k
<0.5mm; l 0.5-0.6mm; m >0.6mm
Table 5 – Comparison of viscosities at strain rates of 0.01 and 0.3s-1. Shear apparent
viscosity at 0.066rpm (app0.066) and 1.98 rpm (app1.98). Biaxial extension: viscosity at
displacement of 5mm and velocities of 0.1mm/s B0.1) and 3mm/s B3).
Mortar
G
Eur3
Eur4
Eur16
Shear
η app0.066 η app1.98
0

Pa
Pa.s
kPa.s
180
347
173
109
3.8
5.7
1.3
3.3
18
35
17
11
Extensional
η B0.1
η B3
Comparison
0.01s-1
0.3s-1
kPa.s
kPa.s
kPa.s
η B/η app
η B/η app
0.6
1.2
0.6
0.4
460
1177
458
430
12
30
12
10
25.6
33.9
26.5
39.4
19.9
25.8
20.8
27.3