Influence of mixing process on mortars rheological behavior through rotational rheometry

Construction and Building Materials 223 (2019) 81–90

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Influence of mixing process on mortars rheological behavior through rotational rheometry Marylinda Santos de França a,⇑, Bogdan Cazacliu b, Fábio Alonso Cardoso a, Rafael Giuliano Pileggi a a b

Escola Politécnica da Universidade de São Paulo, Brazil IFSTTAR, LUNAM, France

h i g h l i g h t s
Mixing process quality affects mortar’s rheological and mechanical behavior.
Bad mixing conditions consist in either short mixing times or slow water loading.
Thixotropic behavior decreases in function of mixing time.
Mortar mechanical properties are mainly influenced by shortest mixing times.

a r t i c l e

i n f o

Article history: Received 3 October 2018 Received in revised form 25 June 2019 Accepted 26 June 2019

Keywords: Mixing behavior Mortar rheology Mechanical performance

a b s t r a c t Mixing is one of the mortar processing steps (the others being proportioning, transport and application) that, due to its apparent operational simplicity, has been somewhat undermined. However, the mixing process quality has direct influence on the mortar’s rheological behavior and its hardened state properties. In this context, this study’s objective is to assess the influence of two mixing parameters (time and water addition rate) on the characteristics of the produced mortar. Both mixing and flow measurements were performed in a rotational rheometer developed at POLI/USP-Brazil. In this equipment, the mixing strain rate is given as a function of torque evolution. Mortar tensile strength, modulus of elasticity and volumetric porosity were evaluated in its hardened state. ‘‘Bad mixing” conditions can arise from too short mixing times or too slow water loading. These bad mixing conditions led to mortars with lower mechanical strength and more unfavorable rheology, meaning high thixotropy and flow curve loops with big hysteretic area. When admixture was employed, mixing resulted in a mortar with higher entrained air leading to negative effects on the mechanical properties of the hardened material. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The preparation of multiphase cementitious systems involves mixing solid constituents with water and sometimes with admixture. Addition of liquid leads to agglomeration events in the system which results in greater effort during mixing. Therefore, depending on how it is performed, mixing can result in systems well homogenized and dispersed. As such, mixing is a fundamental step to produce mortars and concretes. It is the starting point for the consolidation and microstructural development of these products, and as such it directly impacts their fresh and hardened properties. Indeed, changing the mixing process affects the microstructural arrangement of cement pastes [1] and agglomerates initially pre⇑ Corresponding author at: Department of Construction Engineering, Escola Politécnica, University of São Paulo, 05508 900 São Paulo, Brazil. E-mail address: [email protected] (M.S. de França). https://doi.org/10.1016/j.conbuildmat.2019.06.213 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

sent in cement powder can later remain in the paste due to insufficient mixing [2]. The more agglomerates remain in a paste after mixing the less efficient a mixer is [3]. Besides that, for concretes, compressive strength evolution shows that mean values increase with mixing time [4]. Previous studies evaluated parameters that have influence on mixing procedure like mixing sequence [5–10], mixing time [3,4,11–15] and mixer type [1,16,17]. When evaluating refractory concretes, the authors [5,6] showed that adding water in steps is a simple solution to increase the overall process efficiency. The power consumption usually associated with progressive addition of liquid can be reduced by adding liquid in several stages [8]. However, it was observed that it is better to add binder to water when producing lime grout, and that a slow mix velocity and slow liquid addition enhance grout sedimentation and heterogeneity [7]. Furthermore, the mixing sequence effect on cement admixture interactions varies with cement composition, dosage and at which

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time the admixture is added [18]. Laboratory changes in mixing procedures [19] and in the way water is added [20,21] improved mortar dispersion as well as its rheological behavior. High performance concrete (HPC) [22] and ultra-high performance and selfconsolidated concrete (UHPSCC) [13,23,24] were also evaluated to measure their sensibility to the mixing procedure. According to Schiebl et al. [25], mixing time depends on both composition and mixing speed. They have characterized mixing time in three phases (dispersion, optimum and over mixing) using power versus mixing time curves. A high mixing speed can be used to reduce mixing time, but excessive speed can lead to over mixing [3]. Furthermore, mixing time is considered a key process parameter in many dispersion and homogenization processes. It is also dependent on impeller type and operating conditions [12]. Finally, prolonged agitation can also increase the risk of mixture instability in self-consolidating mortars [26] besides promoting fluidity and setting deterioration of grouting mortars [15]. Mixer type and mixing time were evaluated using industrial mortars mixed in two different types of mixers [17]: the typical 120 l drum mixer and the horizontal 60 l blade mixer. For this reason the least efficient mixer had its mixing time increased in order to achieve the required homogenization [17]. Likewise, the mixer type and size can also have great influence on the rheological behavior of concretes [4,16,27]. According to Wallevik and Wallevik [16], mixing concrete in drum mixers with different capacities, (Gustav Eirich 50 l and 150 l), the superplasticizer dosage has to be doubled in order to maintain the self-consolidated concrete (SCC) properties. Therefore, in a planetary mixer the mixing difficulty for a given mix-design is reduced with increased mixer capacity [4]. Additionally, four different types of mixers such as pan, Eirich, drum and ribbon were employed and it was found that the compressive strength and horizontal slump obtained through Eirich mixer is higher when compared to the other mixers [28]. The mixing procedure effect on cementitious grouts was studied employing two mixers (a mechanical and ultrasonic mixer). The ultrasonic mixer improved dispersion and reduced water content compared to high turbulence mixing [29]. In the end, the grouts apparent viscosity was reduced by 50–60% and the yield stress was reduced by more than four times due to ultrasonic mixing. Methods to control particles homogenization and dispersion in mixtures are in general incipient. One classical way to define mixing efficiency is to measure how evenly distributed all constituents are in the mixer vessel. As fine particles tend to agglomerate when in contact with water due to capillary forces or other surface forces, one more relevant way to define mixing efficiency is to measure how agglomerates are breakdown for an effective homogenization and dispersion of fine particles [6,30]. More recent developments allow workability measurements based on power acquisition at several mixing speeds, be it a fixed [23] or truck mounted [27] mixer, and drag force measurements of a particular blade immersed into concrete. In all these applications, mixing power level is most frequently linked to a given mix-design and water proportioning deviations. Variations in other constituents physical features also influence mixing power. In addition, in-line rheology estimation is by itself an aspect of first interest. Even though many studies deal with the mixing of cementitious materials, there are few research works discussing how mixing processes influence the rheological behavior of produced mortars. In this context the current work evaluates mortar mixing behavior (with and without dispersive admixture) through rotational rheometry regarding two main parameters: mixing time and water addition rate. Furthermore, in order to provide a better description of the dispersion final state in the mixture, the following mechanical properties of each produced mortar are also discussed: tensile strength, modulus of elasticity and volumetric porosity.

2. Experimental program 2.1. Materials A reference mortar named ‘‘REF” was formulated using Brazilian raw materials – Portland cement with filler addition (CP-IIF), limestone filler, hydrated lime (CHIII type) and crushed limestone sand (>1.4 mm). The volume proportion of these constituents, the specific surface area measured by Brunauer–Emmett–Teller (BET) technique and density measured by pycnometer are given in Table 1 while the cement and lime chemical composition are presented in Table 2. Filler and sand (97,88% of CaCO3 as determined by thermogravimetry analysis) both came from the same lime stone rock just ground to different particle size distribution. The particle size distribution of raw materials determined by laser diffraction granulometry is shown in Fig. 1. The water vs. solids mass ratio was 0.16. A second composition named ‘‘DIS”, with same granular skeleton and water vs. solids ratio, was studied by adding powder sodium polycarboxylate dispersing admixture MELFLUX with a 0.0125% weight to other solids ratio (polymer to cement weight ratio of 0.08%) in the REF mixture.

2.2. Equipment The rotational rheometer for mortars developed at POLI-USP (Brazil) is presented in Fig. 2a. This equipment is able to evaluate mixing processes and rheological behavior. It performs speed-controlled tests while measuring the electric engine power consumption. This measurement is converted into torque values using a calibration constant. The equipment performs mortars mixing and rheological characterization using the same impeller. Its vessel has a diameter of 214 mm and was filled with about 2.2 L of mortar (filling volume estimated after mixing). The impeller device for mixing and rheological tests has six blades in spiral configuration. The six blades are 135 mm in length and 15.7 mm in diameter, Fig. 2b. For the purpose of this work the impeller turns with an angular velocity that was adjusted to match the different stages of the test. Its vertical axes, off centered by 39 mm from the vessel center, performs a second slow rotational movement (planetary movement). The corresponding gap between impeller device and vessel (bottom and wall) is 1 mm wide.

2.3. Mixing procedure A 4 kg mass of solid materials was premixed in the mixer vessel at nominal speed before liquid loading. The experimental procedure evaluated different batches using four mixing times (17 s; 47 s; 87 s and 297 s) and four water addition rates (8 g/s; 16 g/s; 46 g/s and 128 g/s). The devices shown in Fig. 3 controlled water addition rate. Mixing time was measured from the beginning of water addition. In order to test different mixing times, water addition rate was fixed at 128 g/s, while for testing different water addition rates mixing time was fixed at 297 s. For DIS mortars powder admixture was pre-mixed with solid materials. During mixing the impeller angular speed was kept constant at 13.25 rad/s. The resulting blade’s peripheral linear velocity was 0.89 m/s. The mixing curve (torque evolution versus mixing time) of each test was recorded by the rheometer’s data acquisition system. The same mixing procedure was repeated for both REF and DIS mixtures: one test for rheological evaluation and another one to evaluate hardened mortar properties. A mixing time of 47 s was only used to evaluate the mortar rheological properties Table 3.

2.4. Rheological evaluation The mortar rheological behavior was measured at 15 s after mixing. The flow curve consisted in three consecutive shear cycles. During each shear cycle the equipment rotational speed was increased in steps from 0.66 to 33.1 rad/s and then back to 0.66 rad/s. Every rotational velocity level (0.66–1.33–2.65–3.98–6.63–13.2 5–19.88–26.5–33.1 rad/s) was held for 5 s with 1 s required to reach the next velocity level.

Table 1 Raw materials mix proportions and physical characteristics. Raw material

BET surface area (m2/g)

Specific gravity (g/cm3)

Volume (%)

Filler Cement Hydrated Lime CHIII Crushed sand

1.23 1.43 6.15 0.27

2.76 3.10 2.49 2.79

12.3 14.5 4.5 68.7

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Table 2 Chemical composition of cement and hydrated lime CHIII. Types (%)

Hydrated lime CHIII

Cement

SiO2 Al2O3 Fe2O3 MgO CaO K2O TiO2 SO3 Na2O CO2 Insoluble residue CaO (free) Loss on ignition

4.03 0.40 0.20 28.10 39.10 0.12 0.09 – – – – – 28.00

16.35 3.50 4.25 5.98 55.60 0.24 – 2.26 0.08 8.66 2.32 1.58 8.83

14

Midle sand Filler

Volume (%)

10

Cement Hydrated Lime CHIII

8

2.7. Mechanical properties 2.7.1. Elasticity modulus Elasticity dynamic modulus was measured using a PUNDIT ultrasound equipment with 20 mm diameter transducers and a frequency of 200 kHz. This equipment measures the time (ls) that the sound wave travels along the specimen longitudinal extension. To reduce noise during measurement colorless gel was employed [32]. 2.7.2. Tensile strength Tensile strength was evaluated using a universal testing press INSTRON (model 5569 and load cell of 50 kN) with a load rate application of 3927 N/min. For each mixing condition and mortar seven cylindrical specimens (height 60 mm and diameter 50 mm) were tested using diametrical compression or Brazilian test. This method is not standard for mechanical evaluation of mortars but is well-known and recognized as a better method for determining the tensile strength of brittle materials [33].

Fine sand

12

The specimens were cast vertically into cylindrical shaped forms with two layers. After casting the molds were covered with plastic foil and stored under controlled temperature (T = 22 °C) for two days. After de-molding, specimens were kept in moist curing until reaching the age of 9 and 28 days. After this period in moist curing, specimens were dried (5 days in a drying chamber followed by 5 days in an incubator at 50 °C). After the drying process the mechanical properties were then evaluated.

2.7.3. Porosity Mortar porosity (p) was deduced from the relation between apparent (ad) and
real (rd) density, p ¼ 1  ad [34]. Apparent density (ad) was obtained from the rd relation between dry mass and volume while real density (rd), i.e. mass divided by net volume, was calculated using Powers model [35]. According to this method the material’s real density is calculated considering the chemically combined water and shrinkage. More details can be found in [34,35].

6 4 2 0 0.1

1

10

100

1000

3. Results and discussion

Particle diameter (μm) Fig. 1. Particle size distribution of raw materials measured by laser diffraction granulometry. Torque values measured at the end of each speed level were used to plot rheological (or hysteresis) curves. The equivalent viscosity and yield stress were then measured at descending rotational velocities from 33.1 to 3.98 rad/s by assuming a Bingham behavior and doing a linear regression over the experimental points.

3.1. Air content Fig. 4 shows the air content results. DIS mortars with polycarboxylate admixture incorporated more air than REF mortars. The difference between them was around 2.4%. These results are discussed in the following sections. 3.2. Mixing curves

2.5. Air content The fresh mortar air content was evaluated after rheological measurements via gravimetric method [31].

Fig. 5a shows the torque with mixing time evolution for both REF and DIS mortars for three mixing times and 128 g/s of water

a)

b) 1

ø 220mm Impeller 205

5 3 mm 135 mm

1 mm

3 2

61 mm

4 Mortar

Fig. 2. Rotational rheometer Poli-USP: a) (1) Rotational device, (2) base reaction and console, (3) elevator, (4) mortar vessel, (5) mortar impeller; b) Mortar impeller and vessel details.

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Fig. 3. Devices used to control water addition rate, a) 8 g/s; b) 128 g/s; c) 46 g/s and d) 16 g/s.

Table 3 Mixing configurations tested; F and H indicates fresh and hardened mortar evaluations respectively. For REF and DIS mixtures

Water addition rate (g/s) 8

Mixing time (s)

17 47 87 297

16

F, H

F, H

9

9

6

3

0

Air content (%)

Air content (%)

DIS

128

F, H

F, H F F, H F, H

REF DIS

b)

REF

a)

46

6

3

0 0

100

200

300

400

Mixing time (s)

0

50

100

150

Water-loading rate (g/s)

Fig. 4. Air content measured after mixing and shear cycles in rheometer, a) mixing time and b) water loading rate.

loading. These tests were repeated (Fig. 5b) and the mixing curves repeatability indicates suitable execution of experiments. The mixing curves first 10 s correspond to dry material homogenization. At 10 s, the introduction of liquid lasted for 5 s and produced a torque increase that quickly reached a maximum level. Then torque decreased to an asymptotic limit. This general behavior is described in literature as liquid bridges, formed during water loading, cause torque to considerably increase reaching a maximum level with intense fluctuations [36]. When mixing advances, the liquid-solid compaction gradually produces zones of paste and fluid bonds are incorporated in larger continuous zones of liquid thus reducing torque level [13,23,37–39]. When the mixture becomes a fluid, torque continues to slowly decrease to an asymptotic limit as a consequence of progressive dissolution of agglomerates still present in the paste [13]. Admixture drastically decreased torque level. Similar behavior is reported, for instance, by [40] in cement paste mixing that

incorporated polycarboxylate based superplasticizer. Indeed, polycarboxylate admixture makes the mixing process easier because it improves the dispersion of fine particles through the combined action of electrostatic repulsion and steric hindrance [41–43]. Fig. 5c, d shows REF and DIS mixing curves and their respective studied water addition rates besides indicating at which time water loading ended. When distribution of liquid was delayed, torque remained at higher levels and for a longer period of time. The systems also took longer to reach stable torque values. This is consequence of the competition which occurs between the formation of new liquid bridges and their dissolution during mixture compaction under shear [38,39]. A second peak in the REF mixing curve was observed after water addition, suggesting that clusters were still forming meanwhile water is not fully distributed in the system. This second peak is described in literature as the cohesion point [44].

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REF 17s REF 87s REF 297s

a)

1.5 REF

1.0

8 g/s 16 g/s 46 g/s 128 g/s

c)

2.0 1.5 1.0 0.5

0.5 DIS

0.0 2.5

REF" 17s Time (s) REF" 87s REF" 297s

b) 2.0

Torque (N.m)

2.5

1.5

DIS" 17s DIS" 87s DIS" 297s REF

1.0 0.5

DIS

REF

0.0 2.5

Torque (N.m)

Torque (N.m)

2.0

DIS 17s DIS 87s DIS 297s

Torque (N.m)

2.5

d)

8 g/s 16 g/s 46 g/s 128 g/s

Tempo (s)

2.0 1.5 1.0 0.5

DIS 0.0

0.0 0

50

100

150

200

250

300

Time (s)

0

50

Water addition end

100

150

200

250

300

Water loading rate (g/s)

Fig. 5. Mixing curves torque vs. time for both REF and DIS mortars. Mixing times: 17 s, 87 s and 297 s. a) Mixing curves under analysis. b) Experiments repetition. Mortars mixing curves torque vs. time c) REF and d) DIS with different water addition rates.

In Fig. 5d, events occurring in the mixture tended to anticipate themselves and fluctuation curve levels were lower. However, the maximum torque level was significantly diminished only when water was rapidly loaded into the mixture. This suggests that admixture did not prevent the formation of agglomerates but favored their breaking thus reducing the mixture efforts. Additionally, powder admixture does not play effectively until it is fully dissolved in mixing water. 3.3. Rheological behavior 3.3.1. Shear cycles Rheological behavior measured through three shear cycles curves after four mixing times with 128 g/s of water loading is presented Fig. 6. The admixture that promoted a more fluid and stable material modified the DIS mortar rheological profile. The beginning of the first shearing cycle corresponded to the highest torque level for both REF and DIS mixtures. Torque decreased during the first increase in angular velocity reflecting that the mixture structure strongly evolved under shearing and that the associated decrease in torque is higher than the expected torque increases for a structurally constant mixture. This phenomenon progressively disappears with mixing time and number of cycles. Finally, this phenomenon was not observed during tests with different water loading rates, even at the slowest loading. 3.3.2. Thixotropy Shear cycle curves started with torque values larger than when mixing stopped. However, during the first shear cycles points the mixer angular velocity is much smaller than during mixing. This phenomenon can be associated with the liquid-solid system reagglomeration after mixing and before shear cycles. To analyze the evolution of this thixotropic behavior, the ratio between torque at the beginning of the first shear cycle and at the end of the mixing was introduced. This thixotropic ratio is expected to be (largely) inferior to the unit for a non-thixotropic material. It was observed that thixotropic behavior decreases with mixing time and also when admixture is present in the composition Fig. 7a. Conversely, when the water-loading rate was decreased the thixotropic behavior was not significantly affected. The thixotropic mean values for all water loading rates and their standard

deviation as error bars are represented in Fig. 7b. Thus it is confirmed that admixture significantly decreases the mixture thixotropic behavior, even for long mixing times. This can be explained in terms of admixture adsorption effect. As shown by Lowke [45], thixotropy decreases with increasing surface coverage for similar particle sizes. In addition low thixotropic behavior is influenced by high surface coverage, strong steric interactions and large particle distance [45]. Furthermore, admixture disperses particles by electro steric mechanism, creating a barrier for electrostatic attraction, both by mechanical hindrance due to polymeric chains and by counterbalancing ionic charges [46,47]. 3.3.3. Hysteretic behavior The hysteresis area obtained from the difference between ascending and descending shear curves is a second way to characterize the intensity of time-dependent effects, i.e. the breakdown or buildup of ordered structures within flowing matter [6,21,41,48]. When mixing time was short, the rheological behavior showed larger hysteresis areas mainly for the first shear cycle, Fig. 8 left. Again, this phenomenon was valid for both mixtures while the water-loading rate did not significantly affect their behavior. This last remark is demonstrated by Fig. 8(right) which represents the mean histersis area for different water-loading rates of the same mixture along with the standard deviation of these measures. From this figure is observed that the hysteresis area decreased with the number of cycles and, in case of the DIS mixture, even after the longest mixing time. 3.3.4. Bingham behavior Assuming that the mixture follows a Bingham-like behavior in torque and rotation basis, its flow curve can be represented by a straight line and its rheological parameters determined by linear regressionT ¼ g þ Nh, where g (Nm) and h (Nms) are constants corresponding respectively to the mixture equivalent yield stress and plastic viscosity. An increase in mixing time for REF mixture reduced equivalent yield stress and plastic viscosity as seen in Fig. 9 for the evolution of values corresponding to the first cycle. Higher mixing times promote a more efficient system dispersion by reducing its yield stress. Consequently, lower yield stress requires less stress to

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3

REF 2nd cycle

3 1

297 s

REF 1st cycle

Torque (Nm)

87 s 47 s

2

2 17 s 03

REF 3rd cycle 1

1 2

0 3 0

0 5

10

15

20

25

30

35

3

297 s

Angular velocity (rad/s) DIS 1st cycle

DIS 2nd cycle

1

87 s

Torque (Nm)

47 s 2

2

17 s

0 1

DIS 3rd cycle

1

0

0 0

5

10

15

20

25

30

35

0

5

10

Angular velocity (rad/s)

15

20

25

30

35

Angular velocity (rad/s)

Fig. 6. Shear cycles after different mixing times (17, 47, 87 and 297 s) for REF and DIS mortars.

2

a)

b) REF DIS

Thixotropic factor (-)

Thixotropic factor (-)

2

1

0

REF

DIS

≠ loading rate 1

0 10

100 Mixing time (s)

1000

REF

DIS

30 REF 20 DIS 10

cycle 1 cycle 2 cycle 3 cycle 1 cycle 2 cycle 3

0

Hysteresis area (Nm/s)

Hysteresis area (Nm/s)

Fig. 7. Thixotropic factor evolution (ratio between torque at the beginning of the first shear cycle and the end of mixing) along with the mixing parameters for REF and DIS mixtures.

3

REF

DIS

≠ loading rate

2

1

0 10

100

1000

cycle 1

cycle 2

cycle 3

Mixing time (s) Fig. 8. Hysteresis area evolution (including mixing parameters) for the REF and DIS mixtures.

initiate flow and generally corresponds to higher slump flow, see [49] for instance. However, the mixing time evolutions were small compared, for instance, with the admixture effect. It should be reminded that the Bingham-like parameters are calculated from the descending part of the shear cycle, which accounts for

several tens of seconds of supplementary mixing time before the mixture is evaluated. This certainly reduced the expected effect observed for shorter mixing times. In fact, for REF mixture the Bingham-like parameters also decreased with the number of cycles. For DIS mixture, which globally required a

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0.03

a)

0.02

h (Nm s)

h (Nm s)

0.03

0.01

0

c)

0.02 DIS 0.01

100

1000

5

50

500

Water loading rate (g/s)

Mixing time (s) 1.00

b)

d) g (Nm)

g (Nm)

cycle 1 cycle 2 cycle 3 cycle 1 cycle 2 cycle 3

0 10

1.00

REF

0.50

0.00

0.50

0.00 10

100

1000

Mixing time (s)

5

50

500

Water loading rate (g/s)

Fig. 9. Rheological parameters (h – yield stress coefficient and g – viscosity coefficient) obtained from descending shear cycles curves as a function of the mixing time a), b) and water loading rate c), d).

shorter mixing time, the Bingham-like parameters revealed to be more stable regarding mixing time, water loading rate and number of cycles. The Bingham-like parameters in torque and rotation basis also evolved with water loading rate, but globally, the change is not relevant. Finally, when comparing REF mortars with DIS mortars in Fig. 9 it is seen that DIS mortars display reduced yield stress despite a slight viscosity increase. Normally, for cementitious systems, the presence of admixture induces a decrease in plastic viscosity and yield stress [50]. However the behavior observed in DIS mortars does not follow this pattern. This effect is particular of polycarboxylate admixtures and was extensively observed in the literature [16,51]. Furthermore this relation means also about the mixing stability. If the plastic viscosity is low, the system have a relatively high yield stress to maintain stability [16]. Additionally, higher energy provided by longer mixing times better dispersed the system, since greater shear and impact energies were able to more efficiently destroy the agglomerates. When admixture was employed, dispersing effects performed part of this ‘‘job” due to the electro steric action, therefore the mechanical action provided by mixing was less important. Homogenization of raw materials and water also plays an important role in the mixture rheological behavior regarding shear cycles and hysteresis/thixotropy. A longer mixing time or more efficient mixing provide more homogeneous systems. 3.4. Mechanical properties Fig. 10 shows the relation between porosity and mechanical properties: elasticity modulus and tensile strength. As a general remark, the mortar porosity was lower for REF mixture than for DIS mixture and, for both mixtures, porosity seems to be unaffected by mixing parameters. The only exception was a combination of shorter mixing time and slower water loading for which REF mixture presented higher porosity compared to other mixing conditions Fig. 10. The same atypical behavior was also observed in fresh state measurements. The measures standard deviation

was about 0.5%. This implies that the experimental points indicated in Fig. 10, which are the mean of 7 measurements have a standard deviation of about 0.22%. The results also show that elasticity modulus and tensile strength follow the same trend. The lower the material porosity is, the better its mechanical performance. Regarding mixing time and water addition rate, it was observed that short mixing times, slow water addition rate as well as the presence of admixture increase the porosity. Comparing results at 9 and 28 days, DIS mortars show reduced porosity and improved mechanical properties after 28 days. Despite of REF mortars achieving increased mechanical properties at 28 days the change in porosity was irrelevant. This means that probably the dispersant changed the chemical reaction of hydrates products affecting directly the system’s porosity. Previously it was reported that the superplasticizer dosage could cause a delay in setting time and in the same way inhibit cement hydration [52]. However, after 3 days of hydration, the presence of superplasticizer increases the degree of hydration which results in a decline in pore connectivity as more hydrates fill in the capillary pores [53]. More investigation in this area is recommended. Mortars with lower porosity also presented lower air content in fresh mixing. More precisely, DIS mixture mean air content (5.95%) is 2.4% higher than the mean air content of REF mixture while the mean porosity was 1.3% and 0.7% lower in REF mixtures than in DIS mixtures after 9 and 28 days of curing, respectively. The mean air content of the bad mixing setup (shortest mixing time, slowest water loading rate) is 1% lower for all situations and the difference in porosity was 0.9% and 1% after 9 and 28 days of curing, respectively. One can conclude that air entrained during mixing Fig. 4 explains almost all porosity variations. However, porosity is higher than expected for the bad mixing setups. Tensile strength and elasticity modulus were analyzed after 9 and 28 days of curing for both mixing times and water loading rates parameters, Fig. 11. The statistical treatment of data was done by variance analysis method (ANOVA) using MINITAB 16.0 software. The p-value limit for which the null hypothesis is considered true was fixed at

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22.0

22.0

a)

REF

20.0 19.0 18.0

20.0

19.0

18.0

y = -0.7216x + 40.074 R² = 0.4573

17.0 3.5

y = -0.7881x + 42.194 R² = 0.5623

17.0

b) Tensile strength (MPa)

DIS

21.0

Modulus (GPa)

21.0

Modulus (GPa)

c)

REF DIS

d)

REF DIS

Porosity (%) REF

DIS

3.0

2.5

2.0 y = -0.1158x + 5.6935 R² = 0.2691

1.5 26.5

27

27.5

y = -0.1754x + 7.5398 R² = 0.5421

28

28.5

29

30 26.5

29.5

27

27.5

28

28.5

29

29.5

30

Porosity (%)

Porosity (%)

3.5

Tensile strength (MPa)

Tensile strength (MPa)

Fig. 10. Porosity of hardened REF and DIS mortars and its relation to mechanical properties: elasticity modulus (on top) and tensile strength (bottom) as a function of mixing time a), b) and water addition rate c), d).

a) ±SD 2.5

1.5

3.5

c)

2.5

1.5 0

100

200

300

400

0

100

200

Water loading rate (g/s)

Mixing time (s) 25

25

d)

b) 23

REF28 DIS28 REF9 DIS9

23

±SD

Modulus (GPa)

Modulus (GPa)

REF28 DIS28 REF9 DIS9

21 19 17 15

21 19 17 15

0

100

200

300

400

Mixing time (s)

0

100

200

Water loading rate (g/s)

Fig. 11. Tensile strength and elasticity modulus evaluated at 9 and 28 days for REF and DIS mortars as a function of mixing time a) and b), and water addition rate c) and d).

0.05. The null hypothesis was that the means of the two compared populations were not different. The null hypothesis was rejected for a p-value lower than 0.05. The analysis indicates that, with the exception of DIS mixture at 9 days, a mixing time of 87 s was required to stabilize tensile strength. The water loading rate had no direct influence on it. It was also found that only the lowest water loading rate reduced tensile strength in both mortars.

Elastic modulus analysis gave similar results although some inconsistencies were observed. In fact, some experimental errors in the modulus measurement are questioned. The relation between tensile strength and elastic modulus is represented Fig. 12 where an inferior threshold for samples with smaller tensile strength and, at opposite, a superior threshold for highest strengths is observed. Thus it is very important to take tensile strength into consideration when analyzing the material mechanical response.

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Modulus (GPa)

25 y = 7.9x

20

15 2.0

2.5

3.0

3.5

Tensile strength (MPa) Fig. 12. Correlation between tensile strength and elasticity modulus evaluated at 9 and 28 days for REF and DIS mortars.

REF mortars showed higher tensile strength than mortars with admixture. This fact needs to be considered in relation to air content and porosity. By using Feret’s formula, the mean strength increases of REF mortars compared with DIS mortars, giving the air content measured in their fresh state (Fig. 4), is 7.6%. The measured mean strength increase was 8.6%, i.e. very close to the predicted value. However, an additional phenomenon could explain ‘‘bad mixing” setups (shortest mixing time, slowest water loading rate). The bad mixing tensile strength decrease was about 6.5% while only 3.2% was expected giving the samples higher air content. This is associated to the ‘‘bad mixing” high porosity samples before tensile strength tests, as suggested by the only air content in fresh state. Indeed, bad mixing does not promote good homogenization and clusters of dry particles that were not adequately covered by liquid could be present in the system.

4. Conclusions Mortar mixing behavior was studied by rotational rheometry via changes to process parameters such as mixing time and water loading rate. Mixing time had the biggest influence on mortar behavior with and without water reducer admixture. It continuously affects rheological behavior as observed in the continuous decreasing of torque during mixing. Thixotropic behavior decreased in function of mixing time. Admixture also decreased thixotropy level and this phenomenon seemed to be independent of mixing time. A Bingham-like behavior was systematically observed even when the evolution of rheological parameters was difficult to analyze. Indeed, the required rheological measurement implies the shearing of fresh mortar samples, which changes the mixture microstructure state. This irreversible change in microstructure was indicated by open hysteretic loops in flow curves. Hysteretic area decrease was exponential with mixing time and admixture did not change its characteristic evolution time but lowering the hysteretic area level. Therefore, hysteretic loops closed with a lower mixing time due to admixture. During mixing, the mixture without water reducer entrained less air as long as the mixing time was long enough. This setup also produced hardened mortar with higher porosity in case admixture is used in the formulation. However, the hardened mortar porosity seemed to increase when mixing time is short. Additionally, it was simultaneously observed that the mortar mechanical properties were mainly influenced by shortest mixing times which seemed

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to not provide enough evolution of paste in order to achieve optimal mortar strength. Water loading rate, except for the slowest rate, had no direct influence on mixing results. Regarding the slowest water loading rate, the mechanical behavior effect was significant but not the rheological one. In conclusion, based in this experimental setup, bad mixing conditions can be defined as consisting either in a too short mixing time or a too slow water loading rate. When mixing conditions were good, water loading rate did not significantly affect both the mixture rheological and mechanical properties. Additionally, mixing time induced slow changes to rheological behavior, mainly regarding thixotropy and the hysteretic loops opening level and also regarding the Bingham-like parameters and hysteretic area. Conversely, the mixing time effect on the mortar hardened properties were negligible. Declaration of Competing Interest None. Acknowledgment São Paulo Cátedras Francesa and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 are acknowledged for financing this work. References [1] D. Han, R.D. Ferron, Effect of mixing method on microstructure and rheology of cement paste, Constr. Build. Mater. 93 (2015) 278–288, https://doi.org/ 10.1016/j.conbuildmat.2015.05.124. [2] M. Yang, H.M. Jennings, Influences of mixing methods on the microstructure and rheological behavior of cement paste, Adv. Cem. Based Mater. 2 (1995) 70– 78, https://doi.org/10.1016/1065-7355(95)90027-6. [3] J. Dils, G. De Schutter, V. Boel, Influence of mixing procedure and mixer type on fresh and hardened properties of concrete: a review, Mater. Struct. 45 (2012) 1673–1683, https://doi.org/10.1617/s11527-012-9864-8. [4] D. Chopin, B. Cazacliu, F. de Larrard, R. Schell, Monitoring of concrete homogenisation with the power consumption curve, Mater. Struct. 40 (2007) 897–907, https://doi.org/10.1617/s11527-006-9187-8. [5] R.G. Pileggi, A.R. Studart, V.C. Pandolfelli, J. Gallo, How mixing affects the rheology of refractory castables, part 1, Am. Ceram. Soc. Bull. 80 (2001) 27–31. [6] R.G. Pileggi, A.R. Studart, V.C. Pandolfelli, J. Gallo, How mixing affects the rheology of refractory castables, part 2, Am. Ceram. Soc. Bull. 80 (2001) 38–42. [7] A. Bras, F.M.A. Henriques, The influence of the mixing procedures on the optimization of fresh grout properties, Mater. Struct. 42 (2009) 1423–1432, https://doi.org/10.1617/s11527-008-9461-z. [8] R. Collet, D. Oulahna, A. De Ryck, P.H. Jezequel, M. Martin, Mixing of a wet granular medium: effect of the particle size, the liquid and the granular capacity on the intensity consumption, Chem. Eng. J. 164 (2010) 299–304, https://doi.org/10.1016/j.cej.2010.07.012. [9] J. Zhou, S. Qian, G. Ye, O. Copuroglu, K. van Breugel, V.C. Li, Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence, Cem. Concr. Compos. 34 (2012) 342–348, https://doi.org/10.1016/j.cemconcomp.2011.11.019. [10] M.S. de França, F.A. Cardoso, R.G. Pileggi, Influence of the addition sequence of PVA-fibers and water on mixing and rheological behavior of mortars, Rev. IBRACON Estrut. E Mater. 9 (2016) 226–243, https://doi.org/10.1590/S198341952016000200005. [11] D. Chopin, F. de Larrard, B. Cazacliu, Why do HPC and SCC require a longer mixing time?, Cem Concr. Res. 34 (2004) 2237–2243, https://doi.org/10.1016/ j.cemconres.2004.02.012. [12] S. Foucault, G. Ascanio, P.A. Tanguy, Mixing times in coaxial mixers with Newtonian and non-Newtonian fluids, Ind. Eng. Chem. Res. 45 (2006) 352– 359, https://doi.org/10.1021/ie048761o. [13] B. Cazacliu, J. Legrand, Characterization of the granular-to-fluid state process during mixing by power evolution in a planetary concrete mixer, Chem. Eng. Sci. 63 (2008) 4617–4630, https://doi.org/10.1016/j.ces.2008.06.001. [14] O. Mazanec, D. Lowke, P. Schießl, Mixing of high performance concrete: effect of concrete composition and mixing intensity on mixing time, Mater. Struct. 43 (2010) 357–365, https://doi.org/10.1617/s11527-009-9494-y. [15] K. Takahashi, T.A. Bier, T. Westphal, Effects of mixing energy on technological properties and hydration kinetics of grouting mortars, Cem. Concr. Res. 41 (2011) 1167–1176, https://doi.org/10.1016/j.cemconres.2011.07.005.

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