Effect of mineral admixtures on fluidity and stability of self-consolidating mortar subjected to prolonged mixing time

Construction and Building Materials 40 (2013) 1029–1037

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Effect of mineral admixtures on fluidity and stability of self-consolidating mortar subjected to prolonged mixing time Iman Mehdipour a,b,⇑, Mehran Seyed Razzaghi c, Kamran Amini d, Mohammad Shekarchi b a

Department of Civil Engineering, Buinzahra Branch, Islamic Azad University, Buinzahra, Iran Construction Materials Institute (CMI), School of Civil Engineering, University of Tehran, Tehran, Iran c Department of Civil Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran d Department of Civil Engineering, Concrete Research Center, Qazvin Branch, Islamic Azad University, Qazvin, Iran b

h i g h l i g h t s " Prolonged agitation time can increase the risk of instability of the mixture. " Fly ash increases the fluidity, but mixture is more prone to instability. " MK provides stability retention particularly when prolonged mixing is into consideration. " Effect of MK to stabilize mixtures is more noticeable at higher w/b ratio. " Mixtures containing MK and FA have higher fluid capacity without any signs of segregation.

a r t i c l e

i n f o

Article history: Received 14 June 2012 Received in revised form 12 October 2012 Accepted 22 November 2012 Available online 28 December 2012 Keywords: Self-consolidating mortar Stability Fluidity Prolonged mixing time Fluid capacity Fly ash Metakaolin

a b s t r a c t During continuous mixing of concrete in a truck mixer for its hauling to a construction site, concrete should remain workable without any signs of instability. Good workability at construction site is essential for high quality concrete since concretes of bad workability are prone to yield low strength and poor durability properties. This paper presents the effects of water to binder ratio, binary and ternary blends use of fly ash (FA) and metakaolin (MK) on fluidity, viscosity, and stability of self-consolidating mortars (SCMs) subjected to prolonged mixing time. The obtained results indicate that, by prolonged mixing time, flocculated cement particles are dispersed and as a result, fluidity is increased and so, the risk of instability increases. In addition, increasing FA content from 0 in the reference mixture to 20% and 50% of total binder mass increased the segregation index by about 48% and 160%, respectively. Furthermore, mixtures containing MK and FA have higher fluid capacity without any signs of instability. The addition of 7.5% MK has significantly improved the overall performance of the mixture, including the segregation stability. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Self-consolidating concrete (SCC) is a new category of highperformance concrete characterized by its ability to spread readily into place and self-consolidate without exhibiting any significant separation of constituents. To secure adequate homogeneity necessary for developing proper bond to reinforcement, structural performance, strength and durability, it is imperative to proportion the SCC with high stability. Stability is the ability of a SCC mixture to retain a uniform distribution of all constituent materials during the casting process and once all placement and casting operations ⇑ Corresponding author at: Department of Civil Engineering, Buinzahra Branch, Islamic Azad University, Buinzahra, Iran. Tel.: +98 9121712360. E-mail address: [email protected] (I. Mehdipour). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.108

have been completed. The former is referred to dynamic stability and the latter is referred to static stability [1]. Dynamic instability can be caused by the input of any form of vibration energy into the system during material transport or placement [2]. On the other hand, static stability is the ability of concrete to resist bleeding, segregation, and settlement which are influenced by gravity and time [3]. Immediately after or during its mixing, concrete is transported from its mixing location to the final destination. During its transportation, concrete should remain cohesive and workable without any signs of instability like segregation and bleeding [4]. In this regard, mixing time is another key factor which affects the fresh properties of cement based concretes, especially highly flowable concretes [4,5]. Poor workability of concrete can create several problems including difficulty of concrete placement and

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compaction resulting in concrete having increased voids and lower mechanical strength and durability [6]. However, the increase of both yield stress and viscosity was more remarkable when the concrete mixture was under the standstill condition than that under agitation. When concrete is agitated, flocculated cement particles are dispersed and as a result, fluidity can be increased. Li et al. [7] showed that increasing both the yield stress and apparent viscosity of concrete were more dominant when the mixture was under the rest condition than that under continuous agitation. Felekoglu et al. [8] indicated that, the viscous behavior of selfconsolidating mortar is evident for low rotational speeds of mixing, while at higher speeds flowable behavior becomes dominant. While mixing, the shear thinning effect breaks down the formation of the high viscous behavior of the mix at rest. In other words, when flow is facilitated by local shear or by vibration, the apparent viscosity decreases with the increase in the strain rate [9]. On the other hand, dynamic instability can be caused by the input energy into the system due to the continuous agitation and therefore, results in segregation and bleeding of concrete in fresh state. This may also be the case during the transport of fresh concrete in a truck mixer where some segregation and sedimentation of particles can be induced by the vibration and agitation during transport. It is important to note that a highly flowable concrete that exhibits adequate stability once cast in place may undergo some segregation during the pumping or spread into place. This is because the apparent viscosity at such shear rates can be significantly lower than that at rest because of the pseudo-plastic nature of the concrete. A few studies in the literature [4,5,7,10,11] investigated the effect of the prolonged mixing time on the rheological behavior and stability properties of concrete, especially for SCC. One of the most important differences between SCC and conventional vibrated concrete is the incorporation of supplementary cementitious materials. Many studies about the effects of mineral admixtures on the fresh and hardened properties of SCC have been completed [12–18]. However, there are few published reports regarding the effects of supplementary cementitious materials on the stability of SCC [19,20]. For example, using fly ash (FA) improves the fluidity of SCC mixtures, but some published reports [20] indicate that increasing FA content, slightly increases the risk of instability. On the other hand, some mineral admixtures such as metakaolin (MK) decrease the fluidity and increase the viscosity of the mixture and consequently, it can lead to improved cohesiveness and stability of the mixture. Therefore, by incorporating them within ternary cementitious blends, beneficial effects of one mineral admixture may compensate the shortcomings of the other

Table 1 Chemical compositions and physical properties of cementitious materials. Properties

Cement

Fly ash

Metakaolin

Chemical analysis (%) Insoluble residue SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O Loss of ignition

0.38 20.03 5.53 3.63 62.25 3.42 2.23 0.3 0.73 1.37

– 61.3 28.8 4.98 1.05 0.63 0.13 0.24 1.40 0.70

– 51.85 43.87 0.99 0.20 0.18 0.12 0.01 0.00 0.57

Physical properties Specific gravity (kg/m3) Blain fineness (m2/kg) Initial setting time (min) Final setting time (min)

3150 300 188 240

2200 257 – –

2600 2300 – –

type. Properties of concrete and mortar mixes containing FA or MK were reported comprehensively in literatures [12,13,20,21]. However, there are limited publications to investigate the effect of ternary blends use of FA and MK on the stability of SCC mixtures with different water to binder ratio. The objective of this paper is to investigate the effect of water to binder ratio, binary and ternary blends of FA and MK on fluidity, viscosity, and also stability of self-consolidating mortars (SCMs) subjected to prolonged mixing time. Moreover, this study attempts to investigate the relationship between fluidity and stability of SCMs. A considerable amount of research work has been devoted over the last decade to study the mortar behavior [8,20,21]. Since the properties of the mortar are critical in the production of SCC, it is desirable to be able to predict the characteristics of SCC from the fresh properties of the matrix [22,23]. Therefore, in this study, all experimental tests are conducted on SCMs originated from SCC. 2. Experimental program 2.1. Materials A Type I ordinary Portland cement (OPC), similar to ASTM C150 was used for all mixtures. Also, polycarboxylate-based superplasticizer (SP) with solid content of 36% and specific density of 1.07 were employed for all self-consolidating mortars. The applied sand grading in all SCMs met ASTM C33 to be well-graded with maximum aggregate size of 6 mm, specific gravity of 2.7, fineness modulus of 2.56 and absorption value of 2.8%. Fly ash (FA) and metakaolin (MK) powders were used in binary and ternary systems to develop the SCMs in this study. The chemical compositions and physical properties of applied cementitious materials (OPC, FA and MK) are given in Table 1. 2.2. Mixture proportions In this study, a total of 38 SCM mixtures were designed containing two series of 19 SCM mixtures for two levels of fluidity; water–binder ratio of 0.35 and 0.45. For all mixtures, the total binder content and superplasticizer dosage were kept constant at 700 kg/m3 and 1%, respectively. Two reference mixtures named LR and HR included only OPC as the binder for both w/b of 0.35 and 0.45, respectively. Remaining SCM mixtures were incorporated in binary (OPC + FA, OPC + MK) and ternary (OPC + FA + MK) cementitious blends in which a proportion of Portland cement was replaced with the powders. In binary system, the replacement level for FA was 10%, 20%, 30%, 40% and 50%, while those of MK was 10%, 20% and 30% by the weight of total binder content. On the other hand, in ternary system two different types of incorporation were prepared; first, the replacement levels for both FA and MK were 5%, 10%, 15%, 20% and 25%, while in the second series the ratio of MK to FA were 1:3 by weight of total binder content. The mixture proportions are summarized in Table 2. 2.3. Specimen preparation and testing methods In this study, the mixing sequences were comprised of homogenizing the sand and cementitious materials for approximately 30 s, and then water with superplasticizer was mixed in a flask and gradually added. The mixture was mixed for 5 min at rotation speed of 40 rpm to ensure uniformity. To provide continuous agitation for mixture, a standard rotating drum mixer at a low speed was used; with rotation speed of 4 rpm. Following the SCMs mixing, its fresh properties were evaluated through the mini-slump flow diameter and mini V-funnel flow time in conformity with the standard procedures given by EFNARC [22]. They were selected as their results could approximately demonstrate the yield stress and plastic viscosity of SCMs, respectively [24–28]. To evaluate the time dependency of fresh SCMs, all the mixtures were subjected to prolonged mixing up to 40 min and fresh properties were measured at each 5 min interval. The influence of prolonged mixing time on the dynamic segregation resistance of the mixtures was evaluated through the visual stability index (VSI) during the slump flow test. It rates the quality of a self-consolidating mixture in terms of segregation and bleeding from 0 to 3 as described in Table 3. It should be noted that visual stability index should not be used for the acceptance or rejection of SCC although it provides valuable information on the concrete stability. The S-shaped test was also taken to visually evaluate the stability of the mixtures. Stability of the mixtures was also evaluated in hardened state through hardened visual stability index (HVSI). HVSI is a qualitative measurement of the distribution of aggregate from a sectioned cylinder. This is supported by the hardened visual stability ratings summarized in Table 3 by Fang and Labi [29]. Besides, mini-column segregation was applied to determine static segregation of SCM by measuring the sand aggregate content in the top and bottom portions of a cylindrical specimen. The apparatus used was similar to the one described in ASTM C1610 but in a smaller size. It

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I. Mehdipour et al. / Construction and Building Materials 40 (2013) 1029–1037 Table 2 Mixture proportions. Mixture code

w/b

Aggregate (kg/m3)

OPC (kg/m3)

FA

MK 3

%

kg/m

%

kg/m3

LR LF10 LF20 LF30 LF40 LF50 LM10 LM20 LM30 LF5M5 LF10M10 LF15M15 LF20M20 LF25M25 LF7.5M2.5 LF15M5 LF22.5M7.5 LF30M10 LF37.5M12.5

0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35

1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205 1205

700 630 560 490 420 350 630 560 490 630 560 490 420 350 630 560 490 420 350

0 10 20 30 40 50 0 0 0 5 10 15 20 25 7.5 15 22.5 30 37.5

0 70 140 210 280 350 0 0 0 35 70 105 140 175 52.5 105 157.5 210 262.5

0 0 0 0 0 0 10 20 30 5 10 15 20 25 2.5 5 7.5 10 12.5

0 0 0 0 0 0 70 140 210 35 70 105 140 175 17.5 35 52.5 70 87.5

HR HF10 HF20 HF30 HF40 HF50 HM10 HM20 HM30 HF5M5 HF10M10 HF15M15 HF20M20 HF25M25 HF7.5M2.5 HF15M5 HF22.5M7.5 HF30M10 HF37.5M12.5

0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45

1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136 1136

700 630 560 490 420 350 630 560 490 630 560 490 420 350 630 560 490 420 350

0 10 20 30 40 50 0 0 0 5 10 15 20 25 7.5 15 22.5 30 37.5

0 70 140 210 280 350 0 0 0 35 70 105 140 175 52.5 105 157.5 210 262.5

0 0 0 0 0 0 10 20 30 5 10 15 20 25 2.5 5 7.5 10 12.5

0 0 0 0 0 0 70 140 210 35 70 105 140 175 17.5 35 52.5 70 87.5

Superplasticizer dosage was kept constant at 1% for all mixtures.

Table 3 Description of VSI and HVSI rating. Rating

VSI

HVSI

0 1

No evidence of segregation or bleeding No evidence of segregation and slight bleeding observed as a sheen on the concrete mass A slight mortar halo <10 mm and/or aggregate pile in the center of the concrete mass Clearly segregating by evidence of a large mortar halo >10 mm and/ or a large aggregate pile in the center of the concrete mass

No evidence of segregation in hardened specimen No mortar layer at the top of specimen, but slight variation in size and percent of aggregate distribution from top to bottom A mortar layer, less than 25 mm thick at the top of the specimen and distinguished variation in size and percent of aggregate distribution from top to bottom Severe segregation by evidence of mortar layer greater than 25 mm thick and considerable variation in size and percent of aggregate distribution from top to bottom

2 3

consists of PVC pipes split into three 70 mm sections in height with 75 mm diameter. After letting the mixture rest in the column for 15 min, the mixture in the top and bottom sections of the column are washed over a 300 lm (No. 50) sieve. The aggregate is then dried and weighed, and the static segregation (SI) is calculated according to Eq. (1), where Mtop and Mbot represent the mass of aggregate from the top and bottom sections of the column, respectively.

SI ¼ 2

  M bot  Mtop  100 M bot þ Mtop

ð1Þ

However, there is not any stabilized recommended limit for mortar mixtures. The results of this study show that, mixtures with SI 6 30% exhibit adequate stability. In addition, mortar with 30% < SI < 130% shows semi stable condition in which segregation may occur and finally, mixture with SI P 130% demonstrates severe aggregate segregation and bleeding. These limits are similar with the results obtained by Libre et al. [20].

3. Results and discussion 3.1. Fluidity The results of slump flow versus elapsed time are presented in Figs. 1 and 2 for binary and ternary mixtures, respectively. The obtained results show that both the powder content and w/b ratio have a significant influence on time-dependency of the fluidity. As shown in Fig. 1, incorporating FA in the mixtures leads to higher fluidity especially for higher replacement level of FA. For example, replacing FA by about 50% contributes to increasing the fluidity by about 20% for both w/b ratios of 0.35 and 0.45, compared to reference mixes. Increased fluidity with FA content can be explained by

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Fig. 1. Variation of slump flow versus elapsed time in binary blends (a) w=b ¼ 35% and (b) w=b ¼ 45%.

Fig. 2. Variation of slump flow versus elapsed time in ternary blends (a) w=b ¼ 35% and (b) w=b ¼ 45%.

its lubricating effect of spherical shape and smooth surface characteristics which tend to reduce friction at the interface of aggregate and paste and producing ‘‘ball-bearing effect’’ at the contact point [14,30]. On the other hand, in mixtures incorporating MK, a gradual fall was observed in the fluidity of SCM mixtures. However, the reducing rate of fluidity with higher MK content was more pronounced for mixtures with lower w/b. The decreasing effect of MK on fluidity is probably due to its high chemical activity and its high surface area which results to high water adsorption and free water reduction in the mixture. The fluidity of approximately all SCMs changed during 10– 20 min after mixing as shown in Figs. 1 and 2. On the other hand, for elapsed time less than 10 min and greater than 20 min, the variation of fluidity with time was marginal. The results indicate that, by increasing prolonged mixing time, the fluidity of almost all mixtures was increased. This may be explained by the fact that, when a SCM mixture is agitated, flocculated cement particles are dispersed and as a result, fluidity increases. At the same time, the reduced agglomeration due to such dispersion would increase the amount of excess water (water in excess of that needed to fill up the voids between the solid particles) in the concrete. Since it is the excess water that would provide a thin film of water coating each solid particle to lubricate the concrete, the increase in the amount of excess water would increase the water film thickness and eventually further increase the flowability of the concrete. On the other hand, by increasing the mixing time greater than 30 min the fluidity of mixtures was decreased, especially for mixtures containing MK. Mostly, slump-flow loss occurs as the free mixing water of mixture is absorbed by the hydration reactions, adsorbed on the surfaces of cement-hydrated products, or is evaporated. Visual inspection of mixtures with binary and ternary blends is demonstrated in Figs. 3 and 4. The results show that, by prolonged mixing time, the cohesiveness of the mixtures can be decreased and consequently a lack of cohesiveness may be lead to segregation, surface settlement, and bleeding of the mixture. This may be explained by the fact that when a SCM mixture is agitated,

flocculated cement particles are dispersed and as a result the water film thickness increases. On the other hand, an increase in water film thickness causes a decrease in solid contacts and so cohesiveness of the mixture decreases. Based on the obtained results, for w/ b ratio of 0.35, using FA content up to 20% does not have significant effect on the variation of fluidity with mixing time, and mixtures exhibit stable characteristics. However, for mixtures incorporating FA higher than 20%, the variation of fluidity with time significantly increases, especially at higher w/b ratio. As can be seen in Fig. 3, by increasing the FA content, the prolonged mixing time can lead to higher risk of instability such as accumulation of aggregates at the center and bleeding at the edge of the mixtures. FA particles have a spherical geometry and a coarse particle size, causing a reduction in the surface area to adsorb excess water. As a result

Fig. 3. Visual inspection of mixtures with binary blends.

I. Mehdipour et al. / Construction and Building Materials 40 (2013) 1029–1037

Fig. 4. Visual inspection of mixtures with ternary blends.

of the higher excess water content, the mixture is more prone to instability. As can be seen in Fig. 3, MK blends can control the instability of SCMs by increasing the cohesiveness of the mixture, but it decreases the fluidity of the mixture. Due to the high fineness of MK, the addition of MK would markedly increase the surface area of the solid particles. Hence, the addition of MK could greatly increase the total solid surface area to be coated by the excess water and thereby significantly reduce the water film thickness. Therefore in this case, the cohesiveness of the mixture increases. Similar results have been observed by Fung and Kwan [31] and also Kwan and Ng [32] who noted that at higher silica fume content, the cohesiveness of the mixture is higher due to increase in solid surface area and decrease water film thickness. Therefore based on the obtained results, the combination of MK and FA is a very effective tool for improving the fluidity and also stability of the self-consolidating mixtures during mixing process. Fig. 2 demonstrates the variation of fluidity with mixing time for ternary blends. The results show that, using FA in mixture incorporating MK can compensate the fluidity reduction and also, the variation of fluidity with time at the end of 40 min of agitation is decreased. In addition, no signs of instability like aggregate segregation and bleeding were observed for mixture with equal amount of MK and FA as shown in Fig. 4. On the other hand, using three-component composition with one-third ratio of MK to FA showed different behavior over the fluidity with time-dependency of SCMs. In this case, when the amount of the MK was lower than 7.5% by the weight of total binder, the bleeding and aggregate segregation were obviously observed, while by increasing the MK higher than 7.5% the trend was reversed and it had a significant effect for providing resistance to instability. Similar results were observed for w/b ratio of 0.45 as shown in Fig. 2b. As expected by increasing the w/b ratio to 0.45, the flowability of the mixtures substantially increased, but the mixtures were more prone to instability as shown in Figs. 3 and 4. The addition of water progressively fills the interparticle porosity. When the initial porosity is saturated, the excess water volume disperses the grains and keeps the fine particles in suspension in the mixture, and so cohesiveness and stability of the mixture decrease.

3.2. Viscosity and aggregate blockage The T5 (flow time after 5 min of mixing) and T40 (flow time after 40 min of mixing) of mixtures are presented in Fig. 5. It is

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noted that V-funnel flow time is related to the viscosity of SCC when the mixture discharges uniformly. As expected, incorporation of MK enhanced the cohesiveness of the SCMs due to the high surface area which resulted in higher viscosity. In addition, for w/b ratio of 0.35, using FA in binary blends even at high dosages had marginal reducing effect on the viscosity of the SCMs. On the other hand, during the higher prolonged mixing time, mixtures with higher w/b and also high replacement level of FA are prone to aggregate blockage while flowing narrow section of mini V-funnel nozzle (T40). The reason is that, prolonged mixing time can break down the formation of viscous behavior of mixture to retain the aggregate in the suspension and consequently lead to aggregate blockage. Fig. 5b demonstrates the variation of flow time with mixing time for ternary blends. The results show that, using ternary blends of FA and MK can compensate the viscosity reduction and also aggregate blockage especially for w/b ratio of 0.35. By increasing the w/b ratio, the risk of blockage and segregation of the mixture may be increased. In this case the content of using MK and FA plays an important role for stabilizing the mixture. As the properties of a SCC are characterized with a relatively low yield value to ensure high flowability and a moderate viscosity to resist instability, the combined use of MK with FA seems to be effective to accomplish the aforementioned requirements. Similar results were obtained by Guneyisi and Gesoglu [12] who noted that, ternary use of FA and MK, remarkably diminished a buildup of viscosity of MK. 3.3. Stability 3.3.1. Bleeding and VSI In order to assess the stability of the mixtures, bleeding of SCMs at 5 min (B5) and 40 min (B40) of agitation was measured indirectly through the mini-slump flow test. It should be noted that, at 5 min of agitation, all mixtures exhibited no bleeding except mixtures with high replacement level of FA (LF50 and HF50). The B5 of these mixtures was measured to be 7 cm and 8 cm, respectively. Bleeding of SCMs at 40 min (B40) are summarized in Fig. 6a and b for binary and ternary blends, respectively. The results show that, increasing the replacement level of FA in binary system, leads to higher bleeding during prolonged mixing time. In this case, mixtures with higher w/b are more prone to bleeding. On the other hand, by using MK even at low dosages, bleeding of the mixtures was completely disappeared. For example, in case of w/b of 0.45, addition of only 10% MK resulted in bleeding reduction from 4.5 cm to 0 cm. The reason is probably due to the high surface area of MK that results in higher water adsorbing that could eliminate the bleeding. As shown in Fig. 6b, for ternary blends, the bleeding of the binary mixtures with FA was significantly decreased by addition of MK in the mixtures. In the case of mixtures with equal content of MK and FA in ternary system, no clear evidence of bleeding was determined which resembled the binary blends of MK. Preventing bleeding is attributed to the dominant role of MK over the FA in such mixtures with equal content for MK and FA. For the second series of ternary mixtures, as the amount of the MK was lower than 7.5% by the mass of total binder, the bleeding was appeared. On the other hand, using MK higher than 7.5% had significant effect to prevent bleeding. The influence of prolonged mixing time on the dynamic segregation resistance of the mixtures was also evaluated through the VSI during slump flow test. The VSI of the mortars for elapsed time at 5 min and 40 min of agitation are presented in Table 4. The obtained results show that increasing the replacement level of FA in both w/b ratios of 0.35 and 0.45 reduces the resistance to segregation and bleeding. For instance, for the binary mixture with 30% FA, increasing the w/b ratio from 0.35 to 0.45 increased the va-

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Fig. 5. Flow time of the mixtures (a) binary blends and (b) ternary blends.

Fig. 6. Bleeding of the mixtures after 40 min of agitation (a) binary blends and (b) ternary blends.

lue of VSI from 1 to 2 for elapsed time at 5 min, while for elapsed time at 40 min the VSI had the value of 3 for both w/b ratios. Spherical shape of FA which tends to reduce friction at the interface of aggregate, results in some sedimentation of particles, as shown in Figs. 3 and 4. In addition, results of the S-shaped test which is common for the evaluation of the stability of SCC shows

Table 4 Visual stability index (VSI) of self-consolidating mortars. Code

VSI w/b = 0.35

Reference mix 10FA 20FA 30FA 40FA 50FA 10MK 20MK 30MK 5FA5MK 10FA10MK 15FA15MK 20FA20MK 25FA25MK 7.5FA2.5MK 15FA5MK 22.5FA7.5MK 30FA10MK 37.5FA12.5MK

w/b = 0.45

5 min

40 min

5 min

40 min

0 0 0 1 1 3 0 0 0 0 0 0 0 0 1 1 1 0 0

1 1 2 3 3 3 0 0 0 1 1 1 1 1 3 3 3 2 1

1 1 1 2 2 3 0 0 0 1 0 0 0 0 1 1 3 1 1

3 3 3 3 3 3 0 0 0 2 2 1 1 1 3 3 3 2 2

that mixtures containing FA have not been uniformly filled. In addition, VSI of all SCMs containing binary MK exhibited highly stable (VSI = 0) mixes with no changes in the status of stability with prolonged mixing. Adequate cohesiveness to control bleeding and segregation can be secured by incorporating MK as shown in Fig. 3. In addition, using MK in the ternary blends reduces detrimental effect of FA and it improves stability resistance. On the other hand, increasing the w/b may deteriorate the stabilizing effect of MK, especially for higher replacement level of FA. In this case, the increase of inter-particle distance and reduction of the cohesiveness through an increase in water content can lead to segregation. For example, ternary blends with w/b of 0.35 and incorporating equal value of FA and MK exhibited highly stable state for elapsed time at 5 min (VSI = 0) however, for elapsed time at 40 min, slight bleeding on surface of the mixture was observed (VSI = 1). On the other hand, for w/b ratio of 0.45, increasing the mixing time resulted in bleeding, surface settlement and aggregate segregation (VSI = 2). Therefore, based on the visual inspection can be concluded that mixtures under continuous agitation are more prone to instability as the accumulation of aggregate at the center or bleed water at the edge of mixture. 3.3.2. Segregation index (SI) Static stability of the mixtures was also evaluated by determining the aggregate segregation through the column segregation test. The results of mini-column segregation test after 15 min of agitation are depicted in Fig. 7a and b for binary and ternary blends, respectively. As expected, increasing water content, leads to a reduction of the cohesiveness of the mixture and so, the resistance to aggregate segregation decreases. For example, reference

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mixtures with w/b ratio of 0.35 and 0.45, show very stable (SI < 30%) and instable (SI > 130%) conditions, respectively. In a same level of powder replacement, all mixtures containing higher water content exhibit higher SI than those mixtures with lower w/b, which indicate that w/b has a critical effect on the aggregate segregation. Similar results were reported by Libre et al. [20] who showed that the most significant parameters influencing aggregate segregation is w/c ratio. Fig. 7a shows that mixtures incorporating FA have higher segregation index compared to reference mixture for both w/b ratios of 0.35 and 0.45. For instance, increasing the FA content from 0 in the reference mixture to 20% and 50% of total binder mass in w/b ratio of 0.35 increased the segregation index by about 48% and 160%, respectively. On the other hand, the addition of FA by 50% in the reference mixtures with a w/b of 0.45, led to an increase in segregation index from 166% to 196%. On the other hand, using MK seems to be very affective to enhance segregation resistance of the mixtures. All SCMs with MK exhibited very stable status for both w/b ratios of 0.35 and 0.45 which completely eliminated the static segregation by higher MK replacement. In this case, due to the high fineness of MK, the addition of MK would markedly increase the surface area of the solid particles. Hence, the addition of MK could greatly decrease the water film thickness and also increase cohesiveness and stability of the mixture. Moreover, the effect of MK to stabilize self-consolidating composites was more pronounced at higher w/b ratio. For example, in the mixtures with w/b of 0.45, the addition of MK by 10% of cement mass, led to a decrease in segregation index from 160% to 16.8%, while in the mixtures with a w/b of 0.35, the effect of addition of MK by 10% on the segregation index of SCMs was negligible. In addition, based on the obtained results, the application of MK (10%, 20% or 30%) was more effective in reducing the SI of the sample with w/b = 0.45 compared to the decrease of w/b to 0.35. For example, increasing w/b ratio from 0.35 to 0.45 in the reference mixture increases the SI from 20% to 166% while, addition of 30% MK in the reference mixtures with a w/b of 0.45, leads to decrease in segregation index from 166% to 2%. Fig. 7b demonstrates that, the detrimental effect of FA on the stability of SCM mixtures with binary replacement of FA is reduced by ternary blends of FA and MK. In the case of mixtures with equal content of MK and FA in ternary blends and lower w/b ratio, all the SCMs exhibited very stable states, while at higher water content and low replacement of MK (5%FA5%MK), SCM exhibited semi stable state. On the other hand, while the amount of MK was higher than 10%, all the mixtures met very stable state. Similar results were observed for ternary mixtures with one-third ratio of MK to FA replacement. In this case, test results show that a content of MK around 10% by weight of total binder should be adopted when

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high volume of FA is used in the mixture. Therefore, based on the results, using MK is an effective way to increase stability of the mixture subjected to prolonged mixing time. 3.4. Relationship between fluidity, viscosity and stability of SCMs Fig. 8 shows three regions of stability index as a function of fluidity and viscosity after 40 min of agitation. These regions correspond to SI values ranging lower than 30% (workability region 1), between 30% and 130% (workability region 2), and more than 130% (workability region 3) which exhibit very stable, semi stable and instable mixtures, respectively. Regression analysis of the results shows that there is a close relationship between fluidity and stability of the mixtures. The results show that slump flow diameter of SCMs in the workability region 1 is mostly less than 35 cm, while other mixtures in the workability regions 2 and 3 exhibit fluidity greater than 35 cm and flow time lower than 3 s. In addition, the results show that mixtures with higher segregation index have lower flow time. Therefore, either decreasing fluidity or increasing the flow time has improving effects on stability of SCMs. As shown in previous section, in mixtures with binary blends of FA, as the FA content increases, the flowability of the mixture improves, but at the same time due to the increased free water content, the segregation resistance of the mixtures gradually decreases. On the other hand, in mixtures incorporating binary blends of MK, the cohesion and resistance to instability increases, but the mixtures would suffer from poor fluidity. Based on the results, mixtures containing ternary blends with a balance between FA and MK provide sufficient cohesion to the fresh concrete; it reduces the intrinsic shear strength of the mixture and interparticle friction, and in addition, exhibits high stability resistance. Therefore, to achieve both high flowability and high segregation simultaneously, it is necessary to improve the cohesiveness and reduce the particle interlocking of the concrete mixture. The relationship between fluidity and stability of mixtures after 40 min of agitation is demonstrated in Fig. 9. Besides, in Fig. 9, the filled makers were used to show the mixtures with w/b = 0.35. The results show that segregation index of the mixtures exponentially increases as the fluidity increases. Some researchers [15,20] have found that, there is an exponential relationship between stability and fluidity of SCC. Fig. 9 is divided into four regions as a function of fluidity and segregation index for binary and ternary mixtures. Mixtures in the first region have the fluidity lower than 25 cm and flow time greater than 7 s. In addition, these mixtures have the SI less than 5% and VSI = 0, which correspond to highly stable mixture, while suffering from high viscosity. Test results show that binary mixture incorporating 30% of MK is placed in the first region, but this mixture does not have sufficient fluidity to flow

Fig. 7. Static segregation index of mixtures (a) binary blends and (b) ternary blends.

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Fig. 8. Relationship between fluidity, viscosity and stability of mixtures.

Fig. 10. Aggregate dispersion through a cross section (a) SI 6 30%; (b) 30% < SI < 130% and (c) SI P 130%.

Fig. 9. Relationship between stability and fluidity of mixtures.

readily under their own weight. Therefore, mixtures with a flow time greater than 7 s may suffer from poor workability. Mixtures in the second region (mixtures with fluidity between than 25 cm and 35 cm and also segregation index less than 30%) correspond to stable and highly flowable mixtures. Based on the obtained results from mini-slump flow (Figs. 1 and 2) and minicolumn segregation (Fig. 7) tests, mixtures with equal amount of MK and FA have shown sufficient fluidity and high segregation resistance (SI 6 30%) which are placed in the second region. Furthermore, the obtained results from hardened visual stability index (HVSI) indicate that mixtures in the second region correspond to high segregation resistance and uniform dispersion of aggregates as shown in Fig. 10a. On the other hand, all of the mixtures with flow diameter more than 35 cm have a stability index above 30% and instability may occur in all the cases. This is due to the high water to binder ratio or high replacement level of FA which can increase the fluidity, while decreases the segregation resistance. Mixtures in the third region have the fluidity between 35 cm and 40 cm and are categorized as semi stable mixtures (30% < SI < 130%). Mixtures in this region correspond to HVSI of 2 which is related to the evidence of a mortar layer less than 25 mm thickness at the top of the hardened specimen (Fig. 10b). All placed mixtures in the fourth region, with a slump flow diameter greater than 40 cm are prone to severe segregation (SI P 130%). As shown in Fig. 10c, HVSI of the mixtures with SI higher than 130% rates as 3 exhibiting a non-uniform distribution of aggregates and mortar layer thickness greater than 25 mm. Based on these results, for obtaining high fluidity, increasing the w/b should not be applied due to instability aspects. Moreover, higher w/b may, also, lead to poor mechanical behavior of hardened concrete.

The regression analysis shows that in order to achieve stable mixtures, there exists a threshold value of fluid capacity for each series of binary and ternary mixture. By increasing the fluidity above a threshold value of fluid capacity the risk of instability including bleeding and aggregate segregation increases. Test results show that, mixtures containing ternary blends of MK and FA have higher fluid capacity than binary mixtures incorporating FA. In other words, the detrimental effect of increasing w/b ratio and also FA content on the stability of SCM mixtures can be eliminated by ternary use of MK and FA, and therefore, it leads to higher fluidity with appropriate stability. This may be explained by the fact that using MK can decrease the water film thickness which lead to an increase in the cohesiveness and so increase the stability of the mixture. Therefore, when highly fluid mixtures are needed, using MK in the mixture composition can increase the fluid capacity of the mixture without any signs of instability. 4. Conclusions The objective of this paper is to investigate the effect of water to binder ratio, binary and ternary blends use of FA and MK, and also prolonged mixing time on fluidity, viscosity and stability of self-consolidating mortars. Moreover, this study attempts to investigate the relationship between fluidity and stability of SCMs subjected to prolonged mixing time. Based on the obtained results from this study the following conclusions can be drawn:  By prolonged mixing time, flocculated cement particles are dispersed and as a result, water film thickness and fluidity of the mixture increases. An increase in water film thickness causes a decrease in solid contacts and so cohesiveness and stability of the mixture decreases.  For w/b ratio of 0.35, using FA up to 20% does not have a significant effect on the variation of fluidity with mixing time, and mixtures are very stable. However, incorporation of FA higher than 20%, mixtures are more prone to instability such as segregation and bleeding.  MK blends can strongly control the instability due to high fineness of MK which decreases the water film thickness and so increases the cohesiveness of the mixture. Thus, in order to provide stability retention particularly when prolonged mixing is into consideration, using MK in mixtures incorporating FA can be beneficial.

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 The effect of MK to stabilize self-consolidating mixtures was more pronounced at higher w/b ratio. For example, in the mixtures with w/b of 0.45, the addition of MK by 10% of cement mass, led to a decrease in segregation index from 160% to 16.8%, while in the mixtures with a w/b of 0.35, the effect of addition of MK by 10% on the segregation index of SCMs was negligible.  The application of MK (10%, 20% or 30%) was more effective in reducing the SI of the sample with w/b = 0.45 compared to the decrease of w/b to 0.35. For example, increasing w/b ratio from 0.35 to 0.45 in the reference mixture increases the SI from 20% to 166% while, addition of 30% MK in the reference mixtures with a w/b of 0.45, leads to decrease in segregation index from 166% to 2%.  In order to achieve a stable mixture, there exists a threshold value of fluid capacity for each series of mixtures. By increasing the fluidity higher than the threshold value, the risk of instability increases. When highly fluid mixtures are needed, using MK in the mixture composition can increase the fluid capacity of the mixture without any signs of instability.  Mixture containing ternary blends with a balance between FA and MK, provides sufficient cohesion for the fresh mixture. Furthermore, as the behavior of hardened concrete is greatly influenced by its properties in the fresh state, further research is needed to investigate the effect of stability on mechanical and durability properties of self-consolidating mixtures.

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