Experimental study on segregation resistance of nanoSiO2 fly ash lightweight aggregate concrete

Construction and Building Materials 93 (2015) 64–69

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Experimental study on segregation resistance of nanoSiO2 fly ash lightweight aggregate concrete Gao Yingli ⇑, Zou Chao School of Traffic and Transportation Engineering, Changsha University of Science and Technology, Changsha 410114, China

h i g h l i g h t s  Fly ash gives LWAC mixture with higher fluidity.  NanoSiO2 improves segregation resistance of LWAC.  Combined admixture of nanoSiO2 and fly ash gives LWAC both higher fluidity and higher segregation resistance.  LWAC with nanoSiO2 and fly ash shows higher strength and lower brittleness.

a r t i c l e

i n f o

Article history: Received 6 January 2015 Received in revised form 7 April 2015 Accepted 1 May 2015

Keywords: Lightweight aggregate concrete NanoSiO2 Fly ash Segregation Strength

a b s t r a c t Lightweight aggregate concretes (LWACs) of strength grade raging from LC40 to LC50 were prepared by adding different proportions of fly ash and nanoSiO2. Segregation resistance of LWAC was studied by slump and segregation degree tests. The results indicate that adding of finer fly ash enhances the fluidity of LWAC and enlarges the slump, meanwhile, with the increase of fly ash content, segregation degree of concrete is decreased and floating of lightweight aggregate is somewhat alleviative but still exists; adding of nanoSiO2 improves the segregation resistance of LWAC to a certain extent, but deteriorates the fluidity of concrete mixture; combined admixture of nanoSiO2 and fly ash not only increases the slump of LWAC, but also improves its segregation resistance, prevents the floating of lightweight aggregate and makes concrete show better homogeneity. Mechanical property tests indicate that combined admixture of nanoSiO2 and fly ash improves the compressive strength, increases the tension–compression ratio and reduces the brittleness of concrete. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Lightweight aggregate concrete (LWAC) can reduce the self-weight on the basis of ensuring effective strength, so as to increase the floor height and bridge span due to its light weight, high strength and high durability [1–2]. The density of LWAC is 25–40% smaller than ordinary concrete under the same conditions, so construction of various buildings using LWAC has good economic benefits and technology prospects [3–4]. However, since the densities of most lightweight aggregates are smaller than cement slurries, it makes concrete mixture segregate that the lightweight aggregates in LWAC are easy to float during the vibrating process and on the stationary state [5–6]. Additionally, that lightweight aggregate is easily absorbent in the mixing process due to its porous structure makes the slump of LWAC lose seriously [7]. Li and Ding have studied the effects of different aggregate types on segregation ⇑ Corresponding author. Tel.: +86 15874286258. E-mail address: [email protected] (Y. Gao). http://dx.doi.org/10.1016/j.conbuildmat.2015.05.102 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

resistance of LWAC, and the results show that spherical ceramsite with smooth surface that floating easily has relative poor segregation resistance and the larger the ceramic particle size, the more obvious the floating phenomenon [8]. Wu, Zhang et al. have proposed self-compacting lightweight aggregate concrete technique to solve the floating problem of lightweight aggregate, and have designed two different mix proportions. It is proved that self-compacting lightweight aggregate concrete mixture has good workability and shows no segregation [9]. Chen and Liu have studied the effects of incorporation of different fibers on segregation resistance of LWAC, the results show that incorporation of fibers reduces the aggregate segregation and improves the integrity of LWAC mixture, but decreases the slump of mixture [10]. How to control the floating of aggregate in the condition of without restraining other properties of LWAC has been a hot point of academic research, and also a difficult point in engineering practice. The emergence of nano-materials provides a new perspective to solve this problem [11–12]. In recent years, there are lots of researches on the application of nano-materials in concrete. For

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example, Bastami et al. have researched the high temperature properties of high strength concrete modified by nanoSiO2, the results indicate that mechanical properties of concrete under high temperature can be improved by nano-materials, residual stress and tensile strength of concrete can be enhanced as well [13]. Yu et al. have studied the effect of nano-silica on the hydration of ultra-high performance concrete (UHPC) with a low binder amount, the results reveal that, on one hand, the retardation effect that dormant period of cement hydration is extended by superplasticizer can be significantly compensated by the nucleation effect of nano-silica, on the other hand, with the addition of nano-silica, the of UHPC significantly increases, which causes that more air is entrapped in the fresh mixtures and the porosity of the hardened concrete correspondingly increases [14]. Behfarnia and Salemi have studied the effects of nanoSiO2 and nanoAl2O3 on frost resistance of normal concrete, experimental results show that the frost resistance of concrete containing nano-particles are considerably improved, and it is also concluded that the frost resistance of concrete containing nanoAl2O3 was better than that containing the same amount of nanoSiO2 [15]. Shaikh and Supit have researched the mechanical and durability properties of high volume fly ash concrete containing nanoCaCO3, and results reveal that the addition of nanoCaCO3 not only leads to much denser microstructure in HVFA matrix but also changes the formation of hydration products, hence contributes to the improvement of early-age compressive strength and durability properties of HVFA concretes [16]. However, the study about effect of nano-material and fly ash on segregation resistance of LWAC is still not seen. Hence, this paper presents the LWACs of strength grade raging from LC40 to LC50 by adding different proportions of fly ash and nanoSiO2, studies the influence laws on segregation resistance and mechanical properties of LWAC in the aspects of fly ash content and nanoSiO2 content, and provides a theoretical basis for improving the workability of LWAC in engineering application.

Fig. 1. NanoSiO2.

2. Experiment 2.1. Raw materials The cement used in this study is Ordinary Portland Cement (OPC) PO42.5 with specific surface area of 330 m2/kg, provided by Huaxin Cement (Zhuzhou) Co. Ltd. Fly ash (FA) is offered by Hunan Xiangtan power plant with specific surface area of 425 m2/kg. NanoSiO2 (NS, as shown in Fig. 1) used is produced by Jinhe nano-chemical industry (Shijiazhuang) Co. Ltd, and its specific surface area is 170 m2/g. Coarse aggregate used is high-strength crushed shale ceramsite (as shown in Fig. 2) with continuous grain size of 5–20 mm, and its bulk density is 850 kg/m3, apparent density is 1405 kg/m3, measured cylinder compressive strength is 6.1 MPa, 1 h water absorption rate is 6%. Fine aggregate chosen is Xiang river sand with fineness modulus of 2.75, bulk density of 1480 kg/m3, apparent density of 2650 kg/m3. A naphthalene sulfonate based superplasticizer is used to adjust the workability of LWAC. The chemical components of used OPC and FA are shown in Table 1, respectively. Additionally, the chemical properties of nanoSiO2 are shown in Table 2. 2.2. Test methods The slump test of fresh mixture of LWAC was carried out in accordance with Chinese standard GB/T 50080-2002 ‘‘test method of performance of ordinary concrete mixture’’. The segregation degree test is to measure the weight difference of lightweight aggregate between upper layer and lower layer of segregation degree bucket after 20 s vibration. The segregation degree bucket consists of three conjoint cylindrical buckets with the diameter of 200 mm, height of 200 mm. When measured, the cement mortar is washed away and the lightweight aggregates are picked out then their dried weights are weighed. The formula of segregation degree is shown as follows:

2ðg 1  g 2 Þ SG ¼  100% ðg 1 þ g 2 Þ

ð1Þ

where SG is the segregation degree (%), g 1 the weight of aggregate in upper layer (g), g 2 the weight of aggregate in lower layer (g).

Fig. 2. Crushed shale ceramsite.

Table 1 Chemical components (by mass) of cementitious materials %. Material

SO3

SiO2

Fe2O3

Al2O3

CaO

MgO

K2O

Na2O

OPC FA

2.41 0.42

23.30 50.25

2.77 5.35

5.41 34.20

61.16 4.50

2.65 1.50

0.68 1.20

0.07 0.80

The test of mechanical properties of LWAC was carried out in accordance with Chinese standard GB/T 50081-2002 ‘‘test method of mechanical properties of ordinary concrete’’. LWAC samples were cast in molds with the size of 100 mm  100 mm  100 mm. The specimens were demolded approximately 24 h after casting and then cured in standard curing room of temperature about (20 ± 3)°C. After curing for 7 and 28 days, the compressive strength and splitting tensile strength of the specimens were tested, respectively. Three specimens were tested at each age to compute the average strength.

2.3. Mix design of LWAC Table 3 illustrates the details of mix proportions of LWAC containing different amounts of FA and/or NS as a replacement of cement. The water binder ratio was kept constant as 0.3 and three different proportions of FA (18%, 24%, and 30%) and/or NS (0.5%, 1.0%, and 2.0%) by weight of cement were used for production of LWAC mixes. The control concrete without FA and NS was fabricated along with above mixes.

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Table 2 Chemical properties of NanoSiO2. SiO2 content (%)

Residue on sieve (45 lm)/6%

Total iron content (mg/kg)

SO2 4 content/6%

PH

Specific surface area (m2/g)

Average particle size (nm)

Bulk density (g/L)

95.1

0.20

300

0.2

6.5

170

17

20.5

2.4. Specimen casting The lightweight aggregates were pre-wetted for 24 h before mixing of LWAC. All pre-wetted lightweight aggregates, sand, cement and admixtures (FA or NS) were slowly blended for 30 s in a dry state. Then water reducer and water were slowly added and mixed for another 150 s. For the test of workability, the slump test and segregation degree test were conducted when the fresh mixture of LWAC was obtained. After that, specimens for mechanical property tests were cast.

3. Experimental results and analysis 3.1. Effect of adding FA only on segregation resistance of LWAC The slump and segregation degree results of LWAC containing different contents of FA are shown in Table 4. As seen from Table 4, with the increasing of FA content, slump of LWAC mixture increases, and fluidity of LWAC is accordingly improved. Moreover, segregation degree of LWAC mixture shows a slight downward tendency, and floating of lightweight aggregate is somewhat alleviative. For example, the slump and segregation degree of LWAC containing 18% FA as a replacement of cement increases 16% and decreases by 0.3 percentage point than that of control concrete, respectively; when the adding of FA increases to 24%, the slump has a increase of 22.9% than before and the segregation degree has a reduction of 0.8 percentage point than that of control concrete; when FA content increases to 30%, the segregation degree of LWAC decreases by 1.4 percentage points. Aggregate floating of LWAC is somewhat alleviative but still exists (as shown in Fig. 3). The slump and segregation degree of LWAC versus the amount of FA is depicted in Fig. 4. The reasons why FA has an effect on workability of LWAC mixture are as follows: firstly, it reduces the frictional resistance between concrete particles and enhances the fluidity of mixture consequently that the ball bearing effect of FA changes ‘‘sliding friction’’ into ‘‘rolling friction’’. Secondly, that FA as a replacement of cement reduces the consumption of cement, so the water consumed by cement hydration is reduced, making free water increased accordingly. Furthermore, adding of FA enhances the cohesiveness of concrete mixture, so resistance of aggregate floating increases and floating phenomenon is alleviative. But containing too much FA, the compressive strength of LWAC will decrease. Table 4 shows the 7-day and 28-day compressive strength of LWAC containing different amounts of FA. It can be seen from Table 4 that compressive strengths of LWAC are improved obviously after adding 18% of FA. This is mainly because concrete mixture segregates seriously before FA added. It is difficult to compactly cast the concrete. And the floating of lightweight aggregates restrains the strength. For example, the 7-day and 28-day compressive strength of specimens adding 18% FA reaches to 41.0 MPa and 54.3 MPa, increases by 15.5% and 10.8% than that of control concrete, respectively. But excessive FA will restrain the early strength of LWAC. For example, the 7-day compressive strength of LWAC decreases 5.2 MPa than that of control concrete when FA content reaches to 30%, decreasing rate of which is 14.6%. Relationship between FA content and compressive strength is shown in Fig. 5. 3.2. Effect of adding NS only on segregation resistance of LWAC Based on the results of chapter 3.1, the influence of different NS additions on the segregation resistance of LWAC is studied. Results

indicate that adding of NS restrains the fluidity of LWAC. As seen from Fig. 6, the slump value of LWAC mixture has a tendency of reduction after adding NS. Although previous study [17] shows that NS has a positive effect on the fluidity of concrete that it can fill part of pores between cement particles in the interfacial transition zone of concrete due to its ‘‘small size effect’’ and change ‘‘filling water’’ into ‘‘surface adsorbed water’’ by occupying partially position of ‘‘filling water’’, the finer NS added increases the specific surface area of whole hybrid system, resulting in the more consumption of ‘‘surface adsorbed water’’. This is negative and dominant to the fluidity of concrete. Therefore, with the increase of NS content, the fluidity of LWAC is reduced and the slump of LWAC shows a decreasing tendency. On the other hand, the incorporation of NS prevents the aggregate floating of LWAC to a certain extent and relieves the segregation of LWAC. For example, specimen containing 0.5% NS has a segregation degree of 13.6%, decreases 0.5 percentage point compared to that of control concrete; the segregation degree of specimen containing 1% NS is 12.1%, has a decrease of 2 percentage points than that of control concrete; when the NS content reaches to 2%, the segregation degree reduces to 11.4%, decreasing rate of which reaches to 19.1%. The relationship between NS content and segregation degree is shown in Fig. 7. Reasons are as follows: NS particles having ‘‘dispersion effect’’ can wrap around the aggregate when added in concrete, and aggregates charged with the same electrical after stirring and rubbing repel to dispersion mutually instead of floating on the surface. Therefore, the lightweight aggregates can be relatively evenly distributed in concrete and the heterogeneity of LWAC is reinforced. 3.3. Effect of combined adding of FA and NS on segregation resistance of LWAC On the basis of the above results, 18% FA, taking comprehensive consideration on workability and mechanical properties, and different proportions of NS are added into study the effect on segregation resistance of LWAC. The results indicate that after combined mixing of FA and NS not only the slump value of LWAC is enlarged, but also the aggregate floating is significantly alleviative (as seen in Fig. 8). It also can be seen from the segregation degree test, the segregation degree of LWAC containing 0.5% NS and 18% FA is 11.7%, 2.1 percentage points smaller than that of control concrete, decreasing rate of which is 15.2%; the segregation degree of LWAC containing 1% NS and 18% FA is 9.2%, 4.6 percentage points smaller than that of control concrete, decreasing rate of which is 33.3%; the segregation degree of LWAC containing 2% NS and 18% FA is 7.8%, decreasing rate of which is 43.5%. Compared from Fig. 7, the segregation resistance of combined adding is significantly better than that of single adding. 3.4. Effect of combined adding of FA and NS on mechanical properties of LWAC Table 4 lists the 7-day and 28-day compressive strength and splitting tensile strength of fly ash LWAC containing different amounts of NS. The relationship between age and compressive strength of different samples is illustrated in Fig. 9. As seen from Fig. 9, the early strength and later strength of concrete containing both FA and NS are improved obviously than before. For example,

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Y. Gao, C. Zou / Construction and Building Materials 93 (2015) 64–69 Table 3 Mix proportions of lightweight aggregate concrete. Code

Cement (kg/m3)

Water (kg/m3)

FA (kg/m3)

NS (kg/m3)

Aggregate (kg/m3)

Sand (kg/m3)

Water reducer (%)

Sand ratio (%)

water binder ratio

C0 F1 F2 F3 S1 S2 S3 SF1 SF2 SF3

500 410 380 350 497.5 495 490 407.5 405 400

150 150 150 150 150 150 150 150 150 150

0 90 120 150 0 0 0 90 90 90

0 0 0 0 2.5 5 10 2.5 5 10

603.2 603.2 603.2 603.2 603.2 603.2 603.2 603.2 603.2 603.2

694.3 694.3 694.3 694.3 694.3 694.3 694.3 694.3 694.3 694.3

1 1 1 1 1 1 1 1 1 1

38 38 38 38 38 38 38 38 38 38

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Note: 1. the quantity of water is net water content; 2. sand ratio is made on volume basis.

Table 4 Test results of workability and mechanical properties of lightweight aggregate concrete. Code

0

30 min

60 min

175 203 215 220 173 170 165 210 206 201

110 146 137 141 102 94 80 131 118 106

22 44 50 43 20 — — 30 36 32

Segregation degree (%)

7-day compressive strength (MPa)

28-day compressive strength (MPa)

Splitting tensile strength (MPa)

14.1 13.8 13.3 12.7 13.6 12.1 11.4 11.7 9.2 7.8

35.5 41.0 33.8 30.3 35.7 37.2 38.6 42.3 41.9 42.7

49.0 54.3 51.7 48.6 49.4 50.9 52.5 55.2 58.5 65.9

5.12 5.20 5.07 4.83 5.26 5.82 6.58 5.82 6.74 7.98

20

250 245 240 235 230 225 220 215 210 205 200 195 190 185 180 175 170

slump segregation degree

19 18 17 16 15 14 13 12

segregation degree(%)

slump(mm)

C0 F1 F2 F3 S1 S2 S3 SF1 SF2 SF3

Slump (mm)

11

0

6

12

18

24

10

30

FA content(%)

Fig. 3. Aggregate floating of LWAC with different FA content.

the specimen SF3 containing 2% NS and 18% FA has the highest strength, 28-day compressive strength of which reaches to 65.9 MPa, 21.4% improves than that of specimen F1. The reasons of improvement are as follows: firstly, the incorporation of NS accelerates the hydration of tricalcium silicate particles in cement; secondly, NS particles fill the internal pores of concrete, increase the density of concrete. Furthermore, with the increase of NS

compressive strength% (MPa)

Fig. 4. Relationship between FA content and slump, segregation degree.

66 64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28

7d 28d

0

6

12

18

24

30

FA content(%) Fig. 5. Relationship between FA content and compressive strength.

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Y. Gao, C. Zou / Construction and Building Materials 93 (2015) 64–69

220 210

slump/mm

200 190 180 170 160

0

1

compressive strength(MPa)

70

none FA 18% FA

60 50 40 30 20 10 0

2

C0 F1 F2 F3 SF1 SF2 SF3

0

7

NanoSiO2 content/%

18% FA none FA

segregation degree/%

13 12 11 10 9 8 7

0

1

21

28

2

Fig. 9. Relationship between age and compressive strength of different samples.

0.15 0.14

tension-compression ratio

Fig. 6. Relationship between nanoSiO2 content and slump.

14

14

age(d)

0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.0

NanoSiO2 content/% Fig. 7. Relationship between nanoSiO2 content and segregation degree.

0.5

1.0

1.5

2.0

NanoSiO2 content( %) Fig. 10. Relationship between NanoSiO2 content and tension–compression ratio.

4. Conclusions This paper presents an experimental study on segregation resistance of nanoSiO2 fly ash lightweight aggregate concrete. From the results presented in this study the following conclusions are drawn: (1) Adding of FA only can enhance the fluidity of LWAC. Meanwhile, with the increasing of FA content, the floating of lightweight aggregate is somewhat alleviative but still exists. (2) Adding of NS only can improve the segregation resistance of LWAC to a certain extent, but at the same time, deteriorate the fluidity of concrete mixture. (3) Combined admixture of NS and FA can not only increase the slump of LWAC, but also improve its segregation resistance, prevent the floating of lightweight aggregate and make the concrete show better homogeneity. (4) Compressive strength of LWAC continuously improves with the increasing of NS content. The compressive strength of LWAC containing 2% NS and 18% FA can increase by 21.4%. Furthermore, NS can improve the splitting tensile strength, increase the tension–compression ratio and reduce the brittleness of concrete. It is recommended that 18% FA and 1% NS is optimized for engineering practice comprehensive considering the economic and technical factors. Fig. 8. Aggregate floating of LWAC (18%FA) with different NanoSiO2 content.

Acknowledgments content, the splitting tensile strength of fly ash LWAC increases gradually, and tension–compression ratio of fly ash LWAC has an increasing tendency (as seen in Fig. 10).

This work is supported by the Natural Science Foundation of Hunan Province, China (No. 13JJ3070) and the Scientific Research

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