- Email: [email protected]
Effect of nano-SiO2 on strength, shrinkage and cracking sensitivity of lightweight aggregate concrete
Construction and Building Materials 175 (2018) 115–125
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of nano-SiO2 on strength, shrinkage and cracking sensitivity of lightweight aggregate concrete X.F. Wang, Y.J. Huang, G.Y. Wu, C. Fang, D.W. Li, N.X. Han, F. Xing ⇑ Guangdong Key Laboratory of Durability in Coastal Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen, Guangdong 518060, China
h i g h l i g h t s The early compressive strength of LWAC was improved by adding nano-SiO2. Nano-SiO2 had no significant influence on the long-term shrinkage of LWAC. With nano-SiO2 addition, LWAC reduces the cracking risk at the early age. The simplified models for the compressive strength and shrinkage are proposed.
a r t i c l e
i n f o
Article history: Received 20 July 2017 Received in revised form 4 April 2018 Accepted 13 April 2018
Keywords: Lightweight aggregate concrete (LWAC) Nano-SiO2 Long-term shrinkage Cracking sensitivity Simplified model
a b s t r a c t This paper presents an experimental investigation on the effect of nano-SiO2 on the compressive strength, shrinkage and early cracking sensitivity of lightweight aggregate concrete (LWAC). Two types of ceramsite with different water absorption were used as lightweight aggregate in this study. LWAC with different nano-SiO2 dosage (1%, 2%, 3%) were compared with the reference LWAC to assess the effects of nanoSiO2 on LWAC. Results revealed that the incorporation of 3% nano-SiO2 increased compressive strength of LWAC significantly while the influence of nano-SiO2 on the long-term shrinkage of LWAC was not significant. The total cracking area was decreased with the increase in nano-SiO2 dosages from 1% to 3% by mass of the total binders at early age. The results of scanning electron microscopy (SEM) showed that the interfacial transition zone (ITZ) between lightweight aggregate and paste was enhanced with 3% nano-SiO2 addition. Subsequently, simplified models for compressive strength and shrinkage of LWAC were proposed and a comparison between the experimental data and models was discussed. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introductions In recent years, the development of lightweight aggregate concrete (LWAC) has drawn increasing attention from researchers and engineers. Compared with normal weight concrete, reducing construction costs by decreasing structural dead load and lower thermal conductivity make LWAC increasingly used for housing, bridges and other construction projects [1–3]. However, the properties of lightweight aggregates used in LWAC vary within wide limits and the aggregates exert an important influence on the elastic modulus of concrete [4,5]. Generally speaking, lightweight aggregates have a lower strength and elastic modulus than the mortar matrix and, therefore, than normal weight concrete [6]. The low compressive strength and low elastic modulus of LWAC ⇑ Corresponding author at: Guangdong Key Laboratory of Durability in Coastal Civil Engineering, Shenzhen University, 3688 Nanhai Ave, Shenzhen 518060, Guangdong, China. E-mail address: [email protected] (F. Xing). https://doi.org/10.1016/j.conbuildmat.2018.04.113 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
are often considered as a major shortcoming [2,5] except for some special applications. In order to compensate this deficiency, improve the strength and durability of LWAC and reduce the cement consumption, more and more attentions have been paid on the mineral additions. Similar to normal weight concrete, LWAC can be effectively modified by adding supplementary cementitious materials (SCMs), which can improve the mechanical and durability properties of concrete. Previous studies found that the bulk properties of LWAC might be able to be modified with SCMs addition, such as silica fume [7,8], fly ash [9–11], ground granulated blast furnace slag [12–14] and metakaolin [15,16]. Among these SCMs, silica fume has been proved to be the most effective SCM for the performance enhancement of LWAC [12,16,17]. As a pozzolanic material, silica fume reacts with Ca(OH)2 and produces additional calcium silicate hydrate (C-S-H) gel, which results in a denser microstructure and thereby improving the properties of hardened cementitious materials [18,19]. In general, the improvements were mainly attributed
116
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
to the pozzolanic reaction, as well as the acceleration effect of silica fume on cement hydration and optimizing particle packing of the matrix [17,19,20]. More recently, some studies [21–25] reported that the properties of cement-based materials can be modified effectively by using nano-particles such as nano-SiO2, which has a higher pozzolanic activity than silica fume. A common characteristic of silica fume and nano-SiO2 is that they are both pozzolanic materials which contain over 90% SiO2 (by weight). Ye et al. [26] reported that 3% nano-SiO2 improves the ITZ more effectively than silica fume by digesting Ca(OH)2 crystals, decreasing the orientation of Ca(OH)2 crystals and reducing the crystal size of Ca(OH)2 gathered at the ITZ. Some results [8,27–29] have shown that silica fume could improve the strength and durability of LWAC obviously. Interface bonding between hardened cement paste and lightweight aggregate is improved on account of better packing and pozzolanic reaction [28]. Until now, few studies [21,30] have investigated the effects of nano-SiO2 on the performance of LWAC. In this paper, the effects of nano-SiO2 on the compressive strength, shrinkage and early cracking sensitivity of LWAC were investigated. The effects of nano-SiO2 on microstructure of LWAC at 28 days were also studied. In addition to the experimental work,
simplified models by modifying the ACI-209 model were established by using the experimental results. Moreover, the experimental data was also compared with the model curve in terms of the compressive strength and the shrinkage of LWAC.
2. Experimental 2.1. Materials Two types of ceramsite with different water absorption were used as lightweight aggregate in this study. Ceramsite N (fly ashclay ceramsite) was made in Nantong, China while Ceramsite Y (shale ceramsite) originated from Yichang, China. Main properties of the two ceramsites are given in Table 1, including two different densities. The apparent density and bulk density (GB/T 17431.22010) [31–33] of ceramsite N are 1064.5 kg/m3 and 585.0 kg/m3, respectively, while those of ceramsite Y are 1209.1 kg/m3 and 635.8 kg/m3, respectively. The practicality picture of lightweight aggregate and the SEM images of their pore structures are shown in Fig. 1. The particle size distribution of aggregates used in this study is given in Fig. 2. Amorphous nano-SiO2 was produced by
Table 1 Physical properties of ceramsite. Type
Ceramsite
Cylinder compressive strength (MPa)
Apparent density (kg/m3)
Water absorption 1 h/24 h (%)
Bulk density (kg/m3)
1 2
N (Nantong) Y (Yichang)
4.63 11.66
1064.5 1209.1
9.10/12.70 3.34/5.74
585.0 635.8
Fig. 1. Two types of ceramsite used in this study: (a) ceramsite N; (b) ceramsite Y; (c) SEM image of ceramsite N; (d) SEM image of ceramsite Y.
117
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
Fig. 2. The particle size distribution of aggregates: (a) ceramsite N; (b) ceramsite Y.
Table 2 Physical properties of nano-SiO2. Type
BET-specific area (m2/g)
Average particle size (nm)
SiO2 content (%)
Apparent density (g/m3)
SP1
640 ± 60
20
99.9%
<0.18
Table 3 Physiochemical compositions (% by mass) of fly ash. Compositions
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
SO3
LOI
Content
64
21.32
3.25
4.3
1.03
1.37
0.38
3.05
Table 4 Mix proportions for LWAC (w/b = 0.35). Mix No
N1 N2 N3 N4 Y1 Y2 Y3 Y4 a b
Content (kg/m3) Cement
Nano-SiO2a
Fly ashb
Superplasticizer
Ceramsite
Sand
Water
413.1 408.2 403.3 398.4 413.1 408.2 403.3 398.4
0 (0%) 4.86 (1%) 9.72 (2%) 14.58 (3%) 0 (0%) 4.86 (1%) 9.72 (2%) 14.58 (3%)
72.9 72.9 72.9 72.9 72.9 72.9 72.9 72.9
6.21 6.26 6.29 6.31 6.21 6.26 6.29 6.31
465 465 465 465 465 465 465 465
843 843 843 843 843 843 843 843
170.1 170.1 170.1 170.1 170.1 170.1 170.1 170.1
% of (cement + nano-SiO2 + fly ash) by mass. 15% of (cement + nano-SiO2 + fly ash) by mass.
Mingri Nano-material Co. Ltd, and its physical properties are given in Table 2. The cement used in this study was Portland cement (PII 42.5R), provided by Zhujiang Cement Co., Ltd, and a proper amount of Polycarboxylate-based superplasticizer (SP) was used to adjust the workability of LWAC. Fly ash was offered by Henan Datanggongyi power plant, and the physiochemical compositions of fly ash are shown in Table 3. Natural sand with a fineness modulus of 2.74 was used for concrete mixtures. Tap water was used for concrete mixing. 2.2. Mixing proportions Table 4 summarizes the mix proportions of LWAC used in this study. Identical water to binder ratio (w/b) of 0.35 was used. The amount of the total binders (cementitious materials), lightweight aggregates and fine aggregates in each mix were 486 kg/m3, 465 kg/m3 and 843 kg/m3, respectively. The polycarboxylate-based superplasticizer was used to control the workability for LWAC mix-
tures. Fly ash was added into each mix proportion at 15% by mass of the total binders. The properties of the Fly ash used was in accordance with China National standard (JGJ28-86) [34]. Dosages of nano-SiO2 added in LWAC varied from 0 to 1%, 2% and 3% by mass of the total binders. Cement, fly ash and nano-SiO2 were mixed together for 20 min in a blender, to well-disperse nano-SiO2 into concrete mix. In order to reduce the impact of the difference in moisture content, all lightweight aggregates were pre-wetted in water for 24 h before mixing and the amount of the 24 h absorbed water was included in the moisture content of lightweight aggregates, which was suggested by Ref. [35]. 2.3. Experimental methodology 2.3.1. Test of cubic compressive strength For each LWAC mixing proportion, nine specimens with the size of 100 mm 100 mm 100 mm were cast for the compressive strength test. In order to prevent moisture loss, the molded speci-
118
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
mens were covered with plastic film. After form stripping, the specimens were cured under temperature T = (20 ± 2) °C and humidity RH = 95% until the time of testing. The universal testing machine YAW6306from MTS Systems Co., Ltd was used to load the specimens at a uniform speed of 0.3 MPa/s. Compressive strength of LWAC were determined at 3d, 7d and 28d according to China National Standard (GB/T50081-2002) [36]. The cubic compressive strength was calculated as follows,
f c ¼ 0:95
p A
ð1Þ
where fc is the cubic compressive strength (MPa), P is the failure load (N), A is the area under compression (mm2), and 0.95 is the size conversion coefficient. 2.3.2. Long-term shrinkage test Long-term shrinkage test was conducted for LWAC with the same w/b ratio as the cubic compressive strength test described above, to investigate the effect of nano-SiO2 on shrinkage. The size of the specimens is 100 mm 100 mm 515 mm. The specimens were tested by using the contact shrinkage measuring method and cured for 24 h with plastic film covered. After demold, the specimens were cured under temperature T = (20 ± 2) °C and humidity (RH = (60 ± 5)%) until the time of testing. The shrinkage measuring apparatus were used to test the specimens and the shrinkage was measured by dial indicator with the accuracy of 1 mm. The shrinkage of LWAC were determined at 1d, 2d, 3d, 4d, 5d, 6d, 7d, 14d, 28d, 45d, 60d, 90d, 120d and 150d according to China National Standard (GB/T 50082-2009) [37]. The age was counted from the date of specimens moving into constant temperature and humidity room. The mean value of three replicate specimens was used as the test result of each experimental data. The shrinkage was calculated as follows,
est ¼
l0 lt lb
ð2Þ
where, est refers to the shrinkage strain of concrete at time t, l0 refers to the initial reading, lt refers to the reading at time t and lb refers the gauge length. 2.3.3. Early cracking test For each mixing proportion, two specimens with the size of 800 mm 600 mm 100 mm were cast for early cracking test according to China National Standard (GB/T 50082-2009) [37]. A sketch map of the cracking slab arrangement with dimensions is shown in Fig. 3. The cracking slab was mainly assembled by 7 ribs which act as crack inducers. The restraint effect of the ribs should have a direct impact on the emergence of cracks. The test was carried out under the condition of constant temperature (T = (20 ± 2) °C) and constant humidity (RH = (60 ± 5)%) room. A wind velocity of 5 m/ s parallel to the surface of the concrete plate was applied by adjusting the position and wind speed of the fan after casting concrete in 30 min. The cracks were measured at (24 ± 0.5) h and the test time was count from the concrete mixing with water. The length of the cracks in the concrete plate were measured by a rule while the width of the cracks was measured by reading microscope with the accuracy of 0.02 mm. 2.3.4. Scanning electron microscopy The microstructure of LWAC specimens N1 and N4 was examined to determine the effect of nano-SiO2 addition by using scanning electron microscope (SEM) examination (FEI Quanta TM 250 FEG). After the compressive strength test, the samples of SEM were taken from the fractured surface. Before SEM observation, samples were dried in a vacuum oven. In order to achieve better observation results, the samples were sprayed with a layer of conductive
1.Long side plate; 2. Short side plate; 3. Bolt; 4. Reinforced rib; 5. Crack inducer; 6. Sole-plate. Fig. 3. Sketch map of the early cracking test arrangement.
film and the test was carried out under high vacuum condition. All the samples were tested in accordance with China National Standard (GB/T 16594-2008) [38]. 3. Results and discussion 3.1. Compressive strength 3.1.1. Effect of nano-SiO2 dosage on strength development of LWAC The cubic compressive strength of LWAC with different nanoSiO2 dosages at various ages is shown in Fig. 4. For LWAC made with ceramsite N, the compressive strength continued to increase up to 28 days with the increase in nano-SiO2 dosage. In comparison to the reference LWAC, the compressive strength of concrete at the age of 3 days increased by 14.2%, 18% and 23.5%, with 1%, 2% and 3% nano-SiO2 addition, respectively. At the age of 28 days, an increase of 3.2%, 13.3% and 16.8% was noticed for 1%, 2% and 3% nano-SiO2 addition, respectively. Among these three nano-SiO2 dosages, the compressive strength of LWAC with 3% presented the largest increase at 3d, 7d and 28d, which increased by 23.5%, 23.7% and 16.8%, respectively. Based on the experimental results, it is noted that the rate of compressive strength gain of LWAC with nano-SiO2 addition is fast at the early age and decreases gradually at the later age. Moreover, by comparing Fig. 4(a) and (b), it can be observed that the development tendency of the compressive strength of LWAC made with ceramsite Y is similar to that made with ceramsite N. As expected, the compressive strength of LWAC was noticeably improved at early ages (within 28 days), with 1%–3% nano-SiO2 addition. Such phenomenon for LWAC was similar to what has been found in other research work [30]. Meanwhile, the improvement of cementitious materials in compressive strength due to incorporation of nano-SiO2 was also reported by several researchers [22,24,26]. The mechanism that the nano-SiO2 addition modifies the LWAC can be explained from its physical and chemical effects [30], as well as the amount of nano-SiO2 used in the experiment [26].
119
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125 36
40
34 38
30 28 26 24 22 20
N-3d N-7d N-28d
18 16
Compressive strength (MPa)
Compressive strength (MPa)
32 36 34 32 30 28
Y-3d Y-7d Y-28d
26 24
14
0
1
2
0
3
1
2
3
Nano-SiO2 content (%)
Nano-SiO2 content (%)
(a)
(b)
Fig. 4. Compressive strength of LWAC with different nano-SiO2 dosages: (a) Ceramsite N; (b) Ceramsite Y.
In addition, by comparing the compressive strength between two types of LWAC, it can be found that the difference between two types of LWAC is significant. Herein, the compressive strength difference ratio, which is defined by (fY-fN)/fN and varies from 0 to 1, is used to represent this difference, where fY and fN refer to the compressive strength of LWAC made with ceramsite Y and ceramsite N, respectively. Fig. 5 shows the compressive strength difference ratio varying with the content of nano-SiO2 incorporation. In general, for lightweight aggregates, higher particle density resulted in LWAC with greater compressive strength [2]. This is somewhat consistent with the SEM images shown in Fig. 1, which displays that the microstructure of ceramsite Y is much denser than that of ceramsite N. Thus, to study the effect of nano-SiO2 addition on the mechanical properties of resulting LWAC, the properties of aggregate (such as particle density) must be taken into account. 3.1.2. Simplified model of compressive strength for LWAC Note that, concrete with the pre-wetted lightweight aggregates has the advantage of the internal curing, which is different from the normal weight concrete. The drying shrinkage and the cracking can be decreased effectively by utilizing the porous aggregates with internal curing [39]. Therefore, it is unsuitable to utilize mod-
els for normal weight concrete to estimate the compressive strength of LWAC. After investigating commonly used models, it is found that the ACI209R-92 model [40] is applicable for both ordinary concrete and LWAC. And the law of development over time can be adjusted by modifying the model’s parameters. In this study, a suitable compressive strength simplified model for LWAC is presented by modifying the existing model (ACI209R-92 model), the detail of which is expressed as follows,
t
f c ðtÞ ¼
aþbt
f c ð28Þ
ð3Þ
where, a ¼ 28 28b, a and b are the coefficients, and t in days is the time. From Eq. (3), it can be found that coefficient b is the dominant factor in the model of compressive strength. Based on the foregoing analysis, it can be inferred that coefficient b has close relationship with the moisture content of aggregates and the content of mineral additions. Therefore, this study tries to propose a linear model by modifying the coefficient b. As aforementioned, the content of nano-SiO2 (d) and the moisture content of lightweight aggregate in 1 m3 of concrete (x) was employed in the present study to establish a relationship with the coefficients b. Based on
β = -0.1174ω+0.003δ+0.9913 2
Adj-R =0.8624
3d 7d 28d
0.5
1.00 0.98
0.4
0.96
0.3
β
0.94 2 0.9 0 0.9
0.2
8 0.8 6 0.8
0.0
0.2
43 0.8
0.4
1
2
3
Nano-SiO2 content (%) Fig. 5. The compressive strength difference ratio of two types of LWAC.
0.8
1.0
0
0
1
0.0
Kg /m 3
0.6
ω(
Na noSiO 2 co nte nt δ (% )
)
0.1
2
Compressive strength difference ratio
0.6
1.2
Fig. 6. The relationship between parameters b and (d, x.).
120
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
the regression analysis of the experimental results, a linear model can be proposed as Eq. (4),
b ¼ A þ Bd þ C x
ð4Þ
where, A, B and C are the coefficients. A comparison of the value of b with the analytical surface is shown in Fig. 6. It can be seen that a good agreement (adj-R2 = 0.8624) is demonstrated when A = 0.9913, B = 0.003, C = 0.1174 are employed. In order to improve the accuracy of the model and obtain a more suitable simplified model, similar to the modified parameter b, the content of nano-SiO2 (d) was employed to modify the compressive strength fc (28). A linear relationship between the fc (28) and d can be fitted as follows,
f c ð28Þ ¼ P1 d þ P2
ð5Þ
where d refers to the content of nano-SiO2 (%), P1 and P2 are the coefficients depending on the types of the lightweight aggregate. With utilizing the modified Eqs. (4) and (5), Eq. (3) can be adjusted to a new compressive strength model which is suitable for LWAC. In order to verify the applicability of the model, the experimental data of 28d-compressive strength and the calculated values are compared, which is shown in Fig. 7. In addition to the maximum error 2.01%, it is found that the majority of the calculated values match with the experimental data well, which means that the model has a good accuracy for LWAC. However, it is noted that the proposed model may only applicable to LWAC in this study due to the diversity and complexity of the factors affecting the compressive strength. Hence, it is necessary to further study whether other LWAC is suitable. 3.2. Long-term shrinkage 3.2.1. Effect of nano-SiO2 dosage on shrinkage of LWAC Fig. 8 shows the shrinkage development of LWAC with time for different nano-SiO2 dosages with ceramsites N and Y, respectively. With 1% to 3% nano-SiO2 addition, the similar development tendency of the shrinkage has been observed in LWAC for both ceramsites N and Y. The effect of nano-SiO2 on shrinkage of LWAC is not statistically significant in comparison to the reference LWAC. At the early ages, all the shrinkage curves were almost overlapping. At the age of 90 days, the average measured shrinkage values were increased by 2.2%, 3.9% and 5.4% for LWAC of type N with 1%, 2% and 3% nano-SiO2 addition, respectively. Meanwhile, an increase
34
3.2.2. Simplified model of long-term shrinkage As aforementioned, the moisture content of concrete is the dominating factor on the shrinkage and the higher water absorption of the lightweight aggregate is different from the normal weight concrete. Hence, it can be deduced that a correlation must exist between the internal curing and the shrinkage of LWAC. In this study, the ACI209 model was employed again to estimate the long-term shrinkage, for the same reason mentioned in 3.1.2. The existing ACI209 model suggested by the ACI committee 209 [40] is as follows,
32
esh ðt; tsh;0 Þ ¼
38 2
Adj-R =0.9852
36
Measured strength (MPa)
shrinkage values of 2.5%, 4% and 3.4% was noticed for 1%, 2% and 3% nano-SiO2 addition to LWAC of Y, respectively. The effect of the types of aggregate on the shrinkage development of LWAC is shown in Fig. 9. Regarding to the reference LWAC, as shown in Fig. 9(a), there is some difference between the rates of shrinkage development of LWAC for types N and Y. At the age of 60 days, the shrinkage of LWAC of type N was 2.4% lower than that of type Y, while at the age of 90 days, the former was 3.4% higher than the latter. The development tendency of the shrinkage of LWAC with 3% nano-SiO2 addition, as shown in Fig. 9(b), was similar to what has been observed in the reference LWAC. At the age of 60 days and 90 days, the shrinkage of LWAC of type N was 2.3% lower and 5.4% higher than that of type Y, respectively. Comparing the Fig. 9(a) and (b), it also can be found that the rate of the shrinkage development has a similar trend and does not change very much. From the experimental results, it was revealed that 3% or less nano-SiO2 addition has no significant influence on the long-term shrinkage of LWAC. The shrinkage strain only increased slightly with the increase in nano-SiO2 content. The mechanisms of that the nano-SiO2 addition leads to small variation on the long-term shrinkage of LWAC can be explained from the effect of moisture content. At the early age, the high specific surface of nano-SiO2 particles increased water demand in the mix, which resulted in a decrease of the moisture content in the cement paste [41,42]. The higher rate of shrinkage development was mainly attributed to the moisture migration [42]. As cement hydration continued, the reduction of moisture content decreased the rate of shrinkage development gradually, leading to a lower rate at the later age. It was also observed that the rate of increase in the long-term shrinkage of LWAC decreased with the age of concrete which was in agreement with a previous study [43]. Regarding to the shrinkage of LWAC with different aggregates, LWAC of type N having a lower shrinkage strain at the early ages may be due to the higher water absorption of lightweight aggregate which provided better reservoir effect. Due to the restraining effect of the aggregate, LWAC of type N showed a higher shrinkage. As the degree of restraint offered may be determined by the elastic properties, the relatively lower cylinder compressive strength of ceramsite N correlated to the higher shrinkage strain.
30
ðt t sh;0 Þ esh1 35 þ ðt t sh;0 Þ
esh1 ¼ csh 780 106
28
N-28d Y-28d
26 26
28
30
32
34
36
38
Predicted strength (MPa) Fig. 7. Comparison between model curve and experimental values of 28dcompressive strength.
ð6Þ
ð7Þ
where, esh ðt; t sh0 Þ refers to the shrinkage strain at time t; t sh0 denotes the age of concrete at initiation of drying (days); t means the observation (current) time (days); esh1 signifies the ultimate shrinkage strain determined by experiments; csh represents the correction factor. In order to improve the accuracy of the model and obtain a more suitable simplified model, similar to the modified model of compressive strength of LWAC mentioned in 3.1.2, the internal
121
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125 600
550
550
500
500
450 400
Shrinkage strain (με)
Shrinkage strain (με)
450 400 350 300 250 200
N1 N2 N3 N4
150 100 50 0
0
20
40
60
80
100
120
140
350 300 250 200 150
Y1 Y2 Y3 Y4
100 50 0
160
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
160
600
600
500
500
Shrinkage strain (με)
Shrinkage strain (με)
Fig. 8. Shrinkage of LWAC with different nano-SiO2 dosages: (a) Ceramsite N; (b) Ceramsite Y.
400
300
200
100
400
300
200
100
N1 Y1
0
N4 Y4
0 0
20
40
60
80
100
120
140
160
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
160
600
600
500
500
Shrinkage strain (με)
Shrinkage strain (με)
Fig. 9. Shrinkage of LWAC made with different aggregates: (a) 0% Nano-SiO2; (b) 3% Nano-SiO2.
400
300
200
Experimental data of N1 Experimental data of Y1 Model curve of N1 Model curve of Y1
100
0
0
20
40
60
80
100
120
140
400
300
200
Experimental data of N4 Experimental data of Y4 Model curve of N4 Model curve of Y4
100
160
0
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
Fig. 10. Correlation between the experimental data of shrinkage strain and the model curve: (a) 0% Nano-SiO2; (b) 3% Nano-SiO2.
160
122
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
(a)
(b)
(c)
(d)
* C: The total cracking area of the LWAC plate. Fig. 11. Cracking of LWAC with different nano-SiO2 dosages: (a) ceramsite N, 0% nano-SiO2; (b) ceramsite N, 3% nano-SiO2; (c) ceramsite Y, 0% nano-SiO2; (d) ceramsite Y, 3% nano-SiO2.
44
Total cracking area (mm²/m²)
Total cracking area (mm²/m²)
1.0
0.5
0.0
-0.5
-1.0
0
1
2
3
42 40 38 36 34 32 0
1
2
3
Nano-SiO2 content (%)
Nano-SiO2 content (%)
(a)
(b)
Fig. 12. Cracking areas of LWAC with different nano-SiO2 dosages: (a) ceramsite N; (b) ceramsite Y.
curing of LWAC was taken into account. The modified model of long-term shrinkage is shown as follows:
esh ðt; tsh;0 Þ ¼
ðt tsh;0 Þ esh1 K þ ðt tsh;0 Þ
ð8Þ
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
(a)
(b)
(c)
(d)
(e)
(f)
123
Fig. 13. The SEM micrographs of pastes without and with nano-SiO2 addition at 28 days: (a) 0% nano-SiO2; (b) 3% nano-SiO2; (c) 0% nano-SiO2; (d) 3% nano-SiO2; (e) paste matrix; (f) image of fly ash.
where K is the influence coefficient of shrinkage determined by moisture content of lightweight aggregate. In the equation, a linear relationship can be obtained between K and the moisture content of lightweight aggregate in 1 m3 of concrete (x) after regression analysis:
K ¼ A þ Bx
ð9Þ
where x refers to the moisture content of lightweight aggregate in 1 m3 of concrete (kg), A and B are the coefficients. The coefficients (A = 1.0663, B = 13.0146) can be determined by the regression analysis on the experimental data of long-term shrinkage of LWAC. Based on Eqs. (8) and (9), the model curves
shown in Fig. 10 can be drawn. From Fig. 10, it is revealed that the shrinkage development of both LWAC without nano-SiO2 addition and LWAC with 3% nano-SiO2 addition matched well with the revised model, which indicates that the long-term shrinkage of LWAC can be estimated by using the revised model proposed in this study.
3.3. Early cracking sensitivity Fig. 11 shows the typically cracking morphology of LWAC made with ceramsites N and Y, under different nano-SiO2 dosages at the early ages. From the cracking morphology, it can be seen that 3%
124
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
nano-SiO2 addition has an obvious effect on the early cracking sensitivity for LWAC of type Y. With 3% nano-SiO2 incorporation, the length of the cracks decreased remarkably and the total cracking area decreased by 25% in comparison to the reference LWAC. However, for LWAC of type N, the effect caused by the addition of nanoSiO2 is not obvious on account of the experimental results that no crack was observed both in LWAC with or without nano-SiO2 incorporation. The cracking areas of LWAC with 1%, 2% and 3% nano-SiO2 are shown in Fig. 12. When no nano-SiO2 was used, the total cracking area of LWAC made with ceramsites N and Y are 0 mm2/m2 and 43.6 mm2/m2, respectively. As the dosage of nano-SiO2 increased from 0%, to 1%, 2% and 3%, the total cracking area of LWAC of type Y continued to decrease while the total cracking area of LWAC of type N remained 0 mm2/m2 for all times. Compared with the normal aggregate, the pre-wetted ceramsite has a good performance in internal curing due to the effect of ‘‘reservoir” in the cement paste [44]. Regarding the cracking behavior of LWAC with different lightweight aggregates, it is closely related to the moisture content of the lightweight aggregates [45]. In this study, from Table 1, it can be seen that the water absorption of ceramsites varies significantly, the 24 h water absorption of ceramsite N was about 221% of ceramsite Y. A relatively higher water absorption indicates a better performance for the effect of ‘‘reservoir”, which means it may provide enough water and increase moisture content on the concrete surface in the early shrinkage phase [32,33]. Aggregates with higher moisture content can compensate the reduction of relative humidity in LWAC effectively by capillary suction in cement paste [27], which consequently, reduces the cracking probabilities. Thereby, LWAC of type N showed no visible crack and had a lower cracking sensitivity than that of LWAC of type Y. For LWAC of type Y, 1%–3% nano-SiO2 incorporation significantly reduced the total cracking area. The more the amount of nano-SiO2 used in LWAC, the lower the crack widths or lengths can be observed. Such findings for LWAC of current study is similar to what had been found in silica fume and metakaolin concretes [46]. In addition to the acceleration effect of nano-SiO2 on cement hydration, nano-SiO2, as a micro-filler with extremely fine particle size, can effectively optimize the pore structure of cement paste by decreasing large capillary porosities and increasing medium capillary porosities [24]. With nano-SiO2 addition, the cement hydration products of LWAC can be more compact and homogenous because of the filler effect [30], which may fill the voids between cement and fly ash grains and particularly the ITZ between the paste and the lightweight aggregate, which will be also shown in section 3.4. Thus, the reduced large capillary porosity in the LWAC incorporating nano-SiO2 may efficiently inhibit cracking at the early age. Thereby, relatively less cracking behaviors were observed in LWAC of type Y with the incorporation of nano-SiO2. It should be mentioned that in this study, the effect of nano-SiO2 dosage (1%, 2%, 3%) on early cracking sensitivity of LWAC made with different types of lightweight aggregate was investigated. Whereas, with respect to the performance of nano-SiO2 addition on early cracking sensitivity of LWAC, some relevant chemical analysis or energy dispersive X-ray analysis should be carried out in future studies. 3.4. Microstructure of LWAC The microstructures of the interfacial transition zone (ITZ) between the bulk paste and lightweight aggregate are shown in Fig. 13(a) and (b). By comparing Fig. 13(a) and (b), it can be found that the ITZ’s microstructures of LWAC with 3% nano-SiO2 were more compact than that of the reference LWAC. For LWAC with incorporating 3% nano-SiO2, the borderline between the paste matrix and the ITZ could not be measured specifically and it
appeared that the paste matrix connected with the aggregate together, which is different from that of the reference LWAC. Fig. 13(d) shows the space around the unreacted fly ash particle surrounded by a denser bulk gel, which is different from the reference LWAC shown in Fig. 13(c), where the bulk gel structure is more porous and there are many large pores and gaps around the fly ash particle. Fig. 13(e) shows the unreacted fly ash particle in the paste matrix, and Fig. 13(f) shows the SEM image of fly ash used in this study. From Fig. 13(c) and (d), it can be found that nano-SiO2 with high specific surface has a significant effect on fly ash reaction. For the reference LWAC, amounts of hydration products could be found on the fly ash particle surface and the boundary of the fly ash particle was not clear. However, for the LWAC with 3% nano-SiO2 addition, the fly ash particle surface presents clean and smooth. It is indicated that fly ash in LWAC with 3% nano-SiO2 addition has a lower reaction degree compared with the reference LWAC. The main reason for this could be that the nano-SiO2 and fly ash compete in reacting with Ca(OH)2 and the nano-SiO2 is more reactive than fly ash [47,48]. Therefore, there might be a shortage of Ca (OH)2 in the system, which leads to an impediment in the hydration of fly ash. The results are similar to those using colloidal nano-SiO2 in cement-fly ash cementitious system [49]. Moreover, the influence of nano-SiO2 addition on fly ash/cement hydration is much more complex in nano-SiO2–fly ash–cement systems and it is necessary to conduct more scientific research on these multivariate cementitious material systems. In terms of microstructure modifications, the ITZ and the paste matrix were both much denser and more continuous in contrast to the reference LWAC. This is mainly due to the effects of the nano-SiO2 on the ITZ as discussed in section 3.1.1. The physical and chemical effects can reduce the porosity of cement paste and improve the bonding between the paste and aggregate, both of which can improve the compressive strength.
4. Conclusions In this study, 1%, 2% and 3% dosage nano-SiO2 was used in LWAC to investigate the influence of nano-SiO2 on the compressive strength, long-term shrinkage and early cracking of LWAC. Based on the experimental results, the following conclusions can be drawn: (1) The compressive strength of LWAC was improved obviously at the early ages by adding nano-SiO2 particles. The compressive strength was increased with the increase in nanoSiO2 dosages from 1% to 3% by mass of binders. With 3% nano-SiO2 incorporation, the 3d, 7d and 28d-strength of LWAC made with ceramsite N and ceramsite Y increased by 23.5%, 23.7%, 16.8% and 10%, 9.1%, 9.6%, respectively. (2) Incorporation of 1%-3% nano-SiO2 by mass had no significant influence on the long-term shrinkage of LWAC in comparison to the reference LWAC, although increasing nano-SiO2 content slightly increased the shrinkage of LWAC. (3) In terms of the early cracking sensitivity, with the increasing dosage of nano-SiO2, the total cracking area of LWAC made with ceramsite Y continuously decreased. It reduced from 43.6 mm2/m2 to 32.7 mm2/m2 with 3% nano-SiO2 incorporation. On the other hand, LWAC made with ceramsite N showed no visible crack. (4) The revised simplified models for the compressive strength and the long-term shrinkage are proposed. Though, due to the complexity of the concrete, the current modified model is only applicable to the LWAC studied in the paper, and the applicability to other LWAC needs further study.
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
Conflict of interest The authors declare that there is no conflict of interest. Acknowledgements The authors gratefully acknowledge the financial support provided by the General Program of the National Natural Science Foundation of China (No. 51478272), the joint funds of the National Natural Science Foundation and Guangdong Province of China (U1301241), the International Cooperation and Exchange of the National Natural Science Foundation of China (51520105012), the Science and Technology Foundation for the Basic Research Plan of Shenzhen City (JCYJ20160422095146121), and the Collaborative Innovation Center for Advanced Civil Engineering Materials, Nanjing. References [1] K.S. Chia, M.H. Zhang, Water permeability and chloride penetrability of highstrength lightweight aggregate concrete, Cem. Concr. Res. 32 (4) (2002) 639– 645. [2] H.Z. Cui, T.Y. Lo, S.A. Memon, W.T. Xu, Effect of lightweight aggregates on the mechanical properties and brittleness of lightweight aggregate concrete, Constr. Build. Mater. 35 (10) (2012) 149–158. [3] H.K. Kim, J.H. Jeon, H.K. Lee, Workability, and mechanical, acoustic and thermal properties of lightweight aggregate concrete with a high volume of entrained air, Constr. Build. Mater. 29 (4) (2012) 193–200. [4] R. Wasserman, A. Bentur, Effect of lightweight fly ash aggregate microstructure on the strength of concretes, Cem. Concr. Res. 27 (4) (1997) 525–537. [5] J.M. Chi, R. Huang, C.C. Yang, J.J. Chang, Effect of aggregate properties on the strength and stiffness of lightweight concrete, Cem. Concr. Compos. 25 (2) (2003) 197–205. [6] M.H. Zhang, O.E. Gjørv, Microstructure of the interfacial zone between lightweight aggregate and cement paste, Cem. Concr. Res. 20 (4) (1990) 610–618. [7] A. KılıÇ, C.D. Atisß, E. Yasßar, F. Özcan, High-strength lightweight concrete made with scoria aggregate containing mineral admixtures, Cem. Concr. Res. 33 (10) (2003) 1595–1599. [8] K.S. Youm, J. Moon, J.Y. Cho, J.J. Kim, Experimental study on strength and durability of lightweight aggregate concrete containing silica fume, Constr. Build. Mater. 114 (2016) 517–527. [9] T.Y. Lo, H.Z. Cui, Z.G. Li, Influence of aggregate pre-wetting and fly ash on mechanical properties of lightweight concrete, Waste Manage. 24 (4) (2004) 333–338. [10] C.D. Atisß, A. Kiliç, U.K. Sevim, Strength and shrinkage properties of mortar containing a nonstandard high-calcium fly ash, Cem. Concr. Res. 34 (1) (2004) 99–102. [11] P. Chindaprasirt, S. Homwuttiwong, V. Sirivivatnanon, Influence of fly ash fineness on strength, drying shrinkage and sulfate resistance of blended cement mortar, Cem. Concr. Res. 34 (7) (2004) 1087–1092. [12] S. Akçaözog˘lu, C.D. Atisß, Effect of granulated blast furnace slag and fly ash addition on the strength properties of lightweight mortars containing waste PET aggregates, Constr. Build. Mater. 25 (10) (2011) 4052–4058. [13] K.H. Mo, U.J. Alengaram, M.Z. Jumaat, Utilization of ground granulated blast furnace slag as partial cement replacement in lightweight oil palm shell concrete, Mater. Struct. 48 (8) (2015) 1–12. [14] K.H. Mo, U.J. Alengaram, M.Z. Jumaat, M.Y.J. Liu, Contribution of acrylic fibre addition and ground granulated blast furnace slag on the properties of lightweight concrete, Constr. Build. Mater. 95 (2015) 686–695. [15] H.K. Kim, E.A. Hwang, H.K. Le, Impacts of metakaolin on lightweight concrete by type of fine aggregate, Constr. Build. Mater. 36 (36) (2012) 719–726. [16] R. Siddique, J. Klaus, Influence of metakaolin on the properties of mortar and concrete: A review, Appl. Clay. Sci. 43 (3) (2009) 392–400. [17] B.W. Langan, K. Weng, M.A. Ward, Effect of silica fume and fly ash on heat of hydration of Portland cement, Cem. Concr. Res. 32 (7) (2002) 1045–1051. [18] S.A. Khedr, M.N. Abou-Zeid, Characteristics of silica-fume concrete, J. Mater. Civ. Eng. 6 (3) (1994) 357–375. [19] J.Y. Li, P. Tian, Effect of slag and silica fume on mechanical properties of high strength concrete, Cem. Concr. Res. 27 (6) (1997) 833–837. [20] E. Güneyisi, M. Gesog˘lu, T. Özturan, Properties of rubberized concretes containing silica fume, Cem. Concr. Res. 34 (12) (2004) 2309–2317.
125
[21] E. Güneyisi, M. Gesoglu, O.A. Azez, H.Ö. Öz, Effect of nano silica on the workability of self-compacting concretes having untreated and surface treated lightweight aggregates, Constr. Build. Mater. 115 (2016) 371–380. [22] A.N. Givi, S.A. Rashid, F.N.A. Aziz, M.A.M. Salleh, Experimental investigation of the size effects of SiO2, nano-particles on the mechanical properties of binary blended concrete, Composites Part B 41 (8) (2010) 673–677. [23] R. Yu, P. Spiesz, H.J.H. Brouwers, Effect of nano-silica on the hydration and microstructure development of ultra-high performance concrete (UHPC) with a low binder amount, Constr. Build. Mater. 65 (9) (2014) 140–150. [24] M.H. Zhang, J. Islam, S. Peethamparan, Use of nano-silica to increase early strength and reduce setting time of concretes with high volumes of slag, Cem. Concr. Compos. 34 (5) (2012) 650–662. [25] R. Zhang, X. Cheng, P.K. Hou, Z.M. Ye, Influences of nano-TiO2 on the properties of cement-based materials: Hydration and drying shrinkage, Constr. Build. Mater. 81 (2015) 35–41. [26] Q. Ye, Z.N. Zhang, D.Y. Kong, R.S. Chen, Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume, Constr. Build. Mater. 21 (3) (2007) 539–545. [27] X.M. Liu, K.S. Chia, M.H. Zhang, Development of lightweight concrete with high resistance to water and chloride-ion penetration, Cem. Concr. Compos. 32 (10) (2010) 757–766. [28] K.G. Babu, D.S. Babu, Behaviour of lightweight expanded polystyrene concrete containing silica fume, Cem. Concr. Res. 33 (5) (2003) 755–762. [29] J.A. Bogas, R. Nogueira, N.G. Almeida, Influence of mineral additions and different compositional parameters on the shrinkage of structural expanded clay lightweight concrete, Mater. Design. 56 (4) (2014) 1039–1048. [30] H.J. Du, S.H. Du, X.M. Liu, Effect of nano-silica on the mechanical and transport properties of lightweight concrete, Constr. Build. Mater. 82 (2015) 114–122. [31] GB/T 17431.2-2010, Lightweight Aggregate and Test Method, Standards Press of China, Beijing, 2010. [32] X.F. Wang, C. Fang, W.Q. Kuang, D.W. Li, N.X. Han, F. Xing, Experimental study on early cracking sensitivity of lightweight aggregate concrete, Constr. Build. Mater. 136 (2017) 173–183. [33] X.F. Wang, C. Fang, W.Q. Kuang, D.W. Li, N.X. Han, F. Xing, Experimental investigation on the compressive strength and shrinkage of concrete with prewetted lightweight aggregates, Constr. Build. Mater. 155 (2017) 867–879. [34] JGJ 28-86 Technical Regulations of Application of Fly Ash in Concrete and Mortar, Ministry of Housing and Urban Rural Development of PR China. [35] R. Henkensiefken, D. Bentz, T. Nantung, J. Weiss, Volume change and cracking in internally cured mixtures made with saturated lightweight aggregate under sealed and unsealed conditions, Cem. Concr. Compos. 31 (7) (2009) 427–437. [36] GB/T 50081-2002, Standard for Test Method of Mechanical Properties on Ordinary Concrete, China Architecture & Building Press, Beijing, 2002. [37] GB/T 50082-2009, Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete, China Architecture & Building Press, Beijing, 2009. [38] GB/T 16594-2008, General Rules for Measurement of Length in Micron Scale by SEM, China Standard Press, Beijing, 2009. [39] P. Lura, M. Wyrzykowski, C. Tang, E. Lehmann, Internal curing with lightweight aggregate produced from biomass-derived waste, Cem. Concr. Res. 59 (59) (2014) 24–33. [40] ACI 209R-92, Prediction of creep, shrinkage, and temperature effects in concrete structures, Reported by ACI Committee 209, American Concrete Institute, Farmington Hills, MI, USA, 1992. [41] M. Sonebi, E. García-Taengua, K.M.A. Hossain, J. Khatib, M. Lachemi, Effect of nanosilica addition on the fresh properties and shrinkage of mortars with fly ash and superplasticizer, Constr. Build. Mater. 84 (2015) 269–276. [42] N. Farzadnia, H. Noorvand, The effect of nano silica on short term drying shrinkage of POFA cement mortars, Constr. Build. Mater. 95 (2015) 636–646. [43] G.A. Rao, Long-term drying shrinkage of mortar-influence of silica fume and size of fine aggregate, Cem. Concr. Res. 31 (2) (2001) 171–175. [44] Z.P. Bazˇant, Prediction of concrete creep and shrinkage: past, present and future, Nucl. Eng. Des. 203 (1) (2001) 27–38. [45] D.P. Bentz, K.A. Snyder, Protected paste volume in concrete: extension to internal curing using saturated lightweight fine aggregate, Cem. Concr. Res. 29 (11) (1999) 1863–1867. [46] E. Güneyisi, M. Gesog˘lu, S. Karaog˘lu, K. Mermerdasß, Strength, permeability and shrinkage cracking of silica fume and metakaolin concretes, Constr. Build. Mater. 34 (34) (2012) 120–130. [47] G. Li, Properties of high-volume fly ash concrete incorporating nano-SiO2, Cem. Concr. Res. 34 (6) (2004) 1043–1049. [48] M.H. Zhang, J. Islam, Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag, Constr. Build. Mater. 29 (2012) 573–580. [49] P.K. Hou, K.J. Wang, J.S. Qian, S. Kawashima, D.Y. Kong, S.P. Shah, Effects of colloidal nanoSiO2 on fly ash hydration, Cem. Concr. Compos. 34 (10) (2012) 1095–1103.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of nano-SiO2 on strength, shrinkage and cracking sensitivity of lightweight aggregate concrete X.F. Wang, Y.J. Huang, G.Y. Wu, C. Fang, D.W. Li, N.X. Han, F. Xing ⇑ Guangdong Key Laboratory of Durability in Coastal Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen, Guangdong 518060, China
h i g h l i g h t s The early compressive strength of LWAC was improved by adding nano-SiO2. Nano-SiO2 had no significant influence on the long-term shrinkage of LWAC. With nano-SiO2 addition, LWAC reduces the cracking risk at the early age. The simplified models for the compressive strength and shrinkage are proposed.
a r t i c l e
i n f o
Article history: Received 20 July 2017 Received in revised form 4 April 2018 Accepted 13 April 2018
Keywords: Lightweight aggregate concrete (LWAC) Nano-SiO2 Long-term shrinkage Cracking sensitivity Simplified model
a b s t r a c t This paper presents an experimental investigation on the effect of nano-SiO2 on the compressive strength, shrinkage and early cracking sensitivity of lightweight aggregate concrete (LWAC). Two types of ceramsite with different water absorption were used as lightweight aggregate in this study. LWAC with different nano-SiO2 dosage (1%, 2%, 3%) were compared with the reference LWAC to assess the effects of nanoSiO2 on LWAC. Results revealed that the incorporation of 3% nano-SiO2 increased compressive strength of LWAC significantly while the influence of nano-SiO2 on the long-term shrinkage of LWAC was not significant. The total cracking area was decreased with the increase in nano-SiO2 dosages from 1% to 3% by mass of the total binders at early age. The results of scanning electron microscopy (SEM) showed that the interfacial transition zone (ITZ) between lightweight aggregate and paste was enhanced with 3% nano-SiO2 addition. Subsequently, simplified models for compressive strength and shrinkage of LWAC were proposed and a comparison between the experimental data and models was discussed. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introductions In recent years, the development of lightweight aggregate concrete (LWAC) has drawn increasing attention from researchers and engineers. Compared with normal weight concrete, reducing construction costs by decreasing structural dead load and lower thermal conductivity make LWAC increasingly used for housing, bridges and other construction projects [1–3]. However, the properties of lightweight aggregates used in LWAC vary within wide limits and the aggregates exert an important influence on the elastic modulus of concrete [4,5]. Generally speaking, lightweight aggregates have a lower strength and elastic modulus than the mortar matrix and, therefore, than normal weight concrete [6]. The low compressive strength and low elastic modulus of LWAC ⇑ Corresponding author at: Guangdong Key Laboratory of Durability in Coastal Civil Engineering, Shenzhen University, 3688 Nanhai Ave, Shenzhen 518060, Guangdong, China. E-mail address: [email protected] (F. Xing). https://doi.org/10.1016/j.conbuildmat.2018.04.113 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
are often considered as a major shortcoming [2,5] except for some special applications. In order to compensate this deficiency, improve the strength and durability of LWAC and reduce the cement consumption, more and more attentions have been paid on the mineral additions. Similar to normal weight concrete, LWAC can be effectively modified by adding supplementary cementitious materials (SCMs), which can improve the mechanical and durability properties of concrete. Previous studies found that the bulk properties of LWAC might be able to be modified with SCMs addition, such as silica fume [7,8], fly ash [9–11], ground granulated blast furnace slag [12–14] and metakaolin [15,16]. Among these SCMs, silica fume has been proved to be the most effective SCM for the performance enhancement of LWAC [12,16,17]. As a pozzolanic material, silica fume reacts with Ca(OH)2 and produces additional calcium silicate hydrate (C-S-H) gel, which results in a denser microstructure and thereby improving the properties of hardened cementitious materials [18,19]. In general, the improvements were mainly attributed
116
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
to the pozzolanic reaction, as well as the acceleration effect of silica fume on cement hydration and optimizing particle packing of the matrix [17,19,20]. More recently, some studies [21–25] reported that the properties of cement-based materials can be modified effectively by using nano-particles such as nano-SiO2, which has a higher pozzolanic activity than silica fume. A common characteristic of silica fume and nano-SiO2 is that they are both pozzolanic materials which contain over 90% SiO2 (by weight). Ye et al. [26] reported that 3% nano-SiO2 improves the ITZ more effectively than silica fume by digesting Ca(OH)2 crystals, decreasing the orientation of Ca(OH)2 crystals and reducing the crystal size of Ca(OH)2 gathered at the ITZ. Some results [8,27–29] have shown that silica fume could improve the strength and durability of LWAC obviously. Interface bonding between hardened cement paste and lightweight aggregate is improved on account of better packing and pozzolanic reaction [28]. Until now, few studies [21,30] have investigated the effects of nano-SiO2 on the performance of LWAC. In this paper, the effects of nano-SiO2 on the compressive strength, shrinkage and early cracking sensitivity of LWAC were investigated. The effects of nano-SiO2 on microstructure of LWAC at 28 days were also studied. In addition to the experimental work,
simplified models by modifying the ACI-209 model were established by using the experimental results. Moreover, the experimental data was also compared with the model curve in terms of the compressive strength and the shrinkage of LWAC.
2. Experimental 2.1. Materials Two types of ceramsite with different water absorption were used as lightweight aggregate in this study. Ceramsite N (fly ashclay ceramsite) was made in Nantong, China while Ceramsite Y (shale ceramsite) originated from Yichang, China. Main properties of the two ceramsites are given in Table 1, including two different densities. The apparent density and bulk density (GB/T 17431.22010) [31–33] of ceramsite N are 1064.5 kg/m3 and 585.0 kg/m3, respectively, while those of ceramsite Y are 1209.1 kg/m3 and 635.8 kg/m3, respectively. The practicality picture of lightweight aggregate and the SEM images of their pore structures are shown in Fig. 1. The particle size distribution of aggregates used in this study is given in Fig. 2. Amorphous nano-SiO2 was produced by
Table 1 Physical properties of ceramsite. Type
Ceramsite
Cylinder compressive strength (MPa)
Apparent density (kg/m3)
Water absorption 1 h/24 h (%)
Bulk density (kg/m3)
1 2
N (Nantong) Y (Yichang)
4.63 11.66
1064.5 1209.1
9.10/12.70 3.34/5.74
585.0 635.8
Fig. 1. Two types of ceramsite used in this study: (a) ceramsite N; (b) ceramsite Y; (c) SEM image of ceramsite N; (d) SEM image of ceramsite Y.
117
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
Fig. 2. The particle size distribution of aggregates: (a) ceramsite N; (b) ceramsite Y.
Table 2 Physical properties of nano-SiO2. Type
BET-specific area (m2/g)
Average particle size (nm)
SiO2 content (%)
Apparent density (g/m3)
SP1
640 ± 60
20
99.9%
<0.18
Table 3 Physiochemical compositions (% by mass) of fly ash. Compositions
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
SO3
LOI
Content
64
21.32
3.25
4.3
1.03
1.37
0.38
3.05
Table 4 Mix proportions for LWAC (w/b = 0.35). Mix No
N1 N2 N3 N4 Y1 Y2 Y3 Y4 a b
Content (kg/m3) Cement
Nano-SiO2a
Fly ashb
Superplasticizer
Ceramsite
Sand
Water
413.1 408.2 403.3 398.4 413.1 408.2 403.3 398.4
0 (0%) 4.86 (1%) 9.72 (2%) 14.58 (3%) 0 (0%) 4.86 (1%) 9.72 (2%) 14.58 (3%)
72.9 72.9 72.9 72.9 72.9 72.9 72.9 72.9
6.21 6.26 6.29 6.31 6.21 6.26 6.29 6.31
465 465 465 465 465 465 465 465
843 843 843 843 843 843 843 843
170.1 170.1 170.1 170.1 170.1 170.1 170.1 170.1
% of (cement + nano-SiO2 + fly ash) by mass. 15% of (cement + nano-SiO2 + fly ash) by mass.
Mingri Nano-material Co. Ltd, and its physical properties are given in Table 2. The cement used in this study was Portland cement (PII 42.5R), provided by Zhujiang Cement Co., Ltd, and a proper amount of Polycarboxylate-based superplasticizer (SP) was used to adjust the workability of LWAC. Fly ash was offered by Henan Datanggongyi power plant, and the physiochemical compositions of fly ash are shown in Table 3. Natural sand with a fineness modulus of 2.74 was used for concrete mixtures. Tap water was used for concrete mixing. 2.2. Mixing proportions Table 4 summarizes the mix proportions of LWAC used in this study. Identical water to binder ratio (w/b) of 0.35 was used. The amount of the total binders (cementitious materials), lightweight aggregates and fine aggregates in each mix were 486 kg/m3, 465 kg/m3 and 843 kg/m3, respectively. The polycarboxylate-based superplasticizer was used to control the workability for LWAC mix-
tures. Fly ash was added into each mix proportion at 15% by mass of the total binders. The properties of the Fly ash used was in accordance with China National standard (JGJ28-86) [34]. Dosages of nano-SiO2 added in LWAC varied from 0 to 1%, 2% and 3% by mass of the total binders. Cement, fly ash and nano-SiO2 were mixed together for 20 min in a blender, to well-disperse nano-SiO2 into concrete mix. In order to reduce the impact of the difference in moisture content, all lightweight aggregates were pre-wetted in water for 24 h before mixing and the amount of the 24 h absorbed water was included in the moisture content of lightweight aggregates, which was suggested by Ref. [35]. 2.3. Experimental methodology 2.3.1. Test of cubic compressive strength For each LWAC mixing proportion, nine specimens with the size of 100 mm 100 mm 100 mm were cast for the compressive strength test. In order to prevent moisture loss, the molded speci-
118
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
mens were covered with plastic film. After form stripping, the specimens were cured under temperature T = (20 ± 2) °C and humidity RH = 95% until the time of testing. The universal testing machine YAW6306from MTS Systems Co., Ltd was used to load the specimens at a uniform speed of 0.3 MPa/s. Compressive strength of LWAC were determined at 3d, 7d and 28d according to China National Standard (GB/T50081-2002) [36]. The cubic compressive strength was calculated as follows,
f c ¼ 0:95
p A
ð1Þ
where fc is the cubic compressive strength (MPa), P is the failure load (N), A is the area under compression (mm2), and 0.95 is the size conversion coefficient. 2.3.2. Long-term shrinkage test Long-term shrinkage test was conducted for LWAC with the same w/b ratio as the cubic compressive strength test described above, to investigate the effect of nano-SiO2 on shrinkage. The size of the specimens is 100 mm 100 mm 515 mm. The specimens were tested by using the contact shrinkage measuring method and cured for 24 h with plastic film covered. After demold, the specimens were cured under temperature T = (20 ± 2) °C and humidity (RH = (60 ± 5)%) until the time of testing. The shrinkage measuring apparatus were used to test the specimens and the shrinkage was measured by dial indicator with the accuracy of 1 mm. The shrinkage of LWAC were determined at 1d, 2d, 3d, 4d, 5d, 6d, 7d, 14d, 28d, 45d, 60d, 90d, 120d and 150d according to China National Standard (GB/T 50082-2009) [37]. The age was counted from the date of specimens moving into constant temperature and humidity room. The mean value of three replicate specimens was used as the test result of each experimental data. The shrinkage was calculated as follows,
est ¼
l0 lt lb
ð2Þ
where, est refers to the shrinkage strain of concrete at time t, l0 refers to the initial reading, lt refers to the reading at time t and lb refers the gauge length. 2.3.3. Early cracking test For each mixing proportion, two specimens with the size of 800 mm 600 mm 100 mm were cast for early cracking test according to China National Standard (GB/T 50082-2009) [37]. A sketch map of the cracking slab arrangement with dimensions is shown in Fig. 3. The cracking slab was mainly assembled by 7 ribs which act as crack inducers. The restraint effect of the ribs should have a direct impact on the emergence of cracks. The test was carried out under the condition of constant temperature (T = (20 ± 2) °C) and constant humidity (RH = (60 ± 5)%) room. A wind velocity of 5 m/ s parallel to the surface of the concrete plate was applied by adjusting the position and wind speed of the fan after casting concrete in 30 min. The cracks were measured at (24 ± 0.5) h and the test time was count from the concrete mixing with water. The length of the cracks in the concrete plate were measured by a rule while the width of the cracks was measured by reading microscope with the accuracy of 0.02 mm. 2.3.4. Scanning electron microscopy The microstructure of LWAC specimens N1 and N4 was examined to determine the effect of nano-SiO2 addition by using scanning electron microscope (SEM) examination (FEI Quanta TM 250 FEG). After the compressive strength test, the samples of SEM were taken from the fractured surface. Before SEM observation, samples were dried in a vacuum oven. In order to achieve better observation results, the samples were sprayed with a layer of conductive
1.Long side plate; 2. Short side plate; 3. Bolt; 4. Reinforced rib; 5. Crack inducer; 6. Sole-plate. Fig. 3. Sketch map of the early cracking test arrangement.
film and the test was carried out under high vacuum condition. All the samples were tested in accordance with China National Standard (GB/T 16594-2008) [38]. 3. Results and discussion 3.1. Compressive strength 3.1.1. Effect of nano-SiO2 dosage on strength development of LWAC The cubic compressive strength of LWAC with different nanoSiO2 dosages at various ages is shown in Fig. 4. For LWAC made with ceramsite N, the compressive strength continued to increase up to 28 days with the increase in nano-SiO2 dosage. In comparison to the reference LWAC, the compressive strength of concrete at the age of 3 days increased by 14.2%, 18% and 23.5%, with 1%, 2% and 3% nano-SiO2 addition, respectively. At the age of 28 days, an increase of 3.2%, 13.3% and 16.8% was noticed for 1%, 2% and 3% nano-SiO2 addition, respectively. Among these three nano-SiO2 dosages, the compressive strength of LWAC with 3% presented the largest increase at 3d, 7d and 28d, which increased by 23.5%, 23.7% and 16.8%, respectively. Based on the experimental results, it is noted that the rate of compressive strength gain of LWAC with nano-SiO2 addition is fast at the early age and decreases gradually at the later age. Moreover, by comparing Fig. 4(a) and (b), it can be observed that the development tendency of the compressive strength of LWAC made with ceramsite Y is similar to that made with ceramsite N. As expected, the compressive strength of LWAC was noticeably improved at early ages (within 28 days), with 1%–3% nano-SiO2 addition. Such phenomenon for LWAC was similar to what has been found in other research work [30]. Meanwhile, the improvement of cementitious materials in compressive strength due to incorporation of nano-SiO2 was also reported by several researchers [22,24,26]. The mechanism that the nano-SiO2 addition modifies the LWAC can be explained from its physical and chemical effects [30], as well as the amount of nano-SiO2 used in the experiment [26].
119
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125 36
40
34 38
30 28 26 24 22 20
N-3d N-7d N-28d
18 16
Compressive strength (MPa)
Compressive strength (MPa)
32 36 34 32 30 28
Y-3d Y-7d Y-28d
26 24
14
0
1
2
0
3
1
2
3
Nano-SiO2 content (%)
Nano-SiO2 content (%)
(a)
(b)
Fig. 4. Compressive strength of LWAC with different nano-SiO2 dosages: (a) Ceramsite N; (b) Ceramsite Y.
In addition, by comparing the compressive strength between two types of LWAC, it can be found that the difference between two types of LWAC is significant. Herein, the compressive strength difference ratio, which is defined by (fY-fN)/fN and varies from 0 to 1, is used to represent this difference, where fY and fN refer to the compressive strength of LWAC made with ceramsite Y and ceramsite N, respectively. Fig. 5 shows the compressive strength difference ratio varying with the content of nano-SiO2 incorporation. In general, for lightweight aggregates, higher particle density resulted in LWAC with greater compressive strength [2]. This is somewhat consistent with the SEM images shown in Fig. 1, which displays that the microstructure of ceramsite Y is much denser than that of ceramsite N. Thus, to study the effect of nano-SiO2 addition on the mechanical properties of resulting LWAC, the properties of aggregate (such as particle density) must be taken into account. 3.1.2. Simplified model of compressive strength for LWAC Note that, concrete with the pre-wetted lightweight aggregates has the advantage of the internal curing, which is different from the normal weight concrete. The drying shrinkage and the cracking can be decreased effectively by utilizing the porous aggregates with internal curing [39]. Therefore, it is unsuitable to utilize mod-
els for normal weight concrete to estimate the compressive strength of LWAC. After investigating commonly used models, it is found that the ACI209R-92 model [40] is applicable for both ordinary concrete and LWAC. And the law of development over time can be adjusted by modifying the model’s parameters. In this study, a suitable compressive strength simplified model for LWAC is presented by modifying the existing model (ACI209R-92 model), the detail of which is expressed as follows,
t
f c ðtÞ ¼
aþbt
f c ð28Þ
ð3Þ
where, a ¼ 28 28b, a and b are the coefficients, and t in days is the time. From Eq. (3), it can be found that coefficient b is the dominant factor in the model of compressive strength. Based on the foregoing analysis, it can be inferred that coefficient b has close relationship with the moisture content of aggregates and the content of mineral additions. Therefore, this study tries to propose a linear model by modifying the coefficient b. As aforementioned, the content of nano-SiO2 (d) and the moisture content of lightweight aggregate in 1 m3 of concrete (x) was employed in the present study to establish a relationship with the coefficients b. Based on
β = -0.1174ω+0.003δ+0.9913 2
Adj-R =0.8624
3d 7d 28d
0.5
1.00 0.98
0.4
0.96
0.3
β
0.94 2 0.9 0 0.9
0.2
8 0.8 6 0.8
0.0
0.2
43 0.8
0.4
1
2
3
Nano-SiO2 content (%) Fig. 5. The compressive strength difference ratio of two types of LWAC.
0.8
1.0
0
0
1
0.0
Kg /m 3
0.6
ω(
Na noSiO 2 co nte nt δ (% )
)
0.1
2
Compressive strength difference ratio
0.6
1.2
Fig. 6. The relationship between parameters b and (d, x.).
120
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
the regression analysis of the experimental results, a linear model can be proposed as Eq. (4),
b ¼ A þ Bd þ C x
ð4Þ
where, A, B and C are the coefficients. A comparison of the value of b with the analytical surface is shown in Fig. 6. It can be seen that a good agreement (adj-R2 = 0.8624) is demonstrated when A = 0.9913, B = 0.003, C = 0.1174 are employed. In order to improve the accuracy of the model and obtain a more suitable simplified model, similar to the modified parameter b, the content of nano-SiO2 (d) was employed to modify the compressive strength fc (28). A linear relationship between the fc (28) and d can be fitted as follows,
f c ð28Þ ¼ P1 d þ P2
ð5Þ
where d refers to the content of nano-SiO2 (%), P1 and P2 are the coefficients depending on the types of the lightweight aggregate. With utilizing the modified Eqs. (4) and (5), Eq. (3) can be adjusted to a new compressive strength model which is suitable for LWAC. In order to verify the applicability of the model, the experimental data of 28d-compressive strength and the calculated values are compared, which is shown in Fig. 7. In addition to the maximum error 2.01%, it is found that the majority of the calculated values match with the experimental data well, which means that the model has a good accuracy for LWAC. However, it is noted that the proposed model may only applicable to LWAC in this study due to the diversity and complexity of the factors affecting the compressive strength. Hence, it is necessary to further study whether other LWAC is suitable. 3.2. Long-term shrinkage 3.2.1. Effect of nano-SiO2 dosage on shrinkage of LWAC Fig. 8 shows the shrinkage development of LWAC with time for different nano-SiO2 dosages with ceramsites N and Y, respectively. With 1% to 3% nano-SiO2 addition, the similar development tendency of the shrinkage has been observed in LWAC for both ceramsites N and Y. The effect of nano-SiO2 on shrinkage of LWAC is not statistically significant in comparison to the reference LWAC. At the early ages, all the shrinkage curves were almost overlapping. At the age of 90 days, the average measured shrinkage values were increased by 2.2%, 3.9% and 5.4% for LWAC of type N with 1%, 2% and 3% nano-SiO2 addition, respectively. Meanwhile, an increase
34
3.2.2. Simplified model of long-term shrinkage As aforementioned, the moisture content of concrete is the dominating factor on the shrinkage and the higher water absorption of the lightweight aggregate is different from the normal weight concrete. Hence, it can be deduced that a correlation must exist between the internal curing and the shrinkage of LWAC. In this study, the ACI209 model was employed again to estimate the long-term shrinkage, for the same reason mentioned in 3.1.2. The existing ACI209 model suggested by the ACI committee 209 [40] is as follows,
32
esh ðt; tsh;0 Þ ¼
38 2
Adj-R =0.9852
36
Measured strength (MPa)
shrinkage values of 2.5%, 4% and 3.4% was noticed for 1%, 2% and 3% nano-SiO2 addition to LWAC of Y, respectively. The effect of the types of aggregate on the shrinkage development of LWAC is shown in Fig. 9. Regarding to the reference LWAC, as shown in Fig. 9(a), there is some difference between the rates of shrinkage development of LWAC for types N and Y. At the age of 60 days, the shrinkage of LWAC of type N was 2.4% lower than that of type Y, while at the age of 90 days, the former was 3.4% higher than the latter. The development tendency of the shrinkage of LWAC with 3% nano-SiO2 addition, as shown in Fig. 9(b), was similar to what has been observed in the reference LWAC. At the age of 60 days and 90 days, the shrinkage of LWAC of type N was 2.3% lower and 5.4% higher than that of type Y, respectively. Comparing the Fig. 9(a) and (b), it also can be found that the rate of the shrinkage development has a similar trend and does not change very much. From the experimental results, it was revealed that 3% or less nano-SiO2 addition has no significant influence on the long-term shrinkage of LWAC. The shrinkage strain only increased slightly with the increase in nano-SiO2 content. The mechanisms of that the nano-SiO2 addition leads to small variation on the long-term shrinkage of LWAC can be explained from the effect of moisture content. At the early age, the high specific surface of nano-SiO2 particles increased water demand in the mix, which resulted in a decrease of the moisture content in the cement paste [41,42]. The higher rate of shrinkage development was mainly attributed to the moisture migration [42]. As cement hydration continued, the reduction of moisture content decreased the rate of shrinkage development gradually, leading to a lower rate at the later age. It was also observed that the rate of increase in the long-term shrinkage of LWAC decreased with the age of concrete which was in agreement with a previous study [43]. Regarding to the shrinkage of LWAC with different aggregates, LWAC of type N having a lower shrinkage strain at the early ages may be due to the higher water absorption of lightweight aggregate which provided better reservoir effect. Due to the restraining effect of the aggregate, LWAC of type N showed a higher shrinkage. As the degree of restraint offered may be determined by the elastic properties, the relatively lower cylinder compressive strength of ceramsite N correlated to the higher shrinkage strain.
30
ðt t sh;0 Þ esh1 35 þ ðt t sh;0 Þ
esh1 ¼ csh 780 106
28
N-28d Y-28d
26 26
28
30
32
34
36
38
Predicted strength (MPa) Fig. 7. Comparison between model curve and experimental values of 28dcompressive strength.
ð6Þ
ð7Þ
where, esh ðt; t sh0 Þ refers to the shrinkage strain at time t; t sh0 denotes the age of concrete at initiation of drying (days); t means the observation (current) time (days); esh1 signifies the ultimate shrinkage strain determined by experiments; csh represents the correction factor. In order to improve the accuracy of the model and obtain a more suitable simplified model, similar to the modified model of compressive strength of LWAC mentioned in 3.1.2, the internal
121
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125 600
550
550
500
500
450 400
Shrinkage strain (με)
Shrinkage strain (με)
450 400 350 300 250 200
N1 N2 N3 N4
150 100 50 0
0
20
40
60
80
100
120
140
350 300 250 200 150
Y1 Y2 Y3 Y4
100 50 0
160
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
160
600
600
500
500
Shrinkage strain (με)
Shrinkage strain (με)
Fig. 8. Shrinkage of LWAC with different nano-SiO2 dosages: (a) Ceramsite N; (b) Ceramsite Y.
400
300
200
100
400
300
200
100
N1 Y1
0
N4 Y4
0 0
20
40
60
80
100
120
140
160
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
160
600
600
500
500
Shrinkage strain (με)
Shrinkage strain (με)
Fig. 9. Shrinkage of LWAC made with different aggregates: (a) 0% Nano-SiO2; (b) 3% Nano-SiO2.
400
300
200
Experimental data of N1 Experimental data of Y1 Model curve of N1 Model curve of Y1
100
0
0
20
40
60
80
100
120
140
400
300
200
Experimental data of N4 Experimental data of Y4 Model curve of N4 Model curve of Y4
100
160
0
0
20
40
60
80
Time (d)
Time (d)
(a)
(b)
100
120
140
Fig. 10. Correlation between the experimental data of shrinkage strain and the model curve: (a) 0% Nano-SiO2; (b) 3% Nano-SiO2.
160
122
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
(a)
(b)
(c)
(d)
* C: The total cracking area of the LWAC plate. Fig. 11. Cracking of LWAC with different nano-SiO2 dosages: (a) ceramsite N, 0% nano-SiO2; (b) ceramsite N, 3% nano-SiO2; (c) ceramsite Y, 0% nano-SiO2; (d) ceramsite Y, 3% nano-SiO2.
44
Total cracking area (mm²/m²)
Total cracking area (mm²/m²)
1.0
0.5
0.0
-0.5
-1.0
0
1
2
3
42 40 38 36 34 32 0
1
2
3
Nano-SiO2 content (%)
Nano-SiO2 content (%)
(a)
(b)
Fig. 12. Cracking areas of LWAC with different nano-SiO2 dosages: (a) ceramsite N; (b) ceramsite Y.
curing of LWAC was taken into account. The modified model of long-term shrinkage is shown as follows:
esh ðt; tsh;0 Þ ¼
ðt tsh;0 Þ esh1 K þ ðt tsh;0 Þ
ð8Þ
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
(a)
(b)
(c)
(d)
(e)
(f)
123
Fig. 13. The SEM micrographs of pastes without and with nano-SiO2 addition at 28 days: (a) 0% nano-SiO2; (b) 3% nano-SiO2; (c) 0% nano-SiO2; (d) 3% nano-SiO2; (e) paste matrix; (f) image of fly ash.
where K is the influence coefficient of shrinkage determined by moisture content of lightweight aggregate. In the equation, a linear relationship can be obtained between K and the moisture content of lightweight aggregate in 1 m3 of concrete (x) after regression analysis:
K ¼ A þ Bx
ð9Þ
where x refers to the moisture content of lightweight aggregate in 1 m3 of concrete (kg), A and B are the coefficients. The coefficients (A = 1.0663, B = 13.0146) can be determined by the regression analysis on the experimental data of long-term shrinkage of LWAC. Based on Eqs. (8) and (9), the model curves
shown in Fig. 10 can be drawn. From Fig. 10, it is revealed that the shrinkage development of both LWAC without nano-SiO2 addition and LWAC with 3% nano-SiO2 addition matched well with the revised model, which indicates that the long-term shrinkage of LWAC can be estimated by using the revised model proposed in this study.
3.3. Early cracking sensitivity Fig. 11 shows the typically cracking morphology of LWAC made with ceramsites N and Y, under different nano-SiO2 dosages at the early ages. From the cracking morphology, it can be seen that 3%
124
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
nano-SiO2 addition has an obvious effect on the early cracking sensitivity for LWAC of type Y. With 3% nano-SiO2 incorporation, the length of the cracks decreased remarkably and the total cracking area decreased by 25% in comparison to the reference LWAC. However, for LWAC of type N, the effect caused by the addition of nanoSiO2 is not obvious on account of the experimental results that no crack was observed both in LWAC with or without nano-SiO2 incorporation. The cracking areas of LWAC with 1%, 2% and 3% nano-SiO2 are shown in Fig. 12. When no nano-SiO2 was used, the total cracking area of LWAC made with ceramsites N and Y are 0 mm2/m2 and 43.6 mm2/m2, respectively. As the dosage of nano-SiO2 increased from 0%, to 1%, 2% and 3%, the total cracking area of LWAC of type Y continued to decrease while the total cracking area of LWAC of type N remained 0 mm2/m2 for all times. Compared with the normal aggregate, the pre-wetted ceramsite has a good performance in internal curing due to the effect of ‘‘reservoir” in the cement paste [44]. Regarding the cracking behavior of LWAC with different lightweight aggregates, it is closely related to the moisture content of the lightweight aggregates [45]. In this study, from Table 1, it can be seen that the water absorption of ceramsites varies significantly, the 24 h water absorption of ceramsite N was about 221% of ceramsite Y. A relatively higher water absorption indicates a better performance for the effect of ‘‘reservoir”, which means it may provide enough water and increase moisture content on the concrete surface in the early shrinkage phase [32,33]. Aggregates with higher moisture content can compensate the reduction of relative humidity in LWAC effectively by capillary suction in cement paste [27], which consequently, reduces the cracking probabilities. Thereby, LWAC of type N showed no visible crack and had a lower cracking sensitivity than that of LWAC of type Y. For LWAC of type Y, 1%–3% nano-SiO2 incorporation significantly reduced the total cracking area. The more the amount of nano-SiO2 used in LWAC, the lower the crack widths or lengths can be observed. Such findings for LWAC of current study is similar to what had been found in silica fume and metakaolin concretes [46]. In addition to the acceleration effect of nano-SiO2 on cement hydration, nano-SiO2, as a micro-filler with extremely fine particle size, can effectively optimize the pore structure of cement paste by decreasing large capillary porosities and increasing medium capillary porosities [24]. With nano-SiO2 addition, the cement hydration products of LWAC can be more compact and homogenous because of the filler effect [30], which may fill the voids between cement and fly ash grains and particularly the ITZ between the paste and the lightweight aggregate, which will be also shown in section 3.4. Thus, the reduced large capillary porosity in the LWAC incorporating nano-SiO2 may efficiently inhibit cracking at the early age. Thereby, relatively less cracking behaviors were observed in LWAC of type Y with the incorporation of nano-SiO2. It should be mentioned that in this study, the effect of nano-SiO2 dosage (1%, 2%, 3%) on early cracking sensitivity of LWAC made with different types of lightweight aggregate was investigated. Whereas, with respect to the performance of nano-SiO2 addition on early cracking sensitivity of LWAC, some relevant chemical analysis or energy dispersive X-ray analysis should be carried out in future studies. 3.4. Microstructure of LWAC The microstructures of the interfacial transition zone (ITZ) between the bulk paste and lightweight aggregate are shown in Fig. 13(a) and (b). By comparing Fig. 13(a) and (b), it can be found that the ITZ’s microstructures of LWAC with 3% nano-SiO2 were more compact than that of the reference LWAC. For LWAC with incorporating 3% nano-SiO2, the borderline between the paste matrix and the ITZ could not be measured specifically and it
appeared that the paste matrix connected with the aggregate together, which is different from that of the reference LWAC. Fig. 13(d) shows the space around the unreacted fly ash particle surrounded by a denser bulk gel, which is different from the reference LWAC shown in Fig. 13(c), where the bulk gel structure is more porous and there are many large pores and gaps around the fly ash particle. Fig. 13(e) shows the unreacted fly ash particle in the paste matrix, and Fig. 13(f) shows the SEM image of fly ash used in this study. From Fig. 13(c) and (d), it can be found that nano-SiO2 with high specific surface has a significant effect on fly ash reaction. For the reference LWAC, amounts of hydration products could be found on the fly ash particle surface and the boundary of the fly ash particle was not clear. However, for the LWAC with 3% nano-SiO2 addition, the fly ash particle surface presents clean and smooth. It is indicated that fly ash in LWAC with 3% nano-SiO2 addition has a lower reaction degree compared with the reference LWAC. The main reason for this could be that the nano-SiO2 and fly ash compete in reacting with Ca(OH)2 and the nano-SiO2 is more reactive than fly ash [47,48]. Therefore, there might be a shortage of Ca (OH)2 in the system, which leads to an impediment in the hydration of fly ash. The results are similar to those using colloidal nano-SiO2 in cement-fly ash cementitious system [49]. Moreover, the influence of nano-SiO2 addition on fly ash/cement hydration is much more complex in nano-SiO2–fly ash–cement systems and it is necessary to conduct more scientific research on these multivariate cementitious material systems. In terms of microstructure modifications, the ITZ and the paste matrix were both much denser and more continuous in contrast to the reference LWAC. This is mainly due to the effects of the nano-SiO2 on the ITZ as discussed in section 3.1.1. The physical and chemical effects can reduce the porosity of cement paste and improve the bonding between the paste and aggregate, both of which can improve the compressive strength.
4. Conclusions In this study, 1%, 2% and 3% dosage nano-SiO2 was used in LWAC to investigate the influence of nano-SiO2 on the compressive strength, long-term shrinkage and early cracking of LWAC. Based on the experimental results, the following conclusions can be drawn: (1) The compressive strength of LWAC was improved obviously at the early ages by adding nano-SiO2 particles. The compressive strength was increased with the increase in nanoSiO2 dosages from 1% to 3% by mass of binders. With 3% nano-SiO2 incorporation, the 3d, 7d and 28d-strength of LWAC made with ceramsite N and ceramsite Y increased by 23.5%, 23.7%, 16.8% and 10%, 9.1%, 9.6%, respectively. (2) Incorporation of 1%-3% nano-SiO2 by mass had no significant influence on the long-term shrinkage of LWAC in comparison to the reference LWAC, although increasing nano-SiO2 content slightly increased the shrinkage of LWAC. (3) In terms of the early cracking sensitivity, with the increasing dosage of nano-SiO2, the total cracking area of LWAC made with ceramsite Y continuously decreased. It reduced from 43.6 mm2/m2 to 32.7 mm2/m2 with 3% nano-SiO2 incorporation. On the other hand, LWAC made with ceramsite N showed no visible crack. (4) The revised simplified models for the compressive strength and the long-term shrinkage are proposed. Though, due to the complexity of the concrete, the current modified model is only applicable to the LWAC studied in the paper, and the applicability to other LWAC needs further study.
X.F. Wang et al. / Construction and Building Materials 175 (2018) 115–125
Conflict of interest The authors declare that there is no conflict of interest. Acknowledgements The authors gratefully acknowledge the financial support provided by the General Program of the National Natural Science Foundation of China (No. 51478272), the joint funds of the National Natural Science Foundation and Guangdong Province of China (U1301241), the International Cooperation and Exchange of the National Natural Science Foundation of China (51520105012), the Science and Technology Foundation for the Basic Research Plan of Shenzhen City (JCYJ20160422095146121), and the Collaborative Innovation Center for Advanced Civil Engineering Materials, Nanjing. References [1] K.S. Chia, M.H. Zhang, Water permeability and chloride penetrability of highstrength lightweight aggregate concrete, Cem. Concr. Res. 32 (4) (2002) 639– 645. [2] H.Z. Cui, T.Y. Lo, S.A. Memon, W.T. Xu, Effect of lightweight aggregates on the mechanical properties and brittleness of lightweight aggregate concrete, Constr. Build. Mater. 35 (10) (2012) 149–158. [3] H.K. Kim, J.H. Jeon, H.K. Lee, Workability, and mechanical, acoustic and thermal properties of lightweight aggregate concrete with a high volume of entrained air, Constr. Build. Mater. 29 (4) (2012) 193–200. [4] R. Wasserman, A. Bentur, Effect of lightweight fly ash aggregate microstructure on the strength of concretes, Cem. Concr. Res. 27 (4) (1997) 525–537. [5] J.M. Chi, R. Huang, C.C. Yang, J.J. Chang, Effect of aggregate properties on the strength and stiffness of lightweight concrete, Cem. Concr. Compos. 25 (2) (2003) 197–205. [6] M.H. Zhang, O.E. Gjørv, Microstructure of the interfacial zone between lightweight aggregate and cement paste, Cem. Concr. Res. 20 (4) (1990) 610–618. [7] A. KılıÇ, C.D. Atisß, E. Yasßar, F. Özcan, High-strength lightweight concrete made with scoria aggregate containing mineral admixtures, Cem. Concr. Res. 33 (10) (2003) 1595–1599. [8] K.S. Youm, J. Moon, J.Y. Cho, J.J. Kim, Experimental study on strength and durability of lightweight aggregate concrete containing silica fume, Constr. Build. Mater. 114 (2016) 517–527. [9] T.Y. Lo, H.Z. Cui, Z.G. Li, Influence of aggregate pre-wetting and fly ash on mechanical properties of lightweight concrete, Waste Manage. 24 (4) (2004) 333–338. [10] C.D. Atisß, A. Kiliç, U.K. Sevim, Strength and shrinkage properties of mortar containing a nonstandard high-calcium fly ash, Cem. Concr. Res. 34 (1) (2004) 99–102. [11] P. Chindaprasirt, S. Homwuttiwong, V. Sirivivatnanon, Influence of fly ash fineness on strength, drying shrinkage and sulfate resistance of blended cement mortar, Cem. Concr. Res. 34 (7) (2004) 1087–1092. [12] S. Akçaözog˘lu, C.D. Atisß, Effect of granulated blast furnace slag and fly ash addition on the strength properties of lightweight mortars containing waste PET aggregates, Constr. Build. Mater. 25 (10) (2011) 4052–4058. [13] K.H. Mo, U.J. Alengaram, M.Z. Jumaat, Utilization of ground granulated blast furnace slag as partial cement replacement in lightweight oil palm shell concrete, Mater. Struct. 48 (8) (2015) 1–12. [14] K.H. Mo, U.J. Alengaram, M.Z. Jumaat, M.Y.J. Liu, Contribution of acrylic fibre addition and ground granulated blast furnace slag on the properties of lightweight concrete, Constr. Build. Mater. 95 (2015) 686–695. [15] H.K. Kim, E.A. Hwang, H.K. Le, Impacts of metakaolin on lightweight concrete by type of fine aggregate, Constr. Build. Mater. 36 (36) (2012) 719–726. [16] R. Siddique, J. Klaus, Influence of metakaolin on the properties of mortar and concrete: A review, Appl. Clay. Sci. 43 (3) (2009) 392–400. [17] B.W. Langan, K. Weng, M.A. Ward, Effect of silica fume and fly ash on heat of hydration of Portland cement, Cem. Concr. Res. 32 (7) (2002) 1045–1051. [18] S.A. Khedr, M.N. Abou-Zeid, Characteristics of silica-fume concrete, J. Mater. Civ. Eng. 6 (3) (1994) 357–375. [19] J.Y. Li, P. Tian, Effect of slag and silica fume on mechanical properties of high strength concrete, Cem. Concr. Res. 27 (6) (1997) 833–837. [20] E. Güneyisi, M. Gesog˘lu, T. Özturan, Properties of rubberized concretes containing silica fume, Cem. Concr. Res. 34 (12) (2004) 2309–2317.
125
[21] E. Güneyisi, M. Gesoglu, O.A. Azez, H.Ö. Öz, Effect of nano silica on the workability of self-compacting concretes having untreated and surface treated lightweight aggregates, Constr. Build. Mater. 115 (2016) 371–380. [22] A.N. Givi, S.A. Rashid, F.N.A. Aziz, M.A.M. Salleh, Experimental investigation of the size effects of SiO2, nano-particles on the mechanical properties of binary blended concrete, Composites Part B 41 (8) (2010) 673–677. [23] R. Yu, P. Spiesz, H.J.H. Brouwers, Effect of nano-silica on the hydration and microstructure development of ultra-high performance concrete (UHPC) with a low binder amount, Constr. Build. Mater. 65 (9) (2014) 140–150. [24] M.H. Zhang, J. Islam, S. Peethamparan, Use of nano-silica to increase early strength and reduce setting time of concretes with high volumes of slag, Cem. Concr. Compos. 34 (5) (2012) 650–662. [25] R. Zhang, X. Cheng, P.K. Hou, Z.M. Ye, Influences of nano-TiO2 on the properties of cement-based materials: Hydration and drying shrinkage, Constr. Build. Mater. 81 (2015) 35–41. [26] Q. Ye, Z.N. Zhang, D.Y. Kong, R.S. Chen, Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume, Constr. Build. Mater. 21 (3) (2007) 539–545. [27] X.M. Liu, K.S. Chia, M.H. Zhang, Development of lightweight concrete with high resistance to water and chloride-ion penetration, Cem. Concr. Compos. 32 (10) (2010) 757–766. [28] K.G. Babu, D.S. Babu, Behaviour of lightweight expanded polystyrene concrete containing silica fume, Cem. Concr. Res. 33 (5) (2003) 755–762. [29] J.A. Bogas, R. Nogueira, N.G. Almeida, Influence of mineral additions and different compositional parameters on the shrinkage of structural expanded clay lightweight concrete, Mater. Design. 56 (4) (2014) 1039–1048. [30] H.J. Du, S.H. Du, X.M. Liu, Effect of nano-silica on the mechanical and transport properties of lightweight concrete, Constr. Build. Mater. 82 (2015) 114–122. [31] GB/T 17431.2-2010, Lightweight Aggregate and Test Method, Standards Press of China, Beijing, 2010. [32] X.F. Wang, C. Fang, W.Q. Kuang, D.W. Li, N.X. Han, F. Xing, Experimental study on early cracking sensitivity of lightweight aggregate concrete, Constr. Build. Mater. 136 (2017) 173–183. [33] X.F. Wang, C. Fang, W.Q. Kuang, D.W. Li, N.X. Han, F. Xing, Experimental investigation on the compressive strength and shrinkage of concrete with prewetted lightweight aggregates, Constr. Build. Mater. 155 (2017) 867–879. [34] JGJ 28-86 Technical Regulations of Application of Fly Ash in Concrete and Mortar, Ministry of Housing and Urban Rural Development of PR China. [35] R. Henkensiefken, D. Bentz, T. Nantung, J. Weiss, Volume change and cracking in internally cured mixtures made with saturated lightweight aggregate under sealed and unsealed conditions, Cem. Concr. Compos. 31 (7) (2009) 427–437. [36] GB/T 50081-2002, Standard for Test Method of Mechanical Properties on Ordinary Concrete, China Architecture & Building Press, Beijing, 2002. [37] GB/T 50082-2009, Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete, China Architecture & Building Press, Beijing, 2009. [38] GB/T 16594-2008, General Rules for Measurement of Length in Micron Scale by SEM, China Standard Press, Beijing, 2009. [39] P. Lura, M. Wyrzykowski, C. Tang, E. Lehmann, Internal curing with lightweight aggregate produced from biomass-derived waste, Cem. Concr. Res. 59 (59) (2014) 24–33. [40] ACI 209R-92, Prediction of creep, shrinkage, and temperature effects in concrete structures, Reported by ACI Committee 209, American Concrete Institute, Farmington Hills, MI, USA, 1992. [41] M. Sonebi, E. García-Taengua, K.M.A. Hossain, J. Khatib, M. Lachemi, Effect of nanosilica addition on the fresh properties and shrinkage of mortars with fly ash and superplasticizer, Constr. Build. Mater. 84 (2015) 269–276. [42] N. Farzadnia, H. Noorvand, The effect of nano silica on short term drying shrinkage of POFA cement mortars, Constr. Build. Mater. 95 (2015) 636–646. [43] G.A. Rao, Long-term drying shrinkage of mortar-influence of silica fume and size of fine aggregate, Cem. Concr. Res. 31 (2) (2001) 171–175. [44] Z.P. Bazˇant, Prediction of concrete creep and shrinkage: past, present and future, Nucl. Eng. Des. 203 (1) (2001) 27–38. [45] D.P. Bentz, K.A. Snyder, Protected paste volume in concrete: extension to internal curing using saturated lightweight fine aggregate, Cem. Concr. Res. 29 (11) (1999) 1863–1867. [46] E. Güneyisi, M. Gesog˘lu, S. Karaog˘lu, K. Mermerdasß, Strength, permeability and shrinkage cracking of silica fume and metakaolin concretes, Constr. Build. Mater. 34 (34) (2012) 120–130. [47] G. Li, Properties of high-volume fly ash concrete incorporating nano-SiO2, Cem. Concr. Res. 34 (6) (2004) 1043–1049. [48] M.H. Zhang, J. Islam, Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag, Constr. Build. Mater. 29 (2012) 573–580. [49] P.K. Hou, K.J. Wang, J.S. Qian, S. Kawashima, D.Y. Kong, S.P. Shah, Effects of colloidal nanoSiO2 on fly ash hydration, Cem. Concr. Compos. 34 (10) (2012) 1095–1103.