- Email: [email protected]
Effects of pozzolanic and non-pozzolanic nanomaterials on cement-based materials
Construction and Building Materials 213 (2019) 1–9
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of pozzolanic and non-pozzolanic nanomaterials on cement-based materials Huigang Xiao a,b,⇑, Fengling Zhang b,c, Rui Liu a,b, Rongling Zhang d, Zhiguo Liu e, Hongxia Liu e a Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education and Key Lab of Disaster Smart Prevention and Mitigation for Civil Engineering of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China b School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China c Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, 117576, Singapore d National and Provincial Joint Engineering Laboratory of Road & Bridge Disaster Prevention and Control, Lanzhou Jiaotong University, Lanzhou 730070, China e Shandong Academic of Building Research, Jinan 250031, China
h i g h l i g h t s Nano-TiO2 is more efficient than nano-SiO2 for reducing the harmful pore. Nano-particles, especially nano-SiO2, can accelerate the cement hydration. Reinforcing efficiency of nano-SiO2 is strongly dependent on its size and content.
a r t i c l e
i n f o
Article history: Received 2 November 2018 Received in revised form 4 April 2019 Accepted 8 April 2019
Keywords: Nano-SiO2 Nano-TiO2 Pore structure Strength Cement
a b s t r a c t Although it has been verified that nanomaterials can improve the properties of cement-based materials, the research on the characteristics and mechanism of different types of nanomaterials is still inadequate. Two typical types of nanomaterials, nano-SiO2 (pozzolanic material) and nano-TiO2 (non-pozzolanic material), are utilized in this paper to investigate the combined effects of reactivity, size, and content on the efficiency of nanomaterials in cement-based materials. The experimental results indicate that the size and content effect of nano-SiO2 is obvious. Small size and large content of nano-SiO2 can significantly improve the compressive strength of cement paste. However, the size effect of nano-TiO2 is much weaker than that of nano-SiO2, and the content effect of nano-TiO2 is only obvious at the early age. For the late age compressive strength, the small content of nano-TiO2 shows a similar obviously improving effect as the big content of nano-TiO2. The pore structure analysis shows that both nano-SiO2 and nano-TiO2 can reduce the total porosity of cement paste, but nano-TiO2 is more effective than nano-SiO2 at reducing the harmful pores. Therefore, the small content of nano-TiO2 is more effective to enhance the impermeability of cement-based materials. The results of this paper show that nano-SiO2 and nano-TiO2 have different effects and they should be selected based on the properties requirement of cement-based materials. Ó 2019 Published by Elsevier Ltd.
1. Introduction Because of the advantages of easily casting, moulding, and relatively high compressive strength and durability, concrete has been the most widely used construction material in the world [1]. Great development has been achieved in improving the strength, ductility, and durability of concrete since its invention [2–4]. The property of concrete determines the safe servicing of concrete structures. The emerging needs of building infrastruc⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin, Heilongjiang 150090, China. E-mail address: [email protected] (H. Xiao). https://doi.org/10.1016/j.conbuildmat.2019.04.057 0950-0618/Ó 2019 Published by Elsevier Ltd.
tures at severe environments such as ocean and polar result in an increasing requirement on high-performance concrete (HPC) [5–7]. In concrete, mainly the amorphous calcium silicate hydrate (C-S-H), is the continuous matrix that binds aggregates and fibers together and plays an important role for the mechanical properties and durability of concrete [8–10]. Therefore, modifying the microstructure and properties of cement paste is an efficient way to improve the properties of concrete. Cement paste is actually composed of extremely fine particles in the nanoscale size range, these particles form a poorly ordered structure [11–13]. Hence, using nanotechnology in concrete to manipulate the structure at the nanoscale to get a macroscale superior property is getting
2
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
increasing attention [14]. Remarkable improvements in the mechancial properties and durability of concrete have been realized by incorporating nanomaterials into concrete [15–18]. One typical type of widely used nanomaterials to modify cement-based materials is nano-SiO2, which is a kind of pozzolanic materials that can react with cement hydration products (e.g. Ca (OH)2) [19]. With the addition of nano-SiO2, the performance of cement-based materials has been improved, such as increasing compressive and flexural strength [19–22], enhancing resistance of water penetration [23,24], chloride ingress [24,25] and sulfate attack [5], and reducing calcium leaching [25,26]. The better performance of cement-based materials incorporating nano-SiO2 is attributed to the high pozzolanic reactivity, accelerated cement hydration, refined pore structure, improved interfacial transition zone, low C/S ratio, and higher polymerization of silicate chain [27–29]. The effect of nano-SiO2 on the performance of cementbased materials is dependent on its particle size as well as content [14,22,30]. Nano-TiO2 is another kind of popularly used nanomaterials in cement-based materials, which is nonpozzolanic material [31– 34] but also has similar effects on cement hydration [35]. Some researchers have studied the effect of nano-TiO2 on compressive and flexural strength of cement-based materials [31,36]. The influence of nano-TiO2 aslo depends on its content and particle size [31], but studies on the effect of particle size of nano-TiO2 are limited. Compared to nano-SiO2, nano-TiO2 does not have pozzolanic activity, however, it shows a crystalize effect on the C-S-H gel [37]. It is also found that the content related dispersion state of nano-TiO2 plays an important role on the improvement efficiency of nano-TiO2 [38]. Such works indicate that various kinds of nanomaterials can be used effectively in a bottom-up approach to modify the properties of concrete. However, the effect of nano-TiO2 without pozzolanic activity on the performance of cement composites may be different from nano-SiO2 with pozzolanic activity [39,40], the comparison study is still not so sufficient in the present. While nano-engineered cement-based materials are seen as having tremendous potential, the information of the role of different types of nano-particles, involving pozzolanic, size, and content, on hydration and microstructure of cement and the macroscale properties of cement-based materials is still inadequate [40]. This information is crucial for understanding, predicting, and optimizing the utilization of nanotechnology and the corresponding performance of nano-engineering cement-based materials. This paper aims at providing an insight into the different effects and mechanisms of nano-SiO2 and nano-TiO2 on the properties of cement-based materials.
2. Materials and experimental methods 2.1. Materials and mix proportions Ordinary Portland cement (OPC, P∙O 42.5) and deionized water were used for all cement pastes. Nano-SiO2 (NS) and nano-TiO2 (NT, anatase type) used in this study are in dry powder form. According to the supplier’s data, the NS with a SiO2 content of >99.5% has two different average particle sizes, 15 nm (specific surface area, SSA of 200 m2/g) and 30 nm (SSA of 100 m2/g), denoted as NS15 and NS30, respectively. The NT with a TiO2 content of >99.8% has two different average particle sizes, 10 nm (SSA of 210 m2/g) and 40 nm (SSA of 80 m2/g), denoted as NT10 and NT40, respectively. Silica fume (SF) powder with a SiO2 content of 98.3%, an average particle size of 100–150 nm, and specific surface area of 15–27 m2/g was used for comparison. A naphthalene-based superplasticizer (SP) was used in this study.
The mix proportions of cement pastes are given in Table 1. In total, 16 cement pastes were included. All cement pastes had a water to cementitious materials ratio (w/c) of 0.40. The dosage of NS, NT, and SF was varied from 0.5%, to 1% and 3% by mass of cementitious materials. The dosage of SP was kept as 0.5% by mass of cementitious materials to minimize the effect of SP. For each cement paste, nanoparticles, partial mixing water, and superplasticizer were first mixed, then mechanically stirred for 10 min, and ultrasonicated in a sonicator (power of 400 W) for 30 min. The sonicated solution, remaining water, and cement were mixed in a mixer at a low speed for 120 s, and a high speed for additional 120 s. The fresh cement paste was poured into steel moulds (40 40 40 mm3). An external vibrator was used to facilitate the compaction and decrease the air bubbles. After casting, the moulded specimens were covered with plastic sheet to prevent moisture loss. All specimens were demoulded after 24 h. After demould, all specimens were cured in a standard curing room (temperature of 20 ± 2 °C and relative humidity of > 95%) until the specified testing age (3 days, 7 days, and 28 days). 2.2. Fluidity and mechanical characterization The fluidity of the fresh cement paste was determined by minislump test according to Chinese Standard GB/T 8077-2012. The truncated cone has a height of 30 mm, bottom diameter of 60 mm, and top diameter of 36 mm. The truncated cone was first placed on a flat glass plate, filled with fresh cement paste, and then vertically raised to let the fresh cement paste flow freely and expand into a disc shape in around 30 s. The diameter of the stilled cement paste was measured in two directions at right angles to one another and the average diameter was taken as the fluidity of the cement paste. The compressive strength of the cement paste was determined by 40 40 40 mm3 specimens using a Materials Testing System (MTS) with a loading rate of 1 kN/s at the specified testing age of 3, 7, and 28 days, respectively. At each testing age, at least 6 specimens was used for compressive strength test and the average values and standard deviation were reported. 2.3. Composition characterization The phase compositions of cement pastes containing nanomaterials were analysed by X-ray diffraction (XRD, X’Pert Pro MPD, Panalytical, Netherlands) with a Cu Ka radiation at 40 Kv and 40 Ma. The 2h range was from 5° to 65° with a step size of 0.026°. After compressive strength test at specified ages, the tested specimens were broken into small pieces and samples were collected from the core of the test specimens. The selected samples were immersed in ethanol absolute as fast as possible for at least 1 week to stop cement hydration. Then, the samples were dried in a vacuum oven at 50 °C until constant mass was reached. The samples were grinded into powders that could pass the 80 mm sieve before XRD analysis. Thermo-gravimetric analysis (TGA, TGA/SDTA 851e, Mettler Toledo, USA) was performed to investigate the effect of nanoSiO2 and nano-TiO2 on the Ca(OH)2 content. For each TGA test, approximately 10 mg dried powder was heated from about 30° to 900 °C with a heating rate of 10 °C/min in a nitrogen environment. The degree of cement hydration was also determined by TGA. It was calculated by wb ðtÞ=wb;1 , where wb ðtÞ ¼ ðw105 C w900 C Þ=w900 C is the normalized mass of non-evaporable water at specified ages, w105 C is the mass of samples at 105 °C, w900 C is the mass of samples at 900 °C, wb;1 is the mass of nonevaporable water for fully hydrated cement and 0.23 was selected
3
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9 Table 1 Mix proportions (by weight of cementitious materials). Mixture
Water
Cement
Silica fume (%)
Nanoparticles (%)
Superplasticizer (%)
Reference NS15-0.5 NS15-1.0 NS15-3.0 NS30-0.5 NS30-1.0 NS30-3.0 SF-0.5 SF-1.0 SF-3.0 NT10-0.5 NT10-1.0 NT10-3.0 NT40-0.5 NT40-1.0 NT40-3.0
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
1 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970
0 0 0 0 0 0 0 0.5 1.0 3.0 0 0 0 0 0 0
0 0.5 1.0 3.0 0.5 1.0 3.0 0 0 0 0.5 1.0 3.0 0.5 1.0 3.0
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
[41]. A correction for the mass of non-evaporable water was done by subtracting the mass loss due to the decomposition of CaCO3. According to [42,43], amorphous silica does not consume nonevaporable water to react with Ca(OH)2. Thus, the effect of nanoSiO2 and silica fume on the non-evaporable water content was not considered in this study.
280
Fluidity (mm)
240
2.4. Mercury intrusion porosimetry (MIP) The porosity and pore size distribution of the cement pastes containing nano-SiO2 or TiO2 were analysed by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics, USA) [44,45]. Each pore size is quantitatively determined by the relationship between the volume of intruded mercury and the applied pressure [45]. The relationship between the pore diameter and the applied pressure is generally described by Washburn Eq. (1) as follows [45];
D ¼ 4ccosh=P
Reference SF
NS15 NT10
NS30 NT40
200 160 120 80 0.5
1.0 1.5 2.0 2.5 Content of nanomaterials (%)
3.0
Fig. 1. Fluidity of cement pastes containing nanomaterials.
ð1Þ
where D is the pore diameter (nm), c is the surface tension of mercury (N/m) and 0.485 N/m is used in this study, h is the contact angle between mercury and solid (°) and 141.3 °C is used, and P is the applied pressure (MPa). To prepare samples for MIP evaluation, the cement paste specimens were first broken into smaller pieces (3–5 mm in size) after 3 days and 28 days of curing, respectively, then, three samples were selected from the core of tested specimens and used to measure the pore structure for each case. These samples were immersed in acetone as fast as possible to stop their hydration process. Prior to applying the MIP test, the samples were dried in an oven at about 60 °C for 48 h to remove moisture from their pores before testing.
paste containing nano-TiO2 is much better than that containing nano-SiO2. When the particle size of nano-SiO2 is 100–150 nm (silica fume), the addition of 0.5% and 1% nano-SiO2 shows a slight effect on the fluidity, only when the addition reaches 3%, there is a significantly reduction in fluidity as 35.1%. Therefore, based on the observation of workability, the addition of nano-materials should be lower than 3% by mass of cementitious materials.
3.1. Fluidity of cement paste containing nano-particles
NS15-3.0
Fig. 1 gives the fluidity of cement pastes containing various types and contents of nanomaterials. For nanomaterials, they agglomerate easily due to their high specific surface area and strong Van der Waals forces. The fluidity of the cement paste is reduced by the addition of nanomaterials. The negative effect of nanomaterials on the fluidity is also reported in [30,36]. The reduction degree strengthens with the increase in nanomaterials content and the decrease in nanomaterials size. The 0.5% addition of 15 nm nano-SiO2 reduces the fluidity of cement paste by 58.3%, and when the content reaches 3%, the cement paste nearly totally loses the fluidity. For the similar size and content, the fluidity of the cement
1
TiO2 2 1 2
1-Calcium hydroxide 2-Calcium silicates 3-Ettringite 1 2 1 11
Intensity
3. Results and discussion
NT10-3.0 1 3
Reference
5
10
15
20
25
30 35 40 2 Theta
45
50
55
60
65
Fig. 2. X-ray diffraction spectra of cement pastes containing nanomaterials at 28 days.
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
3.2. XRD and TG analysis results
3.3. Compressive strength Fig. 5 shows the compressive strength of cement pastes containing various sizes and contents of nano-SiO2 and silica fume at 3 days, 7 days, and 28 days. For nano-SiO2, which is a pozzolanic material, it shows an obvious effect on improving the strength of cement paste, and this strengthening effect depends not only on the content of nano-SiO2, but also on the size of nano-SiO2. This observation is consistent with a previous study [30]. Comparing with the compressive strength of reference sample, the compres-
100
0.003
0.000 CC 620-690
85
CSH
DTG -0.006
Free water CH 420-450
105 80
-0.003 TG
0
150
300 450 600 Temperature ( )
750
-0.009 900
Fig. 3. Typical TG/DTG curve of reference cement paste at 28 days.
Mass perc diff (%/
Mass loss (%)
95
)
Non-evaporable water
Ca(OH)2 Content (wt.%)
Fig. 2 shows the X-ray diffraction spectra of the cement pastes containing nano-SiO2 and nano-TiO2 at 28 days. The characteristic diffraction peaks of reference sample, NS15-3.0, and NT10-3.0 are similar, except the unique peak at diffraction angle of 25.3° (belong to TiO2) found for NT10-3.0. The experimental results indicate that the hydration products of the cement pastes containing nanomaterials are the same to the reference cement paste, there is no new products formed. Nano-TiO2 remains in the hydrated cement paste, validating that it is a nonpozzolanic material. The non-pozzolanic nature of nano-TiO2 is also reported in [31]. Although nano-TiO2 does not react with cement hydration products, it can increase the crystalline level and hardness of C-S-H as reported in [37]. Fig. 3 shows a typical thermogravimetric analysis curve of the reference cement paste at 28 days. At 105 °C, free water evaporates. With the further increase of temperature, different components (e.g. Aft, AFm, C-S-H, and Ca(OH)2)will dehydrate at a specific temperature, therefore, the content of each component and the total content of non-evaporable water can be calculated based on the TG and DTG curves [46]. For example, the component of Ca(OH)2 dehydrates at 400–500 °C, the content of Ca(OH)2 is calculated and given in Fig. 4. The hydration degree of three typical cement pastes at 28 days is given in Fig. 4. With respect to the reference cement paste, the hydration degree of NS15-3.0 and NT103.0 is enhanced by 11.7% and 8.8%, respectively, indicating that both nano-SiO2 and nano-TiO2 can improve the cement hydration. The enhanced degree of cement hydration with the incorporation of nanomaterials is also reported in [22,31]. The content of Ca (OH)2 of NT10-3.0 is higher than that of the reference cement paste, validating that hydration degree of NT10-3.0 is higher than the reference cement paste. On the other hand, although the hydration degree of NS15-3.0 is the highest, its Ca(OH)2 content is the lowest among the three cement pastes, which is because the Ca(OH)2 has reacted with nano-SiO2 due to its pozzolanic nature [19].
90
72
20 19
Hydration degree
18
66
17 16 15
69
63 Consumed by SiO2
60
Ca(OH)2 content
57
14 Reference
Hydration degree (%)
4
NS15-3.0
NT10-3.0
Fig. 4. Calcium hydroxide content and hydration degree of cement pastes containing nanomaterials at 28 days.
sive strength of NS15-0.5 at 3 days and 28 days is increased by 41.7% and 14.7%, respectively. The compressive strength of NS153.0 at 3 days and 28 days is increased by 141.3% and 39.4%, respectively. Therefore, the strengthening effect of nano-SiO2 increases with the content but decreases with the increase in size. The small size of nano-SiO2, NS15, is more efficient to enhance the strength of cement paste at early age, the 3-day compressive strength of NS153.0 is 53.8 MPa, which is 2.4 times of that of reference sample (22.3 MPa), and is similar to the 28-day compressive strength (53.1 MPa) of reference sample. On the other hand, the 28-day compressive strength of NS15-3.0 and NS30-3.0 is 74.0 MPa and 70.2 MPa, respectively, which is much higher than that of reference sample (53.1 MPa). Furthermore, the similar compressive strength of NS15-3.0 and NS30-3.0 at 28 days indicates that the effect of different sizes of nano-SiO2 is similar for the later age compressive strength. The experimental results indicate that nano-SiO2 can accelerate the hydration of cement, especially when the size of nano-SiO2 is small as it has a huge specific area and high reactivity. Hence, the compressive strength of cement paste containing high content of small size nano-SiO2 is much higher than that of reference sample at early age. It should be noted that a combination of mechanical stirring and ultrasonication is utilized in this study to ensure the well dispersion of nano-SiO2. In case of using nanoSiO2 with a small size or high content, it may negatively affect the compressive strength if nano-SiO2 does not disperse well as reported by Bolhassani and Samani [30]. Fig. 6 presents the compressive strength of cement pastes containing various sizes and contents of nano-TiO2 at 3 days, 7 days, and 28 days. Nano-TiO2 can also improve the compressive strength of cement paste, but the effect of particle size is not as obvious as the nano-SiO2. Although the high content of nano-TiO2 performs better at the early age, the small content of nano-TiO2 achieves similar strengthening efficiency at the long age, which is consistent with that reported in [36]. As indicated in our previous study [38], the distribution of cement hydration products is strongly associated with the dispersion state of nano-TiO2. Small content of nano-TiO2 can reach a uniform dispersion state and benefit the macroscale properties of cement paste, especially for long term properties. Fig. 7 gives the comprehensive view of the compressive strength enhancing ratio of cement pastes containing various types and contents of nanomaterials at different ages. It is the same for nano-SiO2 and nano-TiO2 that their improving efficiency is more significant at early age, and they are much more efficient than the silica fume, which is attributed to the accelerated hydration of cement paste containing nanomaterials [11]. Because nanoSiO2 can react with cement hydration products, the enhancing efficiency of nano-SiO2 is strongly related to its size and content. For small size and high content, efficiency of nano-SiO2 is much higher
5
80 (a)
Reference NS15-0.5 NS15-1.0 NS15-3.0
60 40 20 0
Compressive strength (MPa)
3d 80
Reference NS30-0.5 NS30-1.0 NS30-3.0
(b)
60 40 20 0
3d
7d
28d
7d
28d
Compressive strength (MPa)
Compressive strength (MPa)
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
80
(c)
Reference SF-0.5 SF-1.0 SF-3.0
60 40 20 0
3d
7d
28d
80
(a)
Reference NT10-0.5 NT10-1.0 NT10-3.0
60 40 20 0
3d
7d
28d
Compressive strength (MPa)
Compressive strength (MPa)
Fig. 5. Compressive strength of cement paste containing nano-SiO2 and silica fume.
80
(b)
Reference NT40-0.5 NT40-1.0 NT40-3.0
60 40 20 0
3d
7d
28d
Enhancement ratio of strength (%)
Fig. 6. Compressive strength of cement paste containing nano-TiO2.
than that of nano-TiO2. Silica fume can also improve the compressive strength of cement paste, but it is relatively weaker than nanoSiO2 at the same content. At age of 28 days, the compressive strength of cement paste containing nano-SiO2 still increases obviously upon the content of nano-SiO2. However, for nano-TiO2, the content of 0.5% reaches the similar enhancing efficiency as the content of 3% at age of 28 days, which agrees well with the reported results shown in [38]. Therefore, for the purpose of enhancing the early-age compressive strength, high content of small size nano-SiO2 is preferred as long as the dispersion is satisfied. For the long-age compressive strength, small content of big size nano-TiO2 is more suitable considering the similar enhancing efficiency and the good workability.
150 NS15 NT10
120 3d
90
NS30 NT40
7d
SF
28d
60 30 0 5 0 0 . 1.
0 5 0 0 5 0 3. 0. 1. 3. 0. 1. Content of nanomaterials (%)
3.
0
Fig. 7. Compressive strength enhancement ratio of various mixtures compared with reference samples.
3.4. Porosity and pore size distribution Fig. 8 shows the pore distribution curve of the cement pastes containing various contents of NS15 at different ages. Fig. 9 shows
0.25 (a) 3d 0.20
Reference NS15-0.5 NS15-1.0 NS15-3.0
0.15 0.10 0.05 0.00 6 10
5
4
10
3
10
2
10
1
10
10
0
10
dV/dlogD Pore Volume (mL/g)
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
dV/dlogD Pore Volume (mL/g)
6
0.25 (b) 28d 0.20
Reference NS15-0.5 NS15-1.0 NS15-3.0
0.15 0.10 0.05 0.00 6 10
5
10
4
10
3
2
10
1
10
10
0
10
Pore Diameter (nm)
Pore Diameter (nm)
0.25 0.20 0.15
(a) 3d Reference NT10-0.5 NT10-1.0 NT10-3.0
0.10 0.05 0.00 6 10
5
10
4
10
3
10
2
10
1
10
0
10
dV/dlogD Pore Volume (mL/g)
dV/dlogD Pore Volume (mL/g)
Fig. 8. Differential curves of pore structure of cement paste containing nano-SiO2。
0.25 0.20 0.15
(b) 28d Reference NT10-0.5 NT10-1.0 NT10-3.0
0.10 0.05 0.00 6 10
Pore Diameter (nm)
5
10
4
10
3
10
2
10
1
10
0
10
Pore Diameter (nm)
Fig. 9. Differential curves of pore structure of cement paste containing nano-TiO2。
the pore distribution curve of the cement pastes containing various contents of NT10 at different ages. Together with Tables 2 and 3, these figures and tables provide the comprehensive information of the pore structures of hardened cement pastes with nano-SiO2 and nano-TiO2. For series of NS15 specimens, the total porosity decreases upon the content of nano-SiO2, the porosity reduction ratio for all ages is higher than 20%, and the maximum porosity reduction degree is reached at 3% content of nano-SiO2 at 3 days, which is about 37.8% as shown in Fig. 10. The reduced porosity with the addition of nano-SiO2 is also reported in [15]. The decrease characteristics of porosity, especially for high content of nano-SiO2 at 3 days, agrees with the enhancing characteristics of compressive strength of cement pastes containing nano-SiO2. The most probable pore diameter of cement pastes containing nanoSiO2 is smaller than that of the reference sample. Unlike the decrease of porosity upon content of nano-SiO2, the average diameter and median diameter increase upon the content of nano-SiO2,
and for most cases, they are higher than that of the reference specimen, which will be discussed below. Nano-SiO2 can accelerate the cement hydration and enhance the hydration degree as indicated in Fig. 4. The accelerating efficiency is more remarkable for large content of nano-SiO2 at early age, therefore, the decrease in porosity of NS15-3.0 is more obvious at 3d in comparison to 28d (Fig. 10). The total porosity of the cement pastes containing nano-TiO2 is obviously lower than that of the reference sample, and it is slightly higher when compared with the cement paste containing nanoSiO2. The decreased porosity with the incorporation of nano-TiO2 is also reported in [31]. Especially for small content of nano-TiO2 at 3 days as shown in Fig. 10, the decrease degree of porosity is not so efficient as the same content of nano-SiO2. On the other hand, at 28 days, the decrease efficiency of nano-TiO2 approaches nano-SiO2, and it is almost independent on content. Hence, the 28-day compressive strength of the cement paste containing vari-
Table 2 Pore structure characteristics of cement paste containing nano-SiO2. Sample
Age
Porosity (%)
Total specific pore volume (mL/g)
Most probable pore diameter (nm)
Average diameter (nm)
Median diameter (nm)
Reference NS15-0.5 NS15-1.0 NS15-3.0
3d
32.19 26.36 24.74 20.02
0.2068 0.1647 0.1545 0.1216
50.38 5.65 50.39 32.40
18.9 16.5 17.6 19.3
31.1 26.9 34.0 31.0
Reference NS15-0.5 NS15-1.0 NS15-3.0
28d
22.58 18.20 16.19 15.55
0.1411 0.1094 0.0964 0.0904
32.39 5.84 32.38 26.30
17.1 16.6 18.2 21.0
28.8 28.2 29.2 30.0
7
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9 Table 3 Pore structure characteristics of cement paste containing nano-TiO2. Sample
Age
Porosity (%)
Total specific pore volume (mL/g)
Most probable pore diameter (nm)
Average diameter (nm)
Median diameter (nm)
Reference NT10-0.5 NT10-1.0 NT10-3.0
3d
32.19 29.44 25.46 24.06
0.2068 0.1849 0.1560 0.1460
50.38 5.84 5.84 6.03
18.9 17.4 15.7 16.2
31.1 24.7 20.9 24.5
Reference NT10-0.5 NT10-1.0 NT10-3.0
28 d
22.58 17.96 17.07 16.35
0.1411 0.1072 0.1012 0.0960
32.39 26.29 26.29 26.30
17.1 16.0 16.3 17.0
28.8 25.7 26.2 27.0
Decrease of porosity (%)
0
NS15-3d NT10-3d
NS15-28d NT10-28d
-10
NT -20
-30
NS -40 0.5
1.0
1.5
2.0
2.5
3.0
Content of nanomaterials (%) Fig. 10. Porosity decrease percentage of cement paste containing various types of nanomaterials.
ous content of nano-TiO2 is similar as discussed in Section 3.3. The average diameter and median diameter of the cement pastes containing various contents of nano-TiO2 are all smaller than those of the reference sample, indicating that the percentage of fine pore dominates the porosity of cement paste containing nano-TiO2. Figs. 11 and 12 show the fractional decrease of the specific volume of the total pores and of different types of pores. Based on the diameter and the corresponding harmful level [47], the pores can be divided into four types: harmless pore (<20 nm), less-harmful pore (20–50 nm), harmful pore (50–200 nm) and more-harmful pore (>200 nm). For the cement paste containing nano-SiO2 as
Decrease of volume ( V/V)
NS15-0.5 NS15-3.0
(b) 28d Decrease of volume ( V/V)
Reference NS15-1.0
(a) 3d 0.2 0.0 -0.2 -0.4 -0.6
o lp
re
0n <2
m 20
n -50
m -2 50
Pore type
n 00
m >2
n 00
Reference NS15-1.0
0.2
NS15-0.5 NS15-3.0
0.0 -0.2 -0.4 -0.6 -0.8
-0.8 ta To
shown in Fig. 11, at 3 days, the decrease level of four types of pores of NS15-0.5 and NS15-1.0 is similar. For NS15-3.0 at 3 days, compared with the reference sample, the amount of less-harmful pore (20–50 nm) does not decrease, the decrease of total porosity is mainly attributed to the reducing amount of harmless pore (<20 nm) and harmful pore (50–200 nm). For all the cement pastes containing nano-SiO2 at 28 days, the decrease of harmless pore (<20 nm) and harmful pore (50–200 nm) is the most significant. The remarkable decrease of amount of harmless pore (<20 nm) results in an increase of the fraction of the big pore, hence, the average pore diameter of the cement paste containing nano-SiO2 is higher than the reference sample (Table 2). For the cement pastes containing nano-TiO2, as shown in Fig. 12, the decrease of total porosity is mainly attributed to the reduced amount of big pores: harmful pores (50–200 nm) and more harmful pores (>200 nm), especially the harmful pores (50– 200 nm). Hence, as shown in Table 3, the average pore diameter of the cement paste containing nano-TiO2 is smaller than that of the reference sample. On the other hand, as given in the previous study [47], although the total porosity of cement-based material containing nano-TiO2 is similar to the cement paste containing nano-SiO2, the impermeability of the cement-based material containing nano-TiO2 is better, which may be attributed to the significant decrease of harmful pores (50–200 nm) as shown in Fig. 12. Hence, the experimental results of this study indicate that nanoTiO2 is more efficient than nano-SiO2 for reducing the amount of harmful pores (50–200 nm). Fig. 13 gives the relationship between compressive strength and porosity. Balshin model and Schiller model [48] are popularly used to describe the change of strength upon porosity. As shown in Fig. 13, the cement paste containing nano-SiO2 agrees well with Balshin model and Schiller model, and the correlation coefficient
m
lp
ta To
ore
0 <2
nm 20
-50
nm
-2 50
00
Pore type
Fig. 11. Fractional decrease of the volume of different types of pores of cement paste containing nano-SiO2.
nm
0 >2
0n
m
8
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
Reference NT10-1.0
(a) 3d
NT10-0.5 NT10-3.0
0.0 -0.2 -0.4 -0.6
l ota
re po
<2
m 0n
-5 20
m 0n
0 0-2
m 0n
5 Pore type
0 >2
m 0n
Rererence NT10-1.0
0.2
NT10-0.5 NT10-3.0
0.0 -0.2 -0.4 -0.6 -0.8
-0.8 T
(b) 28d Decrease of volume ( V/V)
Decrease of volume ( V/V)
0.2
ta To
ore lp
0 <2
nm 20
-50
nm
-2 50
00
nm
0 >2
m
0n
Pore type
85 75
85
(a) SiO2
65 55
Experimental results Balshin model 4.9 =156(1-p) R=0.8994 Schiller model =62ln(0.46/p) R=0.9006
45 35 25 15
0.16
0.20
0.24
0.28
0.32
Compressive strength (MPa)
Compressive strength (MPa)
Fig. 12. Fractional decrease of the volume of different types of pores of cement paste containing nano-TiO2.
75
(b) TiO2
65 55
Experimental results Balshin model 5.6 =176(1-p) R=0.9487 Schiller model =67ln(0.42/p) R=0.9506
45 35 25 15
0.16
0.20
Porosity
0.24
0.28
0.32
Porosity
Fig. 13. Relationship between compressive strength and total porosity of cement pastes containing nanomaterials.
of the two models is 0.8994 and 0.9006, respectively. The relationship between strength and porosity of the cement paste containing nano-TiO2 agrees well with Balshin model and Schiller model too, the correlation coefficient of the two models is 0.9487 and 0.9506, respectively. The good agreement with existing models indicates that the strength and porosity measured in this study is feasible. 4. Conclusion Nano-SiO2 and nano-TiO2 can remarkably improve the compressive strength of cement paste at early age and late age. Being a pozzolanic material, nano-SiO2 is more efficient than nano-TiO2 for improving the early age compressive strength. The efficiency of nano-SiO2 is strongly dependent on the size and content of nano-SiO2. Small size and high content of nano-SiO2 are preferred for improving the compressive strength of cement paste, especially at early age, as long as the dispersion of nano-SiO2 is satisfied. The 3-day compressive strength of NS15-3.0 (53.8 MPa) even exceeds the 28-day compressive strength of reference cement paste (53.1 MPa). For the later age compressive strength, the particle size effect of nano-SiO2 becomes weak. The correlation level between the size and enhancing efficiency of nano-TiO2 is weaker than that of nano-SiO2. The efficiency of nano-TiO2 at early age is lower than nano-SiO2, but it approaches at late age. And at later age, the small content of nano-TiO2 is as effective as big content nano-TiO2 for improving the compressive strength.
The porosity of the cement paste containing nano-SiO2 and nano-TiO2 all decrease with respect to the reference sample, and the porosity of cement paste containing nanomaterials determines the compressive strength. The total porosity decrease efficiency of nano-SiO2 and nano-TiO2 is similar. The big difference is that the porosity decrease of the cement paste containing nano-SiO2 is mainly attributed to the reducing amount of harmless pore (<20 nm) and harmful pore (50–200 nm), but for the cement paste containing nano-TiO2 it is mainly attributed to the reducing amount of harmful pore (50–200 nm) and more-harmful pore (>200 nm). Hence, nano-TiO2 is more efficient for improving the impermeability. For improving the early age properties of cement-based materials, small size and large content of nanoSiO2 is preferred, and small content of nano-TiO2 is suitable for improving the later age properties of cement-based materials. Conflict of interest There is no conflict of interest. Acknowledgements This work was supported by the NSFC [grant number 51678206]; National Key R&D Program of China [grant number 2018YFC0705404]; the Fundamental Research Funds for the Central Universities.
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
References [1] P.C. Aitcin, Cements of yesterday and today: concrete of tomorrow, Cem. Concr. Res. 30 (9) (2000) 1349–1359. [2] Y.S. Zhang, C. Liu, Z.Y. Liu, G.J. Liu, L. Yang, Modelling of diffusion behavior of ions in low-density and high-density calcium silicate hydrate, Constr. Build. Mater. 155 (2017) 965–980. [3] A.S. Inozemtcev, E.V. Korolev, V.A. Smirnov, Nanoscale modifier as an adhesive for hollow microspheres to increase the strength of high-strength lightweight concrete, Struct. Concr. 18 (1) (2017) 67–74. [4] M.Y. Lu, H.G. Xiao, M. Liu, X.J. Li, H. Li, L. Sun, Improved interfacial strength of SiO2 coated carbon fiber in cement matrix, Cem. Concr. Compos. 91 (2018) 21– 28. [5] J.I. Tobón, J. Payá, O.J. Restrepo, Study of durability of Portland cement mortars blended with silica nanoparticles, Constr. Build. Mater. 80 (2015) 92–97. [6] Y. Wang, G. Chen, B. Wan, H. Lin, J. Zhang, Behavior of innovative circular ice filled steel tubular stub columns under axial compression, Constr. Build. Mater. 171 (2018) 680–689. [7] M. Jalal, E. Mansouri, M. Sharifipour, A.R. Pouladkhan, Mechanical, rheological, durability and microstructural properties of high performance self-compacting concrete containing SiO2 micro and nanoparticles, Mater. Des. 34 (2012) 389– 400. [8] I.G. Richardson, The nature of C-S-H in hardened cements, Cem. Concr. Res. 29 (8) (1999) 1131–1147. [9] 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. [10] F. Shaikh, V. Chavda, N. Minhaj, H.S. Arel, Effect of mixing methods of nano silica on properties of recycled aggregate concrete, Struct. Concr. 19 (2) (2018) 387–399. [11] H.G. Xiao, R. Liu, F.L. Zhang, M. Liu, H. Li, Role of nano-SiO2 in improving the microstructure and impermeability of concrete with different aggregate gradations, Constr. Build. Mater. 188 (2018) 537–545. [12] F. Sanchez, K. Sobolev, Nanotechnology in concrete – A review, Constr. Build. Mater. 24 (11) (2010) 2060–2071. [13] Y.L. Wang, Y.S. Wang, B.L. Wan, B.G. Han, G.C. Cai, Z.Z. Li, Properties and mechanisms of self-sensing carbon nanofibers/epoxy composites for structural health monitoring, Compos. Struct. 200 (2018) 669–678. [14] L.P. Singh, S.R. Karade, S.K. Bhattacharyya, M.M. Yousuf, S. Ahalawat, Beneficial role of nanosilica in cement based materials – A review, Constr. Build. Mater. 47 (2013) 1069–1077. [15] R. Liu, H.G. Xiao, H. Li, L. Sun, Z.Y. Pi, G.Q. Waqar, T. Du, L. Yu, Effects of nanoSiO2 on the permeability-related properties of cement-based composites with different water/cement ratios, J. Mater. Sci. 53 (7) (2018) 4974–4986. [16] H.J. Du, S.H. Du, X.M. Liu, Durability performances of concrete with nano-silica, Constr. Build. Mater. 73 (2014) 705–712. [17] G. Quercia, G. Hüsken, H.J.H. Brouwers, Water demand of amorphous nano silica and its impact on the workability of cement paste, Cem. Concr. Res. 42 (2) (2012) 344–357. [18] H. Li, H.G. Xiao, X.C. Guan, Z.T. Wang, L. Yu, Chloride diffusion in concrete containing nano-TiO2 under coupled effect of scouring, Compos. B Eng. 56 (2014) 698–704. [19] Q. Ye, Z. Zhang, D. Kong, R. 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. [20] H. Li, H.G. Xiao, J. Yuan, J.P. Ou, Microstructure of cement mortar with nanoparticles, Compos. B Eng. 35 (2) (2004) 185–189. [21] 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. [22] L.P. Singh, D. Ali, U. Sharma, Studies on optimization of silica nanoparticles dosage in cementitious system, Cem. Concr. Compos. 70 (2016) 60–68. [23] T. Ji, Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2, Cem. Concr. Res. 35 (10) (2005) 1943–1947. [24] H.J. Du, H.J. Gao, S.D. Pang, Improvement in concrete resistance against water and chloride ingress by adding graphene nanoplatelet, Cem. Concr. Res. 83 (2016) 114–123.
9
[25] D. Kong, X. Du, S. Wei, H. Zhang, Y. Yang, S.P. Shah, Influence of nano-silica agglomeration on microstructure and properties of the hardened cementbased materials, Constr. Build. Mater. 37 (2012) 707–715. [26] J.J. Gaitero, I. Campillo, A. Guerrero, Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles, Cem. Concr. Res. 38 (8) (2008) 1112–1118. [27] J. Liu, Q. Li, S. Xu, Influence of nanoparticles on fluidity and mechanical properties of cement mortar, Constr. Build. Mater. 101 (2015) 892–901. [28] L.P. Singh, S.K. Bhattacharyya, S.P. Shah, G. Mishra, S. Ahalawat, U. Sharma, Studies on early stage hydration of tricalcium silicate incorporating silica nanoparticles: Part I, Constr. Build. Mater. 74 (2015) 278–286. [29] R. Liu, H.G. Xiao, J.L. Liu, S. Guo, Y.F. Pei, Improving the microstructure of ITZ and reducing the permeability of concrete with various water/cement ratios using nano-silica, J. Mater. Sci. 54 (1) (2019) 444–456. [30] M. Bolhassani, M. Samani, Effect of type, size, and dosage of nanosilica and microsilica on properties of cement paste and mortar, ACI Mater. J. 112 (2) (2015) 259–266. [31] J. Chen, S.C. Kou, C.S. Poon, Hydration and properties of nano-TiO2 blended cement composites, Cem. Concr. Compos. 34 (5) (2012) 642–649. [32] J. Chen, C.S. Poon, Photocatalytic construction and building materials: from fundamentals to applications, Build. Environ. 44 (9) (2009) 1899–1906. [33] S.S. Lucas, V.M. Ferreira, J.L.B. de Aguiar, Incorporation of titanium dioxide nanoparticles in mortars — Influence of microstructure in the hardened state properties and photocatalytic activity, Cem. Concr. Res. 43 (2013) 112–120. [34] P. Krishnan, M.H. Zhang, L. Yu, H. Feng, Photocatalytic degradation of particulate pollutants and self-cleaning performance of TiO2-containing silicate coating and mortar, Constr. Build. Mater. 44 (2013) 309–316. [35] B. Ma, H. Li, X. Li, J. Mei, Y. Lv, Influence of nano-TiO2 on physical and hydration characteristics of fly ash-cement systems, Constr. Build. Mater. 122 (2016) 242–253. [36] T. Meng, Y. Yu, X. Qian, S. Zhan, K. Qian, Effect of nano-TiO2 on the mechanical properties of cement mortar, Constr. Build. Mater. 29 (2012) 241–245. [37] H. Li, T. Du, H.G. Xiao, Q.Q. Zhang, Crystallization of calcium silicate hydrates on the surface of nanomaterials, J. Am. Ceram. Soc. 100 (7) (2017) 3227–3238. [38] M. Liu, H.G. Xiao, R. Liu, J.L. Liu, Dispersion characteristics of various contents of nano-TiO2 and its effect on the properties of cement-based composite, Struct. Concr. 19 (5) (2018) 1301–1308. [39] J.G. Ren, Y.M. Lai, J.Q. Gao, Exploring the influence of SiO2 and TiO2 nanoparticles on the mechanical properties of concrete, Constr. Build. Mater. 175 (2018) 277–285. [40] M. Oltulu, R. S ß ahin, Pore structure analysis of hardened cement mortars containing silica fume and different nano-powders, Constr. Build. Mater. 53 (2014) 658–664. [41] H.F.W. Taylor, Cement Chemistry, Second Ed., Thomas Telford, London, U.K., 1997. [42] H. Madani, A. Bagheri, T. Parhizkar, The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement, Cem. Concr. Res. 42 (12) (2012) 1563–1570. [43] V.G. Papadakis, Experimental investigation and theoretical modeling of silica fume activity in concrete, Cem. Concr. Res. 29 (1) (1999) 79–86. [44] F. Pacheco-Torgal, S. Jalali, Nanotechnology: Advantages and drawbacks in the field of construction and building materials, Constr. Build. Mater. 25 (2) (2011) 582–590. [45] A.B. Abell, K.L. Willis, D.A. Lange, Mercury intrusion porosimetry and image analysis of cement-based materials, J. Colloid Interface Sci. 211 (1) (1999) 39– 44. [46] Y.R. Zhang, X.M. Kong, Z.B. Lu, Z.B. Lu, Q. Zhang, B.Q. Dong, F. Xing, Influence of triethanolamine on the hydration product of portlandite in cement paste and the mechanism, Cem. Concr. Res. 87 (2016) 64–76. [47] M.H. Zhang, H. Li, Pore structure and chloride permeability of concrete containing nano-particles for pavement, Constr. Build. Mater. 25 (2) (2011) 608–616. [48] A.U. Ozturk, B. Baradan, A comparison study of porosity and compressive strength mathematical models with image analysis, Comput. Mater. Sci. 43 (4) (2008) 974–979.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of pozzolanic and non-pozzolanic nanomaterials on cement-based materials Huigang Xiao a,b,⇑, Fengling Zhang b,c, Rui Liu a,b, Rongling Zhang d, Zhiguo Liu e, Hongxia Liu e a Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education and Key Lab of Disaster Smart Prevention and Mitigation for Civil Engineering of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China b School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China c Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, 117576, Singapore d National and Provincial Joint Engineering Laboratory of Road & Bridge Disaster Prevention and Control, Lanzhou Jiaotong University, Lanzhou 730070, China e Shandong Academic of Building Research, Jinan 250031, China
h i g h l i g h t s Nano-TiO2 is more efficient than nano-SiO2 for reducing the harmful pore. Nano-particles, especially nano-SiO2, can accelerate the cement hydration. Reinforcing efficiency of nano-SiO2 is strongly dependent on its size and content.
a r t i c l e
i n f o
Article history: Received 2 November 2018 Received in revised form 4 April 2019 Accepted 8 April 2019
Keywords: Nano-SiO2 Nano-TiO2 Pore structure Strength Cement
a b s t r a c t Although it has been verified that nanomaterials can improve the properties of cement-based materials, the research on the characteristics and mechanism of different types of nanomaterials is still inadequate. Two typical types of nanomaterials, nano-SiO2 (pozzolanic material) and nano-TiO2 (non-pozzolanic material), are utilized in this paper to investigate the combined effects of reactivity, size, and content on the efficiency of nanomaterials in cement-based materials. The experimental results indicate that the size and content effect of nano-SiO2 is obvious. Small size and large content of nano-SiO2 can significantly improve the compressive strength of cement paste. However, the size effect of nano-TiO2 is much weaker than that of nano-SiO2, and the content effect of nano-TiO2 is only obvious at the early age. For the late age compressive strength, the small content of nano-TiO2 shows a similar obviously improving effect as the big content of nano-TiO2. The pore structure analysis shows that both nano-SiO2 and nano-TiO2 can reduce the total porosity of cement paste, but nano-TiO2 is more effective than nano-SiO2 at reducing the harmful pores. Therefore, the small content of nano-TiO2 is more effective to enhance the impermeability of cement-based materials. The results of this paper show that nano-SiO2 and nano-TiO2 have different effects and they should be selected based on the properties requirement of cement-based materials. Ó 2019 Published by Elsevier Ltd.
1. Introduction Because of the advantages of easily casting, moulding, and relatively high compressive strength and durability, concrete has been the most widely used construction material in the world [1]. Great development has been achieved in improving the strength, ductility, and durability of concrete since its invention [2–4]. The property of concrete determines the safe servicing of concrete structures. The emerging needs of building infrastruc⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin, Heilongjiang 150090, China. E-mail address: [email protected] (H. Xiao). https://doi.org/10.1016/j.conbuildmat.2019.04.057 0950-0618/Ó 2019 Published by Elsevier Ltd.
tures at severe environments such as ocean and polar result in an increasing requirement on high-performance concrete (HPC) [5–7]. In concrete, mainly the amorphous calcium silicate hydrate (C-S-H), is the continuous matrix that binds aggregates and fibers together and plays an important role for the mechanical properties and durability of concrete [8–10]. Therefore, modifying the microstructure and properties of cement paste is an efficient way to improve the properties of concrete. Cement paste is actually composed of extremely fine particles in the nanoscale size range, these particles form a poorly ordered structure [11–13]. Hence, using nanotechnology in concrete to manipulate the structure at the nanoscale to get a macroscale superior property is getting
2
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
increasing attention [14]. Remarkable improvements in the mechancial properties and durability of concrete have been realized by incorporating nanomaterials into concrete [15–18]. One typical type of widely used nanomaterials to modify cement-based materials is nano-SiO2, which is a kind of pozzolanic materials that can react with cement hydration products (e.g. Ca (OH)2) [19]. With the addition of nano-SiO2, the performance of cement-based materials has been improved, such as increasing compressive and flexural strength [19–22], enhancing resistance of water penetration [23,24], chloride ingress [24,25] and sulfate attack [5], and reducing calcium leaching [25,26]. The better performance of cement-based materials incorporating nano-SiO2 is attributed to the high pozzolanic reactivity, accelerated cement hydration, refined pore structure, improved interfacial transition zone, low C/S ratio, and higher polymerization of silicate chain [27–29]. The effect of nano-SiO2 on the performance of cementbased materials is dependent on its particle size as well as content [14,22,30]. Nano-TiO2 is another kind of popularly used nanomaterials in cement-based materials, which is nonpozzolanic material [31– 34] but also has similar effects on cement hydration [35]. Some researchers have studied the effect of nano-TiO2 on compressive and flexural strength of cement-based materials [31,36]. The influence of nano-TiO2 aslo depends on its content and particle size [31], but studies on the effect of particle size of nano-TiO2 are limited. Compared to nano-SiO2, nano-TiO2 does not have pozzolanic activity, however, it shows a crystalize effect on the C-S-H gel [37]. It is also found that the content related dispersion state of nano-TiO2 plays an important role on the improvement efficiency of nano-TiO2 [38]. Such works indicate that various kinds of nanomaterials can be used effectively in a bottom-up approach to modify the properties of concrete. However, the effect of nano-TiO2 without pozzolanic activity on the performance of cement composites may be different from nano-SiO2 with pozzolanic activity [39,40], the comparison study is still not so sufficient in the present. While nano-engineered cement-based materials are seen as having tremendous potential, the information of the role of different types of nano-particles, involving pozzolanic, size, and content, on hydration and microstructure of cement and the macroscale properties of cement-based materials is still inadequate [40]. This information is crucial for understanding, predicting, and optimizing the utilization of nanotechnology and the corresponding performance of nano-engineering cement-based materials. This paper aims at providing an insight into the different effects and mechanisms of nano-SiO2 and nano-TiO2 on the properties of cement-based materials.
2. Materials and experimental methods 2.1. Materials and mix proportions Ordinary Portland cement (OPC, P∙O 42.5) and deionized water were used for all cement pastes. Nano-SiO2 (NS) and nano-TiO2 (NT, anatase type) used in this study are in dry powder form. According to the supplier’s data, the NS with a SiO2 content of >99.5% has two different average particle sizes, 15 nm (specific surface area, SSA of 200 m2/g) and 30 nm (SSA of 100 m2/g), denoted as NS15 and NS30, respectively. The NT with a TiO2 content of >99.8% has two different average particle sizes, 10 nm (SSA of 210 m2/g) and 40 nm (SSA of 80 m2/g), denoted as NT10 and NT40, respectively. Silica fume (SF) powder with a SiO2 content of 98.3%, an average particle size of 100–150 nm, and specific surface area of 15–27 m2/g was used for comparison. A naphthalene-based superplasticizer (SP) was used in this study.
The mix proportions of cement pastes are given in Table 1. In total, 16 cement pastes were included. All cement pastes had a water to cementitious materials ratio (w/c) of 0.40. The dosage of NS, NT, and SF was varied from 0.5%, to 1% and 3% by mass of cementitious materials. The dosage of SP was kept as 0.5% by mass of cementitious materials to minimize the effect of SP. For each cement paste, nanoparticles, partial mixing water, and superplasticizer were first mixed, then mechanically stirred for 10 min, and ultrasonicated in a sonicator (power of 400 W) for 30 min. The sonicated solution, remaining water, and cement were mixed in a mixer at a low speed for 120 s, and a high speed for additional 120 s. The fresh cement paste was poured into steel moulds (40 40 40 mm3). An external vibrator was used to facilitate the compaction and decrease the air bubbles. After casting, the moulded specimens were covered with plastic sheet to prevent moisture loss. All specimens were demoulded after 24 h. After demould, all specimens were cured in a standard curing room (temperature of 20 ± 2 °C and relative humidity of > 95%) until the specified testing age (3 days, 7 days, and 28 days). 2.2. Fluidity and mechanical characterization The fluidity of the fresh cement paste was determined by minislump test according to Chinese Standard GB/T 8077-2012. The truncated cone has a height of 30 mm, bottom diameter of 60 mm, and top diameter of 36 mm. The truncated cone was first placed on a flat glass plate, filled with fresh cement paste, and then vertically raised to let the fresh cement paste flow freely and expand into a disc shape in around 30 s. The diameter of the stilled cement paste was measured in two directions at right angles to one another and the average diameter was taken as the fluidity of the cement paste. The compressive strength of the cement paste was determined by 40 40 40 mm3 specimens using a Materials Testing System (MTS) with a loading rate of 1 kN/s at the specified testing age of 3, 7, and 28 days, respectively. At each testing age, at least 6 specimens was used for compressive strength test and the average values and standard deviation were reported. 2.3. Composition characterization The phase compositions of cement pastes containing nanomaterials were analysed by X-ray diffraction (XRD, X’Pert Pro MPD, Panalytical, Netherlands) with a Cu Ka radiation at 40 Kv and 40 Ma. The 2h range was from 5° to 65° with a step size of 0.026°. After compressive strength test at specified ages, the tested specimens were broken into small pieces and samples were collected from the core of the test specimens. The selected samples were immersed in ethanol absolute as fast as possible for at least 1 week to stop cement hydration. Then, the samples were dried in a vacuum oven at 50 °C until constant mass was reached. The samples were grinded into powders that could pass the 80 mm sieve before XRD analysis. Thermo-gravimetric analysis (TGA, TGA/SDTA 851e, Mettler Toledo, USA) was performed to investigate the effect of nanoSiO2 and nano-TiO2 on the Ca(OH)2 content. For each TGA test, approximately 10 mg dried powder was heated from about 30° to 900 °C with a heating rate of 10 °C/min in a nitrogen environment. The degree of cement hydration was also determined by TGA. It was calculated by wb ðtÞ=wb;1 , where wb ðtÞ ¼ ðw105 C w900 C Þ=w900 C is the normalized mass of non-evaporable water at specified ages, w105 C is the mass of samples at 105 °C, w900 C is the mass of samples at 900 °C, wb;1 is the mass of nonevaporable water for fully hydrated cement and 0.23 was selected
3
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9 Table 1 Mix proportions (by weight of cementitious materials). Mixture
Water
Cement
Silica fume (%)
Nanoparticles (%)
Superplasticizer (%)
Reference NS15-0.5 NS15-1.0 NS15-3.0 NS30-0.5 NS30-1.0 NS30-3.0 SF-0.5 SF-1.0 SF-3.0 NT10-0.5 NT10-1.0 NT10-3.0 NT40-0.5 NT40-1.0 NT40-3.0
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
1 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970 0.995 0.990 0.970
0 0 0 0 0 0 0 0.5 1.0 3.0 0 0 0 0 0 0
0 0.5 1.0 3.0 0.5 1.0 3.0 0 0 0 0.5 1.0 3.0 0.5 1.0 3.0
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
[41]. A correction for the mass of non-evaporable water was done by subtracting the mass loss due to the decomposition of CaCO3. According to [42,43], amorphous silica does not consume nonevaporable water to react with Ca(OH)2. Thus, the effect of nanoSiO2 and silica fume on the non-evaporable water content was not considered in this study.
280
Fluidity (mm)
240
2.4. Mercury intrusion porosimetry (MIP) The porosity and pore size distribution of the cement pastes containing nano-SiO2 or TiO2 were analysed by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics, USA) [44,45]. Each pore size is quantitatively determined by the relationship between the volume of intruded mercury and the applied pressure [45]. The relationship between the pore diameter and the applied pressure is generally described by Washburn Eq. (1) as follows [45];
D ¼ 4ccosh=P
Reference SF
NS15 NT10
NS30 NT40
200 160 120 80 0.5
1.0 1.5 2.0 2.5 Content of nanomaterials (%)
3.0
Fig. 1. Fluidity of cement pastes containing nanomaterials.
ð1Þ
where D is the pore diameter (nm), c is the surface tension of mercury (N/m) and 0.485 N/m is used in this study, h is the contact angle between mercury and solid (°) and 141.3 °C is used, and P is the applied pressure (MPa). To prepare samples for MIP evaluation, the cement paste specimens were first broken into smaller pieces (3–5 mm in size) after 3 days and 28 days of curing, respectively, then, three samples were selected from the core of tested specimens and used to measure the pore structure for each case. These samples were immersed in acetone as fast as possible to stop their hydration process. Prior to applying the MIP test, the samples were dried in an oven at about 60 °C for 48 h to remove moisture from their pores before testing.
paste containing nano-TiO2 is much better than that containing nano-SiO2. When the particle size of nano-SiO2 is 100–150 nm (silica fume), the addition of 0.5% and 1% nano-SiO2 shows a slight effect on the fluidity, only when the addition reaches 3%, there is a significantly reduction in fluidity as 35.1%. Therefore, based on the observation of workability, the addition of nano-materials should be lower than 3% by mass of cementitious materials.
3.1. Fluidity of cement paste containing nano-particles
NS15-3.0
Fig. 1 gives the fluidity of cement pastes containing various types and contents of nanomaterials. For nanomaterials, they agglomerate easily due to their high specific surface area and strong Van der Waals forces. The fluidity of the cement paste is reduced by the addition of nanomaterials. The negative effect of nanomaterials on the fluidity is also reported in [30,36]. The reduction degree strengthens with the increase in nanomaterials content and the decrease in nanomaterials size. The 0.5% addition of 15 nm nano-SiO2 reduces the fluidity of cement paste by 58.3%, and when the content reaches 3%, the cement paste nearly totally loses the fluidity. For the similar size and content, the fluidity of the cement
1
TiO2 2 1 2
1-Calcium hydroxide 2-Calcium silicates 3-Ettringite 1 2 1 11
Intensity
3. Results and discussion
NT10-3.0 1 3
Reference
5
10
15
20
25
30 35 40 2 Theta
45
50
55
60
65
Fig. 2. X-ray diffraction spectra of cement pastes containing nanomaterials at 28 days.
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
3.2. XRD and TG analysis results
3.3. Compressive strength Fig. 5 shows the compressive strength of cement pastes containing various sizes and contents of nano-SiO2 and silica fume at 3 days, 7 days, and 28 days. For nano-SiO2, which is a pozzolanic material, it shows an obvious effect on improving the strength of cement paste, and this strengthening effect depends not only on the content of nano-SiO2, but also on the size of nano-SiO2. This observation is consistent with a previous study [30]. Comparing with the compressive strength of reference sample, the compres-
100
0.003
0.000 CC 620-690
85
CSH
DTG -0.006
Free water CH 420-450
105 80
-0.003 TG
0
150
300 450 600 Temperature ( )
750
-0.009 900
Fig. 3. Typical TG/DTG curve of reference cement paste at 28 days.
Mass perc diff (%/
Mass loss (%)
95
)
Non-evaporable water
Ca(OH)2 Content (wt.%)
Fig. 2 shows the X-ray diffraction spectra of the cement pastes containing nano-SiO2 and nano-TiO2 at 28 days. The characteristic diffraction peaks of reference sample, NS15-3.0, and NT10-3.0 are similar, except the unique peak at diffraction angle of 25.3° (belong to TiO2) found for NT10-3.0. The experimental results indicate that the hydration products of the cement pastes containing nanomaterials are the same to the reference cement paste, there is no new products formed. Nano-TiO2 remains in the hydrated cement paste, validating that it is a nonpozzolanic material. The non-pozzolanic nature of nano-TiO2 is also reported in [31]. Although nano-TiO2 does not react with cement hydration products, it can increase the crystalline level and hardness of C-S-H as reported in [37]. Fig. 3 shows a typical thermogravimetric analysis curve of the reference cement paste at 28 days. At 105 °C, free water evaporates. With the further increase of temperature, different components (e.g. Aft, AFm, C-S-H, and Ca(OH)2)will dehydrate at a specific temperature, therefore, the content of each component and the total content of non-evaporable water can be calculated based on the TG and DTG curves [46]. For example, the component of Ca(OH)2 dehydrates at 400–500 °C, the content of Ca(OH)2 is calculated and given in Fig. 4. The hydration degree of three typical cement pastes at 28 days is given in Fig. 4. With respect to the reference cement paste, the hydration degree of NS15-3.0 and NT103.0 is enhanced by 11.7% and 8.8%, respectively, indicating that both nano-SiO2 and nano-TiO2 can improve the cement hydration. The enhanced degree of cement hydration with the incorporation of nanomaterials is also reported in [22,31]. The content of Ca (OH)2 of NT10-3.0 is higher than that of the reference cement paste, validating that hydration degree of NT10-3.0 is higher than the reference cement paste. On the other hand, although the hydration degree of NS15-3.0 is the highest, its Ca(OH)2 content is the lowest among the three cement pastes, which is because the Ca(OH)2 has reacted with nano-SiO2 due to its pozzolanic nature [19].
90
72
20 19
Hydration degree
18
66
17 16 15
69
63 Consumed by SiO2
60
Ca(OH)2 content
57
14 Reference
Hydration degree (%)
4
NS15-3.0
NT10-3.0
Fig. 4. Calcium hydroxide content and hydration degree of cement pastes containing nanomaterials at 28 days.
sive strength of NS15-0.5 at 3 days and 28 days is increased by 41.7% and 14.7%, respectively. The compressive strength of NS153.0 at 3 days and 28 days is increased by 141.3% and 39.4%, respectively. Therefore, the strengthening effect of nano-SiO2 increases with the content but decreases with the increase in size. The small size of nano-SiO2, NS15, is more efficient to enhance the strength of cement paste at early age, the 3-day compressive strength of NS153.0 is 53.8 MPa, which is 2.4 times of that of reference sample (22.3 MPa), and is similar to the 28-day compressive strength (53.1 MPa) of reference sample. On the other hand, the 28-day compressive strength of NS15-3.0 and NS30-3.0 is 74.0 MPa and 70.2 MPa, respectively, which is much higher than that of reference sample (53.1 MPa). Furthermore, the similar compressive strength of NS15-3.0 and NS30-3.0 at 28 days indicates that the effect of different sizes of nano-SiO2 is similar for the later age compressive strength. The experimental results indicate that nano-SiO2 can accelerate the hydration of cement, especially when the size of nano-SiO2 is small as it has a huge specific area and high reactivity. Hence, the compressive strength of cement paste containing high content of small size nano-SiO2 is much higher than that of reference sample at early age. It should be noted that a combination of mechanical stirring and ultrasonication is utilized in this study to ensure the well dispersion of nano-SiO2. In case of using nanoSiO2 with a small size or high content, it may negatively affect the compressive strength if nano-SiO2 does not disperse well as reported by Bolhassani and Samani [30]. Fig. 6 presents the compressive strength of cement pastes containing various sizes and contents of nano-TiO2 at 3 days, 7 days, and 28 days. Nano-TiO2 can also improve the compressive strength of cement paste, but the effect of particle size is not as obvious as the nano-SiO2. Although the high content of nano-TiO2 performs better at the early age, the small content of nano-TiO2 achieves similar strengthening efficiency at the long age, which is consistent with that reported in [36]. As indicated in our previous study [38], the distribution of cement hydration products is strongly associated with the dispersion state of nano-TiO2. Small content of nano-TiO2 can reach a uniform dispersion state and benefit the macroscale properties of cement paste, especially for long term properties. Fig. 7 gives the comprehensive view of the compressive strength enhancing ratio of cement pastes containing various types and contents of nanomaterials at different ages. It is the same for nano-SiO2 and nano-TiO2 that their improving efficiency is more significant at early age, and they are much more efficient than the silica fume, which is attributed to the accelerated hydration of cement paste containing nanomaterials [11]. Because nanoSiO2 can react with cement hydration products, the enhancing efficiency of nano-SiO2 is strongly related to its size and content. For small size and high content, efficiency of nano-SiO2 is much higher
5
80 (a)
Reference NS15-0.5 NS15-1.0 NS15-3.0
60 40 20 0
Compressive strength (MPa)
3d 80
Reference NS30-0.5 NS30-1.0 NS30-3.0
(b)
60 40 20 0
3d
7d
28d
7d
28d
Compressive strength (MPa)
Compressive strength (MPa)
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
80
(c)
Reference SF-0.5 SF-1.0 SF-3.0
60 40 20 0
3d
7d
28d
80
(a)
Reference NT10-0.5 NT10-1.0 NT10-3.0
60 40 20 0
3d
7d
28d
Compressive strength (MPa)
Compressive strength (MPa)
Fig. 5. Compressive strength of cement paste containing nano-SiO2 and silica fume.
80
(b)
Reference NT40-0.5 NT40-1.0 NT40-3.0
60 40 20 0
3d
7d
28d
Enhancement ratio of strength (%)
Fig. 6. Compressive strength of cement paste containing nano-TiO2.
than that of nano-TiO2. Silica fume can also improve the compressive strength of cement paste, but it is relatively weaker than nanoSiO2 at the same content. At age of 28 days, the compressive strength of cement paste containing nano-SiO2 still increases obviously upon the content of nano-SiO2. However, for nano-TiO2, the content of 0.5% reaches the similar enhancing efficiency as the content of 3% at age of 28 days, which agrees well with the reported results shown in [38]. Therefore, for the purpose of enhancing the early-age compressive strength, high content of small size nano-SiO2 is preferred as long as the dispersion is satisfied. For the long-age compressive strength, small content of big size nano-TiO2 is more suitable considering the similar enhancing efficiency and the good workability.
150 NS15 NT10
120 3d
90
NS30 NT40
7d
SF
28d
60 30 0 5 0 0 . 1.
0 5 0 0 5 0 3. 0. 1. 3. 0. 1. Content of nanomaterials (%)
3.
0
Fig. 7. Compressive strength enhancement ratio of various mixtures compared with reference samples.
3.4. Porosity and pore size distribution Fig. 8 shows the pore distribution curve of the cement pastes containing various contents of NS15 at different ages. Fig. 9 shows
0.25 (a) 3d 0.20
Reference NS15-0.5 NS15-1.0 NS15-3.0
0.15 0.10 0.05 0.00 6 10
5
4
10
3
10
2
10
1
10
10
0
10
dV/dlogD Pore Volume (mL/g)
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
dV/dlogD Pore Volume (mL/g)
6
0.25 (b) 28d 0.20
Reference NS15-0.5 NS15-1.0 NS15-3.0
0.15 0.10 0.05 0.00 6 10
5
10
4
10
3
2
10
1
10
10
0
10
Pore Diameter (nm)
Pore Diameter (nm)
0.25 0.20 0.15
(a) 3d Reference NT10-0.5 NT10-1.0 NT10-3.0
0.10 0.05 0.00 6 10
5
10
4
10
3
10
2
10
1
10
0
10
dV/dlogD Pore Volume (mL/g)
dV/dlogD Pore Volume (mL/g)
Fig. 8. Differential curves of pore structure of cement paste containing nano-SiO2。
0.25 0.20 0.15
(b) 28d Reference NT10-0.5 NT10-1.0 NT10-3.0
0.10 0.05 0.00 6 10
Pore Diameter (nm)
5
10
4
10
3
10
2
10
1
10
0
10
Pore Diameter (nm)
Fig. 9. Differential curves of pore structure of cement paste containing nano-TiO2。
the pore distribution curve of the cement pastes containing various contents of NT10 at different ages. Together with Tables 2 and 3, these figures and tables provide the comprehensive information of the pore structures of hardened cement pastes with nano-SiO2 and nano-TiO2. For series of NS15 specimens, the total porosity decreases upon the content of nano-SiO2, the porosity reduction ratio for all ages is higher than 20%, and the maximum porosity reduction degree is reached at 3% content of nano-SiO2 at 3 days, which is about 37.8% as shown in Fig. 10. The reduced porosity with the addition of nano-SiO2 is also reported in [15]. The decrease characteristics of porosity, especially for high content of nano-SiO2 at 3 days, agrees with the enhancing characteristics of compressive strength of cement pastes containing nano-SiO2. The most probable pore diameter of cement pastes containing nanoSiO2 is smaller than that of the reference sample. Unlike the decrease of porosity upon content of nano-SiO2, the average diameter and median diameter increase upon the content of nano-SiO2,
and for most cases, they are higher than that of the reference specimen, which will be discussed below. Nano-SiO2 can accelerate the cement hydration and enhance the hydration degree as indicated in Fig. 4. The accelerating efficiency is more remarkable for large content of nano-SiO2 at early age, therefore, the decrease in porosity of NS15-3.0 is more obvious at 3d in comparison to 28d (Fig. 10). The total porosity of the cement pastes containing nano-TiO2 is obviously lower than that of the reference sample, and it is slightly higher when compared with the cement paste containing nanoSiO2. The decreased porosity with the incorporation of nano-TiO2 is also reported in [31]. Especially for small content of nano-TiO2 at 3 days as shown in Fig. 10, the decrease degree of porosity is not so efficient as the same content of nano-SiO2. On the other hand, at 28 days, the decrease efficiency of nano-TiO2 approaches nano-SiO2, and it is almost independent on content. Hence, the 28-day compressive strength of the cement paste containing vari-
Table 2 Pore structure characteristics of cement paste containing nano-SiO2. Sample
Age
Porosity (%)
Total specific pore volume (mL/g)
Most probable pore diameter (nm)
Average diameter (nm)
Median diameter (nm)
Reference NS15-0.5 NS15-1.0 NS15-3.0
3d
32.19 26.36 24.74 20.02
0.2068 0.1647 0.1545 0.1216
50.38 5.65 50.39 32.40
18.9 16.5 17.6 19.3
31.1 26.9 34.0 31.0
Reference NS15-0.5 NS15-1.0 NS15-3.0
28d
22.58 18.20 16.19 15.55
0.1411 0.1094 0.0964 0.0904
32.39 5.84 32.38 26.30
17.1 16.6 18.2 21.0
28.8 28.2 29.2 30.0
7
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9 Table 3 Pore structure characteristics of cement paste containing nano-TiO2. Sample
Age
Porosity (%)
Total specific pore volume (mL/g)
Most probable pore diameter (nm)
Average diameter (nm)
Median diameter (nm)
Reference NT10-0.5 NT10-1.0 NT10-3.0
3d
32.19 29.44 25.46 24.06
0.2068 0.1849 0.1560 0.1460
50.38 5.84 5.84 6.03
18.9 17.4 15.7 16.2
31.1 24.7 20.9 24.5
Reference NT10-0.5 NT10-1.0 NT10-3.0
28 d
22.58 17.96 17.07 16.35
0.1411 0.1072 0.1012 0.0960
32.39 26.29 26.29 26.30
17.1 16.0 16.3 17.0
28.8 25.7 26.2 27.0
Decrease of porosity (%)
0
NS15-3d NT10-3d
NS15-28d NT10-28d
-10
NT -20
-30
NS -40 0.5
1.0
1.5
2.0
2.5
3.0
Content of nanomaterials (%) Fig. 10. Porosity decrease percentage of cement paste containing various types of nanomaterials.
ous content of nano-TiO2 is similar as discussed in Section 3.3. The average diameter and median diameter of the cement pastes containing various contents of nano-TiO2 are all smaller than those of the reference sample, indicating that the percentage of fine pore dominates the porosity of cement paste containing nano-TiO2. Figs. 11 and 12 show the fractional decrease of the specific volume of the total pores and of different types of pores. Based on the diameter and the corresponding harmful level [47], the pores can be divided into four types: harmless pore (<20 nm), less-harmful pore (20–50 nm), harmful pore (50–200 nm) and more-harmful pore (>200 nm). For the cement paste containing nano-SiO2 as
Decrease of volume ( V/V)
NS15-0.5 NS15-3.0
(b) 28d Decrease of volume ( V/V)
Reference NS15-1.0
(a) 3d 0.2 0.0 -0.2 -0.4 -0.6
o lp
re
0n <2
m 20
n -50
m -2 50
Pore type
n 00
m >2
n 00
Reference NS15-1.0
0.2
NS15-0.5 NS15-3.0
0.0 -0.2 -0.4 -0.6 -0.8
-0.8 ta To
shown in Fig. 11, at 3 days, the decrease level of four types of pores of NS15-0.5 and NS15-1.0 is similar. For NS15-3.0 at 3 days, compared with the reference sample, the amount of less-harmful pore (20–50 nm) does not decrease, the decrease of total porosity is mainly attributed to the reducing amount of harmless pore (<20 nm) and harmful pore (50–200 nm). For all the cement pastes containing nano-SiO2 at 28 days, the decrease of harmless pore (<20 nm) and harmful pore (50–200 nm) is the most significant. The remarkable decrease of amount of harmless pore (<20 nm) results in an increase of the fraction of the big pore, hence, the average pore diameter of the cement paste containing nano-SiO2 is higher than the reference sample (Table 2). For the cement pastes containing nano-TiO2, as shown in Fig. 12, the decrease of total porosity is mainly attributed to the reduced amount of big pores: harmful pores (50–200 nm) and more harmful pores (>200 nm), especially the harmful pores (50– 200 nm). Hence, as shown in Table 3, the average pore diameter of the cement paste containing nano-TiO2 is smaller than that of the reference sample. On the other hand, as given in the previous study [47], although the total porosity of cement-based material containing nano-TiO2 is similar to the cement paste containing nano-SiO2, the impermeability of the cement-based material containing nano-TiO2 is better, which may be attributed to the significant decrease of harmful pores (50–200 nm) as shown in Fig. 12. Hence, the experimental results of this study indicate that nanoTiO2 is more efficient than nano-SiO2 for reducing the amount of harmful pores (50–200 nm). Fig. 13 gives the relationship between compressive strength and porosity. Balshin model and Schiller model [48] are popularly used to describe the change of strength upon porosity. As shown in Fig. 13, the cement paste containing nano-SiO2 agrees well with Balshin model and Schiller model, and the correlation coefficient
m
lp
ta To
ore
0 <2
nm 20
-50
nm
-2 50
00
Pore type
Fig. 11. Fractional decrease of the volume of different types of pores of cement paste containing nano-SiO2.
nm
0 >2
0n
m
8
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
Reference NT10-1.0
(a) 3d
NT10-0.5 NT10-3.0
0.0 -0.2 -0.4 -0.6
l ota
re po
<2
m 0n
-5 20
m 0n
0 0-2
m 0n
5 Pore type
0 >2
m 0n
Rererence NT10-1.0
0.2
NT10-0.5 NT10-3.0
0.0 -0.2 -0.4 -0.6 -0.8
-0.8 T
(b) 28d Decrease of volume ( V/V)
Decrease of volume ( V/V)
0.2
ta To
ore lp
0 <2
nm 20
-50
nm
-2 50
00
nm
0 >2
m
0n
Pore type
85 75
85
(a) SiO2
65 55
Experimental results Balshin model 4.9 =156(1-p) R=0.8994 Schiller model =62ln(0.46/p) R=0.9006
45 35 25 15
0.16
0.20
0.24
0.28
0.32
Compressive strength (MPa)
Compressive strength (MPa)
Fig. 12. Fractional decrease of the volume of different types of pores of cement paste containing nano-TiO2.
75
(b) TiO2
65 55
Experimental results Balshin model 5.6 =176(1-p) R=0.9487 Schiller model =67ln(0.42/p) R=0.9506
45 35 25 15
0.16
0.20
Porosity
0.24
0.28
0.32
Porosity
Fig. 13. Relationship between compressive strength and total porosity of cement pastes containing nanomaterials.
of the two models is 0.8994 and 0.9006, respectively. The relationship between strength and porosity of the cement paste containing nano-TiO2 agrees well with Balshin model and Schiller model too, the correlation coefficient of the two models is 0.9487 and 0.9506, respectively. The good agreement with existing models indicates that the strength and porosity measured in this study is feasible. 4. Conclusion Nano-SiO2 and nano-TiO2 can remarkably improve the compressive strength of cement paste at early age and late age. Being a pozzolanic material, nano-SiO2 is more efficient than nano-TiO2 for improving the early age compressive strength. The efficiency of nano-SiO2 is strongly dependent on the size and content of nano-SiO2. Small size and high content of nano-SiO2 are preferred for improving the compressive strength of cement paste, especially at early age, as long as the dispersion of nano-SiO2 is satisfied. The 3-day compressive strength of NS15-3.0 (53.8 MPa) even exceeds the 28-day compressive strength of reference cement paste (53.1 MPa). For the later age compressive strength, the particle size effect of nano-SiO2 becomes weak. The correlation level between the size and enhancing efficiency of nano-TiO2 is weaker than that of nano-SiO2. The efficiency of nano-TiO2 at early age is lower than nano-SiO2, but it approaches at late age. And at later age, the small content of nano-TiO2 is as effective as big content nano-TiO2 for improving the compressive strength.
The porosity of the cement paste containing nano-SiO2 and nano-TiO2 all decrease with respect to the reference sample, and the porosity of cement paste containing nanomaterials determines the compressive strength. The total porosity decrease efficiency of nano-SiO2 and nano-TiO2 is similar. The big difference is that the porosity decrease of the cement paste containing nano-SiO2 is mainly attributed to the reducing amount of harmless pore (<20 nm) and harmful pore (50–200 nm), but for the cement paste containing nano-TiO2 it is mainly attributed to the reducing amount of harmful pore (50–200 nm) and more-harmful pore (>200 nm). Hence, nano-TiO2 is more efficient for improving the impermeability. For improving the early age properties of cement-based materials, small size and large content of nanoSiO2 is preferred, and small content of nano-TiO2 is suitable for improving the later age properties of cement-based materials. Conflict of interest There is no conflict of interest. Acknowledgements This work was supported by the NSFC [grant number 51678206]; National Key R&D Program of China [grant number 2018YFC0705404]; the Fundamental Research Funds for the Central Universities.
H. Xiao et al. / Construction and Building Materials 213 (2019) 1–9
References [1] P.C. Aitcin, Cements of yesterday and today: concrete of tomorrow, Cem. Concr. Res. 30 (9) (2000) 1349–1359. [2] Y.S. Zhang, C. Liu, Z.Y. Liu, G.J. Liu, L. Yang, Modelling of diffusion behavior of ions in low-density and high-density calcium silicate hydrate, Constr. Build. Mater. 155 (2017) 965–980. [3] A.S. Inozemtcev, E.V. Korolev, V.A. Smirnov, Nanoscale modifier as an adhesive for hollow microspheres to increase the strength of high-strength lightweight concrete, Struct. Concr. 18 (1) (2017) 67–74. [4] M.Y. Lu, H.G. Xiao, M. Liu, X.J. Li, H. Li, L. Sun, Improved interfacial strength of SiO2 coated carbon fiber in cement matrix, Cem. Concr. Compos. 91 (2018) 21– 28. [5] J.I. Tobón, J. Payá, O.J. Restrepo, Study of durability of Portland cement mortars blended with silica nanoparticles, Constr. Build. Mater. 80 (2015) 92–97. [6] Y. Wang, G. Chen, B. Wan, H. Lin, J. Zhang, Behavior of innovative circular ice filled steel tubular stub columns under axial compression, Constr. Build. Mater. 171 (2018) 680–689. [7] M. Jalal, E. Mansouri, M. Sharifipour, A.R. Pouladkhan, Mechanical, rheological, durability and microstructural properties of high performance self-compacting concrete containing SiO2 micro and nanoparticles, Mater. Des. 34 (2012) 389– 400. [8] I.G. Richardson, The nature of C-S-H in hardened cements, Cem. Concr. Res. 29 (8) (1999) 1131–1147. [9] 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. [10] F. Shaikh, V. Chavda, N. Minhaj, H.S. Arel, Effect of mixing methods of nano silica on properties of recycled aggregate concrete, Struct. Concr. 19 (2) (2018) 387–399. [11] H.G. Xiao, R. Liu, F.L. Zhang, M. Liu, H. Li, Role of nano-SiO2 in improving the microstructure and impermeability of concrete with different aggregate gradations, Constr. Build. Mater. 188 (2018) 537–545. [12] F. Sanchez, K. Sobolev, Nanotechnology in concrete – A review, Constr. Build. Mater. 24 (11) (2010) 2060–2071. [13] Y.L. Wang, Y.S. Wang, B.L. Wan, B.G. Han, G.C. Cai, Z.Z. Li, Properties and mechanisms of self-sensing carbon nanofibers/epoxy composites for structural health monitoring, Compos. Struct. 200 (2018) 669–678. [14] L.P. Singh, S.R. Karade, S.K. Bhattacharyya, M.M. Yousuf, S. Ahalawat, Beneficial role of nanosilica in cement based materials – A review, Constr. Build. Mater. 47 (2013) 1069–1077. [15] R. Liu, H.G. Xiao, H. Li, L. Sun, Z.Y. Pi, G.Q. Waqar, T. Du, L. Yu, Effects of nanoSiO2 on the permeability-related properties of cement-based composites with different water/cement ratios, J. Mater. Sci. 53 (7) (2018) 4974–4986. [16] H.J. Du, S.H. Du, X.M. Liu, Durability performances of concrete with nano-silica, Constr. Build. Mater. 73 (2014) 705–712. [17] G. Quercia, G. Hüsken, H.J.H. Brouwers, Water demand of amorphous nano silica and its impact on the workability of cement paste, Cem. Concr. Res. 42 (2) (2012) 344–357. [18] H. Li, H.G. Xiao, X.C. Guan, Z.T. Wang, L. Yu, Chloride diffusion in concrete containing nano-TiO2 under coupled effect of scouring, Compos. B Eng. 56 (2014) 698–704. [19] Q. Ye, Z. Zhang, D. Kong, R. 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. [20] H. Li, H.G. Xiao, J. Yuan, J.P. Ou, Microstructure of cement mortar with nanoparticles, Compos. B Eng. 35 (2) (2004) 185–189. [21] 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. [22] L.P. Singh, D. Ali, U. Sharma, Studies on optimization of silica nanoparticles dosage in cementitious system, Cem. Concr. Compos. 70 (2016) 60–68. [23] T. Ji, Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2, Cem. Concr. Res. 35 (10) (2005) 1943–1947. [24] H.J. Du, H.J. Gao, S.D. Pang, Improvement in concrete resistance against water and chloride ingress by adding graphene nanoplatelet, Cem. Concr. Res. 83 (2016) 114–123.
9
[25] D. Kong, X. Du, S. Wei, H. Zhang, Y. Yang, S.P. Shah, Influence of nano-silica agglomeration on microstructure and properties of the hardened cementbased materials, Constr. Build. Mater. 37 (2012) 707–715. [26] J.J. Gaitero, I. Campillo, A. Guerrero, Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles, Cem. Concr. Res. 38 (8) (2008) 1112–1118. [27] J. Liu, Q. Li, S. Xu, Influence of nanoparticles on fluidity and mechanical properties of cement mortar, Constr. Build. Mater. 101 (2015) 892–901. [28] L.P. Singh, S.K. Bhattacharyya, S.P. Shah, G. Mishra, S. Ahalawat, U. Sharma, Studies on early stage hydration of tricalcium silicate incorporating silica nanoparticles: Part I, Constr. Build. Mater. 74 (2015) 278–286. [29] R. Liu, H.G. Xiao, J.L. Liu, S. Guo, Y.F. Pei, Improving the microstructure of ITZ and reducing the permeability of concrete with various water/cement ratios using nano-silica, J. Mater. Sci. 54 (1) (2019) 444–456. [30] M. Bolhassani, M. Samani, Effect of type, size, and dosage of nanosilica and microsilica on properties of cement paste and mortar, ACI Mater. J. 112 (2) (2015) 259–266. [31] J. Chen, S.C. Kou, C.S. Poon, Hydration and properties of nano-TiO2 blended cement composites, Cem. Concr. Compos. 34 (5) (2012) 642–649. [32] J. Chen, C.S. Poon, Photocatalytic construction and building materials: from fundamentals to applications, Build. Environ. 44 (9) (2009) 1899–1906. [33] S.S. Lucas, V.M. Ferreira, J.L.B. de Aguiar, Incorporation of titanium dioxide nanoparticles in mortars — Influence of microstructure in the hardened state properties and photocatalytic activity, Cem. Concr. Res. 43 (2013) 112–120. [34] P. Krishnan, M.H. Zhang, L. Yu, H. Feng, Photocatalytic degradation of particulate pollutants and self-cleaning performance of TiO2-containing silicate coating and mortar, Constr. Build. Mater. 44 (2013) 309–316. [35] B. Ma, H. Li, X. Li, J. Mei, Y. Lv, Influence of nano-TiO2 on physical and hydration characteristics of fly ash-cement systems, Constr. Build. Mater. 122 (2016) 242–253. [36] T. Meng, Y. Yu, X. Qian, S. Zhan, K. Qian, Effect of nano-TiO2 on the mechanical properties of cement mortar, Constr. Build. Mater. 29 (2012) 241–245. [37] H. Li, T. Du, H.G. Xiao, Q.Q. Zhang, Crystallization of calcium silicate hydrates on the surface of nanomaterials, J. Am. Ceram. Soc. 100 (7) (2017) 3227–3238. [38] M. Liu, H.G. Xiao, R. Liu, J.L. Liu, Dispersion characteristics of various contents of nano-TiO2 and its effect on the properties of cement-based composite, Struct. Concr. 19 (5) (2018) 1301–1308. [39] J.G. Ren, Y.M. Lai, J.Q. Gao, Exploring the influence of SiO2 and TiO2 nanoparticles on the mechanical properties of concrete, Constr. Build. Mater. 175 (2018) 277–285. [40] M. Oltulu, R. S ß ahin, Pore structure analysis of hardened cement mortars containing silica fume and different nano-powders, Constr. Build. Mater. 53 (2014) 658–664. [41] H.F.W. Taylor, Cement Chemistry, Second Ed., Thomas Telford, London, U.K., 1997. [42] H. Madani, A. Bagheri, T. Parhizkar, The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement, Cem. Concr. Res. 42 (12) (2012) 1563–1570. [43] V.G. Papadakis, Experimental investigation and theoretical modeling of silica fume activity in concrete, Cem. Concr. Res. 29 (1) (1999) 79–86. [44] F. Pacheco-Torgal, S. Jalali, Nanotechnology: Advantages and drawbacks in the field of construction and building materials, Constr. Build. Mater. 25 (2) (2011) 582–590. [45] A.B. Abell, K.L. Willis, D.A. Lange, Mercury intrusion porosimetry and image analysis of cement-based materials, J. Colloid Interface Sci. 211 (1) (1999) 39– 44. [46] Y.R. Zhang, X.M. Kong, Z.B. Lu, Z.B. Lu, Q. Zhang, B.Q. Dong, F. Xing, Influence of triethanolamine on the hydration product of portlandite in cement paste and the mechanism, Cem. Concr. Res. 87 (2016) 64–76. [47] M.H. Zhang, H. Li, Pore structure and chloride permeability of concrete containing nano-particles for pavement, Constr. Build. Mater. 25 (2) (2011) 608–616. [48] A.U. Ozturk, B. Baradan, A comparison study of porosity and compressive strength mathematical models with image analysis, Comput. Mater. Sci. 43 (4) (2008) 974–979.