Microstructural characteristics of sound absorbable porous cement-based materials by incorporating natural fibers and aluminum powder

Construction and Building Materials 243 (2020) 118167

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Microstructural characteristics of sound absorbable porous cement-based materials by incorporating natural fibers and aluminum powder Jinyoung Yoon a, Hyunjun Kim a, Taehoon Koh b,⇑, Sukhoon Pyo a,⇑ a b

School of Urban and Environmental Engineering, Ulsan National Institute of Sciences and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea Korea Railroad Research Institute, 176 Chuldobangmulgwan-ro, Uiwang-si, Gyeonggi-do 16105, Republic of Korea

h i g h l i g h t s
Porous cement-based materials are developed for sound absorption.
Natural fiber and aluminum powder are added to form porous structure.
Workability, compressive strength, density, and porosity are evaluated.
Microporous structure is characterized using optical microscope and X-ray CT.
Efficient sound absorption performance of porous cement-based materials is identified.

a r t i c l e

i n f o

Article history: Received 17 November 2019 Received in revised form 8 January 2020 Accepted 13 January 2020

Keywords: Sound absorbable materials Porous structure Microstructural analysis Natural fiber Aluminum powder

a b s t r a c t As transportation on roads and railways has become an essential means enabling the development of cities, residents living near roads and railways face undesirable noise pollution. To reduce this noise pollution, sound absorbable porous cement-based materials have great potential in terms of high sound absorption ability at a wide frequency range. Because sound absorption performance is primarily related to pore characteristics, this study mainly focuses on the development and evaluation of highly porous structures in cement-based materials by incorporating natural fibers and aluminum powder. The fresh and hardened properties of porous cement-based materials are investigated with respect to workability, compressive strength, and water absorption capacity. Furthermore, the porous network in the materials is characterized by microstructural observation using an optical microscope and X-ray computed tomography. A sound absorption test is also conducted to highlight the influences of the material porosity on sound absorption performance. Experimental results indicate that a combination of natural fibers and aluminum powder has a synergistic effect for forming highly porous structures that improve sound absorption performance. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, as urbanization and industrialization have been accelerated with the development of transportation, residents living near roads and railways face undesirable noise pollution, which can cause serious health problems including annoyance, tinnitus, and sleep disturbance [1]. In particular, railway noise is becoming a serious problem in urban areas due to the increase of train speed and its capacity [2]. Furthermore, the use of concrete slab tracks for high speed lines increases the noise level in comparison to conven⇑ Corresponding authors. E-mail addresses: [email protected] (T. Koh), [email protected] (S. Pyo). https://doi.org/10.1016/j.conbuildmat.2020.118167 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

tionally used ballasted tracks, by approximately 2–4 dB, due to the reflection of the sound wave on the smooth concrete surface [3]. Consequently, adequate noise mitigation measures are needed to solve these problems. Sound absorbable porous materials have commonly been used for noise reduction, consisting of a network of interconnected pores [4]. When porous materials are exposed to an incident sound wave, some portions of the sound wave are reflected and other portions are transmitted through the material. When a sound wave propagates interconnected pores, the energy is dissipated in form of thermal energy due to friction and viscous loss [5,6]. Therefore, the formation of continuous pores and securing sufficient tortuosity in cement-based materials is an important task for reducing noise pollution effectively.

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J. Yoon et al. / Construction and Building Materials 243 (2020) 118167

Compared to commonly used sound absorbing materials such as acoustic foams and fiber composites, porous cementitious composites have several benefits for the application on railways [6]. First, the production cost of cement-based sound absorbing panel is very cheap which can reduce the cost for initial construction and maintenance. In addition, cement-based composites generally have good mechanical properties and long-term durability. Considering harsh environmental conditions in railways such as heat, frost, vibration, and mechanical impact, the high performance and durable cementitious composites are more suitable [7]. However, the in-depth investigation on how to develop and characterize the pore structures in cement-based materials is still required to enhance its sound absorbing performance. Internal porous structures in cement-based materials can be generated by controlling the volume of cement paste as well as by adding pore generation materials [8,9]. It has been reported in the literature that natural fibers and aluminum powder could be effective pore generation materials for cement-based materials [10–12]. Natural fibers obtained from animals (wool, fur) or vegetables (kenaf, jute, abaca, flax) can generate air voids in the matrix during the mixing process [13]. Cement-based materials with natural fibers can form highly porous structures with low density, resulting from numerous interconnected pores along the fibers [14]. In addition, natural fibers have been used to improve the tensile and flexural strength of cement-based materials [15]. On the other hand, the adoption of aluminum powder in cement-based materials promotes a chemical reaction between the powder mixed with water and calcium hydroxide liberated by cement hydration, which generates hydrogen gas and forms internal porous structures [16]. The chemical reaction of aluminum powder with cement paste is provided as [12,17]:

2Al þ 3CaðOHÞ2 þ 6H2 O ¼ 3CaO  Al2 O3  6H2 O þ 3H2 "

ð1Þ

It has been reported in the literature [12] that cellular pores could be generated during this chemical reaction with sizes ranging from 0.1 to 1.0 mm. As a result, the proper use of aluminum powder can produce lightweight porous cement-based materials that have a density of 300–1800 kg/m3 [18]. Even though various experimental results proved that natural fibers and aluminum powder can be used for forming porous structures in cement-based materials, there are no reported results from using the two materials simultaneously and identifying their interacting mechanism. The characteristics of cement-based materials that incorporate both pore generation materials have not fully been studied yet and further microscale investigation is still needed to evaluate the relationship of porous structures with hardened properties for field applications. It has been reported that highly porous cement-based materials generally have poor mechanical properties and long-term durability [1,9]. On the other hand, Zhao et al. [7] showed excellent noise reduction performance of porous lightweight concrete slab, of approximately 4.0 dB. Considering these discrepancies, further in-depth analyses of the pore structure, mechanical properties, and sound absorption performance are needed to develop high performance sound absorbable cement-based materials. The aim of this study is to characterize the porous structure, strength, and sound absorption properties of cement-based materials that incorporate natural fibers and aluminum powder. To fabricate high performance and highly porous cement-based materials, low water-to-binder ratio (w/b) is used. A mini-slump test for cementitious mixtures is conducted to compare the effect of hydrophilic fibers and the water consuming chemical reaction of aluminum powder. The porous structure is investigated using various techniques, such as a water saturation test, microscopic image, and X-ray computed tomography (X-ray CT). Based on the experimental results obtained, the relationships between the

porosity, density, and compressive strength are analyzed. Finally, the sound absorption properties of porous materials are investigated. 2. Experimental program 2.1. Materials and sample preparation Cement used in this study is Type I ordinary Portland cement, which complies with CEM I 42.5 R. The basic physical properties of the cement are 3160 kg/m3 and 330 m2/kg for specific gravity and fineness, respectively. To enhance the strength of cement-based materials, silica fume is used to partially replace the cement. The specific gravity of the silica fume is 2270 kg/m3. The oxide compositions of the binders used, cement and silica fume, are listed in Table 1 and are obtained using X-ray fluorescence analysis (Rigaku ZSX Primus IV, Japan). Fig. 1 shows natural fibers and aluminum powder. Two different kenaf natural fibers are labelled as F1 and F2, having an average length of 10 mm and 1 mm, respectively. The kenaf fibers have the tensile strength of 930 MPa and Young’s modulus of 53 GPa, consisting of 45–57 wt % of cellulose, 22 wt% of Hemicellulose, 8–13 wt% of Lignin, and 3–5 wt% of Pectin. Due to the surface morphology and hydrophilic characteristics of the natural fibers, the conventional water immersion method is not appropriate for measuring their density. Instead, to measure the density of these fibers accurately, the gas pycnometer (Micromeritics AccuPyc 1330, USA) is used for measuring the solid and mass volume by detecting the pressure change of helium gas in the chamber. The specific gravities are 1900 kg/m3 and 1800 kg/m3 for F1 and F2, respectively. The aluminum powder is a flake type with a 99.7% purity. The aluminum powder has a specific gravity of 2700 kg/m3, a covering capacity of 1100–1450 m2/kg, and a mean particle size of 34 mm, with a range of 12–86 mm measured using a laser diffraction (Sympatec Helos, Germany). For enhancing the workability of fresh materials, a polycarboxylate-based superplasticizer is used that has 25% of solid content by its weight. In this study, porous cement-based materials are fabricated by incorporating natural fibers and aluminum powder, for which the mix proportion is provided in Table 2. The w/b for all samples are chosen as 0.20 and 0.30 in order to obtain a high mechanical performance [7,8]. The portion of the used silica fume is fixed at 10% of the cement mass. The amounts of F1 and F2 fibers are selected at 1%, 3%, and 5% of cement-based material mass. Aluminum powder is added at 0%, 0.05%, and 0.10% at the ratio of cement and silica fume mass. The superplasticizer dosages are controlled at 2.5%–3.5% and 2.0% of binder mass for the A and B series, respectively. The mixing process of cement-based materials consists of the dry mixing of binders and aluminum powder, the preparation of mixing water with the superplasticizer, and the paste mixing. If natural fibers are used, they are poured into the superplasticizer-contained mixing water for 1 min prior to the mixing. Subsequently, the powder consisting of cement, silica fume, and aluminum powder is added and the sample is mixed for 10 min using the planetary mixer. Each batch is used for both experiments—the mini-slump flow test and the fabrication of 50 mm cube specimens for the compressive strength test. The fluidity of the cement-based materials is measured after the mixing. Following 24 h of air curing, the cube samples are demolded and immersed into water at a temperature of 23 °C for additional curing. 2.2. Test setup and procedure Because the use of aluminum powder and natural fibers influences the workability, porosity, and mechanical properties of cement-based materials, diverse experimental tests and analyses are conducted in both the fresh and hardened states in order to characterize the effects of these pore generating materials. In the fresh state, a mini-slump flow test is conducted to evaluate workability, considering the hydrophilic fibers and the water consuming reaction of aluminum powder. The mini-slump test uses a Hagermann cone, which is 50 mm in height with 70 mm top and 100 mm bottom diameters, respectively. The steps of the testing procedure are: (1) fill the cone with approximately 0.3 L cement-based material, (2) lift the cone up, and (3) measure the diameter of the mini-slump spread. As a result of this test, the effect of the chemical reaction of aluminum powder and water absorbable fibers can indirectly be identified. Investigations of porous structures, hardened properties, and sound absorption performances are conducted for porous cement-based materials, as summarized in Fig. 2. Using 50 mm cube specimens, analyses of the water absorption capacity, density, compressive strength, and porous internal structure are conducted. The internal porous structure of cement-based materials is analyzed using an optical microscope and an X-ray CT. The samples for the optical microscope are prepared by cutting the middle of hardened samples using the ALLIED PowerCut 10x (USA) cutter. The cross-sectional images are observed using the ZEISS Axio Zoom.V16 and ZEISS PlanApo Z 0.5x (Germany) optical microscopes to examine the solid phase and void with excellent contrast and high resolution. Even though the use of optical microscopes is effective for analyzing the internal porous structure, this method only shows a selected cross section prepared by a sample cutting process. An alternative method is to use an X-ray CT, in this case the Nikon Metrology XT H 320 (Japan), to observe the entire structure of specimens and to analyze the

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J. Yoon et al. / Construction and Building Materials 243 (2020) 118167 Table 1 Oxide composition of cement and silica fume.

Cement Silica fume

CaO

SiO2

Al2O3

SO3

MgO

K2O

Na2O

ZnO

59.9% 0.2%

17.0% 94.8%

4.7% 0.1%

4.1% 0.9%

3.1% 2.2%

1.2% 0.4%

0.2% 0.1%

0.1% 0%

Fig. 1. Materials for forming porous structures: (a) F1 fiber, (b) F2 fiber, and (c) aluminum powder.

porosity without any additional pre-processing of samples [19,20]. In this study, the applied microfocus X-ray source is 230 kV with 300 mA, providing a resolution of 48 mm. The 3D images of the specimens are generated on the basis of 1,140 pictures, taking 354 ms to generate each image. Because X-ray CT is a time-consuming and costly method, only the selected samples for representative series are examined. In the analysis of hardened properties, the water absorption test measures the amount of absorbed water in the sample, indicating the volume of water permeable pores (porosity) in the sample. By following the ASTM C642 [21], the water saturation test is conducted as follows: (a) the dried mass of samples is measured, (b) the sample is submerged in water for 7 days, and (c) the difference of mass between the water-saturated and the dried specimen is calculated. The hardened density of cement-based materials is calculated by dividing the mass of the dried specimen by its volume. Furthermore, the compressive strength of porous cement-based materials at 7 and 28 days is determined using a universal testing machine, according to the ASTM C 109 [22]. It should be noted that the average values of porosity, density, and compressive strength, which were obtained from three specimens, are used. The sound absorption performance of porous cement-based materials is evaluated using an in-situ absorption instrument developed by Microflown Technologies. Using a surface impedance technique, this method determines the reflection coefficient of sound absorbable materials. As shown in Fig. 3, a sound absorption test is conducted by generating continuous sound signals from a loudspeaker and measuring the sound pressure (P) and particle velocity (U) from a PU probe. Using the P and U values, the specific acoustic impedance, Zs, can be calculated:

Zs ¼

P U

ð2Þ

Using a mirror source model, the normal surface impedance, Zn, can be evaluated as the ratio of the measured impedance, Zsample, normal to the field free reference impedance, Zff, without a sample [9,23].

Zn ¼

Z sample eik0 ðx1 x2 Þ ðx1 þ x2 Þ þ eik0 ðx1 þx2 Þ ðx1  x2 ÞR ¼ ik0 ðx1 þx2 Þþ1 ik ðx þx Þ 1 x2 Þ Z ff eik0 ðx1 x2 Þ ðx1 þ x2 Þ  R ikik0 ð0xð1xx e 0 1 2 ðx1  x2 Þ ik0 ðx1 þx2 Þ 2 Þþ1

ð3Þ

where x1 is the distance from the loudspeaker to the sample, x2 is the distance from the PU probe to the sample, R is the reflection coefficient, and k0 is the wave number in air. The reflection coefficient is calculated as [23]:

R¼

Zn  1 2 ik0 ðx1 þx2 Þþ1 Z n xx11 x þx2 ik0 ðx1 x2 Þþ1

eik0 ðx1 x2 Þ ðx1 þ x2 Þ þ 1 eik0 ðx1 þx2 Þ ðx1  x2 Þ

ð4Þ

Then, sound the absorption coefficient a is calculated by

a ¼ 1  jRj2

ð5Þ

The acoustic absorption coefficient of materials varies from 0 to 1, where 0 indicates a full reflection of sound and 1 a perfect sound absorption [1]. The value of the sound absorption coefficient gives the performance of noise mitigation of porous materials.

3. Experimental results and discussion 3.1. Workability of fresh porous cement-based materials The workability of both the A and B series of porous cementbased materials is summarized in Table 3. The experimental results show that the mini-slump spread of mixtures highly depends on the amounts of natural fibers and aluminum powder. Because of hydrophilic fibers and the reaction of aluminum powder, the

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Table 2 Mix proportions of cement-based materials. Label

Mix proportion [g] w/b

Water

Cement

Silica fume

F1

F2

AP§

SP§§

A1-1 A1-2 A1-3 A2-1 A2-2 A2-3 A3-1 A3-2 A3-3 A4-1 A4-2 A4-3 A5-1 A5-2 A5-3 A6-1 A6-2 A6-3 A7-1 A7-2 A7-3

0.20

440

2000

200

0

0

55.0

26.4

0

79.2

0

132.0

0

0

26.4

0

79.2

0

132.0

0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2

B1-1 B1-2 B1-3 B2-1 B2-2 B2-3 B3-1 B3-2 B3-3 B4-1 B4-2 B4-3 B5-1 B5-2 B5-3 B6-1 B6-2 B6-3 B7-1 B7-2 B7-3

0.30

0

0

28.6

0

85.8

0

143.0

0

0

28.6

0

85.8

0

143.0

660

AP§: aluminum powder. SP§§: superplasticizer.

2000

200

0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2 0 1.1 2.2

66.0

66.0

66.0

66.0

77.0

77.0

44.0

spread diameters of the samples drastically decrease as the amount of fibers and aluminum powder increases. Even though the effect of these materials on reducing workability is observed in both the A and B series, the B series—with a higher w/b ratio— are mainly discussed in this section because the fluidity change in the series can obviously be identified with the relatively large amount of free water used in the B series. The B1-1 control sample has a mini-slump flow of 340 mm, the highest value among the tested series. When aluminum powder is added, the workability decreases to 240 mm and 200 mm for B1-2 and B1-3, respectively. This reduction of workability is caused by the consumption of free water during the reaction of aluminum powder with the generation of hydrogen gas bubbles (see Eq. (1)). Even though the B2-1 and B5-1 samples, with 1 wt% fibers, have similar levels of high workability in comparison to the control sample, the addition of 3–5 wt% of natural fibers causes the fluidity to decrease in the materials. It should be noted here that the longer fiber (F1) in B2-1, B3-1, and B4-1 samples influences a significant decrease of fluidity in comparison to with the shorter fiber (F2) in B5-1, B6-1, and B7-1 samples. High workability indicates that an adequate amount of free water remains in mixtures despite the water absorption of fibers and the reaction of aluminum powder. Therefore, the B series with a better workability would have a better homogeneity of well-dispersed fibers and a higher porous structure in comparison to the A series. In the meantime, the A series shows a low fluidity, ranging from 100 to 120 mm with 3–5 wt% natural fibers and 0.05–0.10% aluminum powder, even with relatively higher dosages of the superplasticizer, controlled from 2.5% to 3.5% of binder mass. It can be expected that poor workability samples would lead to poor fiber dispersion and low degree reaction of aluminum powder due to the lack of free water. 3.2. Water absorption capacity and density A water absorption test is conducted to analyze the porosity and density of cementitious samples and its results are summarized in Table 3. The experimental results indicate that the addition of natural fibers and aluminum powder is an effective way to generate porous structures supported by an increase of water absorption capacity and a decrease of sample density. It should be noted that the remarkable change in the B series would be caused by a better fiber dispersion and a reaction of aluminum powder with an adequate amount of free water. More specifically, the B series show a water absorption capacity ranging from 9.6% to 38.0%

Fig. 2. The schematic diagram of the characterization of porous cement-based materials.

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J. Yoon et al. / Construction and Building Materials 243 (2020) 118167

with the lowest water absorption, shows the highest density of 1850 kg/m3, while the densities of the other B series’ samples range from 790 kg/m3 to 1790 kg/m3. These results show that the densities of the B series are drastically decreased as the amount of natural fiber and aluminum powder increases due to the induced pore structures. For example, B7-3, which has the highest water absorption of 38.0%, also has the lowest density of 790 kg/m3 among the B series. However, the A series do not show consistent results in water absorption and density due to the lack of free water for the generation of hydrogen gas and dispersion of fibers. Thus, the A series samples have a lower water absorption capacity with a higher density in comparison to those of the B series. However, the inverse proportional trend between density and porosity is also identified in the A series as it is in the B series. It should be noted that the addition of the same amount of F1 (10 mm long) or F2 (1 mm long) fibers shows no noticeable difference in the water absorption capacity. However, the combination of fibers and aluminum powder can enhance the porosity of cement-based materials significantly.

Fig. 3. The schematic diagram of the sound absorption test.

and the effect of fibers and aluminum powder is compared extensively. The B series with F1 fiber show 11.5%, 25.9%, and 23.4% of water absorption capacity for B2-1, B3-1, and B4-1, respectively. The water absorption results of the B series with F2 fiber, B5-1, B6-1, and B7-1, are 10.0%, 16.1%, and 25.6%, respectively. These results indicate that the samples incorporating natural fibers show a great increase in water absorption capacity in comparison to A11 and B1-1 control samples due to the formation of porous structure. It is also found that hydrogen gas generation of aluminum powder also significantly increases the water absorption capacity. The values are increased with the amount of aluminum powder, 9.6%, 18.0%, and 27.2% for B1-1, B1-2, and B1-3, respectively, in comparison to the control case. Even though these results show that each addition of natural fiber and aluminum powder is an effective way to form a pore system in cement-based materials, a combination of additives would further promote a highly porous structure. For example, the B7-3 sample, with 0.10% aluminum powder and 5% F2 fiber, shows the highest water absorption capacity at 38.0%. The densities of the B series are inversely proportional to their water absorption capacity. For example, the B1-1 control sample,

3.3. Porous microstructure The previous section identified that the addition of natural fibers and aluminum powder plays a role in generating internal porous structures, the results of which are visually characterized in this section using cross-sectional images and X-ray CT analyses. Even though cross-sectional images clearly show the porous internal structures using an optical microscope, this method needs to destroy the samples and is labor-intensive. In addition, only a few numbers of 2D microstructural images can be obtained from one sample. Therefore, an X-ray CT analysis is also conducted to scan the entire body at once, without any special pre-processing prior to the test. Furthermore, the X-ray CT provides 3D scanning images along with pore information [24]. However, only selected samples are tested due to the high cost and the time-consuming post-processing. Using these two methods, the porous structure in cementitious samples is visually identified. 3.3.1. Microscopic image analysis Using the optical microscope, cross sections of samples are analyzed to characterize the internal porous structures. Cross-sectional

Table 3 Fresh and hardened properties of porous cement-based materials. Test results

Mini-slump spread [mm]

Water absorption capacity

Density [kg/m3]

A series

Aluminum powder

B series

0%

0.05%

0.10%

180 140 100 100 170 110 100

110 110 100 100 110 100 100

B1 B2 B3 B4 B5 B6 B7

(F1 (F1 (F1 (F2 (F2 (F2

Aluminum powder 0%

0.05%

0.10%

1%) 3%) 5%) 1%) 3%) 5%)

340 250 110 100 325 210 120

240 210 110 100 210 180 105

200 180 110 100 180 130 110

A1 A2 A3 A4 A5 A6 A7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

200 200 110 100 220 120 110

A1 A2 A3 A4 A5 A6 A7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

6.9% 9.1% 11.0% 10.1% 8.0% 10.3% 10.1%

12.4% 19.7% 8.3% 7.8% 19.1% 9.9% 10.0%

8.4% 14.1% 13.1% 10.3% 11.1% 13.3% 12.6%

B1 B2 B3 B4 B5 B6 B7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

9.6% 11.5% 25.9% 23.4% 10.0% 16.1% 25.6%

18.0% 14.6% 21.7% 23.0% 20.3% 17.2% 23.7%

27.2% 28.5% 27.6% 30.3% 37.4% 36.1% 38.0%

A1 A2 A3 A4 A5 A6 A7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

1700 1410 1437 1670 1520 1380 1480

1290 1190 1610 1720 1160 1420 1460

1560 1070 1570 1620 1310 1480 1280

B1 B2 B3 B4 B5 B6 B7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

1850 1650 860 1130 1790 1330 940

1100 1210 940 1100 1110 1040 920

940 940 960 990 810 860 790

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Fig. 4. Cross-sectional images of (a) A1–1, (b) A1–3, (c) A7–1, and (d) A7-3 samples.

Fig. 5. Cross-sectional images of (a) B1–1, (b) B1–3, (c) B7–1, and (d) B7-3 samples.

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Fig. 6. 3D tomographic and cross-sectional images: (a) and (b) for A4-1; (c) and (d) for B6-3.

(a)

(b)

Fig. 7. The properties of closed pores obtained through X-ray CT analysis: (a) relationship between pore diameter and its volume; and (b) number of pores for given diameters in the A2-2, A4-1, B4-1, and B6-3 series.

Table 4 X-ray CT analysis of pore structures in A2-2, A4-1, B4-1, and B6-3. Label

A2-2 A4-1 B4-1 B6-3

X-ray CT analysis

Water saturation test

Volume of solid phase [mm3]

Volume of closed pores [mm3]

Volume of open pores [mm3]

Total porosity (Open/ Closed)

Water absorption capacity

Volume of samples [mm3]

108,488 115,371 104,615 95,860

14,535 5,968 10,096 18,312

5,277 4,661 8,889 9,528

15.4% (4.1%/11.3%) 8.4% (3.7%/4.7%) 15.4% (7.2%/8.2%) 22.5% (7.7%/14.8%)

19.7% 10.1% 23.4% 36.1%

128,300 126,000 123,600 123,700

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images of all porous cement-based material samples are taken and representative images from the A1-1, A1-3, A7-1, A7-3, B1-1, B1-3, B7-1, and B7-3 samples are provided in Figs. 4 and 5. The cross section of the area observed via the optical microscope is a 28.64  35. 70 mm2 rectangular region, consisting of 12.97  12.97 mm2 pixel. The results show that the A and B series’ samples have a significant difference in their porous structures. First, the A1-1 and B1-1 control samples contain few, small-sized pores, as shown in Figs. 4(a) and 5 (a). These A1-1 and B1-1 porous structures are generated by entrapped air bubbles during the mixing process. With the use of aluminum powder, numerous small and rounded pores are generated. The B1-3 sample, in Fig. 5(b), shows a high porous internal structure in comparison to B1-1. Because of the abundant amount of free water in B1-3, the reaction of aluminum powder generates hydrogen gas, resulting in many spherical pores. On the one hand, the pore structure of A1-3, in Fig. 4(b), does not differ greatly from that of A1-1 due to the limited reaction of aluminum powder with a low w/b ratio. In case of the samples containing F1 or F2, they have similar rounded and irregular-shaped pores along the fibers regardless of lengths of fibers. Because of poor workability of A7-1, the added fibers are not dispersed well, resulting in poor pore generation in comparison to the A1-1 sample. On the other hand, in case of the B7-1 sample, numerous small- and large-sized pores are observed in Fig. 5(c). The combination of aluminum powder and natural fiber has a highly porous structure, as shown in Fig. 5(d) for the B7-3 sample. In comparison to B1-3 and B7-1 samples, B7-3 contains both rounded- and irregular-shaped small and large pores generated by aluminum powder and natural fibers. Consequently, B7-3 has a low density of 790 kg/m3, with water absorption of 38.0%. On the contrary, in case of A7-3, only a limited number of pores is observed due to the lack of free water. Therefore, from the cross-sectional image analysis, it can be identified that porous structures in cement-based materials are developed with different sizes and distributions, according to the usage of aluminum powder and natural fibers. 3.3.2. X-ray CT analysis The X-ray CT technique, providing 3D and cross-sectional images, is also used to analyze various characteristics of pores such as pore size, type of pore, and three-dimensional distribution. In this study, four representative samples, A2-2, A4-1, B4-1, and B63, are tested with X-ray CT analysis to effectively compare the effect of fibers and aluminum powder. Fig. 6 shows the X-ray CT results of A4-1 and B6-3 samples with porous 3D tomographic and cross-sectional images using commercially available software myVGL 3.0. In case of A4-1, the volume of closed pores is colored

according to the pore size e.g., blue (less than 15 mm3), green (15–35 mm3), and red (35–55 mm3). As can be seen in Fig. 6(b), there are numerous blue colored closed pores in A4-1 and similar results are also observed in other samples, generated by the dispersion of fibers or the reaction of aluminum powder. The closed pores in B6-3 are illustrated in a monochrome yellow color, regardless of the size of pores, as shown in Fig. 6(c) and (d). In comparison to A41, more open (black) and closed (yellow) pores are observed in B63 from the tomographic images. It should be noted that the open pores in the sample are filled with black due to their connectivity with the background space. The X-ray CT analysis also provides the characteristics of closed pores regarding their number, size, and volume. Fig. 7 represents the relationship between the pore diameter and the volume as well as the numbers obtained from the X-ray CT. Fig. 7 (a) shows the number and size of these pores in A4-1 and B6-3, which have water absorption capacities of 10.1% and 36.1%, respectively. The majority of the closed pores in A4-1 are less than 4 mm in size, which would be influenced by the dispersion of the added F1 fiber. On the contrary, the B6-3 case exhibits plenty of closed pores, ranging from 0.12 mm to 10 mm in size, due to the synergistic effect of fiber and aluminum powder. Fig. 7(b) also demonstrates the synergistic effect obtained from adopting fibers and aluminum powder together for generating small-sized pores of less than 1 mm. The total numbers of closed pores are 388,458, 92,931, 149,983, and 272,638 for A2-2, A4-1, B4-1, and B6-3, respectively. Based on the volumes of the solid phase and the closed pore, the volume of the open pore and the porosity of samples can be calculated. The porosity results of A4-1, A2-2, B4-1, and B6-3, obtained from the X-ray CT, are listed in Table 4. When the volumes of the solid phase (Vs) and the closed pores (Vc) are obtained, the volume of the open pore (Vo) can be evaluated by subtracting Vs and Vc from the total volume of samples (Vt). It should be noted that the value of Vt is determined by the saturated mass of sample subtracting its mass in water following the ASTM C 642 [21]. Finally, the total, open, and closed porosity can be calculated by dividing their volumes into Vt, respectively. The calculated total porosities of A41, A2-2, B4-1, and B6-3 are lower than the water absorption capacity provided in Table 3. The differences in porosity, estimated by two different methods, are primarily due to the limitation on resolution of the used X-ray CT. The pores having sizes less than 48 mm cannot be detected using the X-ray CT analysis in this study. Nonetheless, X-ray CT analysis has strong advantages with respect to its non-destructive testing method and 3D visualization of the solid phase and pores.

Table 5 Compressive strength of cement-based materials after 7 and 28 days of curing. Test results

Compressive strength at 7 days [MPa]

Compressive strength at 28 days [MPa]

A series

Aluminum powder

B series

0%

0.05%

0.10%

Aluminum powder 0%

0.05%

0.10%

A1 A2 A3 A4 A5 A6 A7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

48.3 26.6 32.6 41.8 30.2 28.4 32.0

18.7 17.9 33.8 40.4 13.8 22.4 25.9

34.0 12.8 39.2 35.4 18.1 36.4 20.5

B1 B2 B3 B4 B5 B6 B7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

52.6 46.2 11.1 12.4 54.6 29.1 11.1

13.0 18.2 8.8 12.2 14.3 11.9 7.7

10.2 11.4 10.2 10.1 6.7 9.5 6.9

A1 A2 A3 A4 A5 A6 A7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

43.3 30.9 28.7 47.0 33.3 28.0 36.2

18.1 15.8 33.3 43.0 15.9 27.1 30.5

32.3 14.7 39.9 41.4 20.1 28.9 22.9

B1 B2 B3 B4 B5 B6 B7

(F1 (F1 (F1 (F2 (F2 (F2

1%) 3%) 5%) 1%) 3%) 5%)

54.6 44.1 9.5 13.3 51.7 30.6 10.1

14.0 20.2 9.9 12.0 14.7 12.7 9.9

10.8 10.7 9.9 11.8 5.5 7.9 6.4

9

J. Yoon et al. / Construction and Building Materials 243 (2020) 118167

60

Compressive strength [ fc, MPa]

Compressive strength [ fc, MPa]

60 7 days (w/b 0.20) 28 days (w/b 0.20) 7 days (w/b 0.30) 28 days (w/b 0.30)

50

40

7 days (w/b 0.20) 28 days (w/b 0.20) 7 days (w/b 0.30) 28 days (w/b 0.30)

50 40

30

30

20

fc =

fc = 3.3·10-7·ρ2.5

20

454.0·W-1.2

10

10

0 0

10

20

30

40

Water absorption capacity [W, %]

50

0 0

500

(a)

1000

1500

Density [ρ, kg/m3]

2000

(b)

Fig. 8. The relationship of compressive strength with: (a) the water absorption capacity and (b) the density.

water. However, it should be noted that the average strength increase ratios of the A and B series at 7 and 28 days are as small as 11.2% and 9.2%, respectively. The limited strength increase with additional curing days shows that the compressive strength of samples mostly depends on their pore structures. The relationships of compressive strength with the water absorption capacity in Fig. 8(a) and with the density of porous cement-based materials in Fig. 8(b) are identified. Water absorption properties and density indicate the porosity of cement-based material and their strong correlations with compressive strength are identified regardless of the curing time. Proposed empirical equations are provided in the figures. The high R-square values of 0.82 and 0.96 are obtained from the equations shown in the figures, respectively. These empirical equations also highlight that the mechanical properties of these cement-based materials are strongly dependent on their porous structures.

Fig. 9. Sound absorption coefficients of B1-1, B6-1, B1-3, and B6-3.

3.4. Compressive strength The porous characteristics of cement-based materials are highly related to their mechanical responses [25–27]. Since the addition of natural fibers and aluminum powder increases porosity, the reduction of compressive strength is obviously observed for the B series, as shown in Table 5. The use of aluminum powder greatly decreases the compressive strength from 54.6 MPa for B1-1 to 14.0 MPa and 10.8 MPa for B1-2 and B1-3, respectively. The similar strength decrease is identified in B2-1, B3-1, and B4-1 containing F1 fiber. On the contrary, the samples having F2 fiber shows the high compressive strength in comparison to F1 fiber samples. It is because the use of F1 fiber in cement-based materials increase the porosity, resulting in the strength decrease. The effect of aluminum power and natural fibers on the compressive strength are not consistent for the A series due to the limited quantity of free

3.5. Sound absorption performance Considering the porous structure and compressive strength of cementitious samples, the B1-1 control sample and the B1-3, B61, and B6-3 porous specimens are selected for the sound absorption test considering their different porosity from 9.6% to 36.1%. These mixtures are fabricated as a form of the 300  300  10 mm3 panel. As shown in Fig. 3, the in-situ sound absorption test is conducted and the acoustic absorption coefficient (see Eq. (5)) is determined in order to evaluate the noise reduction performance. The distance (x1) from speaker to the PU probe and the distance (x2) between the PU probe and the sample are set to 260 mm and approximately 1 mm, respectively. The resulting sound absorption coefficient values are obtained in the range of 100–2500 Hz, covering typical traffic noise frequency range of 500–1500 Hz [24]. To compare sound absorption performances, the noise reduction coefficient (NRC), the sound absorption average (SAA), and the sound absorption area ratio (SAAR) are adopted [1,7]. The NRC calculates the arithmetic average values

Table 6 Evaluation of the sound absorption performance for B1-1, B1-3, B6-1, and B6-3. Sound absorption performance

NRC (Noise reduction coefficient) SAA (Sound absorption average) SAAR (Sound absorption area ratio)

Samples B1-1

B1-3

B6-1

B6-3

0.31 0.32 0.26

0.34 0.35 0.29

0.36 0.37 0.31

0.39 0.39 0.32

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J. Yoon et al. / Construction and Building Materials 243 (2020) 118167

of sound absorption coefficients at 250, 500, 1000, and 2000 Hz, according to the ASTM C 423 [28]:

NRC ¼ ða250 þ a500 þ a1000 þ a2000 Þ=4

ð6Þ

Because the NRC only considers sound absorption coefficients at certain frequencies, this value is practically used. Similarly, the SAA calculates the arithmetic average values of sound absorption coefficients for 12 one-third octave bands ranging from 200 to 2500 Hz. Considering the wider frequency range and more coefficient values, the SAA value could be more robust than the NRC one. On the contrary, the SAAR is calculated by the ratio of the sound absorption coefficient curve surface area to the total background area [8]. Because the SAAR considers all frequency ranges obtained from the sound absorption test, this value can compensate for the NRC limitations.

Z

SAAR ¼ Ai =A0 and Ai ¼

f ð xÞ

ð7Þ

where Ai is the surface area of the sound absorption coefficient curve, A0 is the background area, and f(x) is the absorption coefficient at x frequency. The sound absorption coefficient surface area is calculated using the trapezoidal method for approximate integration. Fig. 9 and Table 6 show the acoustic absorption coefficient curves and the sound absorption performances of B1-1, B6-1, B13, and B6-3 in the 100–2500 Hz frequency range. As expected, the more porous specimens show better sound absorption performance. For example, the highly porous B6-3 sample shows superior sound absorption performance in comparison to the control sample. More specifically, the NRC, SAA, and SAAR of B6-3 are 25.8%, 21.9%, and 23.1% higher than those of B1-1, respectively. Other porous materials, B1-3 and B6-1, also show better sound absorption performance than the B1-1 control sample. This is because higher specific surfaces of highly porous samples dissipate sound energy effectively [8]. The sound absorption test concludes that the combination of aluminum powder and natural fibers has a synergistic effect, fabricating high performance sound absorbable porous cement-based materials.

(2) Microscopic observation and X-ray CT analysis characterize the porous structures formed using aluminum powder and natural fibers. According to the types of pore generation materials, different shape, size, and number of pores are observed. The reaction of aluminum powder forms many spherical shape pores. Meanwhile, a relatively small number of pores is observed in the sample that has natural fibers. The combination of aluminum powder and natural fibers has a synergistic effect in making highly porous structures. (3) Inverse relationships between compressive strength and water absorption capacity as well as density are identified. Using pore generation materials, a highly porous structure of up to 38.0% can be created. Furthermore, it can be concluded from the limited strength increase with additional curing that the compressive strength of porous cementbased materials mostly depends on their pore structures. (4) Using the in-situ sound absorption test instrument, excellent sound absorption performance of highly porous cement-based materials is identified. In comparison to the control sample, the porous cementitious material using aluminum powder and natural fibers shows an approximately 20% higher sound absorption performance. The results of this experimental research study are expected to provide some basic information for evaluating the porous structures of cement-based materials. Additional research should be conducted to investigate fresh properties of porous cement-based materials, such as the relations between the rheological properties and the fresh state volume change of the cementitious materials. Moreover, a three-dimensional analysis on the internal pore structures needs to be conducted to identify interconnected pores (tortuosity) for more accurate analysis of noise mitigation mechanism in sound absorbable cement-based materials. As an extension of the present study, the in-depth analysis of pore ratio, porosity, and pore connectivity will be carried out based on the image processing technique.

CRediT authorship contribution statement 4. Conclusions This experimental study investigates the characteristics of porous cement-based materials by developing porous structures using natural fibers and aluminum powder. To investigate the effects of pore generation materials, the tested experimental parameters are set with two different natural fiber lengths, two water-binder ratios, three levels of fiber contents, and three levels of aluminum powder contents. The porosity and internal pore structures of the cementbased materials are characterized both directly and indirectly, using flowability, water absorption test, density, compressive strength, optical microscope, and X-ray CT. In addition, a sound absorption test is also carried out to evaluate the influences of material porosity on sound absorption performance. The key observations and findings of this study can be summarized as follows: (1) The mini-slump test spread drastically decreases when the amount of natural fibers and aluminum powder increases, which indicates that free water is consumed by the process of mixing the natural fibers and the chemical reaction of aluminum powder. To ensure adequate pore generation using the reactivity of aluminum powder and the dispersion of natural fiber, a w/b ratio higher than 0.30 is required. Otherwise, it is difficult to achieve consistent results in developing internal pore structures due to the lack of free water for the generation of hydrogen gas and the dispersion of fibers.

Jin Young Yoon: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Hyunjun Kim: Validation, Formal analysis, Writing - review & editing, Visualization, Investigation. Taehoon Koh: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration. Sukhoon Pyo: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2019R1F1A1060906). The research described herein was also sponsored by a grant from R&D Program of the Korea Railroad Research Institute, Republic of Korea. The opinions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors.

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References [1] H. Kim, J. Hong, S. Pyo, Acoustic characteristics of sound absorbable high performance concrete, Appl. Acoust. 138 (2018) 171–178. [2] J.Y. Yoon, S. Pyo, A review of mitigation measures for reducing railway rolling noise from an infrastructure point of view, Int. J. Railw. 12 (2019) 1–9. [3] D. Thompson, Mitigation measures for rolling noise, Railw. Noise Vib. (2009) 223–279. [4] J. Hald, W. Song, K. Haddad, C.H. Jeong, A. Richard, In-situ impedance and absorption coefficient measurements using a double-layer microphone array, Appl. Acoust. 143 (2019) 74–83. [5] H. Mamtaz, M.H. Fouladi, M. Al-Atabi, S.N. Namasivayam, Acoustic absorption of natural fiber composites, J. Eng. (United Kingdom). 2016 (2016). [6] L. Cao, Q. Fu, Y. Si, B. Ding, J. Yu, Porous materials for sound absorption, Compos. Commun. 10 (2018) 25–35. [7] C. Zhao, P. Wang, L. Wang, D. Liu, Reducing railway noise with porous soundabsorbing concrete slabs, Adv. Mater. Sci. Eng. 2014 (2014) 1–11. [8] S.B. Park, D.S. Seo, J. Lee, Studies on the sound absorption characteristics of porous concrete based on the content of recycled aggregate and target void ratio, Cem. Concr. Res. 35 (2005) 1846–1854. [9] M. Li, W. van Keulen, E. Tijs, M. van de Ven, A. Molenaar, Sound absorption measurement of road surface with in situ technology, Appl. Acoust. 88 (2015) 12–21. [10] N. Narayanan, K. Ramamurthy, Structure and properties of aerated concrete: a review, Cem. Concr. Compos. 22 (2000) 321–329. [11] H.H. Kim, C.S. Kim, J.H. Jeon, C.G. Park, Effects on the physical and mechanical properties of porous concrete for plant growth of blast furnace slag, natural jute fiber, and styrene butadiene latex using a dry mixing manufacturing, Process. Materials (Basel) (2016). [12] E. Holt, P. Raivio, Use of gasification residues in aerated autoclaved concrete, Cem. Concr. Res. 35 (2005) 796–802. [13] K. Tak Lau, P. Yan Hung, M.H. Zhu, D. Hui, Properties of natural fibre composites for structural engineering applications, Compos. Part B Eng. 136 (2018) 222–233. [14] D. Falliano, D. De Domenico, G. Ricciardi, E. Gugliandolo, Compressive and flexural strength of fiber-reinforced foamed concrete: Effect of fiber content, curing conditions and dry density, Constr. Build. Mater. 198 (2019) 479–493.

11

[15] M. Khan, M. Ali, Effect of super plasticizer on the properties of medium strength concrete prepared with coconut fiber, Constr. Build. Mater. 182 (2018) 703–715. [16] R. Shabbar, Effect of aluminium poder grading on the properties of aerated concrete. In: Proceedings of 36th Cement and Concrete Science Conference, in: Cardiff, 2016. [17] F. Wang, Z. Liu, S. Hu, Early age volume change of cement asphalt mortar in the presence of aluminum powder, Mater. Struct. 43 (2010) 493–498. [18] N. Neithalath, M.S. Sumanasooriya, O. Deo, Characterizing pore volume, sizes, and connectivity in pervious concretes for permeability prediction, Mater. Charact. 61 (2010) 802–813. [19] A. du Plessis, B.J. Olawuyi, W.P. Boshoff, S.G. le Roux, Simple and fast porosity analysis of concrete using X-ray computed tomography, Mater. Struct. 49 (2016) 553–562. [20] E.E. Bernardes, E.V. Mantilla Carrasco, W.L. Vasconcelos, A.G. de Magalhães, Xray microtomography (l-CT) to analyze the pore structure of a Portland cement composite based on the selection of different regions of interest, Constr. Build. Mater. 95 (2015) 703–709. [21] ASTM C642-13, Standard Test Method for Density, Absorption, and Voids in Hardened Concrete, 2013. [22] ASTM C109/C109M-16a, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimen), 2016. [23] E. Tijs, Study and Development of an In Situ Acoustic Absorption Measurement Method, University of Twente, 2013. [24] L. Gao, Z. Wang, J. Xie, Z. Wang, H. Li, Study on the sound absorption coefficient model for porous asphalt pavements based on a CT scanning technique, Constr. Build. Mater. 230 (2020) 117019. [25] M.R. Ahmad, B. Chen, Experimental research on the performance of lightweight concrete containing foam and expanded clay aggregate, Compos. Part B Eng. 171 (2019) 46–60. [26] C. Lian, Y. Zhuge, S. Beecham, The relationship between porosity and strength for porous concrete, Constr. Build. Mater. 25 (2011) 4294–4298. [27] W.S. Kim, J.R. Park, B.K. Kim, D.S. Seo, J.S. Park, Physical properties evaluation of porous concrete according to target porosity and pumice contents ratio for application of the aquatic environment, J. Korea Concr. Inst. 28 (2016) 703–711. [28] ASTM C423-17, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, 2017.