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Enhanced properties of high-silica rice husk ash-based geopolymer paste by incorporating basalt fibers
Construction and Building Materials 245 (2020) 118422
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Enhanced properties of high-silica rice husk ash-based geopolymer paste by incorporating basalt fibers Saloni a, Parveen a,⇑, Thong M. Pham b a b
Civil Engineering Department, DCRUST Murthal-131039, Haryana, India Center for Infrastructural Monitoring and Protection, School of Civil and Mechanical, Engineering, Curtin University, Kent Street, Bentley, WA 6102, Australia
h i g h l i g h t s Basalt fiber enhanced the strength characteristics of the geopolymer composites. Setting times of RHA based geopolymer were found to increase with increase of basalt fiber content. Reduced drying shrinkage was obtained for RHA based geopolymer paste with basalt fiber. The microstructure and properties of the RHA and basalt fiber based paste has been studied. Homogeneous and denser paste may be developed by using basalt fiber and RHA.
a r t i c l e
i n f o
Article history: Received 23 October 2019 Received in revised form 9 January 2020 Accepted 12 February 2020
Keywords: Basalt fiber Geopolymer Bulk density Compressive strength Drying shrinkage
a b s t r a c t In this study, the effect of different basalt fiber contents (0%, 10%, 20%, 30%, 100% by binder mass) as a replacement of rice husk ash on the fresh, hardened and microstructural properties of geopolymer paste has been evaluated. NaOH (12 Molar) and Na2SiO3 were used to produce geopolymers. Different characteristics, i.e. initial setting and final setting times, bulk density, porosity, compressive and flexural strengths were investigated by using various techniques, such as scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Results showed that basalt fibers exhibited positive effects on the fiber-matrix transition zone as increased initial setting time, final setting time, bulk density and compressive and flexural strengths with the increased basalt fiber contents. Meanwhile, chemical analysis showed that basalt fibers acted as reinforcements and thus improved the characteristic of geopolymer paste. The major crystalline phases presented in the resultant geopolymer were quartz, calcium silicate hydrate and magnesioferrite. In addition, reduced values of critical pore size and total porosity were observed with an increase of replacement content in rice husk ash. The results of the present study support basalt fibers and rice husk ash as promising solid waste materials for use in the production of geopolymers. Ó 2020 Published by Elsevier Ltd.
1. Introduction Substantial consumption of energy and massive release of greenhouse gases are resulted from the production of ordinary Portland cement (OPC) which is the main binder for the production of concrete in the construction industry [1]. The carbon dioxide Abbreviations: BD, Bulk density; CSH, Calcium silicate hydrate; CASH, Calcium aluminate silicate hydrate; EDS, Energy dispersive spectroscopy; FST, Final setting time; IST, Initial setting time; NASH, Sodium aluminate silicate hydrate; RHA, Rice husk ash; SEM, Scanning electron microscopy; XRD, X-ray diffraction. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Parveen). https://doi.org/10.1016/j.conbuildmat.2020.118422 0950-0618/Ó 2020 Published by Elsevier Ltd.
(CO2) footprints of the cement are very high as nearly 0.65–0.85 tons of CO2 is produced per ton of cement production [2–4]. Therefore, there should be an alternative construction material that solves the issues related to cost and carbon emission and have excellent properties. Furthermore, wastes generated from various industries are posing a great disposal problem and efforts are made to solve this issue and use them for new advancements [5]. Geopolymer, a commercial name used for the alkali-activation of kaolinite/limestone/dolomite [6,7], can act as a binder for construction materials and it utilizes the industrial waste products. This strategy has shown as an economical option to recycle waste [8]. Energy requirements are also reduced as it avoids burning or extra efforts to be made for waste disposal [9] and commences
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the need for their better utilization. Several waste materials like slag, fly ash, rice husk ash (RHA), and pozzolana has been effectively utilised for cement replacement [10,11]. Recent advancements in the production of geopolymers show that it possesses excellent binder properties and provides potential benefits [12]. It is estimated that the CO2 footprints of the geopolymers are approximately 9% less than the conventional concrete produced using 100% cement [4]. Also, researchers are trying to use waste-based activators in order to further reduce the CO2 footprints [13–15]. In addition, performance of geopolymer concrete structures is comparable to conventional concrete in terms of the compressive, flexural and tensile strengths [16]. Therefore, geopolymer concrete which can be considered as an alternative construction material significantly reduces the greenhouse emission, has attracted many researches nowadays due to its excellent properties and environmental benefits. Many studies have been conducted on geopolymer, such as addition to concrete mixes [17], repair and retrofitting materials, fireproofing the structures [18]. Results of utilizing the geopolymer in the various civil engineering applications are found to be remarkable especially in the field of pavements [19,20], masonry structures [21], reinforced geopolymer composites [22] and repair materials [23–27]. RHA is an agricultural by-product [28,29], in which approximately 22% of the 648.9 million tons of rice produced annually constitutes rice husk ash [30]. There is a boost in the demand for natural resources due to increasing urbanization and development while disposal of wastes in such cases adds to an extra cost, thus strongly initiating the use of wastes for some beneficial purposes [31]. RHA is found to have pozzolanic properties as it is a silica-rich material [32]. When it is burnt, it causes environmental problems and its disposal also causes an extra cost. A simple and effective way to solve the associated problems is to use it for partial cement replacement [33,34]. For the production of geopolymer, RHA requires an activation treatment. Previous studies have found that it can be activated by using a mixture of NaOH and Na2SiO3 as an alkaline activator. Nair et al. [35] studied pozzolanic activity of RHA, and NASH gel was formed which bears a direct relationship with the hardening and setting of the geopolymer based product. Thus, RHA can be utilized to form geopolymer paste and its effect on the various properties like the compressive strength, initial and final setting time, and flexural strength should be studied. The use of RHA in geopolymer paste can help in improving the strength and pore structure of the paste [8]. It possesses a high surface area as indicated by its microstructural studies. Extremely fine basalt fibers which are a modified form of the basalt family have also gained attention as an additive in concrete mixes because its cost is low as compared to carbon fibers [36]. They have good resistance to abrasion, good durability characteristics and excellent insulation properties. They have miraculous results in many areas and have a huge potential for their use in practical applications. A previous study indicates that the use of basalt fibers has led to an improvement in mechanical properties and fire resistance [37]. They also caused no harm to the environment and they are considered as a non-hazardous material. The material itself is not new in the market but more innovative applications of this product in the practical field and thus it is on boom in new research studies. Basalt fibers mainly consist of lime and silica. Calcium to silica ratio is mainly responsible for the increase in the compressive strength of basalt fibers based geopolymer [38]. CSH gel development leads to a dense structure that was found to be responsible for the increase in the compressive strength [39]. The microstructural examination of the basalt fibers from previous studies has proven the reason for its potential properties. This study focusses on using RHA and basalt fibers to form hybrid geopolymer paste and to investigate its properties, such as the setting time, compressive strength, and drying shrinkage.
It is noted that a similar study has been done by Punurai et al. [37] in which fly ash-basalt fiber hybrid paste was investigated concerning its properties. In their study, the effect of partial replacement of fly ash by basalt fibers on setting time, strength and drying shrinkage was examined. Meanwhile, in the current study, a composite paste was formed using RHA and basalt fibers with a fixed concentration of 12 M NaOH to study various properties. There have been no studies on RHA and basalt fiber-based composite paste in the literature, therefore, this study focuses on studying the properties of this hybrid paste. 2. Experimental program 2.1. Material properties and chemical compositions The physical and chemical properties of the materials are shown in Table 1. In this study, the source of RHA was a rice milling plant situated in Delhi, India. The major constituents were SiO2 (90%), K2O (4.60%), and P2O5 (2.43%). Basalt fibers were provided from a manufacturing plant in Delhi. Their composition was SiO2 (46.5%), CaO (31.4%), Al2O3 (13.4%). The high silica content of both RHA and basalt fiber was responsible for the pozzolanic properties. Specific gravities of RHA and basalt fibers were 2.18 and 2.38, respectively. 25 mm and 25.62 mm were the respective values of mean diameters of RHA and basalt fibers. Scanning electron microscope (SEM) images of RHA and basalt fiber particles are shown in Fig. 1. The SEM study of RHA collaborates with its high specific surface area. Irregular shape for RHA particles and a cylindrical shape for basalt fibers was observed. Fig. 2 shows the particle size distribution of the RHA and basalt fibers. X-ray diffraction pattern (XRD) of RHA and Basalt fiber is depicted in Fig. 3. Mix proportions used in this study are shown in Table 2. Commercially available sodium silicate solution with a specific gravity of 1.46 and with a SiO2/Na2O ratio of 2.1 and sodium hydroxide pellets with 96% purity were used for developing activator mixture. Preparation time for the geopolymer pastes was kept 24 h. 2.2. Preparation of sample and testing procedure Basalt fibers were used for replacing RHA at percentage levels of 0, 10, 20, 30 and 100% by weight. A 12 M concentration of sodium hydroxide solution and sodium hydroxide to sodium silica ratio of 1.0 was used as an activator of geopolymer paste. The liquid to binder ratio value was kept as 0.5. The mixing was done using a planetary mixer. The setting time was immediately determined
Table 1 Chemical composition and physical properties of processed rice husk ash and basalt fibers. Physical and chemical properties
Rice husk ash
Basalt fiber
Specific Gravity Mean Particle Size (lm) Specific surface area m2/kg CO2 SiO2 K2O P2O5 CaO MgO Fe2O3 Al2O3 MnO SO3 Loss on Ignition
2.18 25 1434.50 0.10% 90.00% 4.60% 2.43% 1.10% 0.77% 0.43% 0.46% 0.11% – 3.90%
2.38 25.62 – – 46.5% 0.41% 0.28% 31.4% 6.38% 0.79% 13.4% 0.17% 0.67% 0.44%
- Not applicable.
Saloni et al. / Construction and Building Materials 245 (2020) 118422
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(a) Rice husk ash
Fig. 3. X-ray diffraction pattern (XRD) of RHA and Basalt fibers.
[41]. The compressive strength was determined at 7, 28 and 90 days using 50 * 50 * 50 mm3 cubes according to ASTM C109/C109 M 16-a [42]. The flexural strength was determined at the age of 28 days according to ASTM C348-14 [43]. The reported values were averaged from 3 identical samples for the above tests. A chisel was used to obtain samples from the middle section of 50 * 50 * 50 mm3 cubes at 28-day age for carrying out microstructural investigations. Oven drying was done for 24 h post to its immersion in acetone for 24 h. Mercury Intrusion Porosimeter (MIP) was used for porosity determination of the specimens while the Edward W. Washburn equation [44] i.e. (Pressure required to force mercury into a capillary pore quals to 2!cosh/r, where r = radius, ! = surface tension and h = the angle of contact) was used to examine the porosity and critical pore size.
(b) Basalt fiber Fig. 1. SEM images of RHA and Basalt fiber particles.
100 90 Percentage passing
80 70
3. Results and discussion
60 50
3.1. Setting time
40
RHA
BF
30 20 10 0 0.001
0.010
0.100 Particle size (mm)
1.000
Fig. 2. Particle size distribution of RHA and Basalt fiber.
after mixing according to ASTM C191-13 [40]. The average of three samples was reported as the final result of the setting time. Prism molds with various sizes of 50 * 50 * 50 mm3, 40 * 40 * 160 mm3, 25 * 25 * 285 mm3 were used for sample preparation and three identical samples of each size were cast. Plastic sheets were used to seal specimens for avoiding the evaporation of moisture and were kept in a room with 25 °C controlled temperature. After 24 h, the specimens of size 50 * 50 * 50 mm3 and 40 * 40 * 160 mm3 were demolded. Then the post-curing was done at 25 °C after covering them with a vinyl sheet till the ages of 7 and 28 days to determine their density and compressive strength. Meanwhile, 50 * 50 * 50 mm3 cubes were used to determine the bulk density (BD) at 28-day age as per ASTM C138/C138 17-a
The variation in setting times with basalt fiber content is shown in Fig. 4. The setting times increased with the basalt fiber content. The increase in basalt fiber content from 0 to 100% resulted in an increase in the initial setting time (IST) from 25 to 93 min while the final setting time (FST) increased from 72 to 145 min. The increase was nearly 272% for the initial setting time and 101% for the final setting time. The initial setting time showed more sensitive to the addition of basalt fiber content than that of the final setting time with respect to the initial value. Si/Al ratio increased from 1.79 to 2.51 correspond to an increase in basalt fiber content from 0 to 100%. This observation agrees well with the results of previous studies which has shown that setting time increases with the Si/Al ratio [37,45,46]. It is noted that basalt fibers have an amorphous nature and therefore, helped in the more uniform particle distribution within the matrix. A glassy interface is formed due to the leaching of silica, alumina, and calcium ions from RHA particles in the presence of a high concentration of NaOH (12 M). With a cylindrical shape, basalt fiber particles have less surface area than RHA particles and therefore, it extended the setting time of the geopolymer paste. Many previous studies also reported that the setting and hardening have a strong relationship with both the amorphous phase and leaching of the starting materials [38,47]. Therefore, the current study is in good agreement that the setting and hardening can be extended with the inclusion of basalt fibers in geopolymer [20,47].
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Table 2 Mix proportions for the hybrid paste. No.
Mix
RHA (%)
Basalt fiber (%)
Na2SiO3 (g)
NaOH (g)
1 2 3 4 5
RHA 10BF90RHA 20BF80RHA 30BF70RHA BF
100 90 80 70 0
0 10 20 30 100
30 30 30 30 30
30 30 30 30 30
Fig. 4. Setting time of hybrid paste with different basalt fiber contents.
Fig. 6. Compressive strength of hybrid pastes with different basalt fiber contents at different ages.
3.2. Bulk density The bulk density was determined at the age of 28 days. The bulk densities of the samples with different percentages of basalt fibers are shown in Fig. 5. As can be seen from the figure, the bulk density increased from 1.90 to 2.30 g/cm3 corresponding to an increase in basalt fiber content from 0 to 100%. Both RHA and basalt fibers have a different value of specific gravity (1.8 and 2.38, respectively). The high specific gravity of basalt fibers compared to RHA leads to an increase in bulk density values, which were 1.90, 1.93, 1.95, 1.97 and 2.30 at 0, 10, 20, 30 and 100% basalt fiber content, respectively. 3.3. Compressive strength The compressive strengths of all the samples are shown in Fig. 6. The compressive strengths obtained at 7 days were 21.50,
26.93, 29.42, 35.51, and 73.12 MPa with 0, 10, 20, 30 and 100% basalt fiber content, respectively. The corresponding compressive strengths at 28 days were 38.33, 52.41, 58.92, 64.31, 94.12 MPa, respectively. The higher concentration of NaOH helped in the leaching of Si and Al ions from the added materials. Furthermore, CSH, CASH, and NASH gels were formed due to the presence of calcium in the matrix and therefore improved the setting and hardening of the pastes [37]. CNASH gel, a combination of all the gels was formed which is the main cursor for the strength development [38]. The increase in the compressive strength for mix BF at 7 days (240%) was more significant as compared to 28 days (145%). The maximum obtained compressive strengths were 73.12 and 94.12 MPa at respective ages. This improvement is attributed to the improved internal matrix structure with the CNASH gel development and pores which were filled by this gel [37,39]. Similar results are also observed for the strength development in Ordinary Portland Cement [48]. It is also noted that the amorphous nature of basalt fibers contributes to strength development. A more uniform and dense structure is formed due to basalt fiber addition as it behaves as an aggregate. Punurai et al. [37] recommended the NaOH concentration of 10 M for fly ash and basalt fiber based geopolymer paste. Meanwhile, 12 M NaOH solution was used in this study and basalt fiber content was increased from 0 to 100% for studying its effect on the compressive strength.
3.4. Flexural strength
Fig. 5. Bulk density of hybrid pastes with different basalt fiber contents.
The flexural strength of the specimens with various basalt fiber contents is shown in Fig. 7. The primary factor governing flexural strength is basalt fiber content. The outcomes demonstrated that the flexural strength of RHA, 10BF90RHA, 20BF80RHA, 30BF70RHA and BF geopolymer pastes at 28 days were 3.6, 4.2, 4.8, 5.7 and 10.82 MPa, respectively. A significant improvement was observed in the flexural strength as a result of increasing the basalt fiber content. In comparison with RHA, the flexural strength of hybrid
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strength increases with the basalt fiber content. Punurai et al. [37] also recommended the use of basalt fibers in fly ash-based geopolymer paste. This study proved that high compressive strength can be achieved by utilizing basalt fiber in the rice husk ash-based geopolymer paste. 3.6. SEM (Scanning electron Microscopy) study
Fig. 7. Flexural strength of hybrid pastes with different basalt fiber contents at 28 days.
binders was 17%, 33%, 58% higher than that of 10BF90RHA, 20BF80RHA, 30BF70RHA, respectively. The production of CNASH gel leads to an improvement in the flexural strength and microaggregate function improvement (as observed in SEM images). As a result, it led to a denser structure of hybrid paste and thus higher flexural strength. The modulus of rupture was also improved due to this phenomenon.
SEM studies were carried out at the age of 28 days and SEM images are shown in Fig. 8. SEM images were utilized to investigate the interaction between RHA and basalt fibers. When compared with 100% RHA paste, a denser and more homogeneous structure of pastes with 10–30% basalt fiber content is shown in the SEM images. Thus, it can be concluded that the structure of the hybrid paste improved with the incorporation of basalt fibers. As the basalt fiber content increases, Ca content increases and CNASH gel is developed. They react partially as seen in the SEM images. A micro aggregate function is performed by basalt fibers leading to an increase in the compressive strength. In addition, the deposition of CSH, CASH, and NASH gels on basalt fibers can be clearly seen in the SEM image of paste with 100% BF. A compact structure was observed as indicated in the SEM images. Similar improvement of the microstructure of geopolymer paste with basalt fibers has also reported in the previous study [37]. The compact structure was found to be the main reason for improvement in the properties of the geopolymer paste. More homogeneity was observed with the addition of basalt fiber and thus the matrix structure was improved.
3.5. Porosity 3.7. EDS (Energy Dispersive Spectroscopy) Porosity and critical pore size were examined and the results are summarised in Table 3. The porosity of paste with 100% RHA was 36.9% at 28 days. The porosity percentages at 10, 20, 30 and 100% basalt fiber content were 33.12, 31.03, 24.13 and 8.12, respectively corresponding to approximately 15%, 25%, 35%, and 35% decreases compared to 100% RHA based geopolymer paste. A nearly 77% decrease in porosity percentage was observed by when mix RHA and BF were compared. Pores can be categorized into two types including gel pores and capillary pores [23]. Pores with a diameter of less than 10 nm are gel pores while those are greater than 10 nm are capillary pores, which are further divided into two types including medium size (10–50 nm) and large size (80– 10000 nm). Mercury intrusion porosimetry (MIP) plot was used for the determination of largely interconnected pores also, known as critical pores [44]. The critical pore sizes were 20, 17, 15, and 13 nm for paste with 0, 10, 20, and 30% basalt fiber content, respectively. The minimum pore size obtained at 100% basalt fiber content was 8 nm. The compressive strength is governed primarily by the pore structure development in which gel pores play a major role. Accordingly, CNASH gel development helps in filling up the pores. Meanwhile, permeability is greatly affected by the critical pore size as shown by the previous study [24]. Many other properties like shrinkage, water movement, and creep are also affected by the gel pore development [25]. In general, it can be seen from Table 3 that the higher percentage of basalt fibers were used, the denser structure was achieved. Therefore, the compressive
To measure the composition of elements, RHA and basalt fibers interaction, EDS method was carried out. Fig. 9 shows the chemical compositions of the RHA and basalt fiber based geopolymer pastes at 28 days by using the EDS method. It is clear from Fig. 9 that the presence of Ca, Si, Na, and Al generated the reaction products like CSH, CASH, and NASH. This appearance is similar to the fly ash and basalt fiber based geopolymer paste [37]. The relation of Ca/Si and Si/Al ratio with the compressive strength is presented in Figs. 10 and 11, respectively. An increase in these ratios (Ca/Si and Si/Al) leads to an increase in the compressive strength. Compounds based on calcium and silica are formed thus initiating gel development and consequently matrix pores get filled up leading to the formation of a more homogeneous and denser matrix structure. This improvement in the matrix structure leads to an increase in the compressive strength. 3.8. XRD analysis XRD studies were carried out on 100% RHA and 100% BF paste and the obtained XRD patterns are shown in Fig. 12. For paste with 100% RHA, CSH and quartz were the dominating constituents. With an increase in the basalt fiber content, quartz content increased which resulted in a crystalline phase of a higher intensity level. The high peaks obtained in the XRD pattern indicate an increase in quartz and calcite content with an increase in the basalt fiber
Table 3 Porosity and critical pore sizes. No.
Mix
Porosity (%)
Critical Pore Size (nm)
Compressive strength (MPa)
1 2 3 4 5
RHA 10BF90RHA 20BF80RHA 30BF70RHA BF
36.90 33.12 31.03 24.13 8.12
20 17 15 13 8
38.33 52.41 58.92 64.31 94.12
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Geopolymer matrix
Basalt fiber
Pores
a) RHA
b) 10BF90RHA
Basalt fiber
CNASH
Gel
Microcrack
Basalt fiber
CNASH c) 20BF80RHA
d) 30BF70RHA
Basalt fiber
CNASH
e) BF Fig. 8. SEM images of the specimens with 0%, 10%, 20%, 30% and 100% basalt fiber content.
content. They are formed due to the formation of CSH peaks which also play an important role in the strength development of the paste. The figure also shows the XRD patterns obtained for the pastes with 10, 20, 30% basalt fiber content. With increasing the basalt fiber content, the peaks of the XRD pattern increased. It could be attributed to the development of the crystalline phase. As can be seen from the figure, 100% BF paste has a higher CSH peak than that of 100% RHA paste. The large calcium content of basalt fiber was responsible for this high CSH peak. Accordingly, better bonding is achieved when using basalt fibers as it clinkers the gel development and results in the improvement of properties, as confirmed by the existence of the peaks. The composition of oxides is represented by these higher peaks. When CSH and CASH gels development occurred, the compressive strength consequently
increased. With an increase in calcium content, the gel development occurs which is supported by similar observation as reported in the previous study [37]. In addition, some properties like creep and shrinkage are affected by CSH gel as it is porous in nature and have a pore size between 0.5 and 10 nm. 3.9. Drying shrinkage The contrast of drying shrinkage values with respect to curing age is illustrated in Fig. 13. At the age of 28 days, drying shrinkage of RHA, 30BF70RHA, BF geopolymer pastes were 28762, 15220, and 12601 10 6 mm/mm, respectively. Punurai et al. [37] studied the drying shrinkage of fly ash-based geopolymer paste and recorded a drying shrinkage value of around 27690 10 6 mm/mm at the age
Saloni et al. / Construction and Building Materials 245 (2020) 118422
Mix Element C
BF Weight Atomic (%)
(%)
8.56
4.56
O
52.34
69.69
Na
14.56
13.49
Mg
1.22
1.08
Al
1.92
1.51
Si
8.95
3.18
S
0.72
0.48
K
1.42
0.76
Ca
9.87
5.26
Mix Element
RHA Weight Atomic (%)
(%)
C
11.34
6.82
O
37.89
57.01
Na
10.15
10.62
Mg
0.72
0.72
Al
13.46
12.00
Si
16.6
6.66
S
1.67
1.26
K
1.28
0.77
Ca
6.87
4.13
Mix Element
7
30BF70RHA Weight Atomic (%)
(%)
C
7.11
3.86
O
51.83
70.39
Na
9.35
8.83
Mg
1.41
1.28
Al
5.44
4.38
Si
11.88
4.30
S
0.82
0.56
K
2.25
1.22
Ca
9.54
5.18 Fig. 9. EDS image of geopolymer paste at 28 days.
of 28 days. At the age of 28 days, drying shrinkage of RHA based geopolymer was slightly more than fly ash-based geopolymer paste and reason may be due to the high-water demand of rice husk ash. The drying shrinkage of all the pastes was investigated up to the age of 120 days and is shown in Fig. 13. On the other hand, the addition of basalt fibers improved the drying shrinkage,
for example, drying shrinkage of 30BF70RHA at 28 days was 15220 10 6 mm/mm which was much smaller than the corresponding drying shrinkage of RHA (28762 10 6 mm/mm). This finding agrees well with a previous study by Yusuf et al. [49] who used palm oil fuel ash. In general, it can be concluded that various gels such as CSH, CASH, and NASH formed with the addition of
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Fig. 10. Compressive strength with Ca/Si ratio.
(a) XRD Pattern of RHA with 10-30% Basalt Fiber
Fig. 11. Compressive strength with Si/Al ratio.
(b) XRD Pattern of BF and RHA samples Fig. 12. X-Ray Diffraction (XRD) Pattern of the hybrid paste.
calcium-based basalt fibers. In addition, these gels combined together to form CNASH gel and thus improved the microstructure of geopolymer paste [5,50–53]. In the meantime, SEM and EDS analyses also confirmed the development of CNASH gel as a result of high calcium in basalt fibers. A reduction in shrinkage values was observed because of the micro-aggregate function of partially reacted basalt fibers. With an increase in Si/Al and Ca/Si ratios drying shrinkage values improved and agree with the results previously reported [37,49]. 4. Conclusion This study investigates the effect of various replacement percentages of RHA by basalt fibers on different material properties. The findings from this experimental study can be summarized as follows: 1. Both the initial and final setting time increased with the basalt fiber content. The setting time increased with the Si/Al ratio. 2. Initial setting time is more sensitive to the addition of basalt fibers as compared to the final setting time regarding the initial value. The increase was nearly 272% for the initial setting time and 101% for the final setting time. 3. The addition of basalt fibers to geopolymer paste improves the mechanical properties of geopolymer paste.
Fig. 13. Drying shrinkage of geopolymer pastes at different ages.
4. The overall increase in the bulk density was 21.05% with change the in-basalt fiber content from 0 to 100%. This could be attributed to the higher specific gravity of basalt fibers as compared to RHA.
Saloni et al. / Construction and Building Materials 245 (2020) 118422
5. Improvement in the compressive and flexural strengths was observed with an increase in the basalt fiber content. The addition of basalt fibers increased Ca content thus the CNASH gel development. The role of basalt fibers as micro aggregates played an important role in boosting the compressive strength. 6. The increase in the compressive strength was about 240% at 7 days and 145% at 28 days when the basalt fiber content changed from 0 to 100%. From the test results, it can be seen that the compressive strength also increased with the fiber contents. 7. With an increase in the basalt fiber content, porosity and critical pore size decreased. A more homogeneous and denser structure would have formed a compact structure that is responsible for the improvement in its properties. CRediT authorship contribution statement Saloni: Conceptualization, Methodology, Software, Writing original draft. Parveen: Data curation, Writing - original draft, Visualization, Investigation, Supervision. Thong M. Pham: Visualization, Writing - review & editing. 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. References [1] D. Hardjito et al., On the development of fly ash-based geopolymer concrete, Mater. J. 101 (6) (2004) 467–472. [2] T. Rahmiati et al., Effect of solid/liquid ratio during curing time fly ash based geopolymer on mechanical property, Materials Science Forum, Trans Tech Publ, 2015. [3] Parveen et al., Mechanical and microstructural properties of fly ash based geopolymer concrete incorporating alccofine at ambient curing, Constr. Build. Mater. 180 (2018) 298–307. [4] L.K. Turner, F.G. Collins, Carbon dioxide equivalent (CO2-e) emissions: a comparison between geopolymer and OPC cement concrete, Constr. Build. Mater. 43 (2013) 125–130. [5] Parveen, D. Singhal, B.B. Jindal, Preparation of geopolymer concrete (GPC) using high-silica rice husk ash (RHA) incorporating alccofine, Adv. Sci., Eng. Med. 9 (5) (2017) 370–376. [6] J.L. Provis, Geopolymers and other alkali activated materials: why, how, and what?, Mater Struct. 47 (1–2) (2014) 11–25. [7] A. Palomo, M.W. Grutzeck, M.T. Blanco, Alkali-activated fly ashes: a cement for the future, Cem. Concr. Res. 29 (8) (1999) 1323–1329. [8] Parveen, D. Singhal, Development of mix design method for geopolymer concrete, Adv. Concr. Constr. 5 (4) (2017) 377–390. [9] M. Anwar, T. Miyagawa, M. Gaweesh, Using rice husk ash as a cement replacement material in concrete, in: Waste management series, Elsevier, 2000, pp. 671–684. [10] M. Younes, H. Abdel-Rahman, M.M. Khattab, Utilization of rice husk ash and waste glass in the production of ternary blended cement mortar composites, J. Build. Eng. 20 (2018) 42–50. [11] S.D. Parveen, A. Sharma, Rubberized concrete: needs of good environment (overview), Int. J. Emerg. Technol. Adv. Eng 3 (2013) 192–196. [12] Parveen et al., Mechanical properties of ground granulated blast furnace slag based geopolymer concrete incorporating alccofine with different concentration and curing temperature, Adv. Sci., Eng. Med. 9 (11) (2017) 948–958. [13] S.A. Ahmed, M.-E.A. Metwally, S.E. Zakey, Utilizing industrial waste-water as alkali activator in sand-cement kiln dust bricks, Constr. Build. Mater. 182 (2018) 284–289. [14] A. Passuello et al., Evaluation of the potential improvement in the environmental footprint of geopolymers using waste-derived activators, J. Cleaner Prod. 166 (2017) 680–689. [15] J. Moraes et al., New use of sugar cane straw ash in alkali-activated materials: a silica source for the preparation of the alkaline activator, Constr. Build. Mater. 171 (2018) 611–621. [16] Parveen, A. Sharma, D. Singhal. Mechanical properties of geopolymer concrete: A state of the art report. in 5th Asia And Pacific Young Researchers And Graduate Symposium-13, 2013, Jaipur. [17] S.A. Zareei, F. Ameri, N. Bahrami, Microstructure, strength, and durability of eco-friendly concretes containing sugarcane bagasse ash, Constr. Build. Mater. 184 (2018) 258–268.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Enhanced properties of high-silica rice husk ash-based geopolymer paste by incorporating basalt fibers Saloni a, Parveen a,⇑, Thong M. Pham b a b
Civil Engineering Department, DCRUST Murthal-131039, Haryana, India Center for Infrastructural Monitoring and Protection, School of Civil and Mechanical, Engineering, Curtin University, Kent Street, Bentley, WA 6102, Australia
h i g h l i g h t s Basalt fiber enhanced the strength characteristics of the geopolymer composites. Setting times of RHA based geopolymer were found to increase with increase of basalt fiber content. Reduced drying shrinkage was obtained for RHA based geopolymer paste with basalt fiber. The microstructure and properties of the RHA and basalt fiber based paste has been studied. Homogeneous and denser paste may be developed by using basalt fiber and RHA.
a r t i c l e
i n f o
Article history: Received 23 October 2019 Received in revised form 9 January 2020 Accepted 12 February 2020
Keywords: Basalt fiber Geopolymer Bulk density Compressive strength Drying shrinkage
a b s t r a c t In this study, the effect of different basalt fiber contents (0%, 10%, 20%, 30%, 100% by binder mass) as a replacement of rice husk ash on the fresh, hardened and microstructural properties of geopolymer paste has been evaluated. NaOH (12 Molar) and Na2SiO3 were used to produce geopolymers. Different characteristics, i.e. initial setting and final setting times, bulk density, porosity, compressive and flexural strengths were investigated by using various techniques, such as scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Results showed that basalt fibers exhibited positive effects on the fiber-matrix transition zone as increased initial setting time, final setting time, bulk density and compressive and flexural strengths with the increased basalt fiber contents. Meanwhile, chemical analysis showed that basalt fibers acted as reinforcements and thus improved the characteristic of geopolymer paste. The major crystalline phases presented in the resultant geopolymer were quartz, calcium silicate hydrate and magnesioferrite. In addition, reduced values of critical pore size and total porosity were observed with an increase of replacement content in rice husk ash. The results of the present study support basalt fibers and rice husk ash as promising solid waste materials for use in the production of geopolymers. Ó 2020 Published by Elsevier Ltd.
1. Introduction Substantial consumption of energy and massive release of greenhouse gases are resulted from the production of ordinary Portland cement (OPC) which is the main binder for the production of concrete in the construction industry [1]. The carbon dioxide Abbreviations: BD, Bulk density; CSH, Calcium silicate hydrate; CASH, Calcium aluminate silicate hydrate; EDS, Energy dispersive spectroscopy; FST, Final setting time; IST, Initial setting time; NASH, Sodium aluminate silicate hydrate; RHA, Rice husk ash; SEM, Scanning electron microscopy; XRD, X-ray diffraction. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Parveen). https://doi.org/10.1016/j.conbuildmat.2020.118422 0950-0618/Ó 2020 Published by Elsevier Ltd.
(CO2) footprints of the cement are very high as nearly 0.65–0.85 tons of CO2 is produced per ton of cement production [2–4]. Therefore, there should be an alternative construction material that solves the issues related to cost and carbon emission and have excellent properties. Furthermore, wastes generated from various industries are posing a great disposal problem and efforts are made to solve this issue and use them for new advancements [5]. Geopolymer, a commercial name used for the alkali-activation of kaolinite/limestone/dolomite [6,7], can act as a binder for construction materials and it utilizes the industrial waste products. This strategy has shown as an economical option to recycle waste [8]. Energy requirements are also reduced as it avoids burning or extra efforts to be made for waste disposal [9] and commences
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the need for their better utilization. Several waste materials like slag, fly ash, rice husk ash (RHA), and pozzolana has been effectively utilised for cement replacement [10,11]. Recent advancements in the production of geopolymers show that it possesses excellent binder properties and provides potential benefits [12]. It is estimated that the CO2 footprints of the geopolymers are approximately 9% less than the conventional concrete produced using 100% cement [4]. Also, researchers are trying to use waste-based activators in order to further reduce the CO2 footprints [13–15]. In addition, performance of geopolymer concrete structures is comparable to conventional concrete in terms of the compressive, flexural and tensile strengths [16]. Therefore, geopolymer concrete which can be considered as an alternative construction material significantly reduces the greenhouse emission, has attracted many researches nowadays due to its excellent properties and environmental benefits. Many studies have been conducted on geopolymer, such as addition to concrete mixes [17], repair and retrofitting materials, fireproofing the structures [18]. Results of utilizing the geopolymer in the various civil engineering applications are found to be remarkable especially in the field of pavements [19,20], masonry structures [21], reinforced geopolymer composites [22] and repair materials [23–27]. RHA is an agricultural by-product [28,29], in which approximately 22% of the 648.9 million tons of rice produced annually constitutes rice husk ash [30]. There is a boost in the demand for natural resources due to increasing urbanization and development while disposal of wastes in such cases adds to an extra cost, thus strongly initiating the use of wastes for some beneficial purposes [31]. RHA is found to have pozzolanic properties as it is a silica-rich material [32]. When it is burnt, it causes environmental problems and its disposal also causes an extra cost. A simple and effective way to solve the associated problems is to use it for partial cement replacement [33,34]. For the production of geopolymer, RHA requires an activation treatment. Previous studies have found that it can be activated by using a mixture of NaOH and Na2SiO3 as an alkaline activator. Nair et al. [35] studied pozzolanic activity of RHA, and NASH gel was formed which bears a direct relationship with the hardening and setting of the geopolymer based product. Thus, RHA can be utilized to form geopolymer paste and its effect on the various properties like the compressive strength, initial and final setting time, and flexural strength should be studied. The use of RHA in geopolymer paste can help in improving the strength and pore structure of the paste [8]. It possesses a high surface area as indicated by its microstructural studies. Extremely fine basalt fibers which are a modified form of the basalt family have also gained attention as an additive in concrete mixes because its cost is low as compared to carbon fibers [36]. They have good resistance to abrasion, good durability characteristics and excellent insulation properties. They have miraculous results in many areas and have a huge potential for their use in practical applications. A previous study indicates that the use of basalt fibers has led to an improvement in mechanical properties and fire resistance [37]. They also caused no harm to the environment and they are considered as a non-hazardous material. The material itself is not new in the market but more innovative applications of this product in the practical field and thus it is on boom in new research studies. Basalt fibers mainly consist of lime and silica. Calcium to silica ratio is mainly responsible for the increase in the compressive strength of basalt fibers based geopolymer [38]. CSH gel development leads to a dense structure that was found to be responsible for the increase in the compressive strength [39]. The microstructural examination of the basalt fibers from previous studies has proven the reason for its potential properties. This study focusses on using RHA and basalt fibers to form hybrid geopolymer paste and to investigate its properties, such as the setting time, compressive strength, and drying shrinkage.
It is noted that a similar study has been done by Punurai et al. [37] in which fly ash-basalt fiber hybrid paste was investigated concerning its properties. In their study, the effect of partial replacement of fly ash by basalt fibers on setting time, strength and drying shrinkage was examined. Meanwhile, in the current study, a composite paste was formed using RHA and basalt fibers with a fixed concentration of 12 M NaOH to study various properties. There have been no studies on RHA and basalt fiber-based composite paste in the literature, therefore, this study focuses on studying the properties of this hybrid paste. 2. Experimental program 2.1. Material properties and chemical compositions The physical and chemical properties of the materials are shown in Table 1. In this study, the source of RHA was a rice milling plant situated in Delhi, India. The major constituents were SiO2 (90%), K2O (4.60%), and P2O5 (2.43%). Basalt fibers were provided from a manufacturing plant in Delhi. Their composition was SiO2 (46.5%), CaO (31.4%), Al2O3 (13.4%). The high silica content of both RHA and basalt fiber was responsible for the pozzolanic properties. Specific gravities of RHA and basalt fibers were 2.18 and 2.38, respectively. 25 mm and 25.62 mm were the respective values of mean diameters of RHA and basalt fibers. Scanning electron microscope (SEM) images of RHA and basalt fiber particles are shown in Fig. 1. The SEM study of RHA collaborates with its high specific surface area. Irregular shape for RHA particles and a cylindrical shape for basalt fibers was observed. Fig. 2 shows the particle size distribution of the RHA and basalt fibers. X-ray diffraction pattern (XRD) of RHA and Basalt fiber is depicted in Fig. 3. Mix proportions used in this study are shown in Table 2. Commercially available sodium silicate solution with a specific gravity of 1.46 and with a SiO2/Na2O ratio of 2.1 and sodium hydroxide pellets with 96% purity were used for developing activator mixture. Preparation time for the geopolymer pastes was kept 24 h. 2.2. Preparation of sample and testing procedure Basalt fibers were used for replacing RHA at percentage levels of 0, 10, 20, 30 and 100% by weight. A 12 M concentration of sodium hydroxide solution and sodium hydroxide to sodium silica ratio of 1.0 was used as an activator of geopolymer paste. The liquid to binder ratio value was kept as 0.5. The mixing was done using a planetary mixer. The setting time was immediately determined
Table 1 Chemical composition and physical properties of processed rice husk ash and basalt fibers. Physical and chemical properties
Rice husk ash
Basalt fiber
Specific Gravity Mean Particle Size (lm) Specific surface area m2/kg CO2 SiO2 K2O P2O5 CaO MgO Fe2O3 Al2O3 MnO SO3 Loss on Ignition
2.18 25 1434.50 0.10% 90.00% 4.60% 2.43% 1.10% 0.77% 0.43% 0.46% 0.11% – 3.90%
2.38 25.62 – – 46.5% 0.41% 0.28% 31.4% 6.38% 0.79% 13.4% 0.17% 0.67% 0.44%
- Not applicable.
Saloni et al. / Construction and Building Materials 245 (2020) 118422
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(a) Rice husk ash
Fig. 3. X-ray diffraction pattern (XRD) of RHA and Basalt fibers.
[41]. The compressive strength was determined at 7, 28 and 90 days using 50 * 50 * 50 mm3 cubes according to ASTM C109/C109 M 16-a [42]. The flexural strength was determined at the age of 28 days according to ASTM C348-14 [43]. The reported values were averaged from 3 identical samples for the above tests. A chisel was used to obtain samples from the middle section of 50 * 50 * 50 mm3 cubes at 28-day age for carrying out microstructural investigations. Oven drying was done for 24 h post to its immersion in acetone for 24 h. Mercury Intrusion Porosimeter (MIP) was used for porosity determination of the specimens while the Edward W. Washburn equation [44] i.e. (Pressure required to force mercury into a capillary pore quals to 2!cosh/r, where r = radius, ! = surface tension and h = the angle of contact) was used to examine the porosity and critical pore size.
(b) Basalt fiber Fig. 1. SEM images of RHA and Basalt fiber particles.
100 90 Percentage passing
80 70
3. Results and discussion
60 50
3.1. Setting time
40
RHA
BF
30 20 10 0 0.001
0.010
0.100 Particle size (mm)
1.000
Fig. 2. Particle size distribution of RHA and Basalt fiber.
after mixing according to ASTM C191-13 [40]. The average of three samples was reported as the final result of the setting time. Prism molds with various sizes of 50 * 50 * 50 mm3, 40 * 40 * 160 mm3, 25 * 25 * 285 mm3 were used for sample preparation and three identical samples of each size were cast. Plastic sheets were used to seal specimens for avoiding the evaporation of moisture and were kept in a room with 25 °C controlled temperature. After 24 h, the specimens of size 50 * 50 * 50 mm3 and 40 * 40 * 160 mm3 were demolded. Then the post-curing was done at 25 °C after covering them with a vinyl sheet till the ages of 7 and 28 days to determine their density and compressive strength. Meanwhile, 50 * 50 * 50 mm3 cubes were used to determine the bulk density (BD) at 28-day age as per ASTM C138/C138 17-a
The variation in setting times with basalt fiber content is shown in Fig. 4. The setting times increased with the basalt fiber content. The increase in basalt fiber content from 0 to 100% resulted in an increase in the initial setting time (IST) from 25 to 93 min while the final setting time (FST) increased from 72 to 145 min. The increase was nearly 272% for the initial setting time and 101% for the final setting time. The initial setting time showed more sensitive to the addition of basalt fiber content than that of the final setting time with respect to the initial value. Si/Al ratio increased from 1.79 to 2.51 correspond to an increase in basalt fiber content from 0 to 100%. This observation agrees well with the results of previous studies which has shown that setting time increases with the Si/Al ratio [37,45,46]. It is noted that basalt fibers have an amorphous nature and therefore, helped in the more uniform particle distribution within the matrix. A glassy interface is formed due to the leaching of silica, alumina, and calcium ions from RHA particles in the presence of a high concentration of NaOH (12 M). With a cylindrical shape, basalt fiber particles have less surface area than RHA particles and therefore, it extended the setting time of the geopolymer paste. Many previous studies also reported that the setting and hardening have a strong relationship with both the amorphous phase and leaching of the starting materials [38,47]. Therefore, the current study is in good agreement that the setting and hardening can be extended with the inclusion of basalt fibers in geopolymer [20,47].
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Table 2 Mix proportions for the hybrid paste. No.
Mix
RHA (%)
Basalt fiber (%)
Na2SiO3 (g)
NaOH (g)
1 2 3 4 5
RHA 10BF90RHA 20BF80RHA 30BF70RHA BF
100 90 80 70 0
0 10 20 30 100
30 30 30 30 30
30 30 30 30 30
Fig. 4. Setting time of hybrid paste with different basalt fiber contents.
Fig. 6. Compressive strength of hybrid pastes with different basalt fiber contents at different ages.
3.2. Bulk density The bulk density was determined at the age of 28 days. The bulk densities of the samples with different percentages of basalt fibers are shown in Fig. 5. As can be seen from the figure, the bulk density increased from 1.90 to 2.30 g/cm3 corresponding to an increase in basalt fiber content from 0 to 100%. Both RHA and basalt fibers have a different value of specific gravity (1.8 and 2.38, respectively). The high specific gravity of basalt fibers compared to RHA leads to an increase in bulk density values, which were 1.90, 1.93, 1.95, 1.97 and 2.30 at 0, 10, 20, 30 and 100% basalt fiber content, respectively. 3.3. Compressive strength The compressive strengths of all the samples are shown in Fig. 6. The compressive strengths obtained at 7 days were 21.50,
26.93, 29.42, 35.51, and 73.12 MPa with 0, 10, 20, 30 and 100% basalt fiber content, respectively. The corresponding compressive strengths at 28 days were 38.33, 52.41, 58.92, 64.31, 94.12 MPa, respectively. The higher concentration of NaOH helped in the leaching of Si and Al ions from the added materials. Furthermore, CSH, CASH, and NASH gels were formed due to the presence of calcium in the matrix and therefore improved the setting and hardening of the pastes [37]. CNASH gel, a combination of all the gels was formed which is the main cursor for the strength development [38]. The increase in the compressive strength for mix BF at 7 days (240%) was more significant as compared to 28 days (145%). The maximum obtained compressive strengths were 73.12 and 94.12 MPa at respective ages. This improvement is attributed to the improved internal matrix structure with the CNASH gel development and pores which were filled by this gel [37,39]. Similar results are also observed for the strength development in Ordinary Portland Cement [48]. It is also noted that the amorphous nature of basalt fibers contributes to strength development. A more uniform and dense structure is formed due to basalt fiber addition as it behaves as an aggregate. Punurai et al. [37] recommended the NaOH concentration of 10 M for fly ash and basalt fiber based geopolymer paste. Meanwhile, 12 M NaOH solution was used in this study and basalt fiber content was increased from 0 to 100% for studying its effect on the compressive strength.
3.4. Flexural strength
Fig. 5. Bulk density of hybrid pastes with different basalt fiber contents.
The flexural strength of the specimens with various basalt fiber contents is shown in Fig. 7. The primary factor governing flexural strength is basalt fiber content. The outcomes demonstrated that the flexural strength of RHA, 10BF90RHA, 20BF80RHA, 30BF70RHA and BF geopolymer pastes at 28 days were 3.6, 4.2, 4.8, 5.7 and 10.82 MPa, respectively. A significant improvement was observed in the flexural strength as a result of increasing the basalt fiber content. In comparison with RHA, the flexural strength of hybrid
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strength increases with the basalt fiber content. Punurai et al. [37] also recommended the use of basalt fibers in fly ash-based geopolymer paste. This study proved that high compressive strength can be achieved by utilizing basalt fiber in the rice husk ash-based geopolymer paste. 3.6. SEM (Scanning electron Microscopy) study
Fig. 7. Flexural strength of hybrid pastes with different basalt fiber contents at 28 days.
binders was 17%, 33%, 58% higher than that of 10BF90RHA, 20BF80RHA, 30BF70RHA, respectively. The production of CNASH gel leads to an improvement in the flexural strength and microaggregate function improvement (as observed in SEM images). As a result, it led to a denser structure of hybrid paste and thus higher flexural strength. The modulus of rupture was also improved due to this phenomenon.
SEM studies were carried out at the age of 28 days and SEM images are shown in Fig. 8. SEM images were utilized to investigate the interaction between RHA and basalt fibers. When compared with 100% RHA paste, a denser and more homogeneous structure of pastes with 10–30% basalt fiber content is shown in the SEM images. Thus, it can be concluded that the structure of the hybrid paste improved with the incorporation of basalt fibers. As the basalt fiber content increases, Ca content increases and CNASH gel is developed. They react partially as seen in the SEM images. A micro aggregate function is performed by basalt fibers leading to an increase in the compressive strength. In addition, the deposition of CSH, CASH, and NASH gels on basalt fibers can be clearly seen in the SEM image of paste with 100% BF. A compact structure was observed as indicated in the SEM images. Similar improvement of the microstructure of geopolymer paste with basalt fibers has also reported in the previous study [37]. The compact structure was found to be the main reason for improvement in the properties of the geopolymer paste. More homogeneity was observed with the addition of basalt fiber and thus the matrix structure was improved.
3.5. Porosity 3.7. EDS (Energy Dispersive Spectroscopy) Porosity and critical pore size were examined and the results are summarised in Table 3. The porosity of paste with 100% RHA was 36.9% at 28 days. The porosity percentages at 10, 20, 30 and 100% basalt fiber content were 33.12, 31.03, 24.13 and 8.12, respectively corresponding to approximately 15%, 25%, 35%, and 35% decreases compared to 100% RHA based geopolymer paste. A nearly 77% decrease in porosity percentage was observed by when mix RHA and BF were compared. Pores can be categorized into two types including gel pores and capillary pores [23]. Pores with a diameter of less than 10 nm are gel pores while those are greater than 10 nm are capillary pores, which are further divided into two types including medium size (10–50 nm) and large size (80– 10000 nm). Mercury intrusion porosimetry (MIP) plot was used for the determination of largely interconnected pores also, known as critical pores [44]. The critical pore sizes were 20, 17, 15, and 13 nm for paste with 0, 10, 20, and 30% basalt fiber content, respectively. The minimum pore size obtained at 100% basalt fiber content was 8 nm. The compressive strength is governed primarily by the pore structure development in which gel pores play a major role. Accordingly, CNASH gel development helps in filling up the pores. Meanwhile, permeability is greatly affected by the critical pore size as shown by the previous study [24]. Many other properties like shrinkage, water movement, and creep are also affected by the gel pore development [25]. In general, it can be seen from Table 3 that the higher percentage of basalt fibers were used, the denser structure was achieved. Therefore, the compressive
To measure the composition of elements, RHA and basalt fibers interaction, EDS method was carried out. Fig. 9 shows the chemical compositions of the RHA and basalt fiber based geopolymer pastes at 28 days by using the EDS method. It is clear from Fig. 9 that the presence of Ca, Si, Na, and Al generated the reaction products like CSH, CASH, and NASH. This appearance is similar to the fly ash and basalt fiber based geopolymer paste [37]. The relation of Ca/Si and Si/Al ratio with the compressive strength is presented in Figs. 10 and 11, respectively. An increase in these ratios (Ca/Si and Si/Al) leads to an increase in the compressive strength. Compounds based on calcium and silica are formed thus initiating gel development and consequently matrix pores get filled up leading to the formation of a more homogeneous and denser matrix structure. This improvement in the matrix structure leads to an increase in the compressive strength. 3.8. XRD analysis XRD studies were carried out on 100% RHA and 100% BF paste and the obtained XRD patterns are shown in Fig. 12. For paste with 100% RHA, CSH and quartz were the dominating constituents. With an increase in the basalt fiber content, quartz content increased which resulted in a crystalline phase of a higher intensity level. The high peaks obtained in the XRD pattern indicate an increase in quartz and calcite content with an increase in the basalt fiber
Table 3 Porosity and critical pore sizes. No.
Mix
Porosity (%)
Critical Pore Size (nm)
Compressive strength (MPa)
1 2 3 4 5
RHA 10BF90RHA 20BF80RHA 30BF70RHA BF
36.90 33.12 31.03 24.13 8.12
20 17 15 13 8
38.33 52.41 58.92 64.31 94.12
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Saloni et al. / Construction and Building Materials 245 (2020) 118422
Geopolymer matrix
Basalt fiber
Pores
a) RHA
b) 10BF90RHA
Basalt fiber
CNASH
Gel
Microcrack
Basalt fiber
CNASH c) 20BF80RHA
d) 30BF70RHA
Basalt fiber
CNASH
e) BF Fig. 8. SEM images of the specimens with 0%, 10%, 20%, 30% and 100% basalt fiber content.
content. They are formed due to the formation of CSH peaks which also play an important role in the strength development of the paste. The figure also shows the XRD patterns obtained for the pastes with 10, 20, 30% basalt fiber content. With increasing the basalt fiber content, the peaks of the XRD pattern increased. It could be attributed to the development of the crystalline phase. As can be seen from the figure, 100% BF paste has a higher CSH peak than that of 100% RHA paste. The large calcium content of basalt fiber was responsible for this high CSH peak. Accordingly, better bonding is achieved when using basalt fibers as it clinkers the gel development and results in the improvement of properties, as confirmed by the existence of the peaks. The composition of oxides is represented by these higher peaks. When CSH and CASH gels development occurred, the compressive strength consequently
increased. With an increase in calcium content, the gel development occurs which is supported by similar observation as reported in the previous study [37]. In addition, some properties like creep and shrinkage are affected by CSH gel as it is porous in nature and have a pore size between 0.5 and 10 nm. 3.9. Drying shrinkage The contrast of drying shrinkage values with respect to curing age is illustrated in Fig. 13. At the age of 28 days, drying shrinkage of RHA, 30BF70RHA, BF geopolymer pastes were 28762, 15220, and 12601 10 6 mm/mm, respectively. Punurai et al. [37] studied the drying shrinkage of fly ash-based geopolymer paste and recorded a drying shrinkage value of around 27690 10 6 mm/mm at the age
Saloni et al. / Construction and Building Materials 245 (2020) 118422
Mix Element C
BF Weight Atomic (%)
(%)
8.56
4.56
O
52.34
69.69
Na
14.56
13.49
Mg
1.22
1.08
Al
1.92
1.51
Si
8.95
3.18
S
0.72
0.48
K
1.42
0.76
Ca
9.87
5.26
Mix Element
RHA Weight Atomic (%)
(%)
C
11.34
6.82
O
37.89
57.01
Na
10.15
10.62
Mg
0.72
0.72
Al
13.46
12.00
Si
16.6
6.66
S
1.67
1.26
K
1.28
0.77
Ca
6.87
4.13
Mix Element
7
30BF70RHA Weight Atomic (%)
(%)
C
7.11
3.86
O
51.83
70.39
Na
9.35
8.83
Mg
1.41
1.28
Al
5.44
4.38
Si
11.88
4.30
S
0.82
0.56
K
2.25
1.22
Ca
9.54
5.18 Fig. 9. EDS image of geopolymer paste at 28 days.
of 28 days. At the age of 28 days, drying shrinkage of RHA based geopolymer was slightly more than fly ash-based geopolymer paste and reason may be due to the high-water demand of rice husk ash. The drying shrinkage of all the pastes was investigated up to the age of 120 days and is shown in Fig. 13. On the other hand, the addition of basalt fibers improved the drying shrinkage,
for example, drying shrinkage of 30BF70RHA at 28 days was 15220 10 6 mm/mm which was much smaller than the corresponding drying shrinkage of RHA (28762 10 6 mm/mm). This finding agrees well with a previous study by Yusuf et al. [49] who used palm oil fuel ash. In general, it can be concluded that various gels such as CSH, CASH, and NASH formed with the addition of
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Saloni et al. / Construction and Building Materials 245 (2020) 118422
Fig. 10. Compressive strength with Ca/Si ratio.
(a) XRD Pattern of RHA with 10-30% Basalt Fiber
Fig. 11. Compressive strength with Si/Al ratio.
(b) XRD Pattern of BF and RHA samples Fig. 12. X-Ray Diffraction (XRD) Pattern of the hybrid paste.
calcium-based basalt fibers. In addition, these gels combined together to form CNASH gel and thus improved the microstructure of geopolymer paste [5,50–53]. In the meantime, SEM and EDS analyses also confirmed the development of CNASH gel as a result of high calcium in basalt fibers. A reduction in shrinkage values was observed because of the micro-aggregate function of partially reacted basalt fibers. With an increase in Si/Al and Ca/Si ratios drying shrinkage values improved and agree with the results previously reported [37,49]. 4. Conclusion This study investigates the effect of various replacement percentages of RHA by basalt fibers on different material properties. The findings from this experimental study can be summarized as follows: 1. Both the initial and final setting time increased with the basalt fiber content. The setting time increased with the Si/Al ratio. 2. Initial setting time is more sensitive to the addition of basalt fibers as compared to the final setting time regarding the initial value. The increase was nearly 272% for the initial setting time and 101% for the final setting time. 3. The addition of basalt fibers to geopolymer paste improves the mechanical properties of geopolymer paste.
Fig. 13. Drying shrinkage of geopolymer pastes at different ages.
4. The overall increase in the bulk density was 21.05% with change the in-basalt fiber content from 0 to 100%. This could be attributed to the higher specific gravity of basalt fibers as compared to RHA.
Saloni et al. / Construction and Building Materials 245 (2020) 118422
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