Microstructural study of environmentally friendly

Journal of Cleaner Production 189 (2018) 805e812

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Microstructural study of environmentally friendly boroaluminosilicate geopolymers Ali Bagheri a, *, Ali Nazari a, Ailar Hajimohammadi b, Jay G. Sanjayan a, Pathmanathan Rajeev a, Mostafa Nikzad c, Tuan Ngo b, Priyan Mendis b a

Centre for Sustainable Infrastructure, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia Department of Infrastructure Engineering, University of Melbourne, Victoria, 3010, Australia c Department of Mechanical Engineering and Product Design Engineering, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 October 2017 Received in revised form 28 March 2018 Accepted 4 April 2018 Available online 7 April 2018

This paper is investigating a more environment-friendly type of alkali-activated materials so-called boroaluminosilicate geopolymer (BASG). The microstructural and thermal behaviour of boroaluminosilicate geopolymers is studied using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) respectively. The chemical bonds forming the geopolymeric network as well as the reaction of the matrix at the elevated temperatures up to 600 C are investigated. Different combinations of sodium silicate, sodium hydroxide solution and borax are utilised to activate fly ash. The effect of boron ions, from the alkaline solution, on forming geopolymer gel is the main idea of the study. FTIR spectroscopy shows that not only boron ions have an undeniable influence on the formation of geopolymer compounds but also the change in the content of sodium silicate has a significant role in the gel homogeneity of geopolymer products. The existence of certain peaks relating to boron compounds is an evidence to the formation of the BASG binder. These peaks vary as the composition of the activator is changed. In addition, TGA results reveal that the water molecules embedded within the matrix could be one of the main reasons for changes in the mechanical characteristics. Loss of water at the temperatures above 200 C has a similar pattern as of the strength development of the samples. It maintains the significant role of water molecules in the geopolymer structure. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Thermogravimetric Structural analysis Elevated temperature Geopolymer bonds

1. Introduction Ordinary Portland cement (OPC) has been one of the most inseparable industrial productions in our life. It is, however, reported by the International Energy Agency that production of each kilogram of cement emits 0.81 kg of carbon dioxide to the atmosphere (Hendriks, 1998; Huntzinger, 2009). Excessive consumption of energy and contribution of OPC to carbon dioxide emission has been mentioned repeatedly (Hendriks, 1998; Huntzinger, 2009; Nazari et al., 2011, 2012; Duan, 2017). Apart from environmental issues, durability and sustainability of concrete was a concern. Therefore, numerous studies have focused on developing durability and sustainability of concrete by using supplementary

* Corresponding author. E-mail address: [email protected] (A. Bagheri). https://doi.org/10.1016/j.jclepro.2018.04.034 0959-6526/© 2018 Elsevier Ltd. All rights reserved.

cementitious additives (SCA) (Ahari et al., 2015a, 2015b; Aprianti, 2015; Mirzahosseini, 2015; Rakhimova, 2015; Alvarez, 2017). Nonetheless, limitations of SCAs in OPC have led to exploring alternative types of cementitious substances, geopolymer for instance. The term “geopolymer” is used for a category of materials that are produced by alkali-activation of aluminosilicate compounds (ASCs). ASCs including silica fume, fly ash, metakaolin, and ground granulated blast-furnace slag, react with an alkaline solution such as alkali-metals hydroxides and silicate solutions. The proposed model for formation of geopolymers consists of reactions between geopolymeric precursors such as silicon, aluminium, and oxygen atoms (San Nicolas, 2017). Despite many research, these reactions, so called polycondensation and geopolymerisation, are still far from being revealed (Duxson, 2007; Yip, 2008; Diaz, 2010). According to the polysialate model of geopolymers, repeating silicon-oxo-aluminate units (Si O Al OÞ forms a general formula of Mn[(SiO2)ZAlO2]n,wH2O where n is the degree of poly

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A. Bagheri et al. / Journal of Cleaner Production 189 (2018) 805e812

condensation and M is a cation (Phair, 2002; Nazari et al., 2011, 2012; Bagheri, 2014). Many studies have been conducted on the effects of different alkaline solutions on geopolymer properties (Phair, 2002; Bagheri and Nazari, 2014, Bagheri et al., 2017a, 2017b; Xin, 2014; Hajimohammadi, 2016; Huseien, 2016). Among them all, those who are more popular are sodium hydroxide, sodium silicate, sodium aluminate, sodium carbonate, and potassium hydroxide. In a previous study (Bagheri et al., 2017a, 2017b), a new type of geopolymer called boroaluminosilicate was developed. Fly ash as a resource of alumina and silicate as well as a mixture of sodium hydroxide and sodium silicate, which was partially replaced with borax, were utilised to produce the geopolymeric binder. In those papers, the motivation was focused on decreasing the use of sodium silicate in order to decline the cost and energy. It was also claimed that the negative effects of borax on the environment are lower than that of silicate compounds. According to the materials safety data sheet of the both materials, borax is safer to use by the human as well. In the current study, the microstructure of this new generation of binders is investigated. The microstructure of materials can be investigated through many tests such as Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and nuclear magnetic resonance (NMR). FTIR provides useful information about the chemical bonds between molecules and atoms. TGA technique, also, presents the loos of weight as the material is exposed to the high temperatures. NMR determines the characteristics of the structure, the position of the boron ions in the geopolymer matrix, in addition to the coordination of each atom within the matrix. FTIR is highly used to track the nanostructural behaviour of materials (Hajimohammadi et al., 2008, 2016a, 2016b, 2017a, 2017b; Aboulayt, 2017; Peyne, 2017; Wan et al., 2017a, 2017b). Researchers used this method to track the nanostructural evolution in geopolymer gels and precursors of gel in one-part and traditional geopolymers (Hajimohammadi et al., 2017a, 2017b). FTIR outcomes showed creation of a new band at 950 cm1 relating to the formation of new Si-O-T gel representing the aluminosilicate phase in geopolymers (T could be Si or Al). A change to lower wavelength was also observed in both systems. Few years later FTIR was applied to investigate the nanostructure of the geopolymers produced by aluminium foaming agent (Hajimohammadi et al., 2017a, 2017b). In that work FTIR was utilised to monitor the changes in nanostructure of geopolymeric materials. TGA is widely used to understand the structural behaviour of rík, 2017; Nikolov, 2017; materials as well (Douiri, 2016; Kova Peyne, 2017; Ranjbar, 2017; Wan et al., 2017a, 2017b). In 2017, researchers employed this approach to study thermomechanical properties of geopolymers. According to IR spectroscopy results, roughly 11% by weight loss occurred during heat exposure up to rík, 2017). This decline in the weight of the material is 1000 C (Kova connected to evaporating of water, either in the free form or in pores, within the matrix and consequent condensation of hydroxyl groups. Another study used this approach as a confirmation in thermal behaviour of metakaolin-phosphoric acid geopolymers with various content of H3PO4 (Douiri, 2016). It was concluded that thermal behaviour of metakaolin is different to geopolymers, and it is due to the existence of water in geopolymer matrix. The position of atoms and their coordination in the boron-based geopolymer matrix have been investigated by researchers

previously (Nicholson CL, 2005). It is claimed by the author that aluminum is located in tetrahedral positions; silicon is predominately saturated in aluminum and sodium ions are in pores. Moreover, boron atoms have trigonal (BO3) and tetrahedral coordination. They also maintain that the incorporation of boron in geopolymer structure is in the form of substitution with aluminum and silicon atoms. The aim of this study is to determine the structural and thermal behaviour of BASG subjected to the variations in the composition of activator. To achieve this goal, two different methods, including FTIR and thermogravimetric tests, are adopted. A special design of experiments is provided to investigate the effects of sodium silicate variations on the phase evolution of the structure. A comparative study is conducted to investigate the differences between conventional aluminosilicate geopolymers and BASG binder. 2. Experimental procedure 2.1. Materials and mixture proportions Pozzolanic Fly ash class F (according to the ASTM C618, SiO2 þ Al2O3 þ Fe2O3 ˃ 70) from Gladstone power station in Queensland, Australia is utilised as a source of alumina and silica. The contents of reactive SiO2 and Al2O3 in fly ash are 32.3 and 16.8 wt% respectively and SiO2/Al2O3 ratio in both crystalline and amorphous phases is almost 2. Table 1 shows details of the chemical composition of the used fly ash. Different alkaline activators are employed in the system to observe the effect of borax in the structure of the binder. Solutions made from sodium hydroxide, sodium silicate and borax with variable percentage are prepared. Solid sodium hydroxide is from Sigma Aldrich, D-grade sodium silicate solution 30e60% (29.4% SiO2 and 14.7% Na2O by weight) from PQ Australia, and anhydrous borax (Na2B4O7) form BLANTs Wellbeing & Lifestyle. Anhydrous Borax was dehydrated from Decahydrate borax as described in the literature (Bagheri et al., 2017a, 2017b). Table 2 illustrates the mixture details for alkaline activators. A constant alkali activator to fly ash ratio of 0.3 was used in all samples. To study the effect of borax on the structure of geopolymers, nine different alkaline activators were designed. The content of borax in samples FBG1 to FBG7 was increased and samples FG1 to FG7 were prepared to compare boroaluminosilicate with aluminosilicate binders. The only difference between FBG and FG samples is that the former one includes borax. Four samples per

Table 2 Designation and mixture proportion of the used samples. Mixture designation

NaOH (wt.%)

Sodium silicate (wt.%)

Borax (wt.%)

FG0 FG1 FG3 FG5 FG7 FBG1 FBG3 FBG5 FBG7

50 55 65 75 85 52 57 60 63

50 45 35 25 15 43 30 20 11

0 0 0 0 0 5 13 20 26

*An alkali activator to fly ash weight ratio of 0.3 was used to make geopolymers.

Table 1 Oxide percentages of the fly ash (Bagheri et al., 2017a, 2017b). Component

SiO2

Al2O3

CaO

MgO

Fe2O3

Na2O

P2O5

K2O5

MnO

SO3

LOI

TiO2

Content (%)

51.1

25.6

4.30

1.45

12.5

0.77

0.89

0.70

0.15

0.24

0.57

1.32


mixture were prepared for 1-, 3-, 7-, and 28-day air curing. Samples were sealed after casting to minimise carbonation and reactions of the paste with air. 24-hour cured samples were then crushed into maximum particle size of 75 mm using a pestle and mortar. Prepared powder was sieved by the mentioned mesh after crushing. For FTIR testing, after reaching particular age of curing each sample was acetone washed to stop the reaction completely (Alzeer, 2016; Nath, 2016). Coarser crushed particles were collected for TGA testing. 2.2. FTIR testing Attenuated total reflectance FTIR (ATR-FTIR) method was adopted to analyse the microstructural behaviour of geopolymers using Varian FTS 7000 FT-IR machine. The apparatus has a single reflectance diamond ATR attachment and KRS-5 lenses. Scanning specification were 4000 to 100 cm1 of wavelength and 2 cm1 of resolution at 5 kHz of scanning velocity. FTIR was performed on all of mixtures at ages of 1, 3, 7, and 28 days. 2.3. TGA testing A Perkin Elmer Diamond apparatus was used to carry out a thermogravimetric analysis with a heating rate of 5 C/min in the range of 30 Ce600 C and cooling rate of 10 C/min in the nitrogen gas with the rate of no slower than 20 ml/min and no faster than 200 ml/min. The 28-day old samples were kept 10 min at the maximum temperature and then cooled down to the room temperature. 3. Results and discussion 3.1. FTIR The results of the FTIR testing are reported in Figs. 1e5. Fig. 1a and 1b shows the FTIR peaks for fly ash and borax sources respectively. It is important to know the nature of the basic

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materials. We will be using graphs’ information further in the discussion. As it is labelled on the graph of Fig. 1b, there are some important peaks in FTIR spectroscopy of borax. These peaks include wavenumbers 824 cm1, 942 cm1, and 1331 cm1 for stretching of BeO, 1130 cm1 for bending of BOH, and 1000 cm1 for BeO (Saddeek, 2010; Marzouk, 2013; Elbeyli, 2015). Five graphs of Fig. 2 illustrate the infrared spectroscopy of the samples without borax, and four graphs of Fig. 3 show the infrared spectroscopy of the samples with borax. In Fig. 2, FTIR outcomes of geopolymers with different curing ages are presented in each graph. Fig. 2aee report the spectroscopy of geopolymers without borax with 0%e70% of reduction in the sodium silicate solution. For instance, Fig. 2a shows the curves for control samples in 1, 3, 7, and 28 days of curing while Fig. 2b reports the spectra for samples FG1, in which 10% of sodium silicate is reduced, in 1, 3, 7, and 28 days of curing. It is shown in the picture that all of the curves have a peak at about 1075e1100 cm1 which is related to unreacted fly ash particles in samples (Hajimohammadi et al., 2008, 2017a, 2017b). According to Fig. 2, for samples with higher sodium silicate content, such as FG0 and FG1, the shoulder decreases and even fades at later ages of geopolymer. This is attributed to the accelerated dissolution and participation of unreacted particles in geopolymerisation up to 28 days of curing. Nonetheless, in samples with lower content of sodium silicate, such as FG7, the intensity reduction of this band is much slower over time. It is because of the lack of the initial silicate in the solution which is known to be critical in accelerating the dissolution of fly ash particles and forming the geopolymeric networks (Hajimohammadi, 2016). Fig. 3 demonstrates the infrared spectroscopy of samples containing boron. Each graph in Fig. 3 corresponds to the FTIR result of a sample in a particular age of curing. The pattern of graph alignment is analogous to pictures in Fig. 2. Fig. 3aed represent samples containing 10%e70% borax respectively. Each figure is for one mixture at 1, 3, 7, and 28 days of curing. Once again, peak at about 1075e1100 cm1 that is related to

Fig. 1. FTIR spectroscopy of utilised (a) fly ash and (b) borax.

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Fig. 2. FTIR results of geopolymers without borax (a) FG0 (b) FG1 (c) FG3 (d) FG5 (e) FG7.


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Fig. 3. FTIR results of geopolymers with borax (a) FBG1 (b) FBG3 (c) FBG5 (d) FBG7.

unreacted fly ash exists in this plot. As shown in Fig. 3, FBG7 and FBG5 possess unreacted fly ash, which remains up to 28 days. However, in sample FBG3, shoulders disappear after a week and in sample FBG1 the 1075 cm1 peak is hardly seen. Similar to the time release of alumina in geopolymer bulk solution, adequate early release of boron in solution will affect the kinetics of the reaction and delay geopolymer gel formation (Hajimohammadi, 2010). In Fig. 3, the presence of peaks at about 908e910 cm1 refers to a formation of the borosilicate networks in the structure of boroaluminosilicate geopolymers (Elbeyli, 2015). Other changes in infrared spectroscopy of samples with borax happen in peaks around 950-1000 cm1. Where dissolution of borax leads to decline

in the geopolymeric gel homogeneity. Very similar impact on gel homogeneity has been observed in geopolymer systems with high early release of Al species in bulk solution. This impact is shown by broadening and shifting theses peaks to lower wavelengths. Another important change in the graphs of Figs. 2 and 3 is in a peak around 1400 cm1. This peak is attributed to asymmetric vil, 2016). As can be clearly bration band of sodium carbonate (Kro seen, with increasing the sodium hydroxide content in samples, the intensity of this peak also rises. This is related to the unreacted extra alkalinity in the pores, which is prone to carbonation when exposed to the air during the stop reaction process. Fig. 4 provides information about changes that take place in the

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Fig. 4. FTIR results of 28 days-cured geopolymers (a) without borax (b) with borax.

this figure are 28-day cured, and each curve represents a particular sample design. As shown in Fig. 4a, the increase in the intensity of the shoulder at wavelength around 1075 to 1100 cm1 indicates a decline in the participation of fly ash in the corresponding reaction. Therefore, as the content of sodium silicate decreases, the amount of reacted fly ash drops. On the other hand, an increase in the sharpness of the peaks around 950-1000 cm1, which correspond to the geopolymer gel means higher homogeneity in geopolymer gel matrix(Hajimohammadi et al., 2017a, 2017b). According to Fig. 4b, presence of peaks at about 867 and 880 cm1 for FBG1 and FBG3 samples is attributed to carbonate compounds, mainly sodium carbonate l, 2016). This may be due to high alkalinity of mixture. None(Kro theless, decreasing of sodium silicate in FBG5 and FBG7 samples will result in eliminating carbonate. FBG1 and FBG3 samples have higher amount of Na2SiO4 than FBG5 and FBG7; As a result, the dissolution of ash particles and consumption of water and alkali solution accelerates and hence, the formation of preliminary gels increases. This will drop the driving force required for dissolution of borax particles. This is confirmed with appearance of BO3 peaks at about 1200 cm1 in FBG5 and FBG7 specimens (Saddeek, 2010).

3.2. TGA testing

Fig. 5. Thermogravimetric curves of geopolymers (a) aluminosilicate matrix (b) boroaluminosilicate matrix.

structure of geopolymers, with the same curing age, when the content of sodium silicate and borax change. All of the samples in

The thermogravimetric analysis of both aluminosilicate and boroaluminosilicate samples is shown in Fig. 5a and b respectively. Generally, three types of water exist in geopolymers including free water within the network, water molecules in the gel pores, and structural water and hydroxyl groups (Fang, 2013). As can be clearly seen, the weight loss of the geopolymeric samples can be divided in two main stages. The first stage is from ambient temperature to 200 C, which is related to evaporation of water inside the pores. This type of water can be either free water or interstitial absorbed water. This degradation is between 5% and 7% for aluminosilicate geopolymers and 6%e10% for boroaluminosilicate samples. The second step is from 200 C up to 600 C. Reduction of the weight in this temperature range is attributed to elimination of


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Fig. 6. Thermal and mechanical properties comparison of geopolymers (a) aluminosilicate samples (b) boroaluminosilicate samples.

structural water and hydroxyl groups, Si-OH and Al-OH for instance (Aliabdo, 2016; Duan, 2017). Almost 2% of the weight of the samples is decline in this step. The compressive strength of the boroaluminosilicate geopolymers was investigated in the previous work (Bagheri et al., 2017a, 2017b). By comparing the changes in the compressive strength of the boroaluminosilicate geopolymers with the amount of weight loss due to high temperatures, the correlation between the water content and mechanical properties of geopolymers can be obtained. Fig. 6 illustrates the connection between thermal and mechanical properties of geopolymers. As shown is Fig. 6a, there is a resemblance between thermal and mechanical behaviour of aluminosilicate geopolymers. As the amount of sodium silicate is reduced in these samples, changes in compressive strength and weight loss of the aluminosilicate samples display similar trends. Consequently, water has a direct influence on mechanical properties of aluminosilicate binders. It, however, is totally different in boroaluminosilicate geopolymers, according to Fig. 6b. As can be clearly seen, compressive strength and water content have reverse trends as the content of borax raises. This difference should also be related to the differences in the amount of physically bound water in two systems. In the system with Borax this water is mainly the physically bond water which represents weaker gels such as boron precipitates. However in the other system, this water loss difference is mainly related to the chemically bond water which represents strong geopolymer gels. 4. Conclusions Thermal and structural characterisations of geopolymers are investigated via FTIR and TGA tests in this study. The effects of borax as a replacement of sodium silicate on the structure of geopolymers are observed. For this purpose, mixtures containing

different amount of borax are compared with samples comprising the same proportions excluding borax. Samples were exposed to elevated temperatures up to 600 C for TGA testing and phase evolution of binders was conducted up to 28 days by FTIR. According to the obtained results, gel homogeneity of geopolymers is strictly influenced by sodium silicate to sodium hydroxide ratio. Borax as a supply of boron ions has a significant impact on the structure and formation of geopolymeric bonds as well. TGA outcomes also show that the there is a relationship between mechanical properties of geopolymers and content of water in the structure. This is a direct connection for aluminosilicate binders while it is an inverse relation for boroaluminosilicate compounds. As a possible future study plan, authors recommend to investigate the structure elucidation of BASG matrix through NMR. To conclude, the possibility of altering sodium silicate with borax in order to produce a cleaner and less harmful construction production is investigated successfully. References Aboulayt, A., Riahi, M., Touhami, M.O., Hannache, H., Gomina, M., Moussa, R., 2017. Properties of metakaolin based geopolymer incorporating calcium carbonate. Adv. Powder Technol. 28 (9), 2393e2401. Ahari, R.S., Erdem, T.K., Ramyar, K., 2015a. Permeability properties of selfconsolidating concrete containing various supplementary cementitious materials. Construct. Build. Mater. 79, 326e336. Ahari, R.S., Erdem, T.K., Ramyar, K., 2015b. Effect of various supplementary cementitious materials on rheological properties of self-consolidating concrete. Construct. Build. Mater. 75, 89e98. Aliabdo, A.A., Elmoaty, A.E.M.A., Salem, H.A., 2016. Effect of cement addition, solution resting time and curing characteristics on fly ash based geopolymer concrete performance. Construct. Build. Mater. 123, 581e593. Alvarez, G.L., Nazari, A., Bagheri, A., Sanjayan, J.G., De Lange, C., 2017. Microstructure, electrical and mechanical properties of steel fibres reinforced cement mortars with partial metakaolin and limestone addition. Construct. Build. Mater. 135, 8e20.

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