Synthesis and characterization of geopolymers derived from coal gangue, fly ash and red mud

Construction and Building Materials 206 (2019) 287–296

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Synthesis and characterization of geopolymers derived from coal gangue, fly ash and red mud Nevin Koshy a, Kunga Dondrob a, Liming Hu a,⇑, Qingbo Wen a, Jay N. Meegoda a,b a b

State Key Laboratory of Hydro-Science and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing, China Department of Civil and Environmental Engineering, New Jersey Institute of Technology, NJ, USA

h i g h l i g h t s  Synthesis of geopolymers from coal gangue, fly ash and red mud.  High temperature is detrimental to the strength gain in three-part geopolymers.  Transformation of sodalite and amorphous phase into nepheline aids strength gain.  Loosely bound unreacted fly ash reduced the strength of the geopolymers.

a r t i c l e

i n f o

Article history: Received 29 May 2018 Received in revised form 22 October 2018 Accepted 13 February 2019

Keywords: Geopolymer Coal gangue Fly ash Red mud Synthesis Characterization Unconfined compressive strength

a b s t r a c t There is a growing need to utilize large stockpiles of the industrial byproducts including coal gangue (CG), fly ash (FA) and red mud (RM) which can pose potential environmental problems. The CG, FA and RM contain aluminosilicates which can be precursors for the synthesis of geopolymers. This research studied the utilization of CG, FA and RM by silicate activation into geopolymers. Binary mixes of CG and RM and ternary mixes of CG, FA and RM were prepared and cured at temperatures ranging from 80 to 800 °C. The unconfined compressive strength tests were performed on the specimens to determine their mechanical strength. Nitrogen adsorption, mercury intrusion porosimetry, Fourier transform infrared spectroscopy and scanning electron microscopy were used to study the textural, structural and morphological characteristics of the end products. At lower curing temperatures, the ternary mixtures of CG, FA and RM showed higher strength gains when compared to the binary mixtures of CG and RM. The textural characteristics revealed that the reduction in pores and specific surface area do not necessarily result in higher compressive strength values. Furthermore, the cementitious gels formed at high temperatures resulted in well-bonded geopolymers with homogenous structures. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Geopolymers are inorganic aluminosilicate polymers having the composition M2O
mAl2O3
nSiO2, where M is one or more alkali/ alkaline earth metals and generally, m  1 and 2  n  6 [1]. They are stable up to 1200 °C and possess excellent binding property comparable to that of cement [2]. But unlike cement, whose production results in large CO2 emissions, geopolymers are low carbon materials [3]. However, the complete physico-chemical characteristics of the geopolymeric end phases are still not well understood due to the complexity of the binder phases and the presence of large amounts of unreacted raw materials and their impurities

⇑ Corresponding author. E-mail address: [email protected] (L. Hu). https://doi.org/10.1016/j.conbuildmat.2019.02.076 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

[1]. Geopolymers have been synthesized from calcined clays [4,5], natural minerals [6] and industrial byproducts [7,8]. In recent years, there has been a growing interest in the utilization of industrial byproducts such as fly ash [3,7], blast furnace slag [9], rice husk ash [10], red mud [8] and waste glass [11] for the synthesis of geopolymers [12]. Annually, around 350 million tons of coal gangue is produced around the world and in China alone, there is a stockpile of 4.5 billion tons of coal gangue [13,14]. Similarly, 780 million tons of fly ash and 150 million tons of red mud are produced annually around the world [7]. These large stockpiles require urgent utilization for both environmental and economic reasons. Alumina and silica, two basic constituents of geopolymers, are present in coal gangue and fly ash thus making them potential materials for waste valorization [15]. The alkali activation for the synthesis of these geopolymers can be supplied partly using red mud, the byproduct of alumina production, with pH  10 and a

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source of alumina. Although majority of literature show high Na/Al values of 0.6–1 to make geopolymers, other studies [16] have shown that it is possible to synthesize them from fly ash even at low Na/Al values of 0.25 and 0.5. Hence this could be a sustainable solution by reducing the use of other alkali sources in geopolymerization. The strength enhancement of the synthesized geopolymer can be achieved by pre-treatment of coal gangue through mechanical grinding. Finer particle sizes of the raw material would allow easier and quicker dissolution of silica and alumina resulting in faster geopolymerization and early strength development [17]. However, the early dissolution and subsequent crystallization of the finer particles on the surfaces of the unreacted coarser particles could sometimes prevent the latter from joining in any further reaction. Furthermore, calcination at higher temperatures will remove the undesired organic component in the coal gangue thereby increasing the relative amount of alumina and silica available for reaction [18]. Calcination of red mud at 600 °C has been reported yield the best pozzolanic characteristics partly due to its CaO content [19,20]. Curing at higher temperatures has been reported to influence the strength of the geopolymers synthesized using raw materials such as fly ash [5], kaolinite [21], red mud [22,23] and coal gangue [18]. Heat curing is vital for geopolymerization using fly ash due to the endothermic reaction involved in fly ash activation [24]. Earlier studies explored the geopolymer synthesis using fly ash [2], red mud [22], coal gangue [25], fly ash-red mud [8,26] and coal gangue-red mud [15]. However, there are lacunae in the available literature with respect to the simultaneous use of coal gangue and fly ash to complement each other for strength development and also for waste valorization. Furthermore, there are very few studies related to the pore structure characteristics of industrial wastesbased geopolymers and the effects of low and high curing temperatures on their physical characteristics. From the literature review, it is proposed that a novel combination of coal gangue and fly ash, could be used in the synthesis of geopolymers since they are good sources of alumina and silica and fly ash has been found to provide better strength with mullite behaving as a filler material for the coal gangue. Furthermore, the alkali environment supplied by red mud could be utilized along with the aforementioned raw materials. In view of the above, this study focused on the suitability of using coal gangue, fly ash and red mud for geopolymerization. Unconfined compression tests were performed to evaluate the strength of the end products and the structural, textural and morphological characteristics were studied to explain the observed strength gains.

2. Materials and methods Coal gangue (CG) was obtained from Hebei, China and fly ash (FA) was collected from Zhangjiakou Power Plant, Hebei, China. Red mud (RM) was sourced from Zibo, Shangdong, China. All the raw materials were oven dried before use and their chemical compositions are presented in Table 1. CG was initially grounded in a ball mill to pass through 75 mm sieve while the RM was directly sieved to collect fraction passing 75 mm. As observed from Table 1, CG has significant loss on ignition (LOI) which would could be removed through calcination. Hence, calcined coal gangue (CC) was prepared by heating the raw CG at 950 °C in a muffle furnace for 6 h and subsequently grinding to less than 75 mm. Samples were prepared using binary and ternary mixes of industrial wastes viz., coal gangue with red mud and a mixture of coal gangue, fly ash and red mud. Table 2 shows the mix design adopted for the geopolymer synthesis along with their corresponding Si/Al and Na/Al ratios and the sample label is suffixed by the corresponding curing temperature. For the binary mixtures, 80%

Table 1 Chemical composition (wt%) of the raw materials. Oxide

Coal gangue

Fly ash

Red mud

SiO2 Al2O3 CaO Fe2O3 Na2O TiO2 K2O MgO SO3 P2O5 SrO ZrO2 MnO Loss on ignition

48.3 23.1 4.1 4.3 0.1 0.8 1.5 1.7 1.0 0.1 0.0 0.0 0.1 14.7

45.5 38.4 4.7 3.2 0.4 1.6 0.6 0.6 0.8 0.5 0.2 0.2 0.0 3.2

11.0 21.8 1.6 41.0 8.0 7.0 0.1 0.1 0.5 0.2 0.0 0.0 0.0 8.7

Table 2 Synthesis conditions. Proportion (w/w)

CG pretreatment

Sample label

Si/Al

Na/Al

CG:RM 2:8 2:8

Uncalcined Calcined

UC2 CC2

1.06 1.09

0.94 0.91

CG:FA:RM 2:6:2 2:6:2 4:4:2

Uncalcined Calcined Calcined

UC2F6 CC2F6 CC4F4

1.17 1.19 1.48

0.73 0.71 0.47

red mud was selected to introduce sufficient alkalinity in the mixtures and for the ternary samples, fly ash was added to enhance its strength gain. The mixtures had Si/Al ratio varying from 1.06 to 1.48 and Na/Al ratio ranging from 0.47 to 0.94 which have been reported to be adequate for geopolymerization [17,27]. Based on preliminary trial and error, the liquid to solid ratio of 0.36 w/w was found to be adequate wherein the liquid part consisted of 58 wt% of Na2SiO3
5H2O dissolved in deionized water. After homogenization for at least 15 min using a mechanical stirrer, the mixture was poured into a well-greased acrylic mold of 10 cm length and 4 cm diameter, having an aspect ratio of 2.5 in order to reduce the end effects. Molds were vibrated for 5 min and wrapped in a vinyl sheet to prevent evaporation. The UCS samples were oven dried at 80 °C for 2 h followed by curing at different temperatures of 80, 300, 500 and 800 °C for 2 h. A standard unconfined compressive strength (UCS) test apparatus at a constant loading rate of 0.5 mm min 1 was used to test the cured cylindrical specimens. The chemical composition was analyzed using X-ray fluorescence (XRF, Shimadzu XRF-1800, Japan) and the mineralogical characterization was carried out using Xray diffraction (XRD, Rigaku SmartLab, Japan) with a Cu Ka source on well-powdered samples placed on a glass plate. The qualitative analysis was carried out using the International Centre for Diffraction Data PDF2-2004 database and the highest three intensities of the mineral peaks were matched. The inner core samples were used for pore size distribution and surface area analysis since the external surfaces of the original UCS specimens would have undergone higher rates of oxidation and would not be representative of the bulk. Gas adsorption is ideal for analysis of micropores (pore size < 2 nm) and mesopores (pore size ranging from 2 to 50 nm) and a Brunauer, Emmett and Teller (BET) surface area analyzer (Quantachrome NOVA, USA) was employed for the same. Mercury intrusion porosimetry (MIP) is well suited for characterizing macropores (pore size > 50 nm). MIP studies (Quantachrome PoreMaster, USA) were carried out at low and high pressures. The structural properties were determined using a Fourier transform

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infrared spectroscopy (FT-IR) spectrometer (Netzsch PERSEUS TGA 209 F1 Libra, Germany) and the morphological and chemical characteristics were studied using scanning electron microscopy (Zeiss MERLIN VP Compact, Germany) fitted with energy dispersive spectroscopy (EDS). Leaching tests were conducted by interacting the end products with deionized water (liquid to solid ratio = 20) for 24 h. The supernatant was analyzed using inductively coupled plasma – optical emission spectrometry (ICP-OES, Optima 8000, PerkinElmer, USA) and no heavy metal was detected in the end products. 3. Results and discussion

Unconfined compressive strength (MPa)

Fig. 1 shows the variation of unconfined compressive strength of the various geopolymers with increase in curing temperatures. At low curing temperature of 80 °C, the strengths of binary mixture were much weaker than the ternary mixtures due to the higher content of flaky RM particles (80% content) in the binary mixtures which prevented initial strength gain. On the other hand, in the ternary mixtures with 60% FA, the spherical FA particles acted as filler materials in the matrix. In addition, the FA released aluminosilicates to enhance dissolution and subsequent formation of geopolymers, resulting in better strength compared to that of UC2 and CC2. For the CG and RM mixes, the strength showed an overall upward trend with the increase in curing temperature. A steep increase was observed from 80 to 300 °C due to higher dissolution of alumina in red mud contributing to the formation of aluminosilicate gels. However, for all five mixtures, the rise in curing temperature from 300 to 500 °C resulted in a drop in the compressive strength due to the degradation of the previously formed aluminosilicate gels at elevated temperatures [26]. Furthermore, the significant reduction in strength of UC2 (from 7.1 to 3.8 MPa) could also be due to the removal of organic content (see the loss on ignition in Table 1) in CG and RM at elevated temperature which resulted in an increase in void content. However, further heating of the binary mixtures of CG and RM at 800 °C led to a substantial improvement in strength. From 500 to 800 °C, the further formation of geopolymeric gels and subsequent reduction in pores and formation of a well bonded matrix helped the gain in strength. At 300 and 800 °C, binary mixes possessed much higher strength (6.4 to 7.3 MPa) compared to ternary mixes, except for energy cost. Unlike the CG and RM mixes, the ternary mixes possessed the highest compressive strength values at the lowest curing temperature of 80 °C. From 500 to 800 °C, CC2F6 gained 63% increase in

UC2 UC2F6

8

CC2 CC2F6

CC4F4

6

4

2

0

0

200

400

600

800

Curing temperature (°C) Fig. 1. Compressive strength variation with curing temperature of geopolymers.

289

strength probably due to the higher fly ash content with reactive silica at elevated temperatures. For all ternary mixtures, the strength reduction from 300 to 500 °C is much less than that from 80 to 300 °C which could be due to evaporation and the subsequent formation of geopolymeric gel. With increase in curing temperature from 500 to 800 °C, all mixtures gained in strength except UC2F6 which has the least strength at 800 °C. UC2F6 has higher organic content which is completely removed at this elevated temperature leaving behind voids and its lower red mud content would provide lesser alkalinity for geopolymerization resulting in a weakly bonded matrix. At elevated temperature of 800 °C, all three ternary mixtures showed almost same strength although their initial Si/Al and Na/Al varied from 1.17 to 1.48 and 0.47 to 0.73 respectively. It has been reported that high Si concentration need not necessarily ensure better aluminosilicate dissolution and geopolymerization due to the lower concentrations of reactive species in some mixtures [16]. Hence, for the current combination of CG, FA and RM, curing at high temperatures is ineffective, if not detrimental to the strength. From Fig. 1, it is clear that the ternary mixes are better than the binary samples in terms of strength at low temperature of curing (80 °C). Among all mixtures, CC4F4 with equal amounts of CG and FA (40% each), showed the least variation in strength at different curing temperatures. The X-ray diffractograms of the raw materials and the end products (Fig. 2) reveal the significant mineralogical alterations for the different proportions and curing temperatures. Among the two binary mixes, only UC2 shows the characteristic broad amorphous hump between 20 and 37° due to the uncalcined nature of the CG. For the ternary mixes, the glassy (amorphous) phase is seen in UC2F6 and CC2F6 due to the high FA content [28]. The ubiquitous quartz in the raw materials remains as the most dominant mineral for up to 500 °C while its intensity reduces at 800 °C. However, several sharp peaks of the innate minerals such as hematite, perovskite and rutile remain even at 800 °C for the various end products. The formation of nepheline, an aluminosilicate mineral, at 800 °C can be observed in all the end products and it contributes to the strength of the matrix. As reported from earlier literature, at temperatures higher than 600 °C, the sodalite and amorphous aluminosilicates in fly ash-based geopolymers transform into a denser structure of nepheline attaining its highest intensity at 800 °C [2]. This also accounts for the gain in strength with increase in curing temperature from 500 to 800 °C. The cumulative pore volume distribution and the log differential pore intrusion ( dV/dlogD vs. pore diameter) of the end products determined using MIP are shown in Figs. 3 and 4. Unlike gas adsorption experiments, the largest pores are filled first in MIP and hence the cumulative pore volume curve decreased with increase in the pore diameter. The total mercury intrusion volume increases with increase in curing temperature for all the samples. The air voids, capillary pores and gel pores contribute to the total porosity and with a rise in temperature, a general increase in the porosity is observed. UC2 shows the highest porosity at 500 °C due to the removal of the organic content, hence leaving pores in its wake. However, at elevated temperature of 800 °C, the total porosity decreases for UC2, UC2F6 and CC2F6 due to the densification of the matrix as discussed previously. It is a well-known fact that the porosity directly affects the strength and for all the ternary mixes, the porosity at 800 °C is comparable (26.7–27.2%), thus explaining their similar compressive strengths (3.8–4.1 MPa). For up to 500 °C, the pore widths (Fig. 4) are larger for the binary mixes when compared to their ternary counterparts except for CC4F4. Many samples, especially at higher curing temperatures, show bimodal distribution for the differential curves in the pore diameter ranges of 0.03–0.2 mm and 0.2–9 mm, which are indicative of the presence of meso- and macro-pores. The geopolymeric gels and the formation of minerals such as nepheline at elevated temperature

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(a) Raw materials HQ H H MQ M M Q

(b) UC2 Red mud Fly ash Calcined coal gangue

Q C Q Q

D

20

30

K 10

Uncalcined coal gangue 40

50

HH N H RN

10

S S

HH

500 °C

HH

300 °C

10

HH

HA 20

30

20

30

50

500 °C

H

300 °C

Q

10

20

30

M 40

80 °C 50

(f) CC4F4 NN

NH,M 800 °C 500 °C

300 °C Q

NH,M

800 °C

Q 500 °C Q 300 °C Q

80 °C 20

30

40

60

2 (°)

Q

10

60

800 °C

DH

60

Q

Intensity (a.u.)

50

Q

Intensity (a.u.)

Q

40

H

2 (°) (e) CC2F6 NN

80 °C

Q

H 80 °C 40

HH

(d) UC2F6 N N QNN H

800 °C

Q S

300 °C

2 (°)

P,H H

Q H R HQ

HH

A

60

500 °C

Q

Intensity (a.u.)

Intensity (a.u.)

H NN R N

H

Q A

2 (°) (c) CC2

800 °C

Q

Intensity (a.u.)

Intensity (a.u.)

290

50

60

2 (°)

80 °C 10

20

30

40

50

60

2 (°)

Fig. 2. XRD patterns of (a) raw materials and (b-f) end products (A: Anatase, C: Calcium sulphate anhydrite, D: Dolomite, H: Hematite, M: Mullite, N: Nepheline, P: Perovskite, Q: Quartz, R: Rutile, S: Sodalite).

can block the percolated capillary pores leading to two different pore systems. For CC4F4, having equal amounts of CG and FA exhibit bimodal distribution for at all temperatures, albeit at a smaller scale for the 80 °C cured sample. In the bimodal log differential curves, the left peak represents the gel pore structure while the right peak is associated with the capillary pore structure since the pore diameter increases towards the right. Interestingly, the pore diameter generally increases with a rise in temperature due to the loss of capillary water, preventing further geopolymerization while the pore width range simultaneously broadens. Due to the pore shapes and closed pores (ink bottle effect), the MIP results may not represent the precise porosity. But it is a preferred technique due to its ability to analyse a wide range of pores and complementing it with BET analysis can help to understand the pore structure better. Fig. 5 shows the BET surface area characteristics of the geopolymers based on binary and ternary raw materials at various curing temperatures. The surface area decreased with rise in curing temperature primarily due to the loss of physically and chemically bound water. This results in large voids

which make the specimen susceptible to mechanical failure [24]. On the contrary, the formation of aluminosilicate gels and agglomeration of particles can also result in reduction of the pores. This explains the anomalous strength behavior of different samples upon curing at elevated temperatures. However, CC2 is the only outlier showing lower surface area at 80 °C which could be due to an early weak gel formation. At 80 °C curing, the physically bonded water, which contributes to 70% of the total reaction water, is lost thus affecting the microporosity. Furthermore, at different curing temperatures, the pore contribution, pore size and pore network formed are different and will ultimately influence the final strength of specimens [24]. As seen from the compressive strength results and the pore structural characteristics, higher curing temperatures do not necessarily impart higher strength. This is due to the fact that the water evaporates quickly at elevated temperatures before sufficient reaction has taken place, leaving behind large sized void spaces (as compared to smaller micro and mesopores). This also explains the general trend of lower specific surface area at higher

291

0.30

(a) UC2

Total Porosity (%) 80 °C 16.2 300 °C 21.2 500 °C 31.3 800 °C 23.3

0.25 0.20 0.15 0.10 0.05 0.00

0.1

1

10

Cumulative pore volume (cc/g)

Cumulative pore volume (cc/g)

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0.30

0.20 0.15 0.10 0.05

0.1

(c) UC2F6

Total Porosity (%) 80 °C 22.1 300 °C 27.4 500 °C 29.5 800 °C 27.2

0.25 0.20 0.15 0.10 0.05 0.00

0.1

1

10

100

Cumulative pore volume (cc/g)

Pore diameter (µm) 0.30

(e) CC4F4

1

10

100

Pore diameter (µm)

Cumulative pore volume (cc/g)

Cumulative pore volume (cc/g)

Pore diameter (µm) 0.30

Total Porosity (%) 80 °C 8.7 300 °C 18.9 500 °C 22.3 800 °C 29.9

0.25

0.00

100

(b) CC2

0.30

(d) CC2F6

Total Porosity (%) 80 °C 20.3 300 °C 24.5 500 °C 28.8 800 °C 26.7

0.25 0.20 0.15 0.10 0.05 0.00

0.1

1

10

100

Pore diameter (µm)

Total Porosity (%) 80 °C 21.3 300 °C 24.5 500 °C 23.7 800 °C 27.0

0.25 0.20 0.15 0.10 0.05 0.00

0.1

1

10

100

Pore diameter (µm) Fig. 3. Cumulative pore volume distribution and total porosity of the binary and ternary products.

curing temperatures which is due to the presence of voids rather than micropores. The micropores and mesopores can be better understood using BET analysis and the nitrogen sorption isotherms for the five mixes at various curing temperature are shown in Fig. 6. The isotherms are Type IV as per International Union of Pure and Applied Chemistry (IUPAC) classification and are typical of mesoporous sorbents (pore width between 2 nm and 50 nm) [29]. The initial portion of the curve is associated with monolayer-multilayer sorption similar to Type II isotherm. The characteristic hysteresis loop in the multilayer range of the isotherm is linked to capillary condensation in the mesopores and there is a limiting uptake near unity of the relative pressure. The H1 type of hysteresis is seen for all the samples with the adsorption and desorption branches remaining almost vertical over a significant range of p/p0. This is often associated with porous materials made up of rigidly joined particles or agglomerates [29]. The silicate activated geopolymers have a wide variety of chemicals including impurities held together by the

geopolymeric gels. All end products in the binary and ternary mixes show a decrease in sorption with increase in temperature except that for CC2 cured at 80 °C. This anomaly in CC2 is be due to the negligible organic content in the binary mix after precalcination of CG. There is a large disparity in the sorption values of the 80 °C cured specimens when compared to those cured at higher temperatures (300–800 °C), especially for CC4F4. The steep curve near unity (p/p0 = 1) for almost all the specimens indicate the presence of large macropores or voids due to the evaporation of water [28]. The 80 °C cured samples show very high nitrogen sorption near unity (p/p0 = 1) due to the large surface area as also seen from their BET values in Fig. 5 with the exception of CC2. The binary specimens (Fig. 6) contain a well-connected pore network as compared to their ternary counterparts which have partially replaced red mud by FA. Thus, FA plays a major role in the development of the pore structure of the silica activated geopolymers. All specimens cured at 800 °C show very low surface area and hence, the low sorption values. However, the MIP results clearly

0.4

(a) UC2

80 °C 300 °C 500 °C 800 °C

0.3

0.2

0.1

0.0 0.01

0.1

1

10

Log differential intrusion (cc/g)

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Log differential intrusion (cc/g)

292

0.4

(b) CC2

0.3

0.2

0.1

0.0

100

0.01

(c) UC2F6

80 °C 300 °C 500 °C 800 °C

0.6

0.4

0.2

0.0 0.01

0.1

1

10

100

Log differential intrusion (cc/g)

Pore diameter (µm) 0.5

(e) CC4F4

0.1

1

10

100

Pore diameter (µm)

Log differential intrusion (cc/g)

Log differential intrusion (cc/g)

Pore diameter (µm) 0.8

80 °C 300 °C 500 °C 800 °C

0.6

(d) CC2F6

80 °C 300 °C 500 °C 800 °C

0.4

0.2

0.0 0.01

0.1

1

10

100

Pore diameter (µm) 80 °C 300 °C 500 °C 800 °C

0.4 0.3 0.2 0.1 0.0 0.01

0.1

1

10

100

Pore diameter (µm) Fig. 4. Log differential pore volume distribution of the binary and ternary products.

showed the increase in macropores at 800 °C (refer Fig. 4) which was not detected using BET analysis. Furthermore, this feature of reduced pores do not necessarily result in higher compressive strength (see Fig. 1) due to the presence of closed pores (inkbottle phenomenon). The sample with 40% calcined CG resulted in lower surface area and gas sorption as well as better strength compared to their ternary counterparts with 20% CG indicating that calcined CG aids the strength development. The phase transformation of the industrial byproducts upon silica activation can be identified from the FTIR spectra in Fig. 7. In Fig. 7a, the stretching vibrations of inner surface OH groups in kaolinite present in CG are responsible for the strong modes at 3695–3697 cm 1 [30]. The stretching vibration at 1097 and the shoulder at 746 cm 1 in FA are due to the presence of quartz [9]. The bands at 997 cm 1 are associated with the asymmetric stretching vibrations of T–O (T = Si or Al) in the raw RM [31]. The absorptions seen in the region 881 to 650 cm 1 were associated with

different vibrational modes of gibbsite-like sheet of the octahedral crystals with Al3+ ions [32]. The broad, overlapping bands in the end products indicate the amorphous phases, notable with high curing temperatures of 300–800 °C. In UC2 and UC2F6, the strong absorption bands of inner hydroxyl groups at 3695–3697 cm 1 vanished at higher temperatures of 500 and 800 °C. These bands are completely absent in the end products from other mixes due to the removal of hydroxyl groups during pre-calcination of CG. The strong absorption bands at approximately 995 cm 1 for UC2 and CC2 are characteristic of silicate activated geopolymers with very low Na/Al of 0.25 [16]. A very strong band is visible near this wavenumber for UC2 at 800 °C and CC2 at 500 °C. The peak at 3620 cm 1 is characteristic of the stretching vibration of the inner surface –OH groups of the AlVI–O octahedral [33]. The asymmetric narrow band at 3653 cm 1 in the raw CG is attributed to the stretching vibration of Ca–OH from Ca(OH)2 [20]. The bands at 1461, 1467 and

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1473 cm 1 in the three ternary mixes are characteristic of atmospheric carbonation and shows a shift in CC4F4 upon increase in curing temperature [34]. The absorption peaks at 1049, 1039 and 1033 cm 1 can be associated with different stretching vibrations of Si–O–Si in SiIV–O tetrahedral [22]. Geopolymers are known for the characteristic strong band at 1200–900 cm 1 associated with asymmetric stretching vibrations of T–O–Si formed due to TO4, where T = Al, Si [35]. The absorption bands 881–870 cm 1 related to the Si–OH bending disappeared at high temperature of 800 °C due to the decomposition of the mineral phase into amorphous phase. The vibrations near 742 to 694 cm 1 are due to the asymmetric and symmetric vibrations of Si–O–T (i.e. Si–O–Al and Si– O–Si) bonds of the geopolymeric gels of AlO4 and SiO4 tetrahedra [36]. All the end products showed bending vibrations of Si–O–Si and O–Si–O at 470–460 cm 1 [22]. The microstructural development of the geopolymer takes place due to the reaction between the aluminosilicate and alkali-based precursors at different temperatures. For the sake of brevity, selected SEM images of the end products are shown in Fig. 8. Table 3 shows the Na/Al and Si/Al ratios derived from the EDS spec-

BET surface area (m2/g)

25 UC2 CC2 UC2F6 CC2F6 CC4F4

20

15

10

5

0 0

200

400

600

800

Curing temperature (°C) Fig. 5. BET surface area of the geopolymers at different curing temperatures.

40

20

10

0

20

10

(d) CC2F6

80

60

40

20

0

30

0

(c) UC2F6

Quantity adsorbed (cm3/g)

Quantity adsorbed (cm3/g)

(b) CC2

80 °C 300 °C 500 °C 800 °C

Quantity adsorbed (cm3/g)

Quantity adsorbed (cm3/g)

(a) UC2 30

80

60

40

20

0

(e) CC4F4

0

0.2

0.4

0.6

0.8

Quantity adsorbed (cm3/g)

Relative pressure (p/p0) 60

40

20

0 0

0.2

0.4

0.6

0.8

1

Relative pressure (p/p0) Fig. 6. Nitrogen sorption isotherms of the binary and ternary end products.

1

294

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(a) Raw materials

611 779 561 692 594 796 463 430

Calcined CG 1626

Transmittance (a.u.)

3427

Raw CG

3653

(b) UC2

800°C 1438 615 702 576

1091

3695

694 758 1435 937 798 779 1033 538 912 1165 470 1116 1008 875 1639 1413 468

3620

RM 3414

FA

997

1626

3425

746 561475

439

500°C 300°C 881

80°C 3697 3620

1633 1452

3429

659

698 543 879 464 997

1097

(c) CC2

702 561

Transmittance (a.u.)

800°C

(d) UC2F6

800°C

723 515

500°C

500°C

300°C 1448

453

459

300°C

80°C

696

80°C

1635 1458

881

3421

615466 453

993

(e) CC2F6

750 777 798 549 468 881

1631 1467

3697 3622 3431

1033 1010

800°C

515

(f) CC4F4

Transmittance (a.u.)

715 500°C

515 800°C 500°C

455

449 300°C

1012 777

300°C

462 742 796 563 455

617 694 80°C

464 80°C 3415

4000

3500

2972

3000

1631 1461

1049

881

451

740 777 565 798 464 881 451

1635 1473

3421

1039 1003

1500

1000 -1

Wavenumber (cm )

500

3500

3000

1500

1000

500

-1

Wavenumber (cm )

Fig. 7. FTIR spectra of the raw materials and end products.

tra S1 to S4. Both UC2 and CC2 possessing the higher average strength among all the mixes are shown in Fig. 8a-d and UC2F6, which showed the highest strength among the ternary mixes at low temperature is shown in Fig. 8e and f corresponding to 80 and 800 °C respectively. The significant morphological features include solidified aluminosilicate gels formed in geopolymers (Fig. 8b and 5d) and FA particles (Fig. 8e and 5f). These unreacted and partially reacted fly ash of varying sizes, which are embedded in the matrix also act as filler materials thereby enhancing the strength. The loosely bonded structures of CC2 cured at 80 °C is responsible for its reduced strength when compared to that of CC2 cured at 800 °C (see Fig. 8c and d). On the other hand, CC2, cured at 800 °C with the highest compressive strength of 7.3 MPa (see Fig. 1), shows homogeneous surface with void spaces (Fig. 8d). These cementitious gels are responsible for the reduction in the surface area resulting in solidified and well-bonded structure. Interestingly, the uniform microstructure bears striking similarity to the adequately cured geopolymers synthesized from rice husk ash

[10]. Furthermore, the Na/Al ratio of 0.65 in S2 of Fig. 8d is below the previously reported value of near unity for high strength geopolymers [17]. Hence, it is to be inferred that this lower value of Na/Al can produce geopolymers with strength >7 MPa and may not necessarily be indicative of very low strength. When the CG-RM based end products are compared to those of CG-FA-RM, the change in morphology due to the presence of spherical fly ash particles in UC2F6 is evident from the SEM images Fig. 8e and 5f. UC2F6 cured at 80 °C showed a dense matrix with few unreacted FA particles (Fig. 8e) while at 800 °C, it shows homogenous cementitious structures with a large number of voids (Fig. 8f) formed by evaporation of water, removal of organic content and subsequent trapping of these air pockets in the solidified aluminosilicate matrix. These voids are also responsible for the reduction in strength for the ternary mixes at elevated temperatures. Spectra S3 and S4 revealed that Na/Al of 1.47 and 0.58 confirm with previous studies of red-mud based geopolymers with higher Na/Al and higher UCS values [17]. It can be noticed from (Fig. 8e and f) that the complete dissolution of FA particles is not

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Fig. 8. SEM images of major end products: (a) UC2-500, (b) UC2-800, (c) CC2-80, (d) CC2-800, (e) UC2F6-80, (f) UC2F6-800.

Table 3 Si/Al and Na/Al ratios from EDS spectra. Spectrum

Si

Al

Na

O

C

Si/Al

Na/Al

S1 S2 S3 S4

10.02 29.16 17.92 14.13

8.98 7.48 5.94 9.38

9.87 4.85 8.76 5.43

40.63 28.68 38.50 35.56

6.47 29.83 28.89 35.49

1.12 3.90 3.02 1.51

1.10 0.65 1.47 0.58

achieved even at low or very high temperatures, which could be due to the lack of adequate alkali in the mixture. The study shows the valorization of coal gangue, red mud and fly ash into geopolymers using silicate activation. The maximum strength of 6.58 MPa for a combination of uncalcined CG, FA and RM, the least energy consuming mix design, make it a potential road base material [37] and as a soil stabilizer [38].

4. Summary and conclusions This study examined the synthesis of geopolymers using three industrial by-products, viz., coal gangue, fly ash and red mud cured at low and elevated temperatures. The following are the main conclusions:

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 The coal gangue and red mud based geopolymer showed a general increase in strength with increase in curing temperature.  At very high curing temperature (800 °C), the binary specimens of coal gangue and red mud showed much high strength values (up to 7.3 MPa) when compared to ternary mixes of coal gangue, fly ash and red mud.  The geopolymer synthesized by the combination of coal gangue, fly ash and red mud offered higher strength values (up to 5.7 MPa) at lower curing temperature of 80 °C.  Higher temperatures could be detrimental to the strength development of the CG-FA-RM based geopolymers due to the formation of large voids left behind by quick evaporation and removal of organic matter.  The synthesized geopolymers contained cementitious gels which are responsible for the tightly packed structures with reduced surface area, pore volume and average pore sizes.  Loosely bound unreacted fly ash reduced the overall strength of the geopolymer in the ternary mixtures.  The synthesized geopolymers have direct implications on sustainable development by valorizing industrial wastes into geopolymers with potential applications as road base material and soil stabilizers. Future studies will be conducted with higher Na/Al ratios and addition of NaOH in order to improve the compressive strength of coal gangue, fly ash and red mud based geopolymers. Conflicts of interest This research work has no conflicts of interest. Acknowledgements This research was funded by National Natural Science Foundation of China (Grant No. 51323014 and 51579132), Ministry of Education (Grant No. 2015THZ02-2-20161080101 and 2015THZ01-1-20161080079), and the State Key Laboratory of Hydro-Science and Engineering (SKLHSE-2019-D-01). References [1] J.L. Provis, G.C. Lukey, J.S.J. Van Deventer, Do geopolymers actually contain nanocrystalline zeolites? a reexamination of existing results, Chem. Mater. 17 (2005) 3075–3085. [2] X.Y. Zhuang, L. Chen, S. Komarneni, C.H. Zhou, D.S. Tong, H.M. Yang, W.H. Yu, H. Wang, Fly ash-based geopolymer: Clean production, properties and applications, J. Clean. Prod. 125 (2016) 253–267. [3] A. Arulrajah, T.-A. Kua, S. Horpibulsuk, C. Phetchuay, Strength and microstructure evaluation of recycled glass-fly ash geopolymer as lowcarbon masonry units, Construct. Build. Mater. 114 (2016) 400–406. [4] S. Selmani, A. Sdiri, S. Bouaziz, E. Joussein, S. Rossignol, Effects of metakaolin addition on geopolymer prepared from natural kaolinitic clay, Appl. Clay Sci. 146 (2017) 457–467. [5] D.L.Y. Kong, J.G. Sanjayan, K. Sagoe-Crentsil, Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures, Cem. Concr. Res. 37 (2007) 1583–1589. [6] H. Xu, J.S.J. Van Deventer, The geopolymerisation of alumino-silicate minerals, Int. J. Miner. Process. 59 (2000) 247–266. [7] N. Toniolo, A.R. Boccaccini, Fly ash-based geopolymers containing added silicate waste. A review, Ceram. Int. 43 (2017) 14545–14551. [8] M. Zhang, M. Zhao, G. Zhang, D. Mann, K. Lumsden, M. Tao, Durability of red mud-fly ash based geopolymer and leaching behavior of heavy metals in sulfuric acid solutions and deionized water, Construct. Build. Mater. 124 (2016) 373–382. [9] I. Ismail, S.A. Bernal, J.L. Provis, R. San Nicolas, S. Hamdan, J.S.J. Van Deventer, Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash, Cem. Concr. Compos. 45 (2014) 125–135. [10] P. Sturm, G.J.G. Gluth, H.J.H. Brouwers, H. Kühne, Synthesizing one-part geopolymers from rice husk ash, Construct. Build. Mater. 124 (2016) 961–966.

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