Minerals Engineering 144 (2019) 106026

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Minerals Engineering

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One-part geopolymers from residues – Effect of thermal treatment on three different T ⁎ Priyadharshini Perumal , Katri Piekkari, Harisankar Sreenivasan, Paivo Kinnunen, Mirja Illikainen

Fibre and Particle Engineering Research Unit, University of Oulu, Pentti Kaiteran Katu 1, 90014 Oulu, Finland

ARTICLE INFO ABSTRACT

Keywords: Use of mine tailings as an aluminosilicate precursor in alkali activation is becoming an interesting alternative to Mine tailings manage the high-volume of generated from mining industries. However, very few tailings have so far been Mineralogy studied for their mineralogical properties and alkali activation potential. This study aims at understanding the ability of mine tailings from phosphate, kaolinite and lithium mines for their efficient participation in alkali Reactivity activation. Biotite, muscovite, kaolinite, albite, and quartz were found to be the major minerals present in them. Alkali activation The impact of variation in mineralogy on silica and alumina solubility of these tailings was analyzed. The solubility was found to be high in impure kaolinite compared to the other two. Effectiveness of thermal treat- ment (750 °C and 900 °C) on improving the reactivity of these tailings in alkaline condition was also investigated. It was observed from the results that the effect of thermal treatment on the crystalline structure and solubility of an aluminosilicate material mainly depends on the mineral structure of the material, as well as the treatment temperature. Interestingly, thermal treatment reduced the solubility of lithium tailings with albite and quartz mineral. Effort has been made to relate the strength attained by alkali activation of mine tailings to their so- lubility values. However, despite of the higher solubility offered by impure kaolinite, phosphate tailings gives the maximum strength improvement by 62%. This can be due to the presence of calcium compounds in phosphate tailings that resulted in additional hydration products.

1. Introduction availability of aluminum or silicon ions from the precursor can be compensated by adding metakaolin (Provis and Van Deventer, 2013, p. Geopolymers are materials obtained through alkali-activation of 107). Especially when using waste materials as aluminosilicate sources, aluminate and silicate rich precursors. The aluminosilicate precursor thorough monitoring of the raw material properties is imperative. For contains the matrix-forming components that polymerize to form the example, the elemental compositions, alkalinity and granulometry can geopolymer structure, and it can be any aluminosilicate material that be used to evaluate the suitability of a material for geopolymerization. contains soluble silicate and aluminate (Panagiotopoulou et al., 2007; (Davidovits, 2015, pp. 417–422) Van Jaarsveld et al., 1997). For applications, where good mechanical Various natural minerals, such as calcined kaolinite, albite or vol- and chemical properties are needed, the suitable molar Si/Al ratio is canic ashes have been used in geopolymer synthesis (Provis et al., 1–5(Provis and Van Deventer, 2013, p. 95). A lower ratio results in a 2015). Experiments have revealed that some aluminosilicate minerals more zeolitic structure, while with a higher ratio silicate derivative can form geopolymers on their own (i.e. stibilite, sodalite), while other, dominates the matrix (Wan et al., 2017). The most common alumino- less reactive minerals require the supplementary sources (such as me- silicate sources are various industrial byproducts and residues (i.e. fly takaolinite) in order to form proper geopolymer structures during al- ash, slag, mine tailings i.e. from copper, iron and gold ores), natural kali-activation (Xu and Van Deventer, 2000). The group of minerals sources (i.e. sedimentary diatomaceous earth, volcanic glass) and ac- that appears as the most ideal for geopolymerization is the 1:1 layer tivated aluminosilicates (calcined clays like metakaolin) (Provis and lattice aluminosilicates (MacKenzie et al., 2007). Due to the complexity Van Deventer, 2013, pp. 103–112). Sometimes reactive fillers or setting of the dependence between solubility, mineral chemistry and structure, additives are needed to improve the properties of the product material it is not possible to determine the suitability of a given mineral for (Khale and Chaudhary, 2007). For example, too low solubility or geopolymerization theoretically, but experimental validation is always

⁎ Corresponding author at: Inorganic Binders Group, Fibre and Particle Engineering Research Unit, University of Oulu, Pentti Kaiteran Katu 1, 90014 Oulu, Finland. E-mail address: priyadharshini.perumal@oulu.fi (P. Perumal). https://doi.org/10.1016/j.mineng.2019.106026 Received 25 February 2019; Received in revised form 4 September 2019; Accepted 13 September 2019 Available online 27 September 2019 0892-6875/ © 2019 Fibre and Particle Engineering Research Unit, University of Oulu, Finland. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). P. Perumal, et al. Minerals Engineering 144 (2019) 106026 needed (Xu and Van Deventer, 2000). Mine tailings are the mineral residues gained from the refining process at the mining sites. It usually has a fine particle size (> 100 μm) and is stored in dammed ponds. Mine tailings properties (like chemical composition) depend entirely on the mineralogy of the mining site and the used refining method. The tailings compose of a mixture of different minerals, which makes the characterization and utilization of a po- tential material even more difficult (Kinnunen et al., 2018). Due to their varying chemical compositions and commonly high Si/Al ratios, mine tailings are challenging and unpredictable raw materials (Wan et al., 2017). The parameters controlling the material properties are difficult to determine and understanding in this direction is given importance in recent years (Provis et al., 2015). The particle size and chemical dis- tributions of the tailings are non-evenly distributed in the pond due to sedimentation, making the utilization further difficult. Pumping the waste slurry into the pond through a pipeline leads to the larger sized particles settling close to the discharge pipe as the finer material flows further. Industrial processes usually demand homogenous raw material supply to maintain a uniform quality of the product. In general, low alkali-reactivity of the tailings results in the primary problem and raises a need for various pretreatments (Kinnunen et al., 2018). The heterogeneous nature of mine tailings is a challenge regarding their use as raw materials. Mine tailings, as well as other reused , Steatitizated sedimentary carbonite in metadiabase, quartzite and chlorite (Mica) Geology of mining site Glimmerite (phlogopite, richterite andgneiss apatite) bedrock and carbonatite intrusion in granite require more characterization than virgin raw materials (Duxson et al., schist, kaolinite Spodumene pegmatite seams inand epiclastic gneiss, sedimento and and volcanogenic volcanogenic uralite mica and schists plagioclase porphyrite 2007). Furthermore, due to the varying chemical compositions between different batches, robustness of the industrial process becomes im- perative. Geopolymerization process is relatively robust and insensitive to variation of the amounts of the non-reactive components. The most studied mine tailings are so far tungsten mining side streams, copper mine tailings and oil sand tailings (Kinnunen et al., 2018). The main aim of this study is to screen the alkali activation potential for a set of mine tailings samples from three different sources viz., phosphate, kaolinite and lithium mining locations. Solubility tests are used to measure the Si and Al solubility of the samples, and alkali activation of the tailings are evaluated by preparing the paste samples. Thermal treatment is employed as a method to improve mineral solubility in alkaline condition. It is observed from the results that the effect of thermal treatment on the crystalline structure and solubility of an aluminosilicate material depends on the mineral structure of the ma- terial, as well as the treatment temperature.

2. Materials and methodology

2.1. Materials Main components Muscovite (42%), quartz (27%), kaolinite (20.7%) Phlogopite (63%), calcite, (14%),(1.4%), dolomite actinolite (5.9%), (1.0%), potassium albite feldspar (0.97%) spodumene (2.5%), plagioclase (other than albite) (1.2%)

The origin and main mineral components of the mine tailings stu- died in this work are listed in Table 1. The elemental compositions of the mineral components (which constitute the mine tailings) are listed in Table 2. The samples contained varying moisture contents and par- ticle size distributions. The overall elemental compositions of mine tailings based on XRF analysis are listed in Table 3. According to Table 3, samples, impure kaolinite (K) and Lithium tailings (L) pos- Mined product sessed high silica and alumina contents when compared to Phosphate biotite tailings (P). This observation is in agreement with their mineralogical composition as in Table 1. On the other hand, sample P contained re- latively high amounts of MgO, and Fe2O3 (originating from the mica minerals) and CaO (originating from CaCO3). All the samples contained significant portions of potassium oxide (K2O), and L had a high con- centration of sodium oxide (Na2O) originating from albite.

2.2. Methods

2.2.1. Pre-treatment 2.2.1.1. Drying and grinding. The crude samples were placed in aluminum trays and were dried in oven at 100 °C for 24 h. Fig. 1 P Siilinjärvi Siilinjärvi, Finland Phosphate (apatite), lime, Sample Mine Location K Pihlajavaara Puolanka, Finland Kaolinite, talc L Länttä Ullava, Finland Lithium (spodumene) Albite (39%), quartz (36%), microcline (15.9%), muscovite (6.4%),

shows the images of samples after drying. Sample K had initially a very Table 1 Major mineral composition of the studied mine tailings.

2 P. Perumal, et al. Minerals Engineering 144 (2019) 106026

Table 2 Hence, the results represented partly calcined samples, and the possible Elemental composition of various mineral components of the samples. reactions during the bead preparation posed a risk of error in the

Mineral Composition Remark calculations. Thermogravimetric analysis (TGA) was carried out to observe changes in the material during the heating process, and hence Calcite CaCO3 - exposed the possible reactions. The equipment used was PrepASH 129 Phlogopite KMg3(AlSi3O10)(OH)2 Mica mineral (Precisa, Switzerland), and the temperature ranged between room Muscovite KAl (AlSi O )(OH) Mica mineral 2 3 10 2 temperature (23 °C) and 950 °C. The weight of the analyzed Quartz SiO2 -

Kaolinite Al2(Si2O5)(OH)4 - specimens were maintained below 7.7 g. XRF results were corrected Albite Na(AlSi3O8) Feldspar mineral by adjusting the results for a changed total mass. It was assumed that no Microcline K(AlSi3O8) Feldspar mineral measured components, especially Si or Al, was lost during the heating. The equation for calculating corrected values is presented as Eq. (1).

Table 3 LOI m(%)= ⎛ 1 − ⎞ ∗ mm (%) Composition of mine tailings samples expressed as oxides according to XRF (%). ⎝ 100 ⎠ (1)

Sample SiO2 Al2O3 MgO CaO Fe2O3 K2ONa2O Other* LOI where m (%) is the corrected weight percent of a component (%), LOI

P 32.99 7.09 17.27 12.92 7.99 5.53 0.71 1.49 14.01 (loss on ignition) is the percent of sample mass lost in TGA and mm is K 72.52 16.43 0.82 0.05 1.90 3.05 0.08 0.73 4.42 the measured weight percent of a component. L 78.44 12.57 0.14 0.30 0.51 2.80 4.44 0.20 0.59 2.2.2.2. XRD. XRD analysis was performed to observe changes in the *Includes other minor oxides present in the tailings (MnO, P O and SO ). 2 5 3 crystalline structure of samples during thermal treatment. Therefore, the analysis was carried out for each sample prepared in different high moisture content and behaved like clay, but upon drying, it calcination temperatures. Rigaku SmartLab (9 kW) with 40 kV and transformed into one solid, with agglomerated particles. After drying, 135 mA radiations was used, and the crystalline structures were the samples were ground in a vibratory disk mill in order to standardize identified from the data by comparing the diffraction patterns on the the particle size distributions. The grinding was performed at 1500 rpm ICDD database (the International Center for Diffraction Data). Cu-Kβ for 3 min, in batches of 20 g. The vibratory disk mill Retsch RS 200 was radiation was used with step interval of 0.02° per step and scan rate of used with a hardened steel grinding set. 4°/min for a scan range of 5–110° (2θ). 2.2.1.2. Calcination. Batches consisting of 30 g of each sample were placed in corundum crucibles, calcined at 750 °C and 900 °C for 2 h and 2.2.2.3. Alkaline solubility test. The solubility tests were carried out left to cool down in a Controller P320 furnace (Nabertherm, Germany). with 6 M NaOH solution. For each test 0.5 g of sample was mixed with These samples were used for XRD analysis and solubility tests. Later 20 g of alkali solvent. The mixture was then shaken in a shaker mixer fi calcination of new batches of 100 g of each sample for alkali activation (KS 260 basic, IKA) at 150 rpm for 24 h. The solution was ltered were carried out with the same method except for a holding time of 3 h. through a polypropylene membrane with 0.45 µm pore size. The pH of Several hypotheses were involved while considering the samples for the collected solution was set below 2 with 6 M nitric acid solution to calcination. Sample K was presupposed to be the most promising prevent precipitation. The dissolved Si and Al amounts were sample of the series, since kaolinite is known to form highly reactive determined from the collected solution by inductively coupled plasma optical emission spectrometry (ICP-OES) according to standard SFS-EN metakaolin (Al2Si2O7) during calcination (Sabir et al., 2001). Also sample P was assumed to give a strong response to the thermal ISO 11885:2009. treatment, since CaCO3 is known to form lime (CaO) when heated, releasing CO2 in the process (Rodriguez-Navarro et al., 2009). Sample L 2.2.3. Geopolymerization was supposed to be the most inert of the series. Quartz and the feldspar Geopolymers involving the alumino-silicate precursor and activator components usually appear thermally stable under the calcination solution can be referred to as two-part geopolymers. One-part geopo- conditions (Deer et al., 1963, pp. 12, 180; Feng et al., 2012; Heaney lymers are prepared by mixing the solid activators with the alumino- et al., 1994, p. 314). Due to its highly crystalline nature, quartz is silicate precursor and can be activated with the addition of water. known to have a relatively high chemical stability compared to other Hence, one-part geopolymers are ‘just-add-water’ products and suitable silicate minerals (Loughnan, 1969, p. 53). for situations, where the handling of liquid alkali activators would be problematic. In this study one-part geopolymerization process was

2.2.2. Characterization of treated materials adopted. Mine tailings and alkali activators (Na2SiO3 + NaOH or 2.2.2.1. XRF and TGA. X-ray fluorescence analysis (XRF) was done to Na2SiO3) were co-grinded in the ratio as mentioned in Table 4. The determine the elemental compositions. The analysis was made using paste samples were prepared by adding measured quantity of water. fused bead test, where the sample was melted to form glass at 1150 °C. Thinky ARE-250 planetary mixer was employed for 90 s at 1000 rpm

Fig. 1. Mine tailings after drying from (a) phosphate mine, (b) kaolinite mine and (c) lithium mine.

3 P. Perumal, et al. Minerals Engineering 144 (2019) 106026

Table 4 not expected, since the mineral mainly forms in temperatures higher Mix proportion of geopolymer paste. than 1000 °C (Meinhold et al., 1985). On the other hand, some amor- phous intermediate phases are known to form during calcination of Series Mine tailings (g) Na2SiO3 (g) NaOH (g) Solid/liquid ratio kaolinite at higher temperatures (Meinhold et al., 1985). SN 63.41 14.69 1.91 0.25 The XRD graphs for sample L are presented in Fig. 4. The non-cal- – S 63.41 14.69 0.25 cined sample contains quartz, albite and microcline, which corresponds to the initial mineralogical data. The XRD does not show any signs of major phase changes during calcination at 750 °C, but as the treatment speed to obtain a homogeneous mix. Water content was kept constant temperature is raised to 900 °C, albite decomposes partially. There is a throughout the mixtures except for non-calcined (NC) P tailings which reduction in the intensity of Albite peak. The transformation of albite needed 5.5 g extra water to maintain the minimum workability for can be assumed to have occurred due to chemical reaction with other making the specimens. Isothermal calorimetry to understand the re- components, since pure albite is known to be thermally stable in tem- activity of different mine tailings in alkali activation was performed peratures below 1000 °C (Feng et al., 2012). No new crystalline phases with an 8-channel Tam Air isothermal calorimeter (TA Instruments, emerge during the calcination, which indicates that the new compo- USA). nents formed during the albite transformation are amorphous. The samples for each mix were molded in to six cylindrical (25 mm diameter and 25 mm height) plastic molds and vibrated with the 3.2. Effect of pre-treatment on solubility of Si and Al Vortex-Genie 2 vortex mixer (Scientific Industries, Inc., USA) for 10 s to remove the air captured inside them. The molds with the fresh alkali Relative solubility was calculated based on the ICP-OES results and activated paste were placed in sealed plastic bags and cured at 40 °C for XRF analysis, using formula (2). 24 h. After demolding, the specimens were stored at room temperature until testing. Three specimens of each mix were sulphur capped and m cV∗ s (%) ==d dd used to measure the compressive strength. Zwick 100 testing machine ms wmss∗ (2) (Zwick Roell Group, Germany) with the maximum load capacity of 100 where s – relative solubility (%), m – dissolved mass of the component kN and a cross head speed of 3 mm/min was employed. Water ab- d (mg), m – component mass in the solid sample before dissolution (mg), sorption was measured by immersing the specimens in water bath for s c – component concentration in solvent after dissolution (mg/l), V – 24 h in room temperature (23 ± 2 °C). After wiping with wet cloth, d d volume of the solution (l), w – mass percentage of the component in saturated mass of the specimen was measured (W ). The specimens s sat the solid sample and m – total sample mass before dissolution. were then placed in oven at 105 ± 2 °C for 24 h and the oven dried s Fig. 5(a) shows the relative solubility of Si, and Fig. 5(b) shows the mass was measured (W ). Water absorption was then calculated in dry relative solubility of Al of non-calcined and calcined mine tailings percentage by, (result is expressed as % of element released with respect to its initial WWsat− dry content in the sample before solubility test). During the tests, the so- Water absorption(%) = × 100 Wdry (2) lubility of Al from sample P increases drastically with both treatment temperatures (Fig. 5b). The Si solubility improves during calcination at 750 °C but reduces as the temperature increased further to 900 °C 3. Results and discussion (Fig. 5a). Sample K has the highest solubility among the studied sam- ples. An optimal calcination temperature considering both Si and Al 3.1. Effect of pre-treatment on mineralogy solubility exists at 750 °C by a large margin compared to the other calcination stages. Sample L has a low initial solubility, which further The XRD results for sample P are presented in Fig. 2. According to decreases as the calcination temperature was raised. the analysis, the material initially contains phlogopite, calcite, tremo- For all samples, the relative solubility of Al exceeds Si, but due to lite and dolomite. These results are in line with the initial mineralogical the large content of Si compared to Al in all the samples, the total data. Phlogopite is a magnesium rich biotite mineral (Deer et al., 1962). dissolved amount of Si exceeds Al in almost all cases. For non-calcined

As supposed, calcite (CaCO3) decomposes forming CaO when the sample P, elemental solubility of both Si and Al are very low, and sample is calcined at 750 °C. It can be noted that dolomite was also probably not sufficient for a successful geopolymerization. The in- decomposed. Furthermore, the data shows an amorphous hump that creasing solubility of sample P after the first calcination at 750 °C can be does not appear before the calcination. When the sample is calcined at explained with the reduction of crystalline phases as detected with XRD 900 °C, CaO disappears and phases like quartz, cordierite, crystalline (Fig. 2). The decrease in solubility of Si after calcination at 900 °C can silicates, are formed. The formation of CaO and decrease of crystallinity be explained by the formation of inert Quartz and crystalline silicates during the first calcination stage indicates an increase in the reactivity that stabilize silicon ions. However, unlike Si solubility, Al solubility of of the material. On the other hand, the new crystalline quartz and si- sample P increases slightly after calcination at 900 °C. This could mean licate phases formed during the second calcination stage are assumed to that during the thermal treatment, some aluminum containing amor- be stable (Loughnan, 1969, p. 53), indicating a decrease in reactivity. phous phases were formed leading to its increased solubility. Considering the phase changes during the calcination, sample P is the Sample K had high silicate and aluminate contents (73% and 16% most reactive of the three studied samples. respectively) and with the exceptionally high Al solubility of the sample XRD measurements reveal that sample K consists of quartz, mus- calcined at 750 °C, the total dissolved Al is highest among all the covite and kaolinite, as can be seen in Fig. 3. According to the analysis, samples. On the other hand, the solubility of Si in the sample calcined at none of the different resulted in the formation of any new 900 °C is lower than the initial solubility. The increase in solubility crystalline phases. The only observed change is decomposition of kao- during the first calcination stage supports Kaolinite transformation into linite at 750 °C. It is assumed that metakaolin is formed from kaolinite metakaolinite. Also, the increase of total dissolved Al and Si appears during its decomposition. However, metakaolin has amorphous struc- roughly equal in ratio supports the hypothesis, since the Si/Al ratio of ture and hence is not detected in XRD analysis (Wan et al., 2017). metakaolinite is approximately one. The decline in solubility after Metakaolin is known to react further into crystalline, insoluble mullite, calcination at 900 °C may indicate the formation of amorphous inert which begins to appear in temperatures over 800 °C (Provis and Van components from metakaolin. Based on solubility, sample K appears to Deventer, 2013, p. 106), but the XRD analysis does not show traces of be the most promising sample for geopolymerization. mullite in the sample calcined at 900 °C. Large contents of mullite were The experiments with sample L support the hypothesis of the sample

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P P - Phlogopite M - Monticellite T - Tremolite Q - Quartz

C - Calcite K - KAl(SiO4) P D - Dolomite E - MgSiO3 L - Lime R - Cordierite A - Amorphous

C P T T D T P T No calcination

Calcination Intensity (a.u.) A in 750 °C L L

Calcination in 900 °C M K M RE M M M Q M M

5 1015202530354045505560657075 2θ (deg)

Fig. 2. XRD results for sample P at different calcination stages.

Q M - Muscovite Q - Quartz K - Kaolinite

Q M Q No calcination K M Q Q Q Q

Intensity (a.u.) Calcination in 750 °C

Calcination in 900 °C

5 1015202530354045505560657075808590 2θ (deg)

Fig. 3. XRD results for sample K at different calcination stages.

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Q - Quartz Q A - Albite M - Microcline

N - Na 2CO 3

A Q M Q A A Q Q M N A A A A Q M Q No calcination

Intensity (a.u.) Calcination in 750 °C

Calcination in 900 °C

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 2θ (deg)

Fig. 4. XRD results for sample L at different calcination stages. being inert. The partial decomposition of albite observed with XRD (Fig. 6a). Setting of geopolymer happens with crystal reorganization implies that some chemical reactions took place during the thermal and growth of nuclei as it reaches a critical size (Duxson et al., 2007) treatment, and the declining solubility supports the interpretation. and hardening of the matrix takes place as excess water escapes the Therefore, the new phases that formed during the calcination appear to solidifying mixture (Davidovits, 2015, p. 424; Khale and Chaudhary, be amorphous but insoluble. The solubility decreases continuously as 2007). However, this is not the case with other tailings. The K tailings the calcination temperature rises, which implies the reaction occurs calcined at 900 °C does not set after 24 h curing regime (Fig. 7c). This gradually as the temperature is increased. can be related to the mineralogical changes of kaolinite mineral which becomes amorphous at 750 °C and recrystallizes at 900 °C. This is also 3.3. Geopolymer properties supported by the drastic reduction in solubility of S-tailings when cal- cined at 900 °C (Fig. 5a and b). Whereas, any of the alkali activated L 3.3.1. Setting characteristics tailings samples hardened only partially. Extending the curing period The setting of alkali activated mine tailings before and after calci- does not induce setting and the plastic nature of cured samples can be nation was observed visually and represented in Figs. 6–8. Setting observed in Fig. 8. This can be related to the lower Si and Al solubility properties were similar with both SN and S series of mixtures. Hence, S of L-tailings compared to other tailings. Slow polymerization process series are represented in the figures for discussions. The setting of alkali and relatively rapid precipitation of geopolymers, results in incomplete activated mixtures with P tailings does not show any abnormality setting. As a result the gained geopolymer matrix has an amorphous and

14 50 P P K 45 K 12 a b L L 40 10 35

8 30 25 6 20 Release of Si (%) Release of Al (%) Release of Al 4 15 10 2 5 0 0 Non calc. 750 900 Non calc. 750 900 Treatment temperature in °C Treatment temperature in °C

Fig. 5. Effect of calcination on relative solubility of (a) Si and (b) Al.

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Fig. 6. Alkali activated specimen with P tailings (a) NC (b) 750 and (c) 900. semi-crystalline structure (Criado et al., 2005; Pacheco-Torgal et al., reaction product (Ben Haha et al., 2011; Sreenivasan et al., 2017). Also, 2008; Xu and Van Deventer, 2000). The crystallization continues inside P tailings is rich in carbonate content from calcite and dolomite which the hardening material, resulting in a large variation of the final pre- increased the pH with the addition of water. This would help in the cipitation rate among different geopolymer materials based on their dissolution of Al and Si containing minerals and further geopolymer mineralogical composition (Provis et al., 2005). formation. However, there is a reduction in strength when the calci- nation temperature is increased to 900 °C (Fig. 11). Considering the 3.3.2. Water resistance release of Si and Al ions in solubility test, the reduction in solubility of ff The alkali activated SN (Na2SiO3 + NaOH) and S (Na2SiO3) series of Si and increase in solubility of Al at 900 °C a ects the Si/Al ratio. The mine tailings pastes were examined for their resistance in water and the Si/Al ratio in the final geopolymer matrix affects the structure and results are shown in Fig. 9. Specimens from series SN with NaOH were application of the material and has an optimal value which varies based not stable in water. Initially, white crystals were formed on the surface on the precursors used. For example, Wan et al. (2017) observed in of the specimens showing the possible formation of sodium carbonate their study that the optimum Si/Al ratio for metakaolin-based geopo- due to the exposure to atmospheric carbon-dioxide (Slaty et al., 2015). lymers is 2. A smaller ratio leads to a porous, zeolitic structure, while a This is also explained by Barbosa et al. (2000) that the excess Na+ ions larger ratio resulted in formation of a well porous silicate derivatives comes to the surface of the specimen and undergoes carbonation. This forming structures. The heat evolution curves corroborate this argu- can be further justified by the appearance of S series (without NaOH) ment as the heat energy increased at 750 °C and reduced at 900 °C specimens which does not show any carbonation on the surface. Hence, (Fig. 12a). the water resistance of S series specimens was studied and represented K tailings shows the highest strength among the three tailings that in Fig. 10. are studied here. The Muscovite and kaolinite mineral by itself parti- cipate in alkali activation even before any treatment. And calcination at 3.3.3. Compressive strength 750 °C improved the strength by 25%. This can be related to the for- Compressive strength of SN and S series of the alkali activated mine mation of metakaolin with the heat treatment of clay mineral present in tailings specimens are shown in Fig. 11. As specimens of alkali activated this tailings (Fig. 3). However, due to recrystallization that could occur non-calcined and calcined L tailings had setting problem (Fig. 8), with further calcination of metakaolin, the decline in strength was testing its strength was not possible for those mixtures. Due to imperfect noticed at 900 °C (Provis and Van Deventer, 2013). This is also dissolution, some of the precursor material might have left inside the matching with the behavior of this tailings during geopolymerization final material as unreacted residues and worked as an aggregate. This and the heat evolution as shown in Fig. 12b. behavior of the material may either improve or weaken the material properties (Provis et al., 2005; Xu and Van Deventer, 2000). Very low 4. Conclusion heat evolved during the geopolymerization of L tailings measured at initial 70 h (Fig. 12c) and lower solubility (Fig. 5) can be an indication In an effort to improve the reactivity of mine tailings from three of the weak product formation leading to setting problems. different sources, calcination was applied as a treatment method and Calcination of P tailings at 750 °C improved the strength by 62% the effect on reactivity in alkali activation was studied. The following and 75% in SN and S series, respectively. The high magnesium content conclusions are drawn based on the material properties of the tailings of P tailing from Phlogopite mineral could be a reason for this beha- acquired from this study: viour. MgO is already known to improve the mechanical strength of alkali activated materials by the formation of hydrotalcite as the • Mineralogical characterization of the mine tailings can be

Fig. 7. Alkali activated specimen with K tailings (a) NC (b) 750 and (c) 900.

7 P. Perumal, et al. Minerals Engineering 144 (2019) 106026

Fig. 8. Alkali activated specimen with L tailings (a) NC (b) 750 and (c) 900.

Fig. 9. SN series alkali activate specimen (a) after curing (b) under water.

7 16 P-SN P-S P-S )aPM(htgnertsevisserpmoC 14 K-SN 6 K-S K-S L-S 12 L-SN 5 L-S 10

8 4 6

Water absorption (%) 3 4

2 2 0 0 (NC) 750 900 0 (NC) 750 900 0 Treatment temperature in 0C Treatment temperature in C Fig. 11. Average compressive strength of alkali activated mine tailings speci- Fig. 10. Water absorption of S-series specimens. mens after 7 days of curing.

considered as an important identifier to categorize the material from kaolinite (K) even without any heat treatment. However, heat its response to any treatment or its reactivity in alkali activation. treatment resulted in the formation of CaO, MgO and metakaolin The mineralogical composition of different mine tailings affected which helped in the dissolution of Si and Al at high pH and further the Si and Al solubility. P and K tailings responded positively to strength improvement. calcination until 700 °C, beyond which it declined. Whereas, L • Compressive strength improved with calcination of mine tailings tailings showed continuous reduction in the solubility values. with a maximum of 62% for P tailings and 25% for K tailings at • Alkali activation was successful in tailings with carbonates (P) and 750 °C. Increase in solubility of Al ions at 900 °C is likely to affect

8 P. Perumal, et al. Minerals Engineering 144 (2019) 106026

50 50 750 a b

40 40

0 (NC) 30 30

750

20 900 20 0 (NC) Normalized heat (J/g) heat Normalized Normalized heat (J/g)

10 10 900

0 0 0 10203040506070 0 10203040506070 Time in hours Time in hours 50 c 40

30

20 Normalized heat (J/g)

10 0 (NC) 750 900 0 0 10203040506070 Time in hours

Fig. 12. Heat evolved during alkali activation of (a) P (b) K and (c) L tailings (S-series).

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