# Alkali and acid activated geopolymers based on iron-silicate fines -by-product from copper industry

Aleksandar Nikolov

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Aleksandar Nikolov. Alkali and acid activated geopolymers based on iron-silicate fines -by-product from copper industry. INTERNATIONAL SCIENTIFIC JOURNAL "MACHINES. TECHNOLOGIES. MATERIALS", 2020, 14 (1), pp.37-39. hal-04609878

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# Alkali and acid activated geopolymers based on iron-silicate fines - by-product from copper industry

Aleksandar Nikolov¹

¹Institute of Mineralogy and Crystallography, Bulgarian Academy of Science (IMC-BAS), Sofia, Bulgaria

E-mail: drsashko@imc.bas.bg

**Abstract:** Geopolymer based on iron-silicate fines (fayalite slag) were synthesized in alkaline and acidic media using activation solution comprised of respectively alkali silicate and phosphoric acid solutions. The raw material consists of fayalite, magnetite and pyroxene which could be a conglomerate in some particles. The alkali activation occurs very slow at room temperature, while acid activation take place very rapid. The acid activated geopolymer binder phase include cracks probably formed by thermal gradient because of the rapid exothermal reaction. The morphology of the alkali activated geopolymers were presented by porous structure.

**Keywords:** GEOPOLYMER, IRON-SILICATE FINES, FAYALITE, IRON-RICH SLAG, ALKALI, ACID, COPPER

## 1. Introduction

The total copper content of world mine production was 22.046 thousands of short tons copper for 2017 [1]. The slag from the flash furnace and the converters contains residual copper which is extracted through grinding and flotation. Two products are produced at the flotation plant: flotation copper concentrate and flotation product called iron-silicate fines. The flotation product is deposit into flotation pond or been processed as pressing to decrease water content to about 10-12% humidity prior to landfill deposition or utilization. The main mineral phase of the iron-silicate fine material is fayalite, hence the material is commonly named fayalite slag. The fayalite slag is powder material with high content of iron in the form of minerals fayalite, magnetite. There is no economic reason to extract the iron from the slag at this level of technology advance. The slag is barely usable, because of its specific fineness, high weight, contamination of heavy metals, etc. However, fayalite slag is marketable as a road surface, Portland cement production and concrete additive. Still huge amount of the produced iron-silicate fines remains unutilized deposited in landfills. With this in mind, geopolymer technology is potential solution to this rising problem.

Geopolymers are a class of inorganic polymer materials with amorphous or semi-crystalline three-dimensional structure [2]. The geopolymer materials possess high compressive strength, chemical resistance, thermal and fire stability [3], low CO₂ footprint, possibility of utilizing industrial waste materials, etc [4]. There are other names describing the same or similar materials, including the terms: "alkali-activate material", "alkali-bounded-ceramics", "hydroceramics", "inorganic polymer concrete", "alumino-silicate inorganic polymer", etc [5]. However, these materials seem promising and they are potential alternative of conventional Portland cement and ceramics.

The term ‘geopolymer’ was coined by Joseph Davidovits. There are two routes of geopolymer synthesis: in alkaline medium using alkali hydroxides and silicates; and acidic medium with phosphoric acid [2]. Depends on the precursors and synthesis route following terminology was adopted to describe geopolymers comprised by following molecular units (or chemical groups):

-Si-O-Si-O- siloxo, poly(siloxo)

-Si-O-Al-O- sialate, poly(sialate)

-Si-O-Al-O-Si-O- sialate-siloxo, poly(sialate-siloxo)

-Si-O-Al-O-Si-O-Si-O- sialate-disiloxo, poly(sialate-disiloxo)

-P-O-P-O- phosphate, poly(phosphate)

-P-O-Si-O-P-O- phospho-siloxo, poly(phospho-siloxo)

-P-O-Si-O-Al-O-P-O- phospho-sialate, poly(phospho-sialate)

-(R)-Si-O-Si-O-(R) organo-siloxo, poly-silicone

-Al-O-P-O- alumino-phospho, poly(alumino-phospho)

-Fe-O-Si-O-Al-O-Si-O- ferro-sialate, poly(ferro-sialate) [6].

The geopolymers are two component binder: main powder material and activator solution. The most common activation solution is mixture of sodium silicate and potassium/sodium

hydroxide [4], [7]. Recent studies of Nikolov showed suitable activator concentration to prepare alkali-activated geopolymers based iron-silicate fines [8]. The material hardens slowly at normal temperature. Additional, alkali activation of similar slags were also studies by: Komitsas et. al [9]–[11]; Maragkos et. al using Greek ferronickel slag [12]; and the research team of the University of Leuvenusing Belgium fayalite slag [13], [14].

So far limited studies were found on acid activation fayalite or similar slag. Nikolov et. al used same Aurubis fayalite slag [15] to synthesis geopolymers and Katsiki et al. used Belgium fayalite slag [16] to prepare geopolymer (inorganic polymer), both with up to 19 MPa compressive strength. The phosphate activation acts very rapid with setting time within a minute.

In the present study geopolymers based on fayalite slag (iron-silicate fines) were synthesized by two routes: in acid media with phosphoric acid solution and in alkaline media – using sodiumsilicate and potassium hydroxide solution. The purpose of the study is to compare the differences of using acid or alkaline activator solution.

## 2. Experimental

### 3.1. Method of analysis

The chemical composition of the iron-silicate fines (fayalite slag) was determined by XRF using Panalytical Axios Max WD. Powder X-Ray diffractograms (XRD) were produced with a Bruker D-8 Advance instrument with Bragg-Brentano geometry, using a CoKα source. The optical properties of the raw fayalite was studied in polished specimen using light polarized microscope Leitz Orthoplan. SEM images were obtained with different magnification by using electron microscope JEOL 6390, INCA Oxford analysis and energy disruptive spectroscope (EDS).

### 2.2. Materials

The geopolymer precursor was iron-silicate fines from local copper plant – Aurubis, Bulgaria. The iron-rich fayalite precursor was dried to constant mass in oven at 105 °C. The chemical and mineral composition are presented below.

The activator solutions were prepared by using solid KOH pellets, sodium silicate (SiO₂/Na₂O=3), orthophosphoric acid (H₃PO₄) and tap water. The ingredients were mixed by magnetic stirrer and tempered prior usage as activator

### 2.3. Geopolymer synthesis

The geopolymers in the presented study were prepared based on previous works [8], [15]. The composition of the prepared geopolymer pastes is presented by molar ratios as follows: alkali activated (series 15F3) H₂O/M₂O=11.40, Fe₂O₃/M₂O=4.00, Al₂O₃/M₂O=0.50; acid-activated Fe₂O₃/P₂O₅=3.76; H₂O/P₂O₅=1.71, (where M is K + Na). The water to solid ratio of the mixtures were respectively 0.15 and 0.196 in order to obtain workable mixtures. The alkali-activated homogenized mixture was
# Alkali and acid activated geopolymers based on iron-silicate fines - by-product from copper industry

Aleksandar Nikolov¹

¹Institute of Mineralogy and Crystallography, Bulgarian Academy of Science (IMC-BAS), Sofia, Bulgaria

E-mail: drsashko@imc.bas.bg

**Abstract:** Geopolymer based on iron-silicate fines (fayalite slag) were synthesized in alkaline and acidic media using activation solution comprised of respectively alkali silicate and phosphoric acid solutions. The raw material consists of fayalite, magnetite and pyroxene which could be a conglomerate in some particles. The alkali activation occurs very slow at room temperature, while acid activation take place very rapid. The acid activated geopolymer binder phase include cracks probably formed by thermal gradient because of the rapid exothermal reaction. The morphology of the alkali activated geopolymers were presented by porous structure.

**Keywords:** GEOPOLYMER, IRON-SILICATE FINES, FAYALITE, IRON-RICH SLAG, ALKALI, ACID, COPPER

## 1. Introduction

The total copper content of world mine production was 22.046 thousands of short tons copper for 2017 [1]. The slag from the flash furnace and the converters contains residual copper which is extracted through grinding and flotation. Two products are produced at the flotation plant: flotation copper concentrate and flotation product called iron-silicate fines. The flotation product is deposit into flotation pond or been processed as pressing to decrease water content to about 10-12% humidity prior to landfill deposition or utilization. The main mineral phase of the iron-silicate fine material is fayalite, hence the material is commonly named fayalite slag. The fayalite slag is powder material with high content of iron in the form of minerals fayalite, magnetite. There is no economic reason to extract the iron from the slag at this level of technology advance. The slag is barely usable, because of its specific fineness, high weight, contamination of heavy metals, etc. However, fayalite slag is marketable as a road surface, Portland cement production and concrete additive. Still huge amount of the produced iron-silicate fines remains unutilized deposited in landfills. With this in mind, geopolymer technology is potential solution to this rising problem.

Geopolymers are a class of inorganic polymer materials with amorphous or semi-crystalline three-dimensional structure [2]. The geopolymer materials possess high compressive strength, chemical resistance, thermal and fire stability [3], low CO₂ footprint, possibility of utilizing industrial waste materials, etc [4]. There are other names describing the same or similar materials, including the terms: "alkali-activate material", "alkali-bounded-ceramics", "hydroceramics", "inorganic polymer concrete", "alumino-silicate inorganic polymer", etc [5]. However, these materials seem promising and they are potential alternative of conventional Portland cement and ceramics.

The term ‘geopolymer’ was coined by Joseph Davidovits. There are two routes of geopolymer synthesis: in alkaline medium using alkali hydroxides and silicates; and acidic medium with phosphoric acid [2]. Depends on the precursors and synthesis route following terminology was adopted to describe geopolymers comprised by following molecular units (or chemical groups):

-Si-O-Si-O- siloxo, poly(siloxo)

-Si-O-Al-O- sialate, poly(sialate)

-Si-O-Al-O-Si-O- sialate-siloxo, poly(sialate-siloxo)

-Si-O-Al-O-Si-O-Si-O- sialate-disiloxo, poly(sialate-disiloxo)

-P-O-P-O- phosphate, poly(phosphate)

-P-O-Si-O-P-O- phospho-siloxo, poly(phospho-siloxo)

-P-O-Si-O-Al-O-P-O- phospho-sialate, poly(phospho-sialate)

-(R)-Si-O-Si-O-(R) organo-siloxo, poly-silicone

-Al-O-P-O- alumino-phospho, poly(alumino-phospho)

-Fe-O-Si-O-Al-O-Si-O- ferro-sialate, poly(ferro-sialate) [6].

The geopolymers are two component binder: main powder material and activator solution. The most common activation solution is mixture of sodium silicate and potassium/sodium

hydroxide [4], [7]. Recent studies of Nikolov showed suitable activator concentration to prepare alkali-activated geopolymers based iron-silicate fines [8]. The material hardens slowly at normal temperature. Additional, alkali activation of similar slags were also studies by: Komitsas et. al [9]–[11]; Maragkos et. al using Greek ferronickel slag [12]; and the research team of the University of Leuvenusing Belgium fayalite slag [13], [14].

So far limited studies were found on acid activation fayalite or similar slag. Nikolov et. al used same Aurubis fayalite slag [15] to synthesis geopolymers and Katsiki et al. used Belgium fayalite slag [16] to prepare geopolymer (inorganic polymer), both with up to 19 MPa compressive strength. The phosphate activation acts very rapid with setting time within a minute.

In the present study geopolymers based on fayalite slag (iron-silicate fines) were synthesized by two routes: in acid media with phosphoric acid solution and in alkaline media – using sodiumsilicate and potassium hydroxide solution. The purpose of the study is to compare the differences of using acid or alkaline activator solution.

## 2. Experimental

### 3.1. Method of analysis

The chemical composition of the iron-silicate fines (fayalite slag) was determined by XRF using Panalytical Axios Max WD. Powder X-Ray diffractograms (XRD) were produced with a Bruker D-8 Advance instrument with Bragg-Brentano geometry, using a CoKα source. The optical properties of the raw fayalite was studied in polished specimen using light polarized microscope Leitz Orthoplan. SEM images were obtained with different magnification by using electron microscope JEOL 6390, INCA Oxford analysis and energy disruptive spectroscope (EDS).

### 2.2. Materials

The geopolymer precursor was iron-silicate fines from local copper plant – Aurubis, Bulgaria. The iron-rich fayalite precursor was dried to constant mass in oven at 105 °C. The chemical and mineral composition are presented below.

The activator solutions were prepared by using solid KOH pellets, sodium silicate (SiO₂/Na₂O=3), orthophosphoric acid (H₃PO₄) and tap water. The ingredients were mixed by magnetic stirrer and tempered prior usage as activator

### 2.3. Geopolymer synthesis

The geopolymers in the presented study were prepared based on previous works [8], [15]. The composition of the prepared geopolymer pastes is presented by molar ratios as follows: alkali activated (series 15F3) H₂O/M₂O=11.40, Fe₂O₃/M₂O=4.00, Al₂O₃/M₂O=0.50; acid-activated Fe₂O₃/P₂O₅=3.76; H₂O/P₂O₅=1.71, (where M is K + Na). The water to solid ratio of the mixtures were respectively 0.15 and 0.196 in order to obtain workable mixtures. The alkali-activated homogenized mixture was
poured in metal cubic moulds (3.17 mm) and was placed in plastic bags in laboratory conditions (23 °C, w=65%). On 28th day was demoulded because of the slow strength gain. Other hand, acid-activated geopolymers showed rapid setting. The mixtures were homogenized for 30 s and placed in cylindrical plastic container (50x30mm). In 1-2 minute the mixture was hard enough to demould. Still it was demoulded after 1 hour tempering because of high exothermicity of the reaction and possibility of detrimental temperature gradient during cooling.

# 3. Results and Discussion

## 3.1. Characterization of the raw material

The chemical composition of the geopolymer precursor was determined by XRF and the results are presented in Table 1. The results from powder XRD examination showed that the geopolymer precursor consists of fayalite, magnetite and small amount of pyroxene (Figure 1). The main phase fayalite is end member of the olivine group. The Fe and Mg can substitute freely for one another in the mineral's atomic structure. In detailed observation with optical microscope of polished specimens made of powder in epoxy resin, it was visible that some particles were conglomerate of magnetite, fayalite and pyroxene (Figure 2). This phenomena makes the magnetic separation of the different mineral phases inconvenient. The chemical point analysis in SEM-EDS showed the difference in chemical composition of a particle (Figure 3).

<table><caption>**Table 1.** Chemical composition of the geopolymer precursor - iron-silicate fines, determined by XRF, (%).</caption><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>SiO<sub>2</sub></td><td>Al<sub>2</sub>O<sub>3</sub></td><td>CaO</td><td>ZnO</td><td>MgO</td><td>K<sub>2</sub>O</td></tr><tr><td>58.42</td><td>29.34</td><td>4.40</td><td>2.66</td><td>1.32</td><td>0.89</td><td>0.71</td></tr><tr><td>Na<sub>2</sub>O</td><td>CuO</td><td>PbO</td><td>TiO<sub>2</sub></td><td>MoO<sub>3</sub></td><td>SO<sub>3</sub></td><td></td></tr><tr><td>0.58</td><td>0.49</td><td>0.37</td><td>0.30</td><td>0.27</td><td>0.26</td><td></td></tr></table>

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Powder XRD curve of the geopolymer precursor - iron-silicate fines (fayalite) from Aurubis, Bulgaria. F- fayalite, M - magnetite, P - pyroxene.</figcaption></figure>

## 3.2. Alkali and acid activated geopolymers

The XRD results did not show any significant differences between raw material and hardened geopolymer, so they are not presented. The main phases stayed mostly inert, for little exception of fayalite, which probably was partly dissolved based on small decrease of its intensities compared to magnetite.

On Figure 4 are presented SEM-EDS images of hardened acid (a,b) and alkali (c,d) activated geopolymers. In both geopolymers it was observed that particles of magnetite and fayalite act as inert filler. In acid activated geopolymers there was more visible geopolymer binder phase between filler particles characterized by 7-11% phosphorous content (point 1 and 2, Figure 4 – a). The newly formed geopolymer gel was homogeneous with difference in elemental composition which is visible by brighter spots on backscattered scanning electron image (Figure 4 – b). However, the

binder phase include many cracks probably formed by thermal gradient because of the rapid exothermal reaction.

The morphology of the alkali activated geopolymers differentiate significantly from the acid activated. The structure was very porous. The alkali-activation geopolymers represent a heterogeneous material composed of geopolymer gel matrix and partially reacted or unreacted particles. The high alkalinity and long hardening process lead to partly dissolution of the precursor particles. Bigger particle reacted only on surface. On Figure 4 – d, Point 3 was probably geopolymer gel which was characterized by higher K and Na content and homogeneous texture.

<figure><img src="image_2.png" /><figcaption>**Figure 2.** Optical microscope image of the raw material - iron-silicate fines (fayalite) from Aurubis, Bulgaria</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Figure 3.** Backscattered SEM-EDS image of the geopolymer precursor iron-silicate fines</figcaption></figure>

# 4. Conclusion

The presented studies showed that iron-silicate fines form local copper industry could be successfully used as geopolymer precursor synthesized in both acidic or alkaline media. The raw material consists of fayalite, magnetite and pyroxene which could be a conglomerate in some particles, which makes the magnetic separation of the different mineral phases inconvenient

The alkali activation occurs very slow at room temperature, while acid activation takes place very rapid. The acid activated geopolymer binder phase include cracks probably formed by thermal gradient because of the rapid exothermal reaction. The morphology of the alkali activated geopolymers were presented by porous structure.

Further more detailed studies are needed to develop materials with practical application.
poured in metal cubic moulds (3.17 mm) and was placed in plastic bags in laboratory conditions (23 °C, w=65%). On 28th day was demoulded because of the slow strength gain. Other hand, acid-activated geopolymers showed rapid setting. The mixtures were homogenized for 30 s and placed in cylindrical plastic container (50x30mm). In 1-2 minute the mixture was hard enough to demould. Still it was demoulded after 1 hour tempering because of high exothermicity of the reaction and possibility of detrimental temperature gradient during cooling.

## 3. Results and Discussion

### 3.1. Characterization of the raw material

The chemical composition of the geopolymer precursor was determined by XRF and the results are presented in Table 1. The results from powder XRD examination showed that the geopolymer precursor consists of fayalite, magnetite and small amount of pyroxene (Figure 1). The main phase fayalite is end member of the olivine group. The Fe and Mg can substitute freely for one another in the mineral's atomic structure. In detailed observation with optical microscope of polished specimens made of powder in epoxy resin, it was visible that some particles were conglomerate of magnetite, fayalite and pyroxene (Figure 2). This phenomena makes the magnetic separation of the different mineral phases inconvenient. The chemical point analysis in SEM-EDS showed the difference in chemical composition of a particle (Figure 3).

<table><caption>**Table 1.** Chemical composition of the geopolymer precursor - iron-silicate fines, determined by XRF, (%).</caption><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>SiO<sub>2</sub></td><td>Al<sub>2</sub>O<sub>3</sub></td><td>CaO</td><td>ZnO</td><td>MgO</td><td>K<sub>2</sub>O</td></tr><tr><td>58.42</td><td>29.34</td><td>4.40</td><td>2.66</td><td>1.32</td><td>0.89</td><td>0.71</td></tr><tr><td colspan="7"></td></tr><tr><td>Na<sub>2</sub>O</td><td>CuO</td><td>PbO</td><td>TiO<sub>2</sub></td><td>MoO<sub>3</sub></td><td>SO<sub>3</sub></td><td></td></tr><tr><td>0.58</td><td>0.49</td><td>0.37</td><td>0.30</td><td>0.27</td><td>0.26</td><td></td></tr></table>

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Powder XRD curve of the geopolymer precursor - iron-silicate fines (fayalite) from Aurubis, Bulgaria. F- fayalite, M - magnetite, P - pyroxene.</figcaption></figure>

### 3.2. Alkali and acid activated geopolymers

The XRD results did not show any significant differences between raw material and hardened geopolymer, so they are not presented. The main phases stayed mostly inert, for little exception of fayalite, which probably was partly dissolved based on small decrease of its intensities compared to magnetite.

On Figure 4 are presented SEM-EDS images of hardened acid (a,b) and alkali (c,d) activated geopolymers. In both geopolymers it was observed that particles of magnetite and fayalite act as inert filler. In acid activated geopolymers there was more visible geopolymer binder phase between filler particles characterized by 7-11% phosphorous content (point 1 and 2, Figure 4 – a). The newly formed geopolymer gel was homogeneous with difference in elemental composition which is visible by brighter spots on backscattered scanning electron image (Figure 4 – b). However, the

binder phase include many cracks probably formed by thermal gradient because of the rapid exothermal reaction.

The morphology of the alkali activated geopolymers differentiate significantly from the acid activated. The structure was very porous. The alkali-activation geopolymers represent a heterogeneous material composed of geopolymer gel matrix and partially reacted or unreacted particles. The high alkalinity and long hardening process lead to partly dissolution of the precursor particles. Bigger particle reacted only on surface. On Figure 4 – d, Point 3 was probably geopolymer gel which was characterized by higher K and Na content and homogeneous texture.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Optical microscope image of the raw material - iron-silicate fines (fayalite) from Aurubis, Bulgaria</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Backscattered SEM-EDS image of the geopolymer precursor iron-silicate fines</figcaption></figure>

## 4. Conclusion

The presented studies showed that iron-silicate fines form local copper industry could be successfully used as geopolymer precursor synthesized in both acidic or alkaline media. The raw material consists of fayalite, magnetite and pyroxene which could be a conglomerate in some particles, which makes the magnetic separation of the different mineral phases inconvenient

The alkali activation occurs very slow at room temperature, while acid activation takes place very rapid. The acid activated geopolymer binder phase include cracks probably formed by thermal gradient because of the rapid exothermal reaction. The morphology of the alkali activated geopolymers were presented by porous structure.

Further more detailed studies are needed to develop materials with practical application.
<figure><img src="image_4.png" /><figcaption>Figure 4. SEM-EDS images of acid activated geopolymer (a,b) and alkali-activated geopolymer (c,d)</figcaption></figure>

**5. Acknowledgement**

The results in this work have been achieved in fulfilment of a project financed by the National Science Fund of Bulgaria under contract No. DM17/3 from 12.12.2017 (50%) and PMC#577 from 17.08.2018 (50%).

**6. References**

[1] "Annual Data 2018 Copper Supply & Consumption — 1997–2017," 2018.

[2] J. Davidovits, "Geopolymer chemistry and applications," 2008.

[3] A. Nikolov, "Physical properties and powder XRD characterization of fly ash-based geopolymers heated up to 1150 oC," *Rev. Bulg. Geol. Soc.*, vol. 80, no. 3, pp. 36–38, 2019.

[4] J. L. Provis and J. S. J. Van Deventer, *Geopolymers: structures, processing, properties and industrial applications*. Elsevier, 2009.

[5] P. Duxson, A. Fernández-Jiménez, J. L. Provis, G. C. Lukey, A. Palomo, and J. S. J. van Deventer, "Geopolymer technology: the current state of the art," *J. Mater. Sci.*, vol. 42, no. 9, pp. 2917–2933, 2007.

[6] J. Davidovits, "What is a geopolymer? Introduction," 2012. [Online]. Available: https://www.geopolymer.org/science/introduction/. [Accessed: 16-Jan-2020].

[7] A. Nikolov, "GEOPOLYMERS BASED ON METAKAOLIN – LITERATURE REVIEW AND PRELIMINARY STUDIES," 2018.

[8] A. Nikolov, "Alkali-activated geopolymers based on iron-rich waste from copper industry," *Comptes Rendus L'Academie

Bulg. des Sci., 2020.

[9] K. Komnitsas, D. Zaharaki, and V. Perdikatsis, "Geopolymerisation of low calcium ferronickel slags," *J. Mater. Sci.*, vol. 42, no. 9, pp. 3073–3082, May 2007, doi: 10.1007/s10853-006-0529-2.

[10] K. Komnitsas, D. Zaharaki, and G. Bartzas, "Effect of sulphate and nitrate anions on heavy metal immobilisation in ferronickel slag geopolymers," *Appl. Clay Sci.*, vol. 73, pp. 103–109, Mar. 2013, doi: 10.1016/J.CLAY.2012.09.018.

[11] K. Komnitsas, D. Zaharaki, V. P.-J. of H. Materials, and undefined 2009, "Effect of synthesis parameters on the compressive strength of low-calcium ferronickel slag inorganic polymers," *Elsevier*.

[12] I. Maragkos, I. Giannopoulou, D. P.-M. Engineering, and undefined 2009, "Synthesis of ferronickel slag-based geopolymers," *Elsevier*.

[13] S. Onisei, A. P. Douvalis, A. Malfliet, A. Peys, and Y. Pontikes, "Inorganic polymers made of fayalite slag: On the microstructure and behavior of Fe," *J. Am. Ceram. Soc.*, vol. 101, no. 6, pp. 2245–2257, Jun. 2018, doi: 10.1111/jace.15420.

[14] S. Onisei, K. Lesage, B. Blanpain, and Y. Pontikes, "Early Age Microstructural Transformations of an Inorganic Polymer Made of Fayalite Slag," *J. Am. Ceram. Soc.*, vol. 98, no. 7, pp. 2269–2277, Jul. 2015, doi: 10.1111/jace.13548.

[15] A. Nikolov, R. Titorenkova, N. Velinov, and Z. Delcheva, "Characterization of a novel geopolymer based on acid-activated fayalite slag from local copper industry," *Bulg. Chem. Commun.*, vol. 50, no. F, pp. 54–61, 2018.

[16] A. Katsiki, A. Peys, Y. Pontikes, and H. Rahier, "Activation of fayalite slag towards inorganic polymers," in 5th *international slag valorisation symposium. Leuven: Leuven University Press*, 2017.
<figure><img src="image_5.png" /><figcaption>Figure 4. SEM-EDS images of acid activated geopolymer (a,b) and alkali-activated geopolymer (c,d)</figcaption></figure>

## 5. Acknowledgement

The results in this work have been achieved in fulfilment of a project financed by the National Science Fund of Bulgaria under contract No. DM17/3 from 12.12.2017 (50%) and PMC#577 from 17.08.2018 (50%).

## 6. References

[1] "Annual Data 2018 Copper Supply & Consumption — 1997–2017," 2018.

[2] J. Davidovits, "Geopolymer chemistry and applications," 2008.

[3] A. Nikolov, "Physical properties and powder XRD characterization of fly ash-based geopolymers heated up to 1150 oC," *Rev. Bulg. Geol. Soc.*, vol. 80, no. 3, pp. 36–38, 2019.

[4] J. L. Provis and J. S. J. Van Deventer, *Geopolymers: structures, processing, properties and industrial applications*. Elsevier, 2009.

[5] P. Duxson, A. Fernández-Jiménez, J. L. Provis, G. C. Lukey, A. Palomo, and J. S. J. van Deventer, "Geopolymer technology: the current state of the art," *J. Mater. Sci.*, vol. 42, no. 9, pp. 2917–2933, 2007.

[6] J. Davidovits, "What is a geopolymer? Introduction," 2012. [Online]. Available: https://www.geopolymer.org/science/introduction/. [Accessed: 16-Jan-2020].

[7] A. Nikolov, "GEOPOLYMERS BASED ON METAKAOLIN – LITERATURE REVIEW AND PRELIMINARY STUDIES," 2018.

[8] A. Nikolov, "Alkali-activated geopolymers based on iron-rich waste from copper industry," *Comptes Rendus L'Academie

Bulg. des Sci., 2020.

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