Simple Compressive Strength Results of Sodium-Hydroxide- and Sodium-Silicate-Activated Copper Flotation Tailing Geopolymers

Abstract

Geopolymers are created by mixing a source of aluminosilicates, which can be natural or by-products of other industries, with an alkaline solution, which dissolves the aluminates and silicates present in this source, where after a polymerization process, an N-A-S-H gel is formed, which is responsible for providing the properties that characterize geopolymers. Among the variety of existing geopolymers, those based on by-products from other industries stand out since they were demonstrated to be a less-polluting alternative for concrete production than ordinary Portland cement (OPC). Due to the above, it is essential to study copper flotation tailings as raw material to generate geopolymers. The excessive amounts of existing tailing deposits also produce different risks for the nearby communities. Therefore, using this industrial waste as a construction material would provide several environmental and economic benefits. This article reports on the experimental work carried out in the laboratory of the Sustainable Mining Research Center CIMS of the Engineering Consulting Company JRI, where the effect of the alkaline activator type on the compressive strength of geopolymers based on copper flotation tailings was analyzed. For this purpose, two geopolymeric mixtures were made with different kinds of alkaline activators; one activated using 100% NaOH and the other activated with 100% sodium silicate (SS). From the results, it was found that the geopolymers activated with 100% SS obtained the highest compressive strength, reaching 36.46 MPa with 7 days of curing at 90 °C, followed by the geopolymers activated with 100% NaOH, where a compressive strength of 22.98 MPa was obtained under the same curing conditions. On the other hand, it was found that both geopolymers created were not leachable according to the TCLP test performed, and thus, these geopolymers were classified as non-toxic materials. In addition, it was found that both geopolymers presented a high infiltration value, making them practically impermeable.

1. Introduction

Geopolymers have been extensively studied as construction materials. It is a technology that was developed to find a more environmentally friendly option than traditional Portland cement [1]. Generally speaking, a geopolymer is an inorganic synthetic polymer that is generated by aluminosilicate materials and alkaline agents. After curing, a semi-crystalline amorphous material similar to concrete is produced [2]. The curing reaction can occur by adding external heat, i.e., curing in an oven and ambient temperature, depending on the composition of the geopolymer [3]. It should be noted that the curing conditions have a significant effect on the strength development of geopolymers [3].

A wide variety of aluminosilicate reagents can produce geopolymers [2]. The most common sources of aluminosilicates used for geopolymer production are metakaolin and by-products from other industries, such as fly ash, mine tailings, red mud, and slag [4,5,6,7] Within these raw materials, the use of by-products from other industries for the production of geopolymer concrete was recently encouraged since this significantly reduces the amount of CO₂ emitted for the manufacture of these materials [8]. Geopolymer concrete (GPC) is estimated to reduce the carbon footprint of construction projects by 80% compared with ordinary Portland cement (OPC) [9].

Geopolymer precursor materials, both in natural and by-product forms, should be rich in alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) and silica (${\mathrm{SiO}}_2$) contents, preferably in the reactive amorphous form [5], for good dissolution of these compounds upon contact with the alkaline agent. The function of silica and aluminum is to impart strength and set the cement [10]. A concern related to aluminosilicate dissolution is the rate at which it occurs and the amount of total amorphous aluminosilicate material available for geopolymerization [11], as the curing time and the compressive strength subsequently obtained will depend on this [3].

In addition to the aluminosilicate reagent, an alkaline activator is needed to produce a geopolymer. The alkaline activator causes the dissolution of the raw materials [12]. The type and concentration of the alkaline activator should be carefully selected because its composition affects the dissolution of the aluminosilicate source and has different impacts on the properties of fresh geopolymer pastes and the development of compressive strength in hardened geopolymers [13]. The most common activators are alkaline hydroxides and silicate solutions, and within these groups, sodium hydroxide (NaOH) and sodium silicate (${\mathrm{Na}}_2{\mathrm{SiO}}_3$) are the most commonly used, respectively. The ideal concentration of the alkaline activator increases the strength of the geopolymer [3]. In addition, an increase in the concentration of the alkaline activator leads to a rise in the pH of the activating solution. Different authors recommend working with pH values above 13 for the correct dissolution of the aluminosilicates present in the raw material [9,14,15]. Geopolymers’ strength development depends on the raw materials and the alkali-activating solutions [3].

When activating the aluminosilicate source with NaOH, the reaction starts with the dissolution of Al and Si, which are precursor particles in the alkali solution, where reactive aluminate and silicate monomers are released [2]. Then, these monomers interact to form aluminosilicate oligomers, and the latter polymerize in the alkaline environment to form geopolymer gels. At the onset of geopolymerization, an aluminum-rich gel phase is generated, transforming into a final silicon-rich geopolymer gel [2,16].

The result based on a geopolymerization reaction is a semi-crystalline amorphous substance composed of solid phases of aluminosilicates assembled on the basis of ${\mathrm{SiO}}_4{}^{4-}$ and ${\mathrm{AlO}}_4{}^{5-}$ linkages as tetrahedra forming a 3D structure. The main hydration product of low-calcium or calcium-free geopolymers is an N-A-S-H gel, which possesses a 3D structure [17].

From another point of view, geopolymers are a manufactured material that offers several advantages, including good mechanical strength (similar or superior to ordinary Portland cement) and the ability to encapsulate hazardous waste [3,18], as well as being resistant to water and high temperatures [19]. The compressive strength of geopolymers is a critical factor in the construction field; for this reason, geopolymers are being widely studied and show promise as a more environmentally friendly alternative to Portland cement concrete [20].

For the generation of geopolymers, different variables must be considered, such as the source of aluminosilicates, the type of alkaline activator, and the addition of external heat for curing, since these variables have a notable impact on the mechanical properties of the hardened geopolymers.

Therefore, this study sought to analyze the effect of the variation of the alkaline activator on the properties of geopolymers based on copper flotation tailings. For this purpose, two reagents were used, sodium hydroxide (NaOH) and sodium silicate (${\mathrm{Na}}_2{\mathrm{SiO}}_3$), where geopolymers activated with 100% NaOH and geopolymers activated with 100% ${\mathrm{Na}}_2{\mathrm{SiO}}_3$ were made, keeping the tailings/activator ratio constant. The purpose of using these reagents alone without incorporating both reagents together is to analyze the effect that each reagent has on the development of the compressive strength of the geopolymers.

2. Materials

2.1. Tailings Characterization

The copper tailings used in this research came from the San Pedro Mining Company, located in Til Til, Chile, where polymetallic copper and silver concentrates are obtained. The type of deposit from which these concentrates are extracted corresponds to a copper porphyry, where the main element of interest that is extracted is copper. The tailings produced from this mine were filtered, obtaining an average solid content (%Cp) of 85% (Figure 1). The average solids density of the tailings was 2.79 ± 0.12 g/cm³. In addition, process water samples were taken to be used in the formation of the geopolymers.

X-ray diffraction (XRD) analysis of the copper flotation tailing used is shown in Figure 2. This copper tailing is a crystalline material composed mainly of albite (${\mathrm{NaAlSi}}_3{\mathrm{O}}_8$), and to a lesser extent, clinochlore (${\mathrm{Mg}}_5\mathrm{Al}\left({\mathrm{AlSi}}_3{\mathrm{O}}_{10}\right){(\mathrm{OH})}_8$), quartz (${\mathrm{SiO}}_2$), epidote (${\mathrm{Ca}}_2\left({\mathrm{Al}}_2,\mathrm{Fe}\right)\left({\mathrm{SiO}}_4\right)\left({\mathrm{Si}}_2{\mathrm{O}}_7\right)\mathrm{O}\left(\mathrm{OH}\right)$), and orthoclase (${\mathrm{KAlSi}}_3{\mathrm{O}}_8$).

Unlike previous research in which dry tailings were used [21,22,23], for this study, we used tailings with a solids concentration of 83%, which is the method developed in previous research by Castillo et al. [1]. Process water was used for the homogenization of the tailings to reduce the production costs of the geopolymers, which were produced by adding process water until a tailings pulp with a solids concentration Cp of 70% was obtained. After this homogenization, the tailings pulp was filtered to obtain a tailings pulp with a solids concentration Cp of 83% (Figure 3).

The chemical composition of the copper tailings used in this investigation was obtained using an inductively coupled plasma test (ICP), where an Andes Analytical Assay was performed. The results presented in

Table 1. Tailings chemical characterization test results using ICP.

Element	Concentration (%)	Element	Concentration (%)
Si	26.95	K	1.19
Al	9.50	Ti	0.60
Fe	8.05	Mn	0.25
Ca	3.28	S	0.12
Na	2.67	Cr	<0.01
Mg	2.10

corresponded to average values obtained from the analysis of several samples of the tailings used. The ICP analysis was performed under the Accredited Method ISO 17025.Of.2005 INN, LE1386, using an analytical balance with a minimum precision of 1 mg to weigh the sample and prepare it for leaching at a temperature of 23 ± 2 °C for 18 ± 2 h at 30 ± 2 rpm. Then, a precipitation test was performed for which a 20 mL extract was taken and 2 mL of HNO₃ was added; if precipitation was observed, the rest of the extract was not acidified and the measurement was obtained. If the extract showed no precipitation, it was acidified with HNO₃ acid to pH = 2 and stored at 4 °C for subsequent measurement. Subsequently, 2.5 mL of the 20 mL extract was diluted and poured into a 50 mL volumetric flask containing 12.5 mL of quenched aqua regia to perform the ICP-MS measurement with an Aurora Bruker ICP-MS, USA, at the Andes Analytical Assay Laboratory: 3AAA in Chile. From the characterization of the tailings presented in Table 1, it was observed that the main element in its composition was silicon and, to a lesser extent, aluminum and iron, with the first two being fundamental for the synthesis of the geopolymer. From Table 1, it is seen that the tailings used had a Si/Al ratio of 2.83, which, according to previous studies, [3], is close to the optimal range for the formation of geopolymers (Si/Al ratios close to 2.0).

Characterization of the industrial water used for the homogenization of the tailings was also carried out using the inductively coupled plasma (ICP) test, where the same procedure previously described was used to obtain the results. In addition, as in the case of the tailings, the results obtained correspond to the average values obtained from the analysis of several samples. These results are presented in

Table 2. Chemical characterization test results of industrial water using ICP.

Element	Concentration (mg/L)	Element	Concentration (mg/L)
S	215	Ta	<1.00
K	210	Th	<1.00
Na	158	Tl	<1.00
K	28	W	<1.00
Mg	23	As	<0.50
Al	<5	La	<0.50
Fe	<5	Li	<0.50
Ti	<5	P	<0.50
Sr	1.7	Sb	<0.50
Ga	<1.00	Sc	<0.50
Nb	<1.00	Te	<0.50
Sn	<1.00	Ag	<0.25
-	-	Bi	<0.25

.

Table 2 shows that the highest concentration corresponded to sulfur ions. These can react with sodium hydroxide or some component of the tailings, causing impurities or undesired components that alter the results.

The chemical composition of the oxides for the copper flotation tailings of Minera San Pedro, according to official sources from the Chilean National Geology and Mining Service (Sernageomin), is shown in

Table 3. Oxide chemical composition for the tailings.

Oxide Chemical Composition	%
SiO2	52.81
Al2O3	14.26
TiO2	1.01
Fe2O3	10.47
CaO	5.18
MgO	5.35
MnO	0.18
Na2O	3.67
K2O	2.06
P2O5	0.32
PPC	3.62
SO3	<0.01

:

Figure 4 shows the particle size distribution of the copper flotation tailing used, obtained using Ro-Tap tests, which consists of passing the particulate matter through a series of sieves; a laser was used to analyze the smallest particle sizes, which consisted of analyzing the light diffraction of suspended particles. Both tests were performed in the laboratory of the JRI research center (CIMS-JRI).

The Atterberg limits of the tailings, i.e., those water contents at which the behavior of the soil is modified, are presented in

Table 4. Atterberg limits of the tailing used.

Parameter	Value
Liquid limit	23.3%
Plastic limit	19.7%
Shrinkage limit	20.5%
Plasticity index	3.5%

.

Based on the tailings’ particle size and their Atterberg limits, and according to the Unified Soil Classification System (USCS), the tailings were classified as a low plasticity silt loam, as seen in Figure 5, i.e., inorganic silt with low compressibility.

To finish the complete characterization of the tailings, rheology tests were performed at CIMS-JRI using a Haake RheoStress 6000 rheometer with an FL100 paddle probe at a shear rate of 0.1 1/s for 60 s, obtaining the tailings yield stress and viscosity, as well as the pH at different concentrations of solids (Cp). The results are presented in

Table 5. Results of rheological tests on the tailing used.

Cp (%)	Yield Stress (Pa)	Viscosity (m Pa · s)	Temperature (°C)	pH
50.2%	1.85 ± 0.06	9.7 ± 0.2	20.65	7.74
55.1%	3.7 ± 0.1	14 ± 1	20.65	7.81
60.1%	8.1 ± 0.2	21.2 ± 0.5	20.65	7.88
65.2%	20.9 ± 0.6	42 ± 1	20.70	7.93
70.3%	68 ± 2	115 ± 3	20.95	7.94

. The aforementioned measurements were all performed at ambient temperature.

The results shown in Table 5 show the rheological test for the tailing samples by considering different Cp values (solids contents) for which the yield stress was obtained. It was observed that as the solids content Cp increased from 50% to 70%, the yield stress also increased from 1.85 Pa to 68 Pa, respectively, mainly due to the fact that the water present in the tailings decreased.

2.2. Characterization of Sodium Hydroxide NaOH

The sodium hydroxide NaOH used was in the form of beads obtained from Winkler Ltd.a., Santiago, Chile, which had a purity of 99%.

2.3. Sodium Silicate Characterization $N{a}_{2}Si{O}_3$

The sodium silicate used was in an aqueous state acquired from Austral Chemicals Chile S.A., which had a composition of 30.5% ${\mathrm{SiO}}_2$, 13.0% ${\mathrm{Na}}_2\mathrm{O}$, and 56.5% ${\mathrm{H}}_2\mathrm{O}$.

3. Methodology

For the formulation of the geopolymeric mixtures, the type of alkaline activator used was varied, formulating two different mixtures: for the first case, geopolymeric mixtures activated with 100% NaOH were used, and for the second case, geopolymeric mixtures activated with 100% ${\mathrm{Na}}_2{\mathrm{SiO}}_3$ were used, keeping the ratio in weight tailing/alkaline activator constant in both cases to analyze which activating reagent had a more significant influence on the development of the compressive strength of the already cured geopolymers at the same concentration in weight but different molar concentration [1]. The details of the mixtures made and their compositions are shown in

Table 6. Composition of the various geopolymer blends.

Solids in Tailings (Cp) (%)	NaOH (%)	NaOH Molarity (M)	SS (%)	SS Molarity (M)
83	100	15.95	0	0.00
83	0	0.00	100	4.70

.

Figure 6 shows each geopolymer mixture’s chemical compositions (wt.%) in the SiO₂–Na₂O–Al₂O₃ ternary diagram. From this, it can be observed that the composition of the geopolymers varied approximately from 63% silica by weight for the sample activated with sodium hydroxide to 72% silica for the sample activated with sodium silicate due to the addition of soluble silica by this reagent; in addition, the sample activated with sodium hydroxide had a higher percentage of sodium oxide in its composition due to the sodium added by part of the reagent used. On the other hand, both mixtures presented similar alumina amounts in their composition. It should be noted that both mixtures fell in the range of the geopolymeric phase described in the SiO₂–Na₂O–Al₂O₃ ternary phase diagram of the study by Juengsuwattananon et al. [24]. As expected, both blends formed the geopolymer gel since they both hardened after curing; therefore, both blends were in the correct range for geopolymer formation according to their composition.

As mentioned above, the tailings had to be homogenized in the first instance, which was achieved by forming a tailings pulp of Cp 70%. After this, the homogenized tailings were filtered to obtain a tailings pulp of Cp 83%, which was the solids content required in the method used in this study to manufacture geopolymers (method VII of the study by Castillo et al., [1]). The use of dry tailings was avoided, as in previous studies carried out in the same line of study [1], because industrially, the tailing drying process is not very efficient since water has to be added later for the formation of the geopolymeric mixtures. After this, the number of tailings to be used in each mixture was weighed separately and sodium hydroxide and sodium silicate were added in the appropriate amounts to each mixture according to Table 6. Then, each mixture was mechanically agitated for 15 min to obtain a homogeneous geopolymer paste. After agitation, each mixture was poured into cylindrical steel molds that were 10 cm high and 5 cm in diameter, where vibration was used for 5 min to eliminate the air bubbles trapped inside the mold. The molded mixtures were then cured in an oven for 7 days at 90 °C for hardening. After curing, the specimens were demolded and prepared for testing through different characteristic tests.

To analyze the properties of the fresh geopolymer pastes created, rheology, pH, and temperature tests were performed on them prior to curing since these properties are essential to analyze, for example, the eventual transport of these mixtures by pumps for later use, as well as to analyze whether the mixtures are in the correct range of alkalinity; pH measurements of the fresh geopolymer pastes were carried out using a DFRobot digital pH meter, previously calibrated between pH 7 and 13, at a controlled temperature of 20 °C. At the same time as the pH measurements, temperature measurements were performed on the fresh geopolymer pastes using a digital thermometer to obtain the temperature of the mixtures at all times during agitation. Rheological measurements of the geopolymer pastes were carried out using a Haake RheoStress 6000 rheometer with an FL100 paddle probe using a shear rate of 0.1 1/s for 60 s.

Several tests were performed to characterize the hardened geopolymers, i.e., after curing, including compressive strength, permeability, leaching (TCLP), X-ray diffraction, and automated mineralogy, with the latter carried out to analyze the formation of crystalline phases during geopolymerization. Simple compression tests were performed in Geocontrol’s laboratory under ASTM D7012-10 in a concrete press. The hydraulic conductivity (permeability) was obtained using a flexible wall permeameter at constant load, where the test was performed in Geocontrol’s laboratory under ASTM D5084-00. The characteristic leaching test (TCLP) was carried out to guarantee the safety of using this new material called a geopolymer since one of the characteristics that a geopolymer must have is that it must be non-toxic and non-leachable. Andes Analytical Assay Laboratory performed this test under US EPA Method 1311, where an acetic acid solution with a low pH of 2.8 was used as a leaching solution with an extraction cycle of 18 h.

4. Results

The results obtained in this research are presented below; the results are divided into measurements taken before and after the hardening of the geopolymers.

After curing each mixture for 7 days at 90 °C, the specimens were removed from the oven and demolded. In general, it was observed that all the geopolymeric blends had hardened with the application of a moderately high-temperature curing regime, but presented different appearances according to their composition. In Figure 7, the difference in the appearance of a geopolymer based on copper tailings activated with 100% NaOH (Figure 7a) with that of one activated with 100% ${\mathrm{Na}}_2{\mathrm{SiO}}_3$ (Figure 7b) can be observed. It was also observed that the mixtures activated with 100% sodium silicate presented a higher volumetric shrinkage, which was noticed when the specimens were demolded (Figure 8b). This effect of more significant volumetric shrinkage may have occurred since these mixtures had a tremendous amount of water in their composition (due to sodium silicate), and thus, a more significant amount of water is also released during the geopolymerization process occurred [25].

4.1. The Behavior of Fresh Geopolymer Pastes

4.1.1. Rheology Test Results

From the rheology tests, which were performed on the fresh geopolymer pastes of both mixtures after agitation, it was found that the rheological properties increased considerably for both mixtures compared with the rheological properties of the tailings without the addition of reagents (Figure 9). For the geopolymeric mixture activated with 100% NaOH, the yield point of the paste was approximately 1100 Pa and presented monotonous behavior after this yield point since there was no increase or decrease in the resistance exerted by the paste to the rotation of the rheometer paddle after this point. The geopolymeric mixture activated with 100% sodium silicate differed from those mentioned above. In this case, the geopolymeric paste presented a higher yield strength, reaching an 1800 Pa yield strength, and after this limit, the paste strength decreased drastically down to 700 Pa, where it stabilized and began to decrease slowly over the time of the test.

It is possible to observe the evident difference between the rheologies of the tailings and the 100% NaOH and 100% SS geopolymer mixtures, where the tailings themselves displayed more fluid behavior with less resistance, unlike the 100% NaOH, where its behavior was more viscous. On the other hand, the 100% SS mixture has a peak at the beginning and then descended to values much closer to those of the tailings due to the fact that the sodium silicate provided a lower apparent viscosity between the tailing particles.

4.1.2. pH Test Results

The pH measurements of the fresh pastes were carried out during the agitation of the mixtures (Figure 10) to analyze the alkalinity of the systems and also the variation in pH levels from the mixing of the components to the moment when the mixture was poured into the molds; for this reason, the duration of the pH tests was 20 min. The results can be seen in Figure 11.

From Figure 11, it can be seen that the pH of the activated mixture with 100% NaOH increased rapidly to a value close to 14 due to the dissolution of the NaOH in contact with the water coming from the tailings [26]. This indicated that the alkalinity of the system was in the correct range since most authors recommend working in a pH range between 13 and 14 to obtain a good dissolution of the aluminosilicates coming from the source [9,14,15] (in this case, from copper flotation tailings), which subsequently contribute to the formation of an N-A-S-H gel. It should be noted that, even with the correct pH, the dissolution of aluminosilicates from the tailings was expected to be low due to the low reactivity of this source with alkaline agents, which is mainly due to its mineralogical composition [27]. On the other hand, the activated mixture with 100% SS presented a behavior similar to the previous one since its pH level increased quickly in the first minutes of the test; however, in this case, it reached a lower pH of 12. This was because sodium silicate has lower alkalinity than sodium hydroxide such that sodium silicate alone is not enough to reach pH values above 13. Although being in a pH range between 11 and 12, an insufficient dissolution of the aluminosilicates present in the tailings was expected; this was compensated for by the soluble silicon provided by the sodium silicate itself.

4.1.3. Temperature Test Results

At the same time that the pH tests were performed, tests were carried out to measure the temperature of the mixtures during agitation (Figure 10) to analyze the temperature variation from the addition of the reagents to the moment when the mixture was poured into the molds.

Figure 12 shows that the temperature of the mixture activated with 100% NaOH increased rapidly in the first minutes of the test; this was due to the dissolution of NaOH in the industrial water from the tailings, where this dissolution, being an exothermic reaction, generated heat. The maximum temperature reached by the mixture was 51.6 °C. At this temperature, the solubility of NaOH in water was approximately 60% [26], which meant that not all the NaOH added to the mixture was dissolved, leaving an undissolved amount.

On the other hand, the activated mixture with 100% SS presented different behavior, where it did not increase in temperature since exothermic dissolution did not occur, as with the other mixture. For this reason, during the test, the mixture remained at ambient temperature, though increasing slightly during agitation due to the heat generated by the blades’ friction with the mixture.

4.1.4. The Behavior of Hardened Geopolymers

After curing the geopolymers, i.e., after hardening them in a drying oven (Figure 13), different tests were carried out to analyze their physical properties. In order to characterize this product, and thus, verify that it complied with the necessary properties to be used as a construction material, the following tests were carried out.

4.1.5. X-ray Diffraction XRD Test Results

XRD tests were performed to analyze the formation of crystalline phases during the polymerization and curing process in the different geopolymers created and compare these results with those obtained in the XRD of the original tailing prior to adding the reagents. Figure 14 shows the results of the XRD test performed on the mixture activated with 100% NaOH, while Figure 15 shows the XRD results for the sample activated with 100% SS.

From the analysis of the XRD results (

Table 7. Phases present in the tailings and created geopolymers according to XRD testing.

Phases	100% NaOH	100% SS	Tailings
Quartz	5.47	10.07	10.09
Plagioclase	44.99	52.58	52.64
Clinochlorine	37.4	15.45	16.13
Orthoclase	-	7.06	7.09
Kaolinite	-	1.02	0.92
Pyrite	-	-	0.3
Calcite	1.27	4.91	4.83
Epidote	6.91	8.73	7.87
Titanite	1.3	0.18	0.13
Magnetite	2.67	-	0
Total	100	100	100

), it can be observed that the geopolymer activated with 100% SS (Figure 15) presented a mineralogical composition similar to that of the tailing without the addition of reagents (Figure 2), presenting slight variations. In this case, the geopolymer was mainly composed of albite (${\mathrm{NaAlSi}}_3{\mathrm{O}}_8$) (52.58%), and to a lesser extent, clinochlore (${\mathrm{Mg}}_5\mathrm{Al}\left({\mathrm{AlSi}}_3{\mathrm{O}}_{10}\right){(\mathrm{OH})}_8$) (15.45%), quartz (${\mathrm{SiO}}_2$) (10.07%), epidote (${\mathrm{Ca}}_2\left({\mathrm{Al}}_2,\mathrm{Fe}\right)\left({\mathrm{SiO}}_4\right)\left({\mathrm{Si}}_2{\mathrm{O}}_7\right)\mathrm{O}\left(\mathrm{OH}\right)$) (8.73%), and orthoclase (${\mathrm{KAlSi}}_3{\mathrm{O}}_8$) (7.06%), which were found in amounts similar to those of the original tailing. With this result, it was deduced that the activation of the geopolymers with SS did not produce great changes in the mineralogy. On the other hand, the geopolymer activated with 100% NaOH (Figure 14) showed remarkable mineralogical variations, where the amount of albite (44.99%), quartz (5.47%), and epidote (6.91%) decreased; in contrast, the amount of clinochlore (37.4%) increased. In addition, this sample did not present orthoclase minerals, and unlike the tailings and the 100% SS sample, the presence of magnetite (2.67%) was observed.

4.1.6. Automated Mineralogy Test Results (Qemscan)

Automated Qemscan mineralogy tests were performed on the hardened geopolymers to complement the information obtained via X-ray diffraction XRD tests. Figure 16a shows the modal mineralogy obtained from the Qemscan test performed on the 100% NaOH activated mixture and Figure 16b shows the modal mineralogy for the sample activated with 100% SS.

From the results, it was found that the geopolymer activated with 100% NaOH was mineralogically composed mainly of chlorite (27.55%), plagioclase (22.90%), and albite (21.77%), in addition to quartz (8.47%), orthoclase (5.24%), and epidote (4.58%).

On the other hand, the activated geopolymer with 100% SS presented a different mineralogical composition, composed mainly of albite (37.71%) and chlorite (21.38%), and to a lesser extent, quartz (14.04%), plagioclase (8.13%), and orthoclase (5.89%).

Figure 17 shows the images obtained for the mineralogical analysis performed (a) for the geopolymer activated with 100% NaOH and (b) for the geopolymer activated with 100% SS.

4.1.7. Uniaxial Compression Test Results

Compression tests were performed on each of the geopolymeric mixtures created, with a curing time of 7 days at 90 °C. As shown in the results of Figure 18, it was observed that both geopolymeric blends presented good compressive strength, and the strength values obtained were similar to those obtained in previous studies on geopolymers based on copper tailings [3], where it was expected that the compressive strength of geopolymers based on copper tailings was lower than that obtained by geopolymers with other more reactive aluminosilicates sources, such as fly ash and metakaolin. In the case of the geopolymer activated with 100% NaOH, a maximum compressive strength of 22.98 MPa was obtained with an axial deformation of 3.53%. For the geopolymer activated with 100% sodium silicate, a maximum compressive strength of 36.46 MPa was obtained with an axial deformation of 2.36%. Although a lower resistance was obtained for the first case, the elasticity of the material formed stands out since it reached a high deformation before rupture. For the second case, it was obtained that the activation of the geopolymer with sodium silicate increased the compressive strength by 13.5 MPa compared with the sample activated with NaOH.

4.1.8. Characteristic Leaching Test Results

Concerning the results obtained in the characteristic leaching tests, it was found that for the elements Ag, As, Ba, Cd, Cr, Hg, Pb, and Se, both geopolymers presented lower leached amounts than those allowed by Supreme Decree No. 148 (see

Table 8. TCLP test results to geopolymers.

Element	Maximum Allowable Value (D.S. N° 148) (mg/L)	Sample 100% NaOH (mg/L)	Sample 100% SS (mg/L)
Ag	5.0	<0.1	<0.1
As	5.0	<0.1	<0.1
Ba	100.0	4	<2
Cd	1.0	<0.02	<0.02
Cr	5.0	0.3	<0.1
Hg	0.2	<0.05	<0.05
Pb	5.0	<0.1	<0.1
Se	1.0	<0.1	<0.1

). For this reason, neither geopolymer presented the characteristic of toxicity via leaching, i.e., they did not release toxic amounts of heavy elements when a leaching element, such as an acid, was applied to them. This meant that the geopolymers created in this research were not toxic, and thus, their use and handling are safe for humans and the environment.

4.1.9. Permeability Test Results

From the hydraulic conductivity tests, it was found that both mixtures had very low permeability. The results are shown in Figure 19. In the case of the geopolymer activated with 100% NaOH, a permeability coefficient of 2.78 × 10⁻⁷ cm/s was obtained, which is a very low value and represents a practically impermeable material. For the case of the geopolymer activated with 100% SS, this presented a permeability coefficient of 1.17 × 10⁻⁷ cm/s, which also relates to a relatively low permeability value, but is slightly higher than the case of the geopolymer activated with 100% NaOH, but both cases in the same order of magnitude.

As both geopolymers had low permeability, they can be considered suitable for use as backfill in mining shafts, construction material, stability in tailings dams, and other multiple industrial uses in large-scale mining, at least because of this characteristic, since they practically prevent the transport of fluids through their structure.

5. Discussion

According to Glukhovsky [28], there are three stages in the synthesis of geopolymers, and these stages are:

• Dissolution;
• Coagulation–condensation;
• Condensation–crystallization.

During the dissolution stage, the high concentration of ${\mathrm{OH}}^{-}$ in the system is responsible for the breakage of Si-O-Si, Si-O-Al, and Al-O-Al bonds of the soluble glassy aluminosilicate phases, forming Si-OH and Al-OH groups [29], causing an increase in the pH of the medium, as observed in Figure 11. The appearance of ≡ $\mathrm{Si}$-${\mathrm{O}}^{-}$-${\mathrm{Na}}^{+}$ bonds prevents the Si-O-Si bonds from reforming again. The aforementioned alkali silicate bonds can contribute to ion exchange and types of complexes such as Si-O-Ca-OH. With Al-O-Si bonds, the same type of reaction happens and ends up generating predominantly $\mathrm{Al}{\left(\mathrm{OH}\right)}_4{}^{-}$-type complexes. The -$\mathrm{Si}$-$\mathrm{O}$-${\mathrm{Na}}^{+}$ complexes are stable in an alkaline medium, thus fulfilling a transport role and allowing the development of a coagulated structure (colloidal phase) based on the abovementioned units.

In the coagulation–condensation stage, due to the high pH > 11, the disintegration of the Si-O-Si bond gives rise to hydroxylated complexes, with $\mathrm{Si}{\left(\mathrm{OH}\right)}_3{\mathrm{O}}^{-}$ as the most stable component, which condenses and forms a new Si-O-Si bond. The ${\mathrm{OH}}^{-}$ ion acts as a catalyst for the reaction. The polyhydroxylan groups are formed by the polymerization of orthosilicic acid and can grow in all directions, leading to the formation of colloidal particles. The formation of the hydroaluminogel depends on the pH of the medium.

For the condensation–crystallization stage, in addition to the microparticles formed from the condensation, the particles of the solid phases coming from the source (copper flotation tailings in the case of this research) indicate the precipitation of products that are dependent on the mineralogy and chemistry of the initial phase, as well as the nature of the alkaline component and the hardening conditions.

According to the study by Jianhe Xie et al. [30], the simple compressive strength could be improved due to the coupling effect of calcium aluminosilicate gel (C-A-S-H), calcium silicate gel (C-S-H), and sodium aluminosilicate gel (N-A-S-H) in the hydration of alkali-activated fly-ash-based geopolymer products; however, for the present study and given that the percentage of calcium present according to the ICP test was 3.28% in the copper flotation tailings, the same analysis cannot be done. It is suggested to carry out future research by making geopolymeric mixtures with calcium addition and studying their behavior.

The carbonation mechanism in alkali-activated geopolymers is different from that in Portland cement since, in Portland cement pastes, atmospheric CO₂ dissolves in the pore solution and reacts rapidly with portlandite to form CaCO₃ and then with a calcium-silicate-hydrate (C-S-H) gel to form CaCO₃ and silica gel; in contrast, the carbonation of alkali-activated pastes occurs directly in the calcium-aluminosilicate-hydrate (C-A-S-H) gel due to the lack of portlandite, leaving an alumina-containing silica gel remaining, in addition to CaCO₃ [31,32]. Relative humidity of 95% in the curing chambers can inhibit initial carbonation in geopolymers [33].

On the other hand, the dissolution of calcium in sources processed in a low alkalinity system, which was not the case in this research, provides good resistance to simple compression due to the formation of C-S-H gel (calcium-silicon-H₂O) that coexists with the geopolymer gel (N-A-S-H), which complements each other and forms a mostly amorphous structure that provides better mechanical behavior. For aluminosilicate sources, such as copper flotation tailings, very little calcium is dissolved (the percentage of calcium present according to the ICP test was 3.28% in copper flotation tailings), causing a C-S-H gel not to form. The work of Professor Engui Liu [34] on the use of marine shells goes in this direction of taking advantage of the improved characteristics of Portland cement and geopolymers, increasing their hydration of the C-S-H matrix phases and resistance to simple compression; however, the addition of further treatment, selection, and crushing make this technique more expensive for the traditional mining industry.

In high-alkalinity systems, a higher concentration of NaOH or Na₂SiO₃, as in the case of this research, leads to the formation of N-A-S-H gel being predominant, where the role of calcium is less influential in the final product as it cannot generate a C-S-H gel that contributes to the geopolymer gel or a large amount of crystals that interrupt the amorphous gel structure; therefore, the dissolution of calcium does not impact in a determinant way in the simple compressive strength [34].

The work of Yip et al. [35] on geopolymer blends focused on replacing part of the aluminosilicate source with calcium, as did the work of Tian et al. [36], which replaced part of the alkaline activator (NaOH) with industrially produced calcium oxide (CaO) (since the sources of these authors did not have soluble calcium sources) to form a geopolymer based on copper tailings from China with the same compositions in their studies and analyzed the effects of varying the curing temperature [37] and fly ash. It was observed that by substituting 20% of the alkaline activator with CaO, this mixture provided a higher simple compressive strength, but slightly reduced the long-term simple compressive strength (28 days of aging at ambient conditions) since the best strength was obtained at 7 days. In addition, they observed that Si, Al, and Na tended to accumulate in the area corresponding to the N-A-S-H gel and the area with the absence of these elements (Si, Al, and Na) was where the concentration of Ca predominates, concluding that there was no interaction of Ca with the rest of the elements to form a C-S-H gel. On the other hand, unreacted sectors were observed with a high concentration of aluminum, which would indicate that much of the aluminum present in the tailings was in the form of non-soluble crystals; therefore, it was not diluted in the solution, as it had a structure that the alkaline solution was unable to dissolve.

The influence of pH on the development of the compressive strength of geopolymers is directly related to the dissolution of the aluminosilicates present in the raw material, which in this case was copper flotation tailings. This dissolution allowed both silicon and aluminum to go into solution and, therefore, were usable during the geopolymerization process for the formation of the N-A-S-H gel, which was ultimately responsible for providing the strength and stiffness of these materials. At this point, future work to be developed could involve the use of aluminum sulfate and/or fluorine gypsum as a possibility to increase the resistance to simple compression obtained in some percentage to be optimized (2% by weight), as in the work of Professor Yilu Wang [38], enabling optimizations and cost reductions in the generation of geopolymers of copper flotation tailings.

The difference in the rheologies, considering that all of them were performed at the same %Cp, that is, with low water contents, lay mainly in the reagent used, which was higher in the case of sodium hydroxide relative to the sodium silicate rheology; this could have been due to the fact that sodium hydroxide continued to react in the mixture, generating higher coagulations and condensations due to the high pH measured (pH > 14), causing an early solidification of the N-A-S-H gel, as indicated by Lahlou et al. [39], Favier et al. [40], and Riffai et al. [12]. This was not so for the case of Na₂SiO₃, where the pH was between 11 and 12. In addition, as Na₂SiO₃ is soluble, it provides higher apparent viscosity to the tailing particles due to the ease of soluble silica to dissolve and start to polymerize, which allowed for generating a metastable hydrated gel that served as a core for the N-A-S-H gel, as indicated by Favier et al. [40].

6. Conclusions

According to the methodology used for the creation of the geopolymers, their composition, and the temperatures and curing times, it was deduced that these variables were in the correct ranges because, in both cases, hardened geopolymers with good mechanical properties were obtained, which is essential due to the future applications that can be given to this new material.

The characterization of the copper tailings used was fundamental since it could be demonstrated that it was mainly composed of Si and Al, which are elements that are essential for the formation of geopolymers. In addition, the mineralogical characterization and the degree of grinding of the particles was also an important parameter to consider since this indicated how much material from the aluminosilicate source was available for the geopolymerization reactions. On the other hand, the composition of the geopolymeric mixtures created was within the range studied by previous authors.

After curing the geopolymers, it was found that both presented volumetric contractions due to the loss of water that occurred during the geopolymerization process, where this change in volume was more noticeable in the mixture activated with sodium silicate.

Among the geopolymeric mixtures created, the one activated with NaOH had a more specific rheological behavior, presenting an almost linear shape after its yield point. On the other hand, the sample activated with sodium silicate showed a significant decrease in the stress required to deform this pulp after its high yield point.

It was found that the sample activated with NaOH presented higher pH and temperature values during its agitation due to the exothermic reaction of dissolution of this reagent when in contact with water. On the other hand, the mixture activated with SS had lower pH and temperature values because this reagent was not as alkaline as NaOH, and it did not present exothermic reactions during its dissolution.

Based on the mineralogy of the geopolymers, it was found that the one activated with SS presented mineralogy similar to that of the tailings without the addition of reagents. In contrast, the one activated with NaOH presented significant variation in its mineralogy, from which it can be deduced that this reagent caused more significant changes in the raw material, dissolving a greater amount of crystalline phases. The information obtained from the Qemscan tests corroborated these results.

Based on [2], it was identified that the final characteristics of the geopolymer depended on its chemical formation, where the chemical elements Na, Al, and Si, along with H₂O, played a fundamental role in the generation of the N-A-S-H gel, where these first elements are the main compounds of plagioclase, and thus, explaining its decrease in comparative mineralogical analyses.

Based on the uniaxial compression results, it was found that the geopolymer that obtained the highest resistance was the one activated with SS, with 34.46 MPa, and then the one activated with NaOH, with approximately 23 MPa compressive strength.

On the other hand, neither geopolymer presented the characteristic of toxicity via leaching; therefore, these created materials are safe for use. Finally, it was found that both geopolymers presented very low permeability levels, raising their tailing permeability due to geopolymerization.