*Advances in Geopolymer-Zeolite Composites - Synthesis and Characterization*

leads to a mineralogical and granulometric structure highly variable from one pile to another and especially inside the same dump.

Mining has a strong influence on the environment, through various forms and ways affecting most environmental factors, namely: air, water, flora, fauna, landscape and human settlements, cultural heritage, population health, agriculture. Among the most important influences are [22]:


Waste recycling brings many benefits (**Figure 3**), the most important being: conservation of natural resources, reduction of storage space, protection of the environment and recovery of materials deposited in dumps by developing new materials.

The physical and the chemical characteristics of the tailings vary considerably, depending on the mineralogical and geochemical composition, sedimentation characteristics, specific gravity of the particles, rheology and viscosity, hydraulic permeability and conductivity, the evolution of the cementation process, the chemical composition of the water in the pores, the degree of contamination of the external

**Figure 3.** *Advantages of mine tailing recycling.*

### *The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*

surface or underground environment etc. The mineralogical and geochemical composition of the solid material is strictly dependent on the paragenetic characteristics of the processed deposits. This aspect gives the tailings ponds uniqueness because there aren't two ore deposits with the same characteristics. On top of, depending on the quality and performance of the technology used in the separation process, tailings ponds can sometimes contain large amounts of metal sources.

The effects produced by the mine tailings facilities on the soil are, also, related to the raising type of storage (**Figure 4**). As can be seen, over 40% of the tailing dumps worldwide are upstream, mostly, because this is the cheapest design, unfortunately, this is also the most susceptible to failure design, which can result in huge environmental consequences [23]. Downstream is the following inline, being used at over 30% of the tailing's facilities. This design eliminates the disadvantages specific to the upstream designs, yet, the production cost is higher and the occupied space is considerably increased. Moreover, the amount of concrete building materials is significantly higher. Centerline design is a mix between upstream and downstream, this shows a lower failure coefficient than upstream and can be realized with fewer materials than downstream. The designs used in lower percentage (single-stage, dry stack, other) are usually specific to small mines, in terms of the extracted volume, while the in-pit method requires an empty/closed mine that can be filled with the resulted waste. A simplified schematic representation of the upstream, downstream and centerline design can be seen in **Figure 5**.

Although new construction designs have been developed, due to the fact that mines producing greater amounts of mine tailings, as lower grades of ore must be processed, the waste facilities are filled overcapacity, therefore, serious tailings dam failure occur (until 2027, globally, more than 15 catastrophic failures are predicted) [24]. Accordingly, by introducing and encouraging the use of mine tailings in geopolymerisation technology, a convenable source of raw materials for civil construction applications will be developed, while the requirements for a circular economy of the mining sector will be fulfilled.

#### **Figure 5.**

*Schematic representation of the construction design of tailings facilities [23].*

Only in Romania, there are over 1100 tailings dumps and industrial landfills, distributed on the territory of 29 counties, respectively 13 counties for waste dumps (**Figure 6**). Of these, over 990 come from mining activities, while about 190 are located near protected areas. Moreover, more than 40 of them present serious stability problems [25]. Moreover, according to the report published by the Ministry of Economy, Romania is the country with the highest percentage (over 85%, the average in Europe being 25%) of waste resulting from the extractive industry.

Currently, the global stored volume of mine tailings is close to 55 billion cubic meters, and an increase of 23% is expected until 2025 [26]. Accordingly, the use of mine tailings in concrete can support the conservation of natural resources, specific to these activities, for 4 to 5 years [27].

#### *2.3.1 Cooper mine tailings*

Considering the fact that copper tailings are available in large volumes worldwide, and those increase considerable every day [25, 28], multiple authors focused their study on introducing these types of aluminosilicates in geopolymers technology, as precursors or partial replacements of conventional resources. Furthermore, this section aims to summarize the results obtained in this study and the main parameters that influence the feasibility of mine tailings synthetization in geopolymers.

Paiva et al. [29] developed geopolymers based on two types of metakaolin which incorporates fine particles of high-sulfidic Mine Tailings (MT) which comes from a copper and zinc mine. According to their study, the compressive strength of the geopolymers decreases with the increase of the substitution percentage despite the

**Figure 6.** *Distribution of mine tailing facilities on Romania territory [25].*

#### *The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*

curing temperature. Therefore, after replacing 38% of MetaKaolin (MK) mass with mine tailings a decrease of 44% of compressive strength was obtained, while after 50% of metakaolin was replaced a decrease of 69% was observed, for the samples cured at room temperature (**Figure 7**). Based on the XRD evaluation (phases identified: Pyrite FeS2, Anhydrite Ca(SO4), Caldecahydrite CaAl2O4 10H2O, Quartz SiO2), they stated that the analyzed mine waste cannot be used as a precursor for geopolymerisation because no reactive aluminosilicates are present in its composition. However, it can be used in blended systems, even its incorporation results in a more compact structure (bulk density increases from 1.7 to 1.9) with lower mechanical characteristics. Moreover, when Blast Furnace Slag (BFS) was used as a precursor, the bulk density decreases and so does the compressive strength, however, better values can be obtained by curing at high temperature (**Figure 8**).

The structural analysis of a cooper-barium mine tailing activated with 10 M NaOH for geopolymers synthesis exhibits a heterogeneous matrix with a partially dissolved structure, full of voids and unreacted particles (**Figure 9**).

The reactivity of mine tailings in alkaline activators was reported by Obenaus-Emler et al. [30]. According to their publication, the amount of dissolved species from metakaolin reaches 80%, for granulated furnace slag the value was close to 60%, while for copper mine tailing the dissolved amount was lower than 5%. However, better results were obtained after increasing the curing time or the NaOH concentration (**Table 2**). The compressive strength of the obtained geopolymers seems to depend on the same parameters, accordingly, an increase of 40% was obtained for samples cured at 60°C (compared with those cured at room temperature), while by increasing the concentration of water glass from 10–30%, four times higher compressive strength was obtained for room temperature cured samples, and 6 times higher for 60°C cured samples, respectively. Moreover, one more parameter that showed promising results on improving mechanical characteristics was the addition of finer particles or BFS, however, this also affects the pore size distribution of the hardened product.

In another study, Ahmari et al. [31] successfully synthesized geopolymers with copper mine tailings as precursors. According to their study, satisfying compressive strength can be obtained by customizing the curing parameters and the activator (**Figure 10**). The samples synthesized with a 15 M NaOH solution exhibit the optimum value when cured at 90 °C, while for 5 M NaOH and 10 M NaOH, 75°C was the optimum temperature.

Moreover, by introducing sodium silicate in the composition, the highest compressive strength can be obtained at SiO2 to Na2O equal to 1, for the geopolymer activated with 10 M NaOH and cured for 7 days at 60°C (sample 1SS). When the ratio is increased the mechanical characteristics will decrease because this solution

**Figure 7.**

*The effect of precursors replacing with copper mine tailings on compressive strength (\* - approximate value).*

#### **Figure 8.**

*Compressive strength of 50°C cured samples for different periods (\*\* - for sealed samples).*

#### **Figure 9.**

*The microstructure of cooper-barium mine tailing based geopolymer.*

provides the amount of silicon needed in the early stages of geopolymerisation, but in too high concentrations it prevents water evaporation, while encapsulates the particles of the raw material in a film that does not allow the catalyst to advance. Also, for 10 M NaOH activate geopolymers, cured at 90°C, the addition of sodium aluminate in a mass ratio of 1.25 (sodium aluminate/NaOH solution) showed an impressive increase of compressive strength from ≈6 MPa to ≈17 MPa (sample 1SA). The activator and curing parameters also affect the microstructure of the obtained samples, consequently, a denser matrix was observed at higher NaOH concentration


*The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*

#### **Table 2.**

*The amount of dissolved species after 24 h leaching.*

**Figure 10.** *Activator and curing parameters effects on compressive strength in cooper mine waste-based geopolymers.*

or curing temperature. As can be seen from the graph, better results can be obtained by curing the 1SS samples at 90°C, or by increasing the molar concentration of the NaOH solution from 10 M to 15 M, in the case of geopolymers with additions.

According to another study [32], if appropriate manufacturing conditions (NaOH concentration, initial water content, forming pressure, and curing parameters) are selected, the copper mine tailings are feasible to produce eco-friendly bricks by a simple three steps method: (i) mixing the mine tailing with the activator, (ii) compress the mixture in a mold at a specific pressure, (iii) cure the products at slightly elevated temperature.

#### *2.3.2 Tungsten mine tailings*

Usually, tailings from many tungsten extraction facilities contain only small quantities of heavy metals and fatty acids, therefore, those aren't classified as class A facilities. However, due to their large amount available (the production of tungsten reached 87,000 metric tons in 2015, therefore, the amount of waste is even higher) long-term processing and managements solution must be developed [33].

Tungsten Waste-based Geopolymers (TWG) has been successfully synthesized by mixing the cementitious powder with different percentages of calcium hydroxide and mixes of NaOH and waterglass solution. According to previous studies, this type of geopolymers has good adhesion properties, low water absorption and high mechanical characteristics. In their study, Pacheco-Torgal et al. [34], evaluated the influence of aggregates (sand) and calcium hydroxide (Ca(OH)2) introduction in the matrix of TWG on compressive strength evolution. The optimum Ca(OH)2

concentration was 10%, while the ratio of 1 for sand to binder mass showed the most promising results. Moreover, when water to sodium hydroxide ratio was evaluated (**Figure 11a**) it was observed that higher NaOH concentrations result in better compressive strength, while higher water content badly affects the mechanical strength evolution (**Figure 11b**).

In another study [35], the authors evaluated the adhesion characteristics of tungsten-based geopolymers to the surface of pretreated OPC concrete. According to their study, the samples exhibit a high bond strength and monolithic failure even after 1 day of curing. Moreover, they stated that this type of geopolymers are three times stronger than C30/37 strength class OPC concrete, in terms of abrasion resistance, and, almost the same difference, in terms of sulfuric acid resistance. This can be related to the low unrestrained shrinkage and capillary water absorption coefficients, respectively.

#### *2.3.3 Gold mine tailings*

Considering the fact that the majority of the processed gold comes from the open querries, and their activities include substrate removal and grounding of high amounts of rock (almost 20 tones of waste is generated for a standard 18-karat ring). Gold products obtained following mining activities are through the most tailing generative.

Waste resulted from gold mining activities was also approached in geopolymers obtaining. Kiventera et al. [36] synthesized Gold Mine Tailing (GMT) based geopolymers or blended systems of GMT and BFS activated with different NaOH solutions. According to their study, due to the fact that the water quantity constrains the efficiency of the polycondensation stage, the workability was affected by the molarity of NaOH solution and solid to liquid ratio. Additionally, it was observed that a higher water quantity can result in multiples cracks and pores which

#### **Figure 11.**

*The influence of different parameters on compressive strength of TWG, depending on curing time: a) water to sodium hydroxide mass ratio; b) solid to liquid ratio; [34].*

#### *The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*

significantly affects the mechanical characteristics of the samples. Therefore, for optimal properties, a high NaOH quantity must be dissolute in the binder because the Na/Al and Na/Si ratios depend on this, while the strength of the products depends on the volume of binder created during the dissolution stage. However, this need is limited by the NaOH solubility in water, previous studies [31] show that a molar concentration higher than 24 is hard to achieve.

The compressive strength of GMT bricks, measured after 28 days of curing, exhibit a 35% increase, when the NaOH concentration was changed from 5 M to 15 M, also, an even higher effect (at 5 M the compressive strength increased from 1.4 MPa to 2.2 MPa, at 10 M it increases from 2.2 MPa to 3 MPa, while at 15 M NaOH the value increases from 3 to 3.5 MPa) was observed when the liquid to solid ratio was decreased from 0.25 to 0.15 [36].

Demir et al. [37] studied the possibility of obtaining GMT geopolymers for the removal of toxic and radioactive waste by realizing tests of Pb2+ adsorption from aqueous solutions. Firstly, the reaction degree depending on the curing temperature and Al2O3 addition was analyzed, accordingly, it was observed that higher Al2O3 content results in better reaction degree, while curing temperature showed minimum effects. Secondly, the Langmuir isotherm model and second-order kinetic model were identified as being the most suitable to evaluate the Pb2+ absorption. Accordingly, they state that the highest absorption efficiency obtained was 94% for GMT geopolymers with alumina addition.

In another study [38], the author evaluated the potential of obtaining geopolymeric concrete by replacing different percentages of type I Ordinary Portland Cement (OPC). The evaluation has been performed by compressive strength test on samples with 50% by wt. GMT (50GMT), 70%, wt. GMT (70GMT), 80% wt. GMT (80GMT) and 90% by wt. GMT (90GMT), respectively. According to their results, the presence of high percentages of tailing particles significantly increases the mechanical characteristics of the concrete, in 12 h cured samples at 70°C, despite the aging time (**Figure 12**).

Moreover, when the effects of harsh environment conditions (H2SO4, HNO3, MgSO4, Na2SO4 acids attack or high-temperature exposure) was evaluated, it was concluded that the increase of GMT also has good effects on these characteristics. However, if 55 days of immersion in acids reduce the compressive value to almost half, the high-temperature exposure at 1000°C almost destroyed the structure of the samples.

#### *2.3.4 Phosphate mine tailings*

The amount of tailings generated by a potassium mine depends primarily on the configuration of the potassium vein, the stability of the rocks and the mineral composition. All these are natural conditions that vary from one mine to another, from one warehouse to another and sometimes even within the same warehouse. Therefore, there is no standard model of mines in terms of processing and generation of finished products and tailings. Each mine has its specific conditions regarding the generation of liquid and solid tailings and their management. Thus, these specific conditions can vary throughout the life of a mine. However, for economic reasons, operators will seek to minimize the amount of mined tailing and processed [39].

Regarding the solid tailing facilities, waste management involves discharging it into the dump sites and refilling underground mines. The tailings from hot distillation and flotation, with sodium chloride as the main component, are dehydrated using centrifuges and filters, and then transported, employing conveyor belts, to the tailings dump. Besides, the dry process of electrostatic separation allows the dry management of the tailings at the tailings dump [40].

Regarding the liquid tailings, waste management involves discharge into groundwater (under certain geological conditions) and/or discharge into surface waters.

Fine Phosphate Sludge (PS) from the Moroccan Youssoufia plants were used for blended geopolymers obtaining by Moukannaa et al. [41]. According to their publication, PS rich in fluorapatite, quartz, calcite, dolomite, illite, palygorskite and hematite, needs supplementary aluminum content to present geopolymerisation potential, therefore, highly amorphous metakaolin was introduced in addition. Moreover, the reactivity increase was also performed by fusion activation at 550°C and 800°C, respectively. Based on the XRD analysis, it was observed that due to the presence of NaOH and high temperature, hematite decomposition occurs, while sodium aluminum silicate, sodium-calcium silicate, periclase and titanite formation is confirmed. Also, it was stated that the 550°C activations slightly reduce the content of dolomite, palygorskite, calcite and fluorapatite, while the quartz content seems to remain constant. However, at 800°C dolomite was greatly reduced, while the content of Na-rich phases significantly increases. On top of that, when the NaOH content was increased from 10–20%, the effects obtained at 800°C, were obtained at much lower fusion temperature (550°C). Surprisingly, based on the compressive strength results, the samples, cured at 28°C, made with raw materials fused at 550°C with 10% NaOH showed higher values than those with 20% NaOH or those fused at 800°C. Based on the microstructural analysis, it was stated that the poor strength, of 800°C fused samples, is related to the heterogeneous distribution of the sand particles, which acts as barriers for crack propagation in the matrix of 550°C fused samples (**Figure 13**). This phenomenon was also reported in previous studies [17, 42]. However, higher mechanical properties can be obtained by increasing the metakaolin content or by curing the samples at temperatures lower than 350°C.

The recycling possibility through geopolymerisation technology was also confirmed in [43]. According to the study, for optimum physical (low water absorption, high density etc.) and mechanical properties (compressive strength up to 62 MPa) a curing temperature of 83.33°C, a curing time of 14.50 days, and NaOH concentration of 12.5 M must be selected for phosphate sludge-based geopolymers mixed with fly ash or metakaolin.

#### *2.3.5 Aluminum mine tailings (red mud)*

Red Mud (RM) is the main secondary product (waste) that results from the Bayer process of refining bauxite to produce alumina. Generally, this process

*The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*

**Figure 13.**

*Particle distribution effects on geopolymers microstructure: a) uniform distribution; b) heterogeneous distribution [41].*

treats the aluminum ore (bauxite) with concentrated NaOH solutions at elevated temperature and pressure aiming to extract the gibbsite (Al(OH)3), diaspore (AlO(OH)) and/or boehmite (γ-AlO(OH)). Therefore, RM is a strong alkaline waste due to the incomplete removal of NaOH. Accordingly, its composition depends on several factors but mainly on the composition of bauxite. The resulting amount of red mud also depends on the composition of the bauxite, however, it ranges from 0.33 up to 2 t of red mud per ton of alumina produced [5, 44]. Apart from the huge amount which is stored in tailing lakes, the main problem related to the environmental effects produced by this waste is its alkalinity. Moreover, considering that from Jan 1974 to Oct 2020 the global amount of extracted alumina was ≈2,814,091 thousand metric tonnes by multiplying with 1.1 (the average value of generated waste), the total amount of waste generated due to alumina extraction is ≈3,095,500 thousand metric tonnes [45]. According to the literature, only 2–3% of the produced waste is used in industrial-scale application, especially in civil engineering applications, therefore, almost 98% is being deposited [46].

Although, the methods of depositing and securing red mud facilities have been improved significantly over time, currently, worldwide more than 60 refineries extract alumina through the Bayer process depositing the waste in lakes [47, 48].

As can be seen from the literature, red mud is the most studied mine tailing for geopolymers obtaining [49, 50]. As stated in [49], Wagh et al. [51] published the most systematic study on the red mud use in alkali-activated materials. Accordingly, even from preliminary studies promising compressive strength (between 35 and 45 MPa) results have been obtained for RM as a single solid precursor. Moreover, by 15% metakaolin addition [52], RM becomes acts as a filler, and the compressive strength increase more than four times, compared with the geopolymer with RM as a single precursor. Depending on the replaced percentages, other authors obtained even better results [53, 54]. In terms of mechanical compressive, the addition of ground granulated blast furnace slag [55], rice husk ash [56] or fly ash [57] also showed significant improvements.

### **3. Conclusions**

Based on the analyzed studies, the following conclusions can be drawn regarding the possibility to use mine tailings as precursors or components for geopolymers:


*The Physical and Mechanical Characteristics of Geopolymers Using Mine Tailings as Precursors DOI: http://dx.doi.org/10.5772/intechopen.97807*
