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Dieser Artikel geht der Untersuchung und Bewertung von Nebenprodukten aus der Verarbeitung von Spodumen, insbesondere Aluminiumsilikat und Gips, für ihren potenziellen Einsatz in der Baustoffindustrie nach. Die Studie konzentriert sich auf die chemische und mineralogische Zusammensetzung dieser Nebenprodukte, ihre Korngrößenverteilung, pH-Werte und Hochtemperatureigenschaften. Bei den experimentellen Verfahren werden diese Materialien als Zusätze für Zement und Beton getestet, wobei besonderes Augenmerk auf ihr Abbindeverhalten, Druckfestigkeit und Biegefestigkeit gelegt wird. Die Ergebnisse deuten darauf hin, dass sowohl Aluminiumsilikat als auch Gips vielversprechendes Potenzial als Zusätze aufweisen, wobei Aluminiumsilikat die Festigkeit von Zement und Beton erhöht und Gips als Abbindemittel dient. Die Schlussfolgerung hebt die Vorteile der Nutzung dieser Nebenprodukte hervor, darunter verringerte CO2-Emissionen, minimierte Rohstoffgewinnung und geringere Notwendigkeit, die Nebenprodukte selbst zu deponieren.
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Abstract
The most common method of obtaining lithium from spodumene is by extracting it through a chemical treatment process, which produces various by-products, including aluminosilicates and a gypsum-like material. Instead of disposal, various applications for these products are feasible, depending on their properties. Therefore, two such by-products from an industrial processing plant were investigated for their mineralogical and physical properties with respect to different application options. The aluminosilicate was used as an additive for cement and concrete, demonstrating that even an addition of up to 20 wt. % to ordinary Portland cement does not reduce strength or negatively affect the setting behaviour. The use of the gypsum material as a setting control additive also showed no disadvantages. Both materials appear to be promising raw materials for the building materials industry.
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1 Introduction
Lithium is an essential material in many key applications related to sustainability—amongst others the production of Li-Ion batteries—and, due to its limited availability, also on the list of critical raw materials of the European Union [1]. The share of lithium in the earth’s crust is about 0.006%. Due to its high reactivity, it is not found in its elemental state but mineral rocks like pegmatites as well as in saline lacustrine environments [2]. The most important lithium-bearing mineral in pegmatites is spodumene LiAl[Si2O6], which can theoretically contain up to 8 wt. % of lithiumoxide. For the extraction of lithium various processing techniques are employed, including dense media separation, magnetic separation, ore sorting, and flotation [3]. A common method for producing spodumene concentrate involves crushing, sieving, conditioning, flotation, and thickening [4, 5]. The concentrate is then heat-treated to convert α‑spodumene to β‑spodumene at temperatures between 1000–1100 °C prior to leaching. This conversion is necessary because the β‑phase is more reactive and has a lower density, simplifying the leaching process [2, 6, 7]. The leaching is carried out using acids (such as sulfuric acid, hydrofluoric acid, hydrochloric acid, or nitric acid) or alkaline solutions (e.g. limestone) [2, 7]. When sulfuric acid is used, the β‑spodumene is mixed with it and heated to 175–250 °C, producing Li2SO4 and an aluminosilicate (pyrophyllite) as a by-product, according to the following equation [2]:
The products are recovered through water leaching and filtration. In a further purification step, calcium carbonate (CaCO3) is used to neutralize excess sulfuric acid, resulting in gypsum as an additional by-product [2, 7]. Given the average Li2O content of approximately 6–7 wt. % in spodumene concentrate, the amount of pyrophyllite generated as a by-product during the described processing steps is substantial. Significant quantities of gypsum are also produced. To align with modern efforts toward responsible production, landfill disposal of these materials is not a viable option, and alternative, meaningful uses must be identified.
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Due to its properties—such as a low thermal expansion coefficient, low thermal and electrical conductivity, and high corrosion resistance to liquid metals—pyrophyllite is suitable for numerous industrial applications, including the use in refractories, ceramics, fiberglass, and the cosmetics industry [8]. Several studies have explored pyrophyllite as a precursor for geopolymers. MacKenzie et al. [9] state the dehydroxilation temperature of pyrophyllite as 800 °C, whereas Sanches-Soto et al. [10] suggest high-energy milling to disrupt the pyrophyllite crystal structure. Both facilitating leaching of aluminium for the formation of geopolymers [11]. Ettahiri et al. [12] investigated calcined natural pyrophyllite as a starting material, but their results were unsatisfactory. Karrech et al. [13] compared the performance of an aluminosilicate-rich by-product from spodumene processing with fly ash as raw material for geopolymer production. The by-product increased the setting time and workability of the geopolymer. Similarly, in [14], the use of an aluminosilicate residue from lithium processing was compared with granulated blast furnace slag, fly ash, metakaolin, and kaolin. The aluminosilicate showed lower reactivity during the alkaline activation process, which delayed the setting of the geopolymer. However, when combined with granulated blast furnace slag in a 1:1 ratio, compressive strength after 28 days was three times higher than those of conventional concrete. Thus, pyrophyllite not only enhances the performance of alternative binders but can also partially replace conventional binders.
The use of pyrophyllite as a supplementary cementitious material was studied in [15], where 10 wt. % of cement in self-compacting concrete was replaced by calcined pyrophyllite. The substitution slightly decreased the workability compared to a reference without additives. Additionally, the calcined pyrophyllite slowed the hardening process, leading to lower compressive, flexural, and tensile strength values after 28 days. A similar outcome was observed in [16], where polypropylene fibres were added to concrete along with calcined pyrophyllite for reinforcement. The use of pyrophyllite powder in insulating cements is already patented [17].
Woskowski et al. [18] investigated the use of leached spodumene concentrate as supplementary cementitious material leading to lower early compressive strength values after seven days, but increased values in case of a 20 wt. % addition after 28 days compared to a reference. A similar strength increasing effect was discovered in [19] using lithium slag (containing gypsum, spodumene, and quartz) for the fabrication of a lithium composite cement.
The primary application of the gypsum-rich by-product from spodumene processing can potentially be in the building materials industry, replacing natural gypsum or synthetic gypsum (e.g. from flue gas desulfurization | FGD gypsum). As described in [20], it is crucial to analyse the contents of dihydrate, hemihydrate, and anhydrite, as well as any impurities, to determine the appropriate field of application.
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In summary, both by-products—aluminosilicate and gypsum—show great potential for applications in the cement and concrete industries. Although many studies have already investigated alternative applications for by-products from spodumene processing (especially pyrophyllite as a geopolymer precursor), some areas still lack sufficient information. Therefore, the objective of the investigations presented here was to identify and evaluate the potential of both by-products as additives for cement and concrete.
2 Materials Characterisation
For the investigations, two by-products originating from a spodumene ore processing pilot plant operated by Nemaska Lithium Inc. were used. In the process, the spodumene concentrate is calcined, milled, and then leached with sulfuric acid, as shown in Eq. 1. After filtration, the first by-product (referred to here as aluminosilicate AlSi) is obtained. Following an additional purification step of the lithium concentrate, the second by-product (referred to here as gypsum) is produced. Since these materials originate from an industrial process, they first need to be characterised in terms of their chemical and mineralogical composition. Additionally, their grain size distribution, pH value, and high-temperature properties are analysed.
2.1 Chemical Composition
The chemical composition of both samples was determined. The samples were first dried at 105 °C, and the loss on ignition (LOI) was measured after heat treatment at 1000 °C in a platinum crucible. The oxide compositions, as determined by X‑ray fluorescence (XRF), are presented in Table 1. The lithium oxide (Li2O) content was analysed using ICP-MS, yielding 0.57 wt. % for the aluminosilicate sample and 0.85 wt. % for the gypsum sample.
TABLE 1
Chemical composition of the investigated materials in wt. %
[wt. %]
AlSi
Gypsum
Na2O
0.71
0.05
MgO
0.09
0.02
Al2O3
22.08
7.36
SiO2
71.55
0.79
P2O5
0.037
0.09
SO3
0.05
47.47
K2O
0.72
0.02
CaO
0.18
33.52
TiO2
0.03
< 0.01
Cr2O3
< 0.01
< 0.01
MnO
0.1
0.03
Fe2O3
0.8
2.00
LOI
2.75
8.76
Sum
99.10
100.11
Li2O
0.57
0.85
2.2 Mineralogical Composition
Both samples were analysed for their mineralogical composition using X‑ray diffraction (XRD; Bruker D‑8 Advance) with CuKα radiation. The aluminosilicate primarily contains pyrophyllite and quartz, along with small quantities of various feldspar silicates (e.g. albite, microcline) (see Fig. 1). Although small amounts of spodumene are present, its main peak overlaps with that of pyrophyllite, making it difficult to discern. The gypsum sample, according to XRD analysis, is identified as a pure dihydrate (see Fig. 2).
Polished sections were prepared from both samples by embedding a small amount of the material in resin with subsequent polishing. These samples were then examined using a scanning electron microscope (SEM; Carl Zeiss Evo MA 15), and the phases were analysed using energy-dispersive X‑ray spectroscopy (EDX; Oxford Instruments Ultim MAX 65). An overview of the samples’ microscopic appearance is shown in Fig. 3. From the images, it is clear that the aluminosilicate sample (AlSi) contains several distinct minerals (Fig. 3a), whereas the gypsum sample is nearly a single-phase material (Fig. 3b).
Fig. 3
Back scatter electron images of the materials showing a microstructural overview of the aluminosilicate, b microstructural overview of the gypsum, c microstructural detail of the aluminosilicate 1) Pyrophyllite, 2) Alkali feldspar, 3) Quartz, 4) Orthoclase, 5) Plagioclase and d microstructural detail of the gypsum 1) CaSO4*2H2O, 2) Iron aluminiumhydroxide
The grain size distribution was determined with a Helos (H2383) & Rodos laser particle sizer and is shown in Fig. 4. The aluminosilicate (AlSi) exhibits a coarser texture than the gypsum material, and its distribution is relatively broad, as illustrated in the microscopic images (Fig. 3). For the aluminosilicate, the X50 is 12 µm, while the X90 is approximately 99 µm. In contrast, the gypsum has an X50 of 10 µm and an X90 of 41 µm.
Fig. 4
Grain size distribution of the delivered materials
The pH-value was determined by mixing 10 g of each by-product with 100 g of distilled water. The pH-value for the gypsum is around 7. For the aluminosilicate it is between 6 and 7, so it can be considered as slightly acidic.
2.4 Transformation Behaviour
Both materials were analysed for their thermal transformation behaviour using a simultaneous thermal analyser (Netzsch, Jupiter 449 F3). Concurrently, the total mass loss was measured. Based on their mineralogical compositions, the gypsum sample was studied up to 500 °C, while the aluminosilicate (AlSi) was examined up to 1200 °C. The DTA/TG curves are presented in Figs. 5 and 6, with the reactions associated with the observed peaks listed in Table 2. After the heat treatment process, both samples were re-evaluated for their mineralogical composition.
Phase reactions during heat treatment of the by-products
T [°C]
Reaction
Mass loss [wt. %]
Gypsum
111.4
Loss of moisture
21.09
153.9
CaSO4*2H2O → CaSO4*1/2H2O
19.32
280.4
CaSO4*1/2H2O → CaSO4
1.91
Aluminosilicate
~100
Loss of moisture
0.43
538–850
Dehydration of the hydroxides
3.22
573
β‑quartz → α-quartz
–
995
Mullite (Al6Si2O13) formation
–
> 1050
α‑quartz → α-cristobalite
–
1116
α‑quartz → α-tridymite
–
In the gypsum sample, all peaks correspond to dehydration processes. The XRD analysis of the heat-treated sample confirms the complete dehydration of the material, resulting in a total weight loss of approximately 42 wt. % up to 500 °C. A subsequent XRD analysis of the heat-treated aluminosilicate confirmed the phase reactions listed in Table 2. During cooling, some of the silica solidified in a glassy state, while the pyrophyllite peak disappeared as it reacted to form mullite. This change made the spodumene peak visible, whose analysis of the original powder was obscured by the dominant pyrophyllite peak. The amount of cristobalite formed was not yet sufficient to be detected by the XRD equipment used for this analysis.
3 Experimental Procedure and Results
3.1 Gypsum
3.1.1 Investigation of the Suitability as a Raw Material for the Building Industry
Initially, since the dihydrate is not hydraulically active, it was dried at 150 °C to produce reactive phases, primarily hemihydrate. The hemihydrate was examined according to the European standard EN 13279, which specifies the properties and performance characteristics of powdered gypsum binder-based products for use in construction. The amount of litter added was 164.2 g per 100 ml of water, resulting in a water-to-gypsum (W/G) ratio of 0.61. This ratio allowed for the determination of the setting time, which was found to be seven minutes. All values indicate a multiphase gypsum suitable for applications such as the production of plaster of Paris. Using the determined W/G ratio, samples (40 × 40 × 160 mm) were produced (see Fig. 7a) and tested for compressive and flexural strength after seven and 28 days of storage in a humidity chamber (85% rel. humidity; see Table 3). For comparison, a commercially available adhesive plaster was also investigated using the same methodology (see Fig. 7b, c). The yellowish colour of the gypsum is attributed to the iron-aluminium hydroxide present in the material.
Fig. 7
Samples for bending tensile and compressive strength determination a gypsum, b reference, c fractured surface of reference after testing
Compressive and bending tensile strength values of the gypsum and the reference (average of three samples)
Sample
W/G
7 days
28 days
Bending tensile strength [N/mm2]
Compressive strength [N/mm2]
Bending tensile strength [N/mm2]
Compressive strength [N/mm2]
Gypsum
0.61
1.5
2.44
1.35
2.54
Reference
0.5
1.1
1.88
1.92
4.35
3.1.2 Investigation of the Gypsum as Cement Additive
To assess the suitability of the gypsum as a cement additive, Portland cement clinker was co-milled with 5 wt. % of the gypsum (dried at 35 °C), referred to as the test sample. For comparison, the same clinker was co-milled with 5 wt. % of standard gypsum referred to as reference. Both materials were then evaluated for strength and setting behaviour according to the standards EN 196‑1 and 196‑3. After milling, the specific surface area (SSA) of the two test materials was measured using the Blaine method, revealing very similar results. The SSA of the reference sample was 4421 cm2/g, while that of the test material was 4524 cm2/g. The setting behaviour is depicted in Fig. 8, and the results obtained during the tests for setting and hardening are summarised in Table 4. The strength development of the two materials is illustrated in Fig. 9.
Fig. 8
Start of setting and hardening of the investigated materials
Compared to the reference, the clinker with the gypsum material shows higher compressive strength and very similar setting behaviour This indicates that the gypsum from spodumene processing can be used as setting controlling agent for the fabrication of Portland cement.
3.2 Aluminosilicate
3.2.1 Investigation of the Aluminosilicate as Cement Additive
To evaluate the suitability of the aluminosilicate as a cement additive, Portland cement (CEM I 52.5 R) was co-milled for 20 min in a ball mill with 10 and 20 wt. % of the aluminosilicate, respectively. The resulting binders were tested for strength and setting behaviour in accordance with the standards EN 196‑1 and 196‑3. For comparison, both pure Portland cement (CEM I 52.5 R) and a commercially available Portland composite cement (CEM II/A‑M (S, L) 42.5 N) were also investigated using the same methods. The specific surface area, which influences the behaviour of the materials, was determined using the Blaine method. The results are listed in Table 5. The setting behaviour was tested and compared to that of the references (Fig. 10). The parameters determined are summarised in Table 6. The strength development of the different binders tested according to the standard is illustrated in Fig. 11. The corresponding compressive strength values and the error are given in Table 7.
TABLE 5
Specific surface area of the tested binders
Specific surface area [cm2/g]
CEM I 52.5 R
4900
CEM II/A‑M (S, L) 42.5 N
4500
CEM I + 10% AlSi
4947
CEM I + 20% AlSi
4977
Fig. 10
Start of setting and hardening of the investigated binders
Mean compressive strength values (cs) and error (e) of the tested hydraulic binders
24 h
48 h
72 h
7 d
28 d
[MPa]
cs
e
cs
e
cs
e
cs
e
cs
e
CEM I 52.5 R
–
–
–
–
–
–
55.7
2.33
68.5
1.89
CEM II/A‑M (S, L) 42.5 N
12.1
0.72
25.1
0.47
29.1
0.49
34.5
0.57
45.7
0.30
CEM I + 10% AlSi
21.2
1.74
32.9
0.38
40.3
0.32
37.5
0.32
57.2
3.02
CEM I + 20% AlSi
18.3
0.29
30.8
0.81
35.2
0.27
48.9
0.97
69.3
0.80
Although pure Portland cement (CEM I) was “diluted” by up to 20 wt. % with AlSi, its strength after 28 days of curing is not significantly different to that of the pure cement. More importantly, when compared to CEM II—which also contains, by definition, up to 20% of an additional material (in this case, limestone)—both prepared binders exhibit superior strength values. This indicates that the addition of AlSi to cements does not disadvantage strength performance to commercially available products. It is important to note the slightly lower specific surface area of CEM II compared to the other binders, which suggests that its reactivity is also lower. Regarding setting behaviour, both binders with AlSi began to set later than pure CEM I, but not later than CEM II. The end of setting (the start of hardening) was the same as for CEM I.
3.2.2 Investigation of the Aluminosilicate as Concrete Additive
The suitability of AlSi as a concrete additive was tested using mortar prisms fabricated according to EN 196‑1. In this study, the binder or aggregate was partially replaced with 10 wt. % aluminosilicate. CEM II/A‑M (S, L) 42.5 N served as the reference. In one trial, the aluminosilicate was first ground to achieve a higher specific surface area.
To clearly distinguish this experiment from those described in Sect. 3.2.1, it is important to note that, in this section, only the solid fractions of the recipes were mixed. In contrast, Sect. 3.2.1 involved co-milling the aluminosilicate with the binder in a ball mill. The investigated recipes are summarised in Table 8, while the resulting compressive strength values are depicted in Fig. 12.
TABLE 8
Recipes for mortar specimens
Recipe
CEM II/A-M [g]
Standard sand [g]
Water [g]
AlSi ground [g]
AlSi original [g]
1
450
1350
225
–
–
2
450
1215
225
–
135
3
405
1350
225
45
–
4
405
1350
225
–
45
SSA [cm2/g]
4500
–
–
7380
3509
Fig. 12
Compressive strength after 1, 7, and 28 days of storage of the mortar specimens (average of three samples)
After one day, the compressive strength is consistent across all recipes. By day seven, only the recipe where the binder was partially replaced by the delivered AlSi exhibits slightly lower strength than the reference. However, after 28 days, all recipes containing AlSi demonstrate higher compressive strength compared to the reference.
Despite the significantly smaller specific surface area of the original AlSi compared to the ground one, the 28 day compressive strength is only 10% lower. The compressive strength is higher when AlSi is used as an aggregate, attributable to the increased amount of fines introduced in the mix. Overall, the results indicate that the addition of AlSi does not adversely affect strength development in concrete materials.
4 Discussion
Based on the results discussed in this paper, it can be concluded that both materials are promising candidates for applications in the cement and concrete industry. To validate these findings, large-scale experiments and additional testing are recommended, including assessments of the heat of hydration, alkali-aggregate reaction, sulphate resistance, and interactions with concrete reinforcement. It is crucial for both materials to maintain the Li2O content at levels similar to those investigated in this study, or even lower. Lithium acts as an accelerator, and higher concentrations—even slight increases—could significantly affect the behaviour of the cement, potentially resulting in unusable hydraulic binders. Additionally, the levels of alkalis and alkaline earth metals should also be kept low. A grinding or drying process for these materials is not necessarily required.
4.1 The Aluminosilicate By-Product
The aluminosilicate can be used as a cement additive, allowing for the dilution of pure CEM I by at least 20 wt. %. The resulting strength values and setting behaviour still meet the standard specifications for Portland composite cement. After 28 days, the strength is nearly equivalent to that of CEM I 52.5 R and is higher than that with a 10% admixture. This is attributed to the high specific surface area of the aluminosilicate, which acts as a filler and thereby enhances strength. A similar effect is observed in the mortar prisms, where the partial replacement of standard sand by aluminosilicate increases the overall fineness of the mixture.
Portland composite cements typically contain up to 20% of an additional material, which is usually blast furnace slag (BFS) and/or limestone. The advantage of using AlSi over limestone is that it does not require extraction from a quarry, thus minimising environmental disruption.
Regarding blast furnace slag, considerations of price and availability are critical. It is important to understand origin and costs of the raw materials for cement plants. In terms of availability, it is necessary to assess the future trajectory of the steel industry. With ~1.7 tons of CO2 produced per ton of steel (blast furnace route), significant efforts are currently undertaken to explore alternative production routes. One potential option is the direct reduction method, which could reduce the availability of BFS and compel the cement industry to seek suitable alternatives. The AlSi investigated here may serve as one of those alternatives.
Additionally, the AlSi may be suitable as a component for concrete production. It could partially replace the binder, leading to reduced cement consumption, and could also substitute portions of the aggregates, thereby decreasing the need for mining sand or aggregates.
4.2 The Gypsum By-Product
Gypsum could be utilised as a setting-controlling agent in the cement industry, serving as a replacement for mined gypsum materials or as an alternative to FGD gypsum, which may not be available in the near future. The Li2O content of the material used in this study was sufficiently low to avoid any accelerating effect on the setting behaviour.
5 Conclusion
The approach of utilising industrial by-products is becoming increasingly important in light of limited availability of natural raw materials. The building materials industry, in particular, faces new challenges in reducing CO2 emissions from cement manufacturing while also seeking alternatives to fly ash and blast furnace slag, which are expected to decline in availability due to advancing technological changes.
The results of the present investigations conducted here help to address these challenges by showing that aluminosilicate can be effectively used as a cement additive and concrete filler, while gypsum can serve as a setting-controlling agent in the cement industry.
The main benefits of using these by-products from spodumene processing in the building materials industry include the potential for reduced CO2 emissions due to decreased clinker demand in cement production. Additionally, this approach can minimise raw material extraction, thereby reducing environmental impact. Another significant advantage is the reduction in the need for depositing the by-products themselves.
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Die im Laufe eines Jahres in der „adhäsion“ veröffentlichten Marktübersichten helfen Anwendern verschiedenster Branchen, sich einen gezielten Überblick über Lieferantenangebote zu verschaffen.
