Effects of Spodumene Flotation Tailings on Mechanical Properties of Acid-Based Geopolymer Mortar

: This study focuses on using spodumene ﬂotation tailings (SFT) to prepare phosphoric acid-activated metakaolin geopolymer, in which the replacement of metakaolin (MK) by a high percentage (up to 75 wt.%) of tailings was achieved. The compressive strength of geopolymer mortar was signiﬁcantly improved with SFT as aggregates. In addition, the mechanical properties could also be enhanced by an increased concentration of phosphoric acid (H 3 PO 4 ) solution or a decreased aggregate particle size. The optimized geopolymer mortar composite was SFT:MK = 3:1, which was activated by H 3 PO 4 solution with a concentration of 51 vol%, followed by curing at 55 ◦ C for 24 h. On the other hand, properties of the geopolymer mortar could also be affected by the morphology of the aggregates. For example, SFT as aggregates could produce more interconnected pores compared to standard sand. The major chemical structural units of geopolymer mortar were -P-O-Al- and AlPO 4 , which could be spontaneously generated according to the thermodynamic calculation results. Finally, many aluminum ions and a small amount of silicon ions could be leached from the tailings under acidic conditions.


Introduction
Spodumene is one of the principal sources of lithium, commonly used in aeronautics, energy and optical domains due to its outstanding physical and chemical properties [1].Consequently, the efficient separation of spodumene from gangue minerals has gained significant attention [2].The main gangue minerals of spodumene are mica, feldspar and quartz [3].However, spodumene production generates a high volume of industrial by-products, such as spodumene tailings [4].
With respect to spodumene flotation tailings, both Lemougna [5,6] and Yang et al. [7] used lithium tailings as the green materials for the preparation of ceramics and porous ceramics.Although this represents a viable option for reutilization, it consumes significant energy in the sintering process.In addition, the output of tailings was more than the demand of the market.Therefore, it is urgent to develop an energy-effective method that can handle large quantities of tailings.The preparation of building materials from tailings is expected to be one of the significant methods of dealing with bulk tailings.However, Yang et al. [8] found that cement mortar's mechanical properties would be seriously reduced by lithium tailings because of its fine particle size and complex components such as muscovite.On the other hand, tailings could also be recycled via a technique called geopolymerization, which converts the tailings into valuable construction materials [9].
Geopolymers are new inorganic materials that can be consolidated at room or slightly elevated temperatures.They can be synthesized through the reaction between a powder having pozzolanic activity and an alkaline or acid solution, forming a tridimensional network [10,11].The preparation, microstructure, performance and corrosion resistance of geopolymers have been researched by numerous investigators over the past several decades [12][13][14].Geopolymers demonstrate good physical properties, such as high earlyage strength, good chemical attack resistance and acceptable thermal insulation [15][16][17].Metakaolinite is one of the most commonly used raw materials in preparing geopolymer.It is prepared by calcining kaolinite (500~900 • C) to improve its pozzolanic activity [18].Metakaolin is expected to be soluble in these systems due to the typical presence of a substantial amorphous alumino-silicate phase, which is easily dissolved in sodium hydroxide solutions [19].
Acid-based geopolymer is synthesized through the reaction between aluminosilicates and acid solution (phosphoric acid or humic acid) [20,21].It exhibits better properties than alkali-based geopolymers, such as higher temperature resistance and better mechanical strength [22,23].The acid most commonly used in these studies is phosphoric acid, and the aluminosilicate sources reported are mostly metakaolin [24,25].The product obtained through the phosphoric acid solution as a hardener is called poly(phospho-siloxo) -Si-O-P-O-Si-O- [26].Lin et al. [27] investigated the reaction mechanism of metakaolin activated by phosphoric acid with various concentrations of H 3 PO 4 and curing temperatures.They concluded that the major structural units of the silico-aluminate phosphate include Si-O-P, Al-O-P, Si-O-Al and Si-O-Si.
Tailings are often used as the raw material for concrete or cement because of their consolidation effect, especially for the heavy-metal-contaminated tailings [28].On the other hand, acid-based geopolymers have stronger bonding and thus higher compressive strength compared to alkali-based geopolymers [24].However, spodumene flotation tailings have not been used in acid-activated geopolymer mortar aggregates.In addition, although there are many reports on the effects of aggregates on geopolymer mortar, most of them focus on alkali-activated geopolymer and few on acid-based geopolymer [29].Therefore, we combined acid-activated geopolymer and spodumene flotation tailings to prepare high-strength geopolymer mortar.The correlation between various factors and the performance of geopolymer mortar was also studied.The properties and reaction mechanism of geopolymer mortar were analyzed by X-ray diffraction (XRD) measurements, nanoscale pore distribution analysis, scanning electron microscopy (SEM) measurements, Fourier transform infrared spectroscopy (FT-IR) measurements, X-ray photoelectron spectroscopy (XPS) measurements, ion leaching analysis, computed tomography (CT) measurements and thermodynamic calculation.This study can provide some guidance for applying tailings to acid-activated geopolymer mortar.

Raw Materials
Spodumene flotation tailings were obtained from a concentrator in Ganzi Prefecture, Sichuan Province.MK was produced by Chen Yi Antifriction Material Co., Ltd.(Zhengzhou, China) and prepared by calcining kaolin at 750 • C for 3 h whose particle size was less than 38 µm.The standard sand (SS) was purchased from Xiamen ISO Standard Sand Co., Ltd.(Xiamen, China) SS with three different particle sizes was obtained by sieving (<0.3 mm, 0.3~0.5 mm, and >0.5 mm).Table 1 presents the chemical compositions of flotation tailings, metakaolin and standard sand, which were examined by an Axios X-ray fluorescence (XRF) spectrometer (PANalytical B.V from Almelo, Holland).Figure 1 presents the particle size distribution of SFT, which was tested by a laser particle size analyser (Malvern 2000), where the median particle size of flotation tailings was about 305 µm.

Experimental Procedure
Figure 2 shows the experimental procedure for the preparation of tailings-based geopolymer mortar.SFT and MK and H3PO4 solution were mixed for 6 min (NJ-160 cement paste mixer).The mass ratio of solution/material was about 50 wt.%,and there were three groups of parallel samples for each test.The geopolymer mortar was shaped in a rectangular plastic mold (40 × 40 × 40 mm) and placed in an electric blast dryer activated at 60 °C for 24 h [27].After demolding, it was placed in a cement standard curing box (TKY-A70, Nanjing Jiedi Electronic Equipment Manufacturing Co., Ltd.) with a 25 °C curing temperature (the moisture content of the environment was kept at 98% for the unsealed samples).

Experimental Procedure
Figure 2 shows the experimental procedure for the preparation of tailings-based geopolymer mortar.SFT and MK and H 3 PO 4 solution were mixed for 6 min (NJ-160 cement paste mixer).The mass ratio of solution/material was about 50 wt.%,and there were three groups of parallel samples for each test.The geopolymer mortar was shaped in a rectangular plastic mold (40 × 40 × 40 mm) and placed in an electric blast dryer activated at 60 • C for 24 h [27].After demolding, it was placed in a cement standard curing box (TKY-A70, Nanjing Jiedi Electronic Equipment Manufacturing Co., Ltd., Nanjing, China) with a 25 • C curing temperature (the moisture content of the environment was kept at 98% for the unsealed samples).
The composition of raw materials is presented in Table 2. Firstly, through the previous experimental exploration, we found that the concentration of phosphoric acid was too low to realize geopolymerization, and higher concentration of phosphoric acid could cause higher costs.Therefore, the variation range of phosphoric acid concentration was chosen as 34~68 vol% (5.86~11.75mol/L).On the other hand, a lower activation temperature cannot experimentally realize the solidification of geopolymer mortar, while a higher temperature will cause higher energy loss.Therefore, the activation temperature range was selected as 50~80 • C. Finally, the effects of the content of tailings and the particle size of aggregates on the properties of geopolymer mortar were explored.The composition of raw materials is presented in Table 2. Firstly, through the previous experimental exploration, we found that the concentration of phosphoric acid was too low to realize geopolymerization, and higher concentration of phosphoric acid could cause higher costs.Therefore, the variation range of phosphoric acid concentration was chosen as 34~68 vol% (5.86~11.75mol/L).On the other hand, a lower activation temperature cannot experimentally realize the solidification of geopolymer mortar, while a higher temperature will cause higher energy loss.Therefore, the activation temperature range was selected as 50~80 °C.Finally, the effects of the content of tailings and the particle size of aggregates on the properties of geopolymer mortar were explored.

Methods
A microcomputer-controlled electro-hydraulic servo pressure testing machine was used for the measurement of the compressive strength of geopolymer mortar (YAW4106, Meister Co., Ltd from Eden Prairie, Minnesota, USA).X-ray diffractometer (XRD) (D/max IIIA, Rigaku Co., Ltd from Akishima-shi, Tokyo, Japan) was used to test the phase composition of the sample.A specific surface area analyzer was used for the test of pore size

Methods
A microcomputer-controlled electro-hydraulic servo pressure testing machine was used for the measurement of the compressive strength of geopolymer mortar (YAW4106, Meister Co., Ltd. from Eden Prairie, MN, USA).X-ray diffractometer (XRD) (D/max IIIA, Rigaku Co., Ltd. from Akishima-shi, Tokyo, Japan) was used to test the phase composition of the sample.A specific surface area analyzer was used for the test of pore size distribution of geopolymer mortar (Autosorb-1MP, Quantachrome (Boynton Beach, Florida, USA)).The micro-morphology of Geopolymer mortar was tested by scanning electron microscopy (SEM; LEO440, Leica Cambridge Ltd. from Wezler, Germany).FTIR of the sample was tested by a Fourier infrared spectrometer (Perkin Elmer Instruments Co., Ltd.(Norwalk, CA, USA)).XPS analysis was performed by a Thermo Scientific K-Alpha produced by Thermo Fisher Scientific (Cleveland, OH, USA).The computed tomography of the geopolymer mortar was tested by a nanoVoxel-type X-ray three-dimensional micro-CT produced by Sanying Precision Co., Ltd.(Tianjin, China).Inductively coupled plasma optical emission spectroscopy (ICP-OES; Agilent 5110, Agilent Company (Santa Clara, CA, USA)) was used to test the ion mass concentration in the leaching solution.

Macroscopic Appearance of Materials
Figure 3 presents the appearance of the geopolymer mortar under different conditions.Cracks were increasingly prevalent with an increased activation temperature (G-T50, G-T55, G-H51, and G-T80) and mass ratio of MK (G-S25, G-S50, G-S75).However, the surface of geopolymer mortar was softened when the concentration of H 3 PO 4 reached 68 vol%.
(Norwalk, California USA)).XPS analysis was performed by a Thermo Scientific K-Alpha produced by Thermo Fisher Scientific (Cleveland, OH, USA).The computed tomography of the geopolymer mortar was tested by a nanoVoxel-type X-ray three-dimensional micro-CT produced by Sanying Precision Co., Ltd.(Tianjin, China).Inductively coupled plasma optical emission spectroscopy (ICP-OES; Agilent 5110, Agilent Company (Santa Clara, California, USA)) was used to test the ion mass concentration in the leaching solution.

Macroscopic Appearance of Materials
Figure 3 presents the appearance of the geopolymer mortar under different conditions.Cracks were increasingly prevalent with an increased activation temperature (G-T50, G-T55, G-H51, and G-T80) and mass ratio of MK (G-S25, G-S50, G-S75).However, the surface of geopolymer mortar was softened when the concentration of H3PO4 reached 68 vol%.Figure 3b shows the geopolymer mortar with the addition of SS with different particle sizes.From this, it is seen that the pores of samples were increased with an increasing particle size of SS, which may be due to accumulated pores formed by the accumulation of large particles.

Compressive Strength of Materials
Figure 4 presents the compressive strength of geopolymer mortar under different conditions.Figure 4a shows that the curing conditions had some influence on the compressive strength.The best curing condition was to seal the samples with film and place them in a standard curing box compared with the other two curing conditions (curing in the air with no film and curing in water with no film).Figure 4b shows that the compressive strength was increased with increasing activation time.Therefore, the activation time was set as 24 h, and film mulching was chosen as the curing condition.Figure 4c presents Figure 3b shows the geopolymer mortar with the addition of SS with different particle sizes.From this, it is seen that the pores of samples were increased with an increasing particle size of SS, which may be due to accumulated pores formed by the accumulation of large particles.

Compressive Strength of Materials
Figure 4 presents the compressive strength of geopolymer mortar under different conditions.Figure 4a shows that the curing conditions had some influence on the compressive strength.The best curing condition was to seal the samples with film and place them in a standard curing box compared with the other two curing conditions (curing in the air with no film and curing in water with no film).Figure 4b shows that the compressive strength was increased with increasing activation time.Therefore, the activation time was set as 24 h, and film mulching was chosen as the curing condition.Figure 4c presents that the compressive strength was increased with an increasing concentration of H 3 PO 4 solution caused by the changed molar ratio of P/Al.Dissolved Al directly reacted with PO 4 3− to form an Al-O-P structure at a low P/Al ratio.However, the metastable intermediate P-O-P structure appeared at a relatively high P/Al ratio, which promoted the formation of Si-O-P-O-Al networks eventually, increasing the strength of geopolymer mortar [18,27].
cracks were formed at a higher activation temperature (See Figure 3), which led to a reduction in the mechanical properties of geopolymer mortar.Figure 4f shows that the compressive strength increased with a decreasing SS particle size, which could be related to a reduction in large pores in geopolymer mortar, as presented in Figure 3b.As a result, SFT was suitable as a raw material for geopolymer mortar to improve its mechanical properties.

XRD Analysis
To better identify the sample structure, XRD patterns were collected (See Figure 5).Figure 5a presents that the XRD diffraction peak of MK was dispersed (there was no obvious diffraction peak), and MK mainly consisted of amorphous materials with an irregular molecular arrangement [20,33,34].There were no obvious crystalline diffraction peaks in geopolymer mortar G-S0, which also means the geopolymer mortar was mainly composed of amorphous material.The second peak decreased obviously, showing that the geopolymerization products were mainly amorphous, and their structure had Figure 4d illustrates that the compressive strength of geopolymer mortar increased with an increasing mass fraction of SFT.However, aggregates commonly played the role of unreacted materials (and generally had a detrimental effect on the strength of concrete [30]) to reduce the content of cement and save costs.It could be speculated that some physical chemical reaction in the interface between SFT and geopolymer might have happened, Minerals 2023, 13, 150 7 of 18 which improved the strength of the sample [31,32].Figure 4e shows that the compressive strength of geopolymer mortar was the highest with an activation temperature of 55 • C. When the activation temperature was further increased, the mechanical properties of geopolymer mortar decreased significantly.This phenomenon might be related to the production of cracks in the specimen at higher temperatures (Figure 3) because of the accelerated chemical reaction caused by increased temperature [26].A large number of cracks were formed at a higher activation temperature (See Figure 3), which led to a reduction in the mechanical properties of geopolymer mortar.Figure 4f shows that the compressive strength increased with a decreasing SS particle size, which could be related to a reduction in large pores in geopolymer mortar, as presented in Figure 3b.As a result, SFT was suitable as a raw material for geopolymer mortar to improve its mechanical properties.

XRD Analysis
To better identify the sample structure, XRD patterns were collected (See Figure 5).Figure 5a presents that the XRD diffraction peak of MK was dispersed (there was no obvious diffraction peak), and MK mainly consisted of amorphous materials with an irregular molecular arrangement [20,33,34].There were no obvious crystalline diffraction peaks in geopolymer mortar G-S0, which also means the geopolymer mortar was mainly composed of amorphous material.The second peak decreased obviously, showing that the geopolymerization products were mainly amorphous, and their structure had changed after geopolymerization.

XRD Analysis
To better identify the sample structure, XRD patterns were collected (See Figure 5).Figure 5a presents that the XRD diffraction peak of MK was dispersed (there was no obvious diffraction peak), and MK mainly consisted of amorphous materials with an irregular molecular arrangement [20,33,34].There were no obvious crystalline diffraction peaks in geopolymer mortar G-S0, which also means the geopolymer mortar was mainly composed of amorphous material.The second peak decreased obviously, showing that the geopolymerization products were mainly amorphous, and their structure had changed after geopolymerization.were the main phase compositions of flotation tailings.The phase compositions of geopolymer mortar with the addition of SFT had no obvious change, but the intensity of the diffraction peaks was decreased.This was thought to be attributed to the partial dissolution of minerals under acidic conditions [35,36].The dissolution of aluminosilicate minerals, especially the Al ions (Table 5), might be one of the factors [37,38].

Brunauer-Emmett-Teller (BET) and Nanoscale Pore Distribution Analysis
Figure 6a,b presents the pore width distribution and the cumulative pore distribution of geopolymer mortar, respectively.Small broken block samples (38 µm~2 mm) were used to measure specific surface area and pore size distribution.Figure 6a shows that the content of pore volume gradually decreased with the addition of standard sand and spodumene tailings.With the addition of SFT or SS, the specific surface area of geopolymer mortar decreased noticeably.It could be hypothesized that SS and SFT as aggregates could make geopolymer mortar particles more compact.In addition, the shape of standard sand particles was more regular and smoother than SFT particles.This may lead to different reactions at the junction with the slurry, which affects the distribution of nanopores [39].
3.3.2.Brunauer-Emmett-Teller (BET) and Nanoscale Pore Distribution Analysis Figure 6a,b presents the pore width distribution and the cumulative pore distribution of geopolymer mortar, respectively.Small broken block samples (38 μm~2 mm) were used to measure specific surface area and pore size distribution.Figure 6a shows that the content of pore volume gradually decreased with the addition of standard sand and spodumene tailings.With the addition of SFT or SS, the specific surface area of geopolymer mortar decreased noticeably.It could be hypothesized that SS and SFT as aggregates could make geopolymer mortar particles more compact.In addition, the shape of standard sand particles was more regular and smoother than SFT particles.This may lead to different reactions at the junction with the slurry, which affects the distribution of nanopores [39].

SEM-Mapping Analysis
SEM images of the samples are presented in Figure 7. Figure 7(a-1,b-1) shows that many pores and cracks were formed in geopolymer mortar, and the shape of pores with SFT as aggregates was more regular.Internal bonding of geopolymer particles was looser with an increased activation temperature (G-T50 and G-S75), which was consistent with the surface morphology results of geopolymer mortar.The micromorphology of geopolymer mortar was very rough, with MK as the raw material (G-S0).

SEM-Mapping Analysis
SEM images of the samples are presented in Figure 7. Figure 7(a-1,b-1) shows that many pores and cracks were formed in geopolymer mortar, and the shape of pores with SFT as aggregates was more regular.Internal bonding of geopolymer particles was looser with an increased activation temperature (G-T50 and G-S75), which was consistent with the surface morphology results of geopolymer mortar.The micromorphology of geopolymer mortar was very rough, with MK as the raw material (G-S0).
However, the content of large pores was decreased by SFT (G-S75), and the shapes of pores were also more regular than G-S0.The defects caused by pores could be one of the reasons for the lower mechanical properties of G-S0.However, both G-S0 and G-S75 had many cracks.Figure 7(d-1-d-3) shows that the bonding among SFT particles was obviously looser than the geopolymer mortar of a high phosphoric acid concentration.It was consistent with the results of previous tests on compressive strength.
The corresponding mapping images of geopolymer mortar are shown in Figure 7(a-4d-4).Figure 7(a-4) presents that the distribution of elements in geopolymer mortar was very uniform, where Si and Al were mainly from MK(2SiO 2 •Al 2 O 3 ) and P was mainly from H 3 PO 4 .The granular distribution of Si in Figure 7(b-4-d-4) was primarily from SFT mineral particles, while Al and P were mainly distributed around the Si particles.It could be inferred that the geopolymerization reaction between slurry and aggregates may be concentrated more on the surface of the mineral.On the other hand, the mapping images in Figure 7(c-4) present that the chemical composition of the gel was Al, O and P, which might be AlPO 4 [40].3 shows the position and assignment of the infrared vibration bands.Figure 8a illustrates that a new diffraction peak of the P-O bond appeared at 920 cm −1 after acid activation.On the other hand, it could also indicate that the PO 4 3-participated in the geopolymerization reaction.Figure 8b demonstrates that with the increase in H 3 PO 4 concentration, three new absorption bands appeared at the position of 1050, 930 and 880 cm −1 .They represent the stretching vibration of the P-O and Si-O bonds, which was due to the changed molar ratio of Si, Al, P.
H3PO4.The granular distribution of Si in Figure 7(b-4-d-4) was primarily from SFT mineral particles, while Al and P were mainly distributed around the Si particles.It could be inferred that the geopolymerization reaction between slurry and aggregates may be concentrated more on the surface of the mineral.On the other hand, the mapping images in Figure 7(c-4) present that the chemical composition of the gel was Al, O and P, which might be AlPO4 [40].

FTIR Analysis
Figure 8a,b presents the FTIR spectra of MK and geopolymer mortar.Table 3 shows the position and assignment of the infrared vibration bands.Figure 8a illustrates that a new diffraction peak of the P-O bond appeared at 920 cm −1 after acid activation.On the other hand, it could also indicate that the PO4 3-participated in the geopolymerization reaction.Figure 8b demonstrates that with the increase in H3PO4 concentration, three new absorption bands appeared at the position of 1050, 930 and 880 cm −1 .They represent the stretching vibration of the P-O and Si-O bonds, which was due to the changed molar ratio of Si, Al, P.

XPS Analysis
Figure 9a shows the XPS spectra obtained from MK and geopolymer mortar No. G-S0.Characteristic elements such as O 1s , C 1s , Si 2p and Al 2p were detected in each sample, but P 2p only appeared in geopolymer mortar.Figure 9b-d shows that the peak of Si 2p , Al 2p , and O 1s exhibited a significant shift, and the peak strength of Si 2p and Al 2p decreased obviously after geopolymerization.This reflects a typical presentation of a substantial amorphous alumino-silicate phase [19].Soluble aluminosilicate in MK was dissolved, and stable geopolymer mortar gels were gradually formed during geopolymerization.Hence, the binding energy of Al-O and Si-O bonds increased, which shifted upwards to 75.15 and 103.16 eV, respectively [46].
P2p only appeared in geopolymer mortar.Figure 9b-d shows that the peak of Si2p, Al2p, and O1s exhibited a significant shift, and the peak strength of Si2p and Al2p decreased obviously after geopolymerization.This reflects a typical presentation of a substantial amorphous alumino-silicate phase [19].Soluble aluminosilicate in MK was dissolved, and stable geopolymer mortar gels were gradually formed during geopolymerization.Hence, the binding energy of Al-O and Si-O bonds increased, which shifted upwards to 75. 15

CT Analysis
Geopolymer mortar with SFT and quartz was, respectively, tested by CT.Table 4 presents that the porosity of geopolymer mortar with quartz was higher than that of the SFT sample.The average pore radius and pore volume were also larger than that of geopolymer mortar with SFT.In addition, the number of channels representing the connected part among pores with SS as aggregates was much less than when using the SFT as aggregates.The pink materials in Figure 10 were extracted tailings particles and standard sand particles, respectively.The distribution of aggregates was uniform.The pore distribution images show that the pore structure of tailings geopolymer mortar was irregular, with a lot of connected pores.However, the pores were almost isolated and more uniformly distributed with the addition of quartz sand.Therefore, the angular shape of tailings mineral particles could promote the formation of connected pores, and the regular shape of standard sand could help to generate more unconnected pores.

Ion Leaching Analysis
Tailings and standard sand were soaked in the phosphoric acid solution for 24 h at 60 °C to further explore the chemical reaction of the aggregates with the H3PO4 solution.Contents of Si and Al ions in the solution were measured after leaching (See Table 5).The results show that the leaching rate of the aluminum ion in tailings is significantly higher than that of silicon ions, and its leaching mass fraction is as high as 0.1 wt.%.On the other

Ion Leaching Analysis
Tailings and standard sand were soaked in the phosphoric acid solution for 24 h at 60 • C to further explore the chemical reaction of the aggregates with the H 3 PO 4 solution.Contents of Si and Al ions in the solution were measured after leaching (See Table 5).The results show that the leaching rate of the aluminum ion in tailings is significantly higher than that of silicon ions, and its leaching mass fraction is as high as 0.1 wt.%.On the other hand, the leaching content of the silicon ion in tailings is also higher than that of standard sand.These results show that the content of unstable substances in tailings is higher than in the sand, which could also participate in the geopolymerization reaction to form chemical bonds such as Al-O and Si-O.The soaked mineral particles after washing with distilled water were tested with a scanning electron microscope to observe the surface morphology changes.Figure 11 shows that the surface morphology of mineral particles changed before and after phosphoric acid immersion.Before leaching, the surface of tailings and standard sand mineral particles was smooth, but the surface was rougher after leaching.That means that phosphoric acid could corrode the surface of mineral particles in the aggregate to a certain extent.The rough surface may promote the combination of the aggregate and cementitious materials, improving the mechanical properties of geopolymer mortar [47,48].The soaked mineral particles after washing with distilled water were tested with a scanning electron microscope to observe the surface morphology changes.Figure 11 shows that the surface morphology of mineral particles changed before and after phos phoric acid immersion.Before leaching, the surface of tailings and standard sand minera particles was smooth, but the surface was rougher after leaching.That means that phos phoric acid could corrode the surface of mineral particles in the aggregate to a certain extent.The rough surface may promote the combination of the aggregate and cementi tious materials, improving the mechanical properties of geopolymer mortar [47,48].

Thermodynamic Calculation
The thermodynamic calculation of the reaction process and its reactants was carried out using FactSage software (version 8.1).Equations ( 1)-( 10) were calculated at 55 °C, 1 The thermodynamic calculation of the reaction process and its reactants was carried out using FactSage software (version 8.1).Equations ( 1)-( 10) were calculated at 55 • C, 1 atm using the Equilib module.The content of AlPO 4 was increased with the increased content of H 3 PO 4 , which would be the reason for the enhanced compressive strength of geopolymer mortar with high concentration H 3 PO 4 solution.In addition, more water was generated with the increased content of H 3 PO 4 , which was in line with the phenomenon of softening for geopolymer mortar in Figure 3.
(a) Influence of H 3 PO 4 : The content of AlPO 4 was not changed when the content of Al 2 O 3 was increased.However, when the mass ratio of Al 2 O 3 to H 3 PO 4 reached two, no extra materials were formed, which means the content of Al 2 O 3 was excessive.Therefore, it could be inferred that excessive Al 2 O 3 might prevent the increase in geopolymer mortar strength; that is, one of the functions of aggregate should include the content of diluted alumina.
(b) Influence of Al 2 O 3 : The calculation results show that in this system, silica can theoretically participate in the reaction to generate kaolin and orthosilicic acid, which should also affect the final performance of geopolymer mortar.According to the chemical formula of kaolin (2SiO 2 •Al 2 O 3 •2H 2 O), the content of Al 2 O 3 and SiO 2 participating in the reaction was designed to be 1 and 2 mol, respectively.As it was a solution system, 1 mol H 2 O was added, and the calculated reaction formula was (11).The reaction equation (11) was calculated with the reaction module of FactSage, and Figure 12 presents the results.It can be seen that the reaction was exothermic (∆H < 0) and spontaneous (∆G < 0).The ∆G was lower with the increased temperature, which means the degree of spontaneous reaction was increased.These results were in line with the results of Tchakouté and Lin et al. [26,27], which represent that a metastable intermediate P-O-P structure appeared during the reaction and was finally transformed into a P-O-Al structure at relatively high P/Al ratios.

Mechanistic Analysis of Geopolymer Mortar
Figure 13 presents the proposed mechanism of tailings geopolymer mortar.FT-IR and XPS results have corroborated the presence of several silica-and aluminophosphatebased networks in geopolymer mortar.According to prior research [18,40,44] on the mechanism of geopolymerization, it can be divided into three steps: (i) Metakaolinite was   On the other hand, the compressive strength of geopolymer mortar can be significantly improved by adding aggregates such as SFT or SS.Geopolymer mortar with fine standard sand (<0.3 mm) had the highest strength.Pore structure distribution of SFT-geopolymer mortar was considerably different from standard-sand-geopolymer mortar due to their different particle shapes.The mineral particles of SFT were mostly angular, so there were a lot of connected pores in the geopolymer mortar prepared by SFT, while the quartz sand mostly contained independent closed pores because of its smooth particles.Finally, some Al 3+ and a small amount of Si 4+ from SFT would also be leached under acidic conditions whose concentration was much higher than that of standard sand.On the other hand, the compressive strength of geopolymer mortar can be significantly improved by adding aggregates such as SFT or SS.Geopolymer mortar with fine standard sand (<0.3 mm) had the highest strength.Pore structure distribution of SFT-geopolymer mortar was considerably different from standard-sand-geopolymer mortar due to their different particle shapes.The mineral particles of SFT were mostly angular, so there were a lot of connected pores in the geopolymer mortar prepared by SFT, while the quartz sand mostly contained independent closed pores because of its smooth particles.Finally, some Al 3+ and a small amount of Si 4+ from SFT would also be leached under acidic conditions whose concentration was much higher than that of standard sand.

Mechanistic Analysis of Geopolymer Mortar
According to the compressive strength test results in Section 3.2, the properties of the aggregate have a significant impact on the properties of geopolymer mortar.On the one hand, the aggregate could impede the extension of matrix cracks [49].On the other hand, there were complex chemical reactions within the interfacial transition zone between aggregate and paste [50] (including the release of ions in the aggregate [51]).In addition, AlPO 4 could be spontaneously generated according to the thermodynamic calculation results.

Conclusions
Geopolymer mortar was prepared through acid-activation with MK as the raw material, whose compressive strength was greatly improved with the addition of SFT.These results were different from the traditional alkali-activated geopolymer mortar and also differed in the range of compressive strength variation observed [52].Optimized geopolymer mortar composites were 25 wt.%MK and 75 wt.%SFT, activated by H 3 PO 4 solution with a concentration of 51 vol% at 55 • C for 24 h.The compressive strength was higher when it was cured and covered with film.The compressive strength of geopolymer mortar was increased with an increasing concentration of H 3 PO 4 solution.However, the surface of the geopolymer mortar was softened when the concentration reached 68 vol%.According to the thermodynamic calculation results, this should be due to the high concentration of phosphoric acid solution promoting the formation of water molecules.The degree of geopolymerization of geopolymer mortar was more intense and more cracks were formed with an increased activation temperature.The compressive strength of geopolymer mortar was increased with a decreasing particle size of standard sand.These results were also in contrast to the results of alkali-activated geopolymer mortar with the addition of aggregates of various sizes [53].
Although SFT mainly played a role of aggregates, the nanopores and large pores in geopolymer mortar also decreased significantly with the addition of aggregates.Moreover, the particle size and morphology of aggregates also greatly impacted the performance of geopolymer mortar.Multiangular structure SFT could cause the geopolymer mortar to produce more connected pores than SS.On the other hand, some Al 3+ and a small amount of Si 4+ from SFT leached under acidic conditions could also affect the properties of geopolymer mortar, which might promote the formation of -Al-O-P-.In addition, AlPO 4 could be spontaneously generated according to the thermodynamic calculation results.

Figure 3 .
Figure 3. Appearance of geopolymer mortar with the addition of (a) SFT at various conditions and (b) geopolymer mortar with only MK and geopolymer mortar with MK and SS (with various particle sizes).

Figure 3 .
Figure 3. Appearance of geopolymer mortar with the addition of (a) SFT at various conditions and (b) geopolymer mortar with only MK and geopolymer mortar with MK and SS (with various particle sizes).

Figure 4 .
Figure 4. Compressive strength of geopolymer mortar under different conditions: (a) 3 days compressive strength with different activation conditions, which was in air, in film, and in water, respectively; (b) 3 days compressive strength with thermal activation time between 12 and 36 h; (c) concentration of H 3 PO 4 with 34~68 vol%, mass ratio of SFT = 66 wt.%;(d) mass ratio of SFT in geopolymer mortar with 0~75 wt.%; (e) activation temperature with 50~60 • C; (f) particle size of SS with less than 0.5 mm, 0.3~0.5 mm, and more than 0.5 mm (75 wt.%).

Figure 5 .
Figure 5. XRD patterns of (a) MK and geopolymer mortar No. G-S0, and (b) SFT and geopolymer mortar No. G-T50, G-H51 and G-36 h (the raw materials were the same with sample G-T50, but the activation was from 24 h extended to 36 h).

Figure 5 .
Figure 5. XRD patterns of (a) MK and geopolymer mortar No. G-S0, and (b) SFT and geopolymer mortar No. G-T50, G-H51 and G-36 h (the raw materials were the same with sample G-T50, but the activation was from 24 h extended to 36 h).

Figure
Figure 5b indicates that quartz (SiO2 ), muscovite (KAl 2 (AlSi 3 O 10 ) (OH) 2 ), albite (Na 2 O•Al 2 O 3 •6SiO 2 ), microcline (KAlSi 3 O 8 ) and spodumene (Li 2 O•Al 2 O 3 •4SiO 2 )were the main phase compositions of flotation tailings.The phase compositions of geopolymer mortar with the addition of SFT had no obvious change, but the intensity of the diffraction peaks was decreased.This was thought to be attributed to the partial dissolution of minerals under acidic conditions[35,36].The dissolution of aluminosilicate minerals, especially the Al ions (Table5), might be one of the factors[37,38].

Figure 10 .
Figure 10.CT-scanning reconstruction image of geopolymer mortar with the addition of (a) SFT and (b) Qtz (<0.3 mm).PD means diameter of the sample from the front place.

Figure 10 .
Figure 10.CT-scanning reconstruction image of geopolymer mortar with the addition of (a) SFT and (b) Qtz (<0.3 mm).PD means diameter of the sample from the front place.

Figure 11 .
Figure 11.SEM of aggregate before and after phosphoric acid immersion.

Figure 11 .
Figure11.SEM of aggregate before and after phosphoric acid immersion.

Figure 13
Figure 13 presents the proposed mechanism of tailings geopolymer mortar.FT-IR and XPS results have corroborated the presence of several silica-and aluminophosphate-based networks in geopolymer mortar.According to prior research [18,40,44] on the mechanism of geopolymerization, it can be divided into three steps: (i) Metakaolinite was decomposed and released Al 3+ in phosphoric acid solution.(ii) Phosphoric acid reacted with Al 3+ and dealuminated metakaolinite to form AlPO 4 and Si-O-P bonds, respectively.(iii) A tridimensional network structure was formed through [Si-O-P-O-Al] n polycondensation.The major structural units of geopolymer mortar included Al-O-P, Si-O-P, Si-O-Si and Si-O-Al [27].Minerals 2023, 13, x FOR PEER REVIEW 16 of 19

Figure 13 .
Figure 13.Reaction schematic diagram of geopolymerization (Ms is the abbreviation of muscovite, Ab is the abbreviation of albite, Kfs is the abbreviation of potassium feldspar, Qtz is the abbreviation of quartz).

Figure 13 .
Figure 13.Reaction schematic diagram of geopolymerization (Ms is the abbreviation of muscovite, Ab is the abbreviation of albite, Kfs is the abbreviation of potassium feldspar, Qtz is the abbreviation of quartz).
* LOI means loss on ignition.

Table 3 .
Position and assignment of MK and geopolymer mortar.

Table 3 .
Position and assignment of MK and geopolymer mortar.

Table 4 .
Results of CT quantitative analysis of channels and pores.

Table 5 .
Ion leaching amount in raw material.