Fusion of phosphate by-products and glass waste for preparation of alkali-activated binders

Landfilling of mine and industrial waste streams leads to environmental and economic issues. Sustainable management methods through valorization in manufacturing green construction materials are a current research interest. Here, a promising process for recycling mine tailings, such as phosphate sludge, is proposed. A mixture of phosphate sludge, kaolin clay (Al source)


Introduction
Phosphate sludge (Psl) is the main phosphate mine tailing generated after ore processing.It represents one of the most largely produced phosphate mine wastes, especially in Morocco.The worldwide evolution of phosphate demand and production implies intensifying generated mine tailings, which induces several environmental issues.In general, about 1 tonne of tailings composed of clays, dolomite, calcite, quartz, and fluorapatite is generated for each tonne of phosphate ore produced; these tailings are deposited in impoundment facilities, leading to longterm durability and environmental issues, such as the occupation of vast areas, an enormous amount of water losses, physical change of the surrounding environment, the elution of phosphate, and significant challenges in terms of long-term storage capacity [1].Hence, developing sustainable solutions for managing and valorizing phosphate mine wastes has recently attracted scientists' attention.Several investigations have been carried out to assess the valorization potential of phosphate tailings for filtrate membranes synthesis [2], ceramic and lightweight aggregate production [3], fired brick manufacturing [4], and as raw materials for alkali-activated material manufacturing [5,6].
At present, different industries are moving toward cleaner production by adapting or converting to fossil-free processes.The steel industry is one of them, adopting hydrogen-based iron production with a direct reduction, which has induced a shortage of blast furnace slag in several countries.In addition, in several countries, coal fly ash is not available because of the coal-free fossil used in thermal powerplants.Therefore, research into new raw materials for alkali-activated materials and processes for their activation is becoming necessary [7].The conversion of natural aluminosilicates and industrial wastes into high-added value binders by designing and developing alkali-activated materials has become one of the most attractive solutions for industrial waste management [8].However, incorporating unheated industrial by-products, such as mine tailings, in alkali-activated materials has been challenging due to their low reactivity.Different pretreatment methods have been developed and proved to be efficient for increasing the reactivity of unheated by-products, such as i) mechanical [9]; ii) thermal [10]; iii), mechanochemical [11], and iv) thermochemical [12] treatments.
Phosphate sludges are characterized by low alumina and silica content from a chemical point of view [13].Their mineralogy consists of crystalline phases, mainly minerals that are inert toward alkaline activation [13,14].Hence, the low aluminosilicate content and high crystallinity of phosphate mine tailings limit the formation of a geopolymer structure when alkali-activated [14].Calcination at 750 • C was proved to enhance the reactivity and increase the compressive strength of alkali-activated phosphate mine tailing, from 4 to 7 MPa [15].In another study, the alkali activation of calcined Psl at 700 • C resulted in a 28-day compressive strength of 11 MPa.Furthermore, the mechanochemical activation of phosphate mine tailing has been tested to enhance its reactivity.The results showed that this activation method improved Si and Al dissolution; however, the resulting alkali-activated Psl binder had a low compressive strength of 4.4 MPa at 7 days [16].
The phosphate mine tailings from Finland used in previous studies contain a high amount of phlogopite KMg 3 (AlSi 3 O 10 )(OH) 2 and some natural zeolites and clays that can contribute to the alkaline activation reaction [15].In contrast, Psl from Morocco and China was not suitable as a binder even after pretreatment due to its low aluminosilicate content and high amount of inert mineral phases.Hence, other sources of Al, Si, and Ca, like fly ash, metakaolin, and slag, have been added to enhance the properties of the alkali-activated binders [13,14].Thermochemical treatment or alkaline fusion consisting of adding an alkali source to the raw material and then calcining it at a temperature higher than the alkali source melting point was tested in earlier investigations [14,17].This pretreatment was successful in improving the reactivity of the Psl.Wu et al. [14] used 12 wt% NaOH as a fluxing agent and successfully prepared an alkali-activated binder with a 40 wt% slag addition having 30 MPa compressive strength.In our previous study, fusion with 10-20 wt% of NaOH for the activation of Psl was investigated [17].It was proved that this pretreatment using sodium hydroxide as a fluxing agent and sodium silicate as an activator results in an increase in the different properties of Psl-based alkali activated materials (AAMs).However, using sodium hydroxide as fluxing and sodium silicate as an activation solution increases the environmental burdens of AAMs, even when produced using local waste materials [18].
Several attempts have therefore been made to partially or entirely substitute sodium silicate with other waste materials and to develop alternative activators with less environmental impact using biomass ashes [19], glass waste [20], and other silica-rich wastes [21].Adding a silica-rich material during the alkaline fusion of Psl might be a novel pathway to enhance reactivity, reduce the environmental impact, and remove soluble silicates from the mix design of PS-based alkali-activated materials.
The main goal of the present study was to synthesize a raw material for alkali-activated material production based on a ternary mixture of Psl, glass waste (as a Si source), and kaolin clay (as an Al and Si source).Thermochemical treatment was adopted to enhance the reactivity of the raw materials, and different fluxing agents (NaOH, Na 2 CO 3 , Na 2 SO 4 ) were tested and compared.The effects of the fluxing agent type on the mineralogy and reactivity of the synthesized material were first evaluated, and the more suitable fluxing agent was selected.Then, the synthesized ternary mixture was alkali activated, and the effects of the alkaline activator concentrations on the mechanical properties of the obtained pastes were assessed.In addition, as part of the hardened paste characterization, the nature of the reaction products was investigated using XRD, FTIR, SEM/EDX, and magic-angle spinning nuclear magnetic resonance (MAS NMR) techniques.

Characterization methods
Chemical analysis of the raw materials was carried out by employing a PHILIPS PW-1004 X-ray spectrometer.The mineralogical characterizations of the raw material synthesized fused powders, and final alkaliactivated materials were performed by XRD analysis using Bruker-AXS D8 Advance equipment.The scans covered a 2θ range of 5 • -60 • , with a nominal step size of 0.011973 • and 0.5 s/step.For the quantitative XRD analysis, the specimens were scanned with a step size of 0.01973 • and 5 s/step from 5 • to 70 • , a 0.5 • divergence slit, and a 3 mm antiscatter slit.Thermal analysis of the raw and alkali-activated materials was addressed through TG/DTA analysis using a Stanton Redcroft STA 781 thermal analyzer at a heating rate of 10 • C/min.The mechanical properties of the alkali-activated materials were assessed via compressive strength measurement performed on an IBERTEST AUTOTEST-200/ 10 SW test frame, as the arithmetic means of five specimens.A sixth specimen was prepared for microstructural imaging.The microstructural observation via SEM and elemental analysis of the alkali-activated materials by EDX were examined using a JEOL JSPM-5400 microscope coupled with an Oxford-Link-Isis model EDX detector.The FTIR spectra of the fused and alkali-activated materials were obtained using an ATIMATTSON FTIR-TM series spectrophotometer operating in the range of 4000-400 cm − 1 at a resolution of 1 cm − 1 ; 300 mg of KBr was mixed with 1 mg of the sample to prepare the specimens for the analysis.
Isothermal calorimetry was considered to monitor released heat and heat flow, using an isothermal calorimeter of TAM AIR-TA Instruments, at 25 • C, for 5 g of solid sample mixed with the appropriate amount of activator (NaOH solution). 29Si and 27 A1 MAS NMR was performed using an Avance-400 Bruker apparatus.For 29 Si, the resonance frequency was 79.5 MHz with a spinning rate of 10 kHz, a pulse sequence of 5 μs, 10 s of recycling delay, and 5200 transients, using tetramethyl silane (TMS) as an external standard.For 27 Al, the resonance frequency used was 104.3 MHz; a spinning rate of 10 kHz, 5 s recycle delay, pulse sequence single pulse (2 μs), 400 transients, and corundum external standard were employed.The Gaussian function was used for deconvolution of 29 Si MAS NMR.Spectra were fitted with the minimum possible peaks.The full width at half height (FWHH) was considered less than 10 ppm [22].
The chemical analysis of the raw materials obtained by XRF analysis (Table 1) shows that the Psl comprises calcium oxide, silicon, and phosphate oxides with a shallow aluminum content.The kaolin

Alkaline fusion procedure and precursor preparation
The aim was first to assess the effect of the fusion treatment by using three fluxing agents on the material's structure; second, to evaluate the effect of alkaline activation on the mineralogy of the materials and analyze the reaction products.Based on the chemical composition of the raw materials, a mixture of Psl, kaolin, and glass waste was designed in order to produce a precursor with theoretical specific ratios: SiO 2 /Al 2 O 3 ≅ 4; SiO 2 /CaO ≅ 3, and CaO/Al 2 O 3 ≅ 1.5, according to the literature [14,23].
The precursors' synthesis in the current study was performed according to the elaboration methodology depicted in Fig. 2. The first step of the preparation protocol consisted of grinding the starting powders after adding different fluxing agents, the selection of which was generally based on their melting point, price, and efficiency in reducing the melting point of minerals [24,25].In our case, the fluxing agents were NaOH, Na 2 SO 4, and Na 2 CO 3 .The resulting powders were denoted as PN, PS, and PC, respectively.The content of the fluxing agents used was set to incorporate similar quantities of alkalis in all mixes of approximately 4 wt% of Na 2 O in order to use the minimum possible content of alkali and avoid efflorescence issues.In parallel, the resulting powdered materials were characterized using TGA analysis and later calcined at 1000 • C and rapidly cooled.The fused samples were analyzed using XRD (Fig. 2).

Preparation of alkaline-activated binders
To choose the optimum fluxing agent, the reactivity of the fused samples PN, PC, and PS was tested by alkaline activation technology.The samples were alkali activated with a 4 M NaOH solution (activator/ binder = 0.28) to form consistent pastes and stirred until homogenization.The resulting fresh pastes were cast into prismatic molds of 10*10*60 mm 3 .Preliminary tests showed that preliminary curing at room temperature (RT) is mandatory to avoid the formation of cracks.Two curing processes were tested: A = 20 h at RT in a plastic bag; B = 20 h at RT + 6 h at 85 • C in a plastic bag.After curing, the samples were removed from the mold and stored in the curing chamber (22 • C and relative humidity >90%) until the test age (3 days).The second curing procedure (B) was selected because with curing A, the level of hardening was low at 20 h at RT.To investigate the effect of the NaOH solution concentration, the synthesized materials with the chosen fluxing agent were alkali activated using sodium hydroxide solutions at different concentrations (2, 4, and 8 M).The NaOH solution/precursor ratios used were 0.27, 0.28, and 0.3 for 2, 4, and 8 M, respectively, to keep similar consistency.Prismatic samples were prepared following the B curing procedure.Then, the samples were stored in the curing chamber until testing age (3, 7, 14, and 28 days).The mechanical properties of the samples were assessed, then the samples were frozen by stopping hydration using isopropyl alcohol, and their mineralogical composition and microstructural properties were investigated and discussed.

Effects of fluxing agents on the microstructure and reactivity of the elaborated precursors
In accordance with the literature, economic parameters, and tests previously carried out in our laboratory with different possible fluxes, in this work, three were selected: NaOH, Na 2 SO 4 , and Na 2 CO 3 (samples PN, PS, and PC, respectively).For more information, see section 2.3 and Fig. 2.
Fig. 3 presents the TGA/DTG results of the designed materials, mixed with the different fluxing agents.The TGA/DTG curves highlight the presence of several distinct weight losses below 1000 • C. The first weight loss below 100 • C is attributed to the moisture departure, followed by the dihydroxylation of kaolinite, which occurs at around 493 • C for the samples PC and PS, while it corresponds to a broad peak centered at a lower temperature of 445 • C for the sample PN.In fact, this weight loss in the PN sample is a combination of kaolinite dihydroxylation and the melting of NaOH salt reactions.The melting temperature of NaOH (318 • C) is shallow compared to that of the other salts; this implies the presence of free alkali at a low temperature, decreasing the dihydroxylation temperature compared to the other fluxing agents [17].Kaolinite dihydroxylation generally takes place at about 540 • C and ends at around 675 • C in normal conditions [26].The presence of alkali, especially NaOH, alters the thermal decomposition of kaolin and lowers the temperature of its dihydroxylation by about 100 • C.
The weight loss in the temperature range of 670-696 • C could be attributed to the decarbonization reaction or overlapped weight losses corresponding to the decomposition of dolomite and calcite from the Psl.The fluxing agents prompted the decomposition of dolomite and calcite at a lower temperature.This decomposition, which commonly occurs between 650 and 850 • C, took place at a 100 • C lower temperature, showing the effect of alkali on the thermal decomposition of such minerals [17].Above 800 • C, new thermal transformations were detected due to the presence of fluxing agents, except for the sample PN, for which no noticeable weight variation was observed.At about 850 and 860 • C, the melting of Na 2 SO 4 and Na 2 CO 3 respectively occur [27,28].Several other studies have reported the role of fluxing agents in   modifying the thermal decomposition and mineralogical composition during the fusion process [25].In general, fluxing agents alter the different thermal decompositions and transformation of Psl, kaolin, and glass waste mixtures.The alkalis diminish the dihydroxylation and decarbonization reaction temperatures, with NaOH having a higher effect on the dihydroxylation reaction of kaolin.In the present case, the fluxing agent addition induced a decrease in the characteristic Fig. 6.Quantitative analysis of the XRD analysis of the synthesized PC powder using Rietveld refinement.Fig. 4 presents the XRD patterns of the fused materials.The alkaline fusion of the designed material had a significant effect on its structure and mineralogical composition.The original crystalline phases of the starting powders decomposed partially or wholly, and new phases were formed.
The fusion at 1000 • C induced the partial decomposition of quartz for the PS and PC samples and the complete dissolution of this phase for the PN sample.Meanwhile, the calcite, dolomite, kaolin, and muscovite phases were decomposed for all used fluxing agents (Fig. 4).XRD patterns highlight the persistence of fluorapatite to alkaline fusion for all the samples, but the intensity of their characteristic peaks slightly decreased, showing their possible partial decomposition.Furthermore, using NaOH (PN sample) resulted in a low crystalline material consisting of only fluorapatite and wollastonite phases besides the amorphous phase (evidenced by the hump between 20 and 35 2 teta degrees).Wollastonite (CaSiO 3 /COD # 96-900-5778) was newly formed, probably as a result of the combination between the calcium and silica, originating from the decomposed Psl and glass waste, respectively.This reaction generally requires a higher temperature, which was promoted by the presence of fluxing agents that probably reduced the temperature of its formation [29].The samples PS and PC showed higher crystallinity.More crystalline phases were newly formed in these samples, such as diopside (Ca  (SO 4 ) 2 /COD # 00-042-1312) was also formed in the PS sample due to the presence of sulfate released by the thermal decomposition of sodium sulfate.The diopside phase could be formed from the reaction between calcium carbonate and magnesium oxide from Psl and silica from glass or decomposed kaolin [30].It is worth noting that no mullite phase was detected at 1000 • C; alternatively, sodium-rich phase nepheline was found.Moreover, the amorphous phase characteristic hump was apparent for all the samples, confirming the partial dissolution of the original minerals into the amorphous phase after fusion at 1000 • C.
A previous study reported the new formation of crystalline phases such as anorthite, nepheline, and wollastonite in alkali-activated materials after exposure to high temperatures up to 1000 • C due to the recrystallization process during sintering [31].In fact, the formation of new crystalline phases in the fused materials could be beneficial to the material, depending on the reactivity of the phases.Also, some crystalline phases, such as anorthite, could be valuable in strengthening the alkali-activated material due to the hardness of their crystals, as reported by Zhao et al. [32].
The compressive strength of alkali-activated samples at 3 days (Fig. 5) showed that the paste made with curing A revealed lower strength compared to curing B. Although 3 days is a short time to harden well the material, for the hardened pastes using curing B, the compressive strength improved significantly.Similar behavior has been observed in alkali-activated fly ash, where the curing temperature improved the early development of mechanical strengths [33] due to the acceleration of the dissolution process of the precursor.With curing B, PC and PN binders exhibited approximately similar compressive values of 17.94 and 19.72 MPa, respectively, contrary to the PS sample that clearly presented minor strength (9.48 MPa).
Considering the cost-effectiveness of sodium carbonate [34] and based on the obtained mechanical test results (Fig. 5), sodium carbonate as a fluxing agent and curing B were adopted for the next step and throughout the remainder of this work.

Effect of alkali concentration on the properties of the alkali-activated PC binder
Enough quantity of PC powder (≈1 kg) was synthesized in the laboratory according to the process indicated in Fig. 2. The material was ground to obtain a particle size lower than 50 μm.The material was characterized by XRD using a corundum internal standard to determine the quantitative crystalline and amorphous phases.EVA and TOPAS software were utilized for phase identification and quantification using the Rietveld refinement method [35].The results in Fig. 6 show that the PC precursor has, as a crystalline phase, quartz (2.97%), nepheline (1.40%), wollastonite (11.14%), anorthite sodian (23.70%), diopside (7.25%), and fluorapatite (13.22%).The amorphous content was 40%, which is lower than the content generally found in coal fly ash type F (>50%) [36].
The synthesized material PC was mixed with a sodium hydroxide solution at different concentrations (2, 4, and 8 M).Heat flow and total heat at RT in the first 3 days (72 h) were determined by isothermal condition calorimetry.The compressive strength and the characterization of the reaction products at 3, 7, 14, and 28 days were determined for the hardened pastes (see section 2.4).

Reaction kinetics
The heat flow and total heat are shown in Fig. 7.The heat flow curves have a similar shape to those for cement hydration [37].The paste activated with 8 M shows the most intense heat flow peak, while 2 M represents relatively the lowest peak intensity.Increasing the alkalinity of the medium increased the heat rate by accelerating the reaction at an early age (dormant period longer for 4 M and 2 M).A similar behavior has been observed in the alkaline activation of fly ash [38].It was found that at 20 • C, NaOH-activated fly ash showed a single exothermic heat response; increasing alkalinity accelerated the dissolution reaction and increased the heat release.A similar observation can be drawn in the cumulative heat curves (Fig. 7-B); the total heat was improved with alkalinity.This can be assigned to increasing the dissolution and transformation of dissolved species into gels.As can be seen from Fig. 7-B, the effect of the activation was more pronounced for relatively high NaOH concentrations.The ultimate released heat decreased with the decrease of the activator's alkalinity, from about 0.35 kJ/g for the 8 M sample to 0.19 kJ/g for the 2 M specimen.Similar to the heat flux rate, the variation of the released heat was not proportional to the activator content.

Effect of alkali concentration on mechanical strength development
The compressive strength development according to curing B, as a function of time and the activating solution concentration, is depicted in Fig. 8. Binder activated with 4 and 8 M exhibited high mechanical properties, 19 and 21 MPa, at 3 days.However, PC-2M showed mediocre compressive strengths due to the low pH.These results confirm that low alkali concentrations have a detrimental effect on hydration, preventing the solubility of a high amount of the reactive phases and leading to fewer alkali activation products [39].The results also show that the hardening process progresses from 3 to 28 days, evidenced by the increasing compressive strength evolution with time for all the samples.Mechanical characterization seemed to confirm the evolution of reactions with time, as indicated by calorimetric results (Fig. 7-B).The results confirmed the positive effect of alkalinity on the evolution of  compressive strength; this latter is more important when the NaOH concentration increases and improves the early age reaction rates.Indeed, the increase of hydroxide ion concentration increases the solubility of the reactive phases, producing more monomers necessary for ensuring the reactions and strengthening the material's structure [40].
Overall, the mechanical properties of the samples proved that the synthesized powder was successfully alkali activated using only sodium hydroxide as an activation solution, leading to high mechanical properties of up to 46 MPa at 28 days.

Mineralogical characterization of the hardened pastes by XRD
In turn, the results of the XRD analysis are gathered in Fig. 9. Similar crystalline phases were detected for all samples, regardless of the NaOH solution concentration used.No zeolite phases or semi-crystalline calcium-rich phases were identified despite the high calcium content of the raw material.However, the intensities of the wollastonite (CaSiO 3 /COD # 96-900-5778) and anorthite (Al 1.55 Ca 0.55 Na 0.45 O 8 Si 2.45 /COD # 01-071-0748) were slightly decreased, which foreshadows their partial participation in the alkaline reaction.Feldspars such as the anorthite phase have been reported to be reactive in an alkaline medium.González-García et al. [41] showed that the alkali activation of natural pozzolan feldspars leads to the dissolution of anorthite feldspars to form a geopolymeric gel.However, after 14 days, the anorthite phase reappears due to recrystallization or precipitation processes.In addition, Lemougna et al. [42] reported that anorthite, nepheline, and all the crystalline phases from four volcanic ashes were partially dissolved and involved in the alkali activation reactions to form a semi-crystalline geopolymer phase after alkali activation with a 7-12 M NaOH solution.In another study, Kumar et al. [43] indicated that wollastonite and diopside phases from silicomanganese slag were dissolved after alkaline activation, and new crystalline phases, such as hydrotalcite and tobermorite, were formed.
Generally, the amorphous hump location shifting indicates the kind of reaction products formed.In the case of low calcium systems like alkali-activated fly ash, the hump shifts toward high 2θ degrees, indicating the formation of alkaline aluminosilicate gel or a mixture of gels (N-A-S-H/C-A-S-H) [33,44].However, no clear shifting of the amorphous phase was noted in the present alkali-activated samples.

Microstructural characterization of alkali-activated PC precursors by FTIR
The IR spectrum of the fused powder PC before alkaline activation (Fig. 10) illustrates the presence of quartz by means of the bands located at 797 and 543 cm − 1 .The presence of fluorapatite is responsible for two bands at around 576 and 603 cm − 1 , corresponding to the P-O asymmetric stretching vibration of PO 3 [13,45].Moreover, O-Ca-O nonbridging bending vibrational modes and O-Mg-O nonbridging bending modes were observed at 429 and 509 cm − 1 , respectively, for both the original and alkali-activated powder [30].These bands suggest the presence of the diopside phase, which is in accordance with the XRD analysis results.
The band at 1039 cm − 1 in the original fused powder is attributed to the asymmetric stretching vibrations of T-O-T (T = Si or Al) groups of the amorphous phase of the fused PC.This broadband shifted toward lower wavenumbers after alkaline activation, indicating a modification in the chemical environment of the Si-O bands by substitution of the silica tetrahedron by aluminum or calcium ones [46].
The effect of the NaOH concentration on the chemical structure changes during the alkali activations is illustrated in Fig. 10, which compares the alkali-activated materials using different NaOH concentrations at 28 days.The main variation arises in the band of T-O-T, which shifted toward lower wavenumbers with increasing NaOH concentration, indicating a higher dissolution of the amorphous part from the original powder to form the alkali activation reaction products.Also, the appearance of the shoulder at about 870 cm − 1 corresponds to Si-O - terminal vibration, which is a proof of the initial powder reaction forming the geopolymer gel chains [47].
The apparition of the stretching vibrations of O-C-O and C-O in the range of 1400-1500 cm − 1 in the alkali-activated samples indicates the carbonation of samples, possibly from the interaction of the alkali metal with atmospheric CO 2 [48].

Effect of alkali concentration on the morphology and microstructure of the pastes
Fig. 11 shows the BSEM micrographs of PC-2M and PC-8M binders at 28 days.EDX analysis was performed (over 50 points for each sample).The average chemical composition of the different phases observed by BSEM in the alkali-activated systems is summarized in Table 2.In both systems, the coexistence of unreacted particles from the fused material and the reaction products was observed.Quartz particles were embedded in the binding gel (Fig. 11-b and -c), while the fluorapatite phase precipitated in the alkali activation products (Fig. 11-e).Furthermore, acicular-type crystals (a needle-like microstructure) that are wollastonite were observed in both systems [49].
The reaction products were mainly N-A-S-H gel and intermixed (N, C)-A-S-H gels.The observed N-A-S-H gel was not 100% pure (Table 2) due to the presence of calcium in the system, allowing the partial substitution of sodium by calcium [50].The (N, C)-A-S-H gels present in the PC-2M sample had higher calcium content; however, the N-A-S-H gels observed in the PC-8M sample had more homogeneous chemical compositions.The ternary Ca-Si-Al atomic plots leaned toward a higher Si atomic percentage (Fig. 12).This is due to the higher dissolution of silica in high-alkaline conditions.The PC-8M micrographs show higher density and more reaction products surrounding the unreacted particles, which might justify the greater strength development revealed by this sample.

Investigation of the reaction products by 29Si and 27Al MAS NMR
Fig. 13-A, -B, and -C show the 27 Al spectra and deconvoluted 29 Si spectra of raw fused powder PC and the alkali-activated one with 8 M NaOH at 28 days.
The spectra of 27 Al, of both the precursor PC and the PC-8M, show the presence of a broad signal of Al (IV) around +59 ppm (Fig. 13).The alkaline activation of PC induced a slight shift of the signal to high values; these positions are consistent with the presence of only Al (IV) sites from the tetrahedral aluminum environment both in the remaining unreacted raw materials and in the main reaction product after the chemical activation process.The results showed that the activation of alkali impacted the raw PC Al (IV) species, as the intensity in this region (low chemical changes) experienced a clear reduction with exposure to a medium with high alkali content.
Based on the 27 Al spectra, both raw PC and PC-8M samples highlight the presence of one board and isolated Al (IV) pic located at 59 ppm.The alkali activation of PC powder induced a slight shift in the resonance to higher shifts; these positions are consistent with the presence of only tetrahedral aluminum environment Al (IV) sites in both unreacted remaining raw materials and the main reaction product after the chemical activation process.The results exhibited that alkali activation impacted the raw PC Al (IV) species, as the intensity in this region (low chemical shifts) underwent a clear reduction with exposure to high alkali medium.
The Si spectrum of the PC precursor shows a broad overlapped peak in the range between − 70 and − 120 ppm, where 12 silicon sites are identified at − 74, − 76, − 79, − 82, − 84, − 89, − 93, − 97, − 101, − 105, − 109, − 114, and − 117 ppm.These sites are assigned to the Q n Si atoms, connected via oxygen bridges to aluminum atoms, related to Q 0 , Q 1 , Q 2 , and Q 3 , and the Q 4 (mAl) groups with m = 0-4.These groups originate from both raw amorphous and crystalline phase contributions.Alkaliactivated binder PC-8M also shows overlapping resonance peaks from − 70 to − 120 ppm.The results of the deconvolution are summarized in Table 3.The − 73 ppm resonance is assigned to Q 0 sites; the one located at − 77 ppm is attributed to the end of chain Q 1 , regarding which many works consider that two kinds can be identified (Q 1 a and Q 1 b ) [22,51].The Q 2 (1Al) characteristics peak is located at the chemical shift of − 80 ppm, while the Q 2 one is located at − 84 ppm.From this latter, two Q 2 units can often be observed, assigned to Q 2 p and Q 2 b (the paired and bridging sites, respectively) [51,52].The − 88 ppm peak is attributed to the overlapped Q 3 (1Al) and Q 4 (4Al) sites, while that at − 94 ppm characterizes the overlapped peaks of Q 3 and Q 4 (1Al); these crosslinked sites were the subject of several studies in the literature referring to alkali-activated materials [53].Furthermore, additional crosslinking sites of four connected silicon Q 4 (2Al), Q 4 (1Al), and Q 4 (0Al) species were detected at − 98, − 104, and − 110 ppm, respectively [54].
The increase of Al(IV) after alkali activation suggests the formation of a more crosslinked and polymerized structure, which agrees with the previous XRD and FTIR findings.In addition, the detection of Q 3 (1Al) units in the structure formed after the alkaline attack may indicate the existence of Al(IV) in bridging positions as a result of the possible formation of more crosslinked structures [55].This observation announces the formation of non-negligible new silicon structures, assigned to the C-(A)-S-H type gels with dreierketten-type structure and similar to a tobermorite-like structure [53,56], in addition to Q 4 (m Al, m = 0,1,2,3, 4), the dominant structure gel characterizing the main reaction product of the geopolymerization N-A-S-(H), coexisting with (N, C)-A-S-H Fig. 13.MAS NMR spectra of 27 Al (A) of the fused powder and alkali-activated binders and 29 Si of the fused material PC (B) and alkali-activated PC samples with 8 M NaOH (C).structures given that the medium is highly rich in calcium; this latter may readily displace sodium by ion exchange on the N-A-S-H gels, as reported in the literature [57]. 29Si and 27 Al NMR MAS results were in good agreement with the results described by SEM observations and in accordance with previous findings in the literature [51,55].It is worth mentioning that the observed resonance sites also overlap with the remaining unreacted raw materials, at the range of shifts between − 85 and − 100 ppm, including the diopside, anorthite, wollastonite, and nepheline [58].Peaks below − 110 ppm are assigned to unreacted crystalline quartz and mullite [59] Overall, the present study introduces a novel solution for synthesizing aluminosilicate precursors based on phosphate mine wastes combined with other industrial by-products that can be activated with alkaline solutions alone.It is well known that the stability of silicate solutions and their viscosity present a significant challenge for the practical implementation of alkali-activated materials in the construction field.The strong ion pair interactions between the alkali metal cations and the silicate oligomers present in solution are responsible for solution stability.However, these ion pairs preventing the condensation of silicate oligomers induce an increase in viscosity.Furthermore, the chemical composition of silicate solutions is crucial to their metastability; Vail (1952) [60] presented the Na 2 O-SiO 2 -H 2 O ternary diagram.He assigned different regions depending on the composition and described the properties and application of each.The precipitation of Na 2 SiO 3 ⋅9H 2 O is expected in most regions that can be highly prone to crystallization as hydrated sodium metasilicates, metastable, or highly viscous.Most silicate solutions used in alkali-activated materials belong to two small regions, A and B, as reported by Vail (1952), also representing a precipitation risk.Several attempts have been made to develop new alternatives to silicate solutions by using other soluble silicate-rich materials such as biomass ashes, glass waste, or other industrial waste solutions.Hence, the combination of glass waste and Psl to synthesize a new precursor represents a double solution for mine tailing management and activation solution stability.

Conclusions
The present study attempts to offer solutions for industrial side streams management, such as phosphate mine waste and glass waste.An alkaline fusion method was tested to synthesize a novel aluminosilicate precursor for geopolymer elaboration, and the alkaline fusion conditions and fluxing agent type were optimized.Then, the effects of the alkaline solution concentration and curing conditions on the properties of the prepared binders were studied.The results elucidated that, among the four studied fluxing agents, sodium carbonate was the most efficient and cost-effective, resulting in higher reactive precursors and strong alkaliactivated binders.The optimum fusion temperature was 1000 • C, which led to more reactive and amorphous precursors.
Furthermore, the selected fused precursor was successfully alkali activated using different molarities of NaOH solutions resulting in strong binders with a maximum compressive strength of 46 MPa at 28 days.The results of the elaborated binder's characterization by XRD, FTIR, DEM/EDX, and solid-state NMR revealed the coexistence of alkali activation reaction products N-A-S-H and (N, C)-A-S-H due to the hybrid nature of the synthesized precursor (coexistence of reactive Si and Ca).
Overall, the present study offers a simple process for Psl or any other similar side stream valorization in alkali-activated materials while solving the problem of the stability of the activation solution by adapting the alkaline solution as the sole activator.Although the energy consumption of the proposed process is due to fusion, the developed solution can find its practical use by exploring clean energy sources.

Table 3
Deconvolution a results from 29 Si MAS NMR analysis.a The deconvolution results also include the contribution of the unreacted raw materials.

Fig. 2 .
Fig. 2. Scheme illustrating the methodology of raw material activation by alkaline fusion method.

Fig. 3 .
Fig. 3. Thermal analysis of the designed material mixed with the different fluxing agents.A: Thermogravimetry analysis (TG); B: Differential thermal analysis (DTG).

Fig. 5 .
Fig. 5. Compressive strength of the alkali-activated binders at 3 days, with 4 M NaOH, at two different curing processes (A = 20h at RT in a plastic bag; B = 20h at RT +6h at 85 • C in a plastic bag).

Fig. 7 .
Fig. 7. Heat flux (A) and released Heat (B) of the alkali-activated samples during early age at different NaOH concentrations.

Fig. 8 .Fig. 9 .
Fig. 8. Mechanical properties of the alkali-activated PC powder at different curing times and NaOH concentrations.A: Compressive strength.

Fig. 10 .
Fig. 10.FTIR spectra of the PC-8M at different ages and the alkali-activated samples with different NaOH concentrations at 28 days.

Table 1
Chemical composition of the different raw materials.

Table 2
Chemical composition (average values) of the different phases from BSM/EDX analysis (in atm %).