Synthesis of Geopolymers Incorporating Mechanically Activated Fly Ash Blended with Alkaline Earth Carbonates: A Comparative Analysis

: The objective of this study is to perform a comparative analysis of the impact of incorporating alkaline earth metal carbonates (MCO 3 , where M–Mg, Ca, Sr, Ba) into low-calcium fly ash (FA) on the geopolymerization processes and the resultant properties of composite geopolymers. Mechanical activation was employed to enhance the reactivity of the mixtures. The reactivity of the mechanically activated (FA + alkaline earth carbonate) blends towards NaOH solution was experimentally studied using XRD analysis and FTIR spectroscopy. In agreement with thermodynamic calculations, MgCO 3 demonstrated the most active interaction with the alkaline solution, whereas strontium and barium carbonates exhibited little to no chemical interaction, and calcite was situated in the transition region. As the calcite content in the mixture with FA increased, the compressive strength of the geopoly-mers continuously improved. The addition of Mg, Sr, and Ba carbonates to the FA did not enhance the strength of geopolymers. However, the strength of geopolymers based on these blends was comparable with that of geopolymers based on 100% FA. The strength of geopolymers synthesized from the 100% FA and from the (90% FA + 10% MCO 3 ) blends, mechanically activated for 180 s, at the age of 180 days was 11.0 MPa (0% carbonate), 11.1 MPa (10% MgCO 3 ), 36.5 MPa (10% CaCO 3 ), 13.6 MPa (10% SrCO 3 ), and 12.4 MPa (10% BaCO 3 ) MPa, respectively. The influence of carbonate additives on the properties of the composite geopolymers was examined, highlighting filler, dilution, and chemical effects. The latter determined the unique position of calcite among the carbonates of alkaline earth metals.


Introduction
Geopolymer materials, a subset of inorganic polymers, have emerged as a groundbreaking area of research and application in the field of materials science.These materials are created through the aluminosilicate raw materials, such as metakaolin, fly ash (FA), and others.The synthesis of geopolymers typically involves the reaction of aluminosilicate sources with alkaline activators like sodium hydroxide and sodium silicate.The resulting geopolymer structure, represented by the sodium aluminosilicate hydrogel (N-A-S-H gel), offers exceptional mechanical properties [1].Unlike traditional Portland cement, the synthesis of geopolymers involves significantly lower greenhouse gas emissions, rendering them more environmentally friendly alternatives.This reduced environmental impact stems from the fact that geopolymers do not require the high-temperature calcination of limestone, a process central to Portland cement production that releases substantial amounts of CO 2 [2][3][4][5].Furthermore, geopolymer materials exhibit a versatile range of applications owing to their unique chemical and physical properties.They can be used in the production of refractories and fireproof materials due to their excellent thermal stability and resistance to high temperatures [1,[6][7][8][9].Geopolymers have proven effective in the Minerals 2024, 14, 726 2 of 13 immobilization of heavy metals and radioactive waste, as their dense microstructure and chemical composition can encapsulate these hazardous substances, preventing leaching and environmental contamination [10][11][12][13].In the field of wastewater treatment, geopolymers can aid in the removal of contaminants and pollutants, contributing to cleaner water resources [14,15].Additionally, in radiation shielding applications, geopolymers can effectively attenuate various forms of radiation, making them suitable for use in medical and nuclear facilities [16,17].These diverse applications underscore the potential of geopolymers to contribute significantly to sustainable development across various industries.
The disadvantage of low-calcium FA is its low reactivity during geopolymer synthesis.In order to increase the reactivity of FA towards an alkaline agent, and consequently, to improve the strength and other physical and mechanical properties of geopolymers, the mechanical activation is a well-established method [18][19][20][21][22][23].In this paper, the terms "mechanical activation" (MA) and "milling" are used interchangeably to refer to the same process, which is the mechanical treatment of a material using a planetary mill.Another approach to enhancing the performance of geopolymers involves the incorporation of various additives into the FA, including Ca/Mg carbonates.Specifically, it has been found that the addition of dolomite or limestone to the FA leads to an increase in the geopolymer characteristics [24,25].The beneficial impact of incorporating Ca/Mg carbonate minerals into FA to enhance geopolymer performance can be elucidated by considering filler, dilution, and chemical effects [26].
In our previous study, we synthesized geopolymers based on mechanically activated mixtures of FA with alkaline earth carbonates: CaCO 3 [27], MgCO 3 [28], SrCO 3 [29], and BaCO 3 [29].A sodium hydroxide solution served as the alkaline activator, and the curing process was conducted at ambient temperature.The product of geopolymerization of the mechanically activated mixtures of FA with alkaline earth carbonates was the N-A-S-H gel.The geopolymers based on FA blended with magnesite, strontianite, and witherite have potential applications in the production of fire protection materials with improved properties, the immobilization of radioactive strontium, and the formulation of radiationshielding materials, respectively.
In this paper, we present a comparative analysis of the previously published results on the strength of geopolymers incorporating mechanically activated FA blended with alkaline earth carbonates [27][28][29].The standard Gibbs energy of reactions between the carbonates and sodium hydroxide solution depending on the ionic radius of an alkaline earth metal was calculated using reference data.Additionally, the reactivity of the carbonates present in mechanically activated mixtures of (FA + MCO 3 ) (M-Mg, Ca, Sr, Ba) in relation to NaOH solution was experimentally investigated.The following main factors determining the influence of MCO 3 additives to FA on the strength of composite geopolymers were considered: the chemical effect and the effects of filler and dilution.This allowed us for the first time to reveal a special position of calcite in this series of alkaline earth carbonates.
The primary component of the FA was the glass phase, along with crystalline phases such as α-quartz and mullite.Calcite comprised admixtures of augite and a minor quantity of feldspar.Magnesite contained approximately 3% quartz and minor quantities of dolomite, chlorite, talc, and pyrite as impurities.X-ray diffraction (XRD) patterns of SrCO 3 and BaCO 3 are shown in Figure 1.The XRD patterns of FA and other carbonates as well as a more comprehensive overview of the raw material characteristics can be found in [27][28][29].The primary component of the FA was the glass phase, along with crystalline phases such as α-quartz and mullite.Calcite comprised admixtures of augite and a minor quantity of feldspar.Magnesite contained approximately 3% quartz and minor quantities of dolomite, chlorite, talc, and pyrite as impurities.X-ray diffraction (XRD) patterns of SrCO3 and BaCO3 are shown in Figure 1.The XRD patterns of FA and other carbonates as well as a more comprehensive overview of the raw material characteristics can be found in [27][28][29].

Mechanical Activation
MA of the (FA + carbonates) blends was carried out using an AGO-2 laboratory planetary mill (Novic, Novosibirsk, Russia) under atmospheric conditions at a centrifugal force of 40 g for 180 s.Steel vials and steel balls with diameters of 7-8 mm were employed as milling bodies.Further details regarding the MA process can be found in [27][28][29].

Reactivity Test
To evaluate the reactivity of carbonates in the blends with respect to the alkaline activator, experiments were conducted using blends consisting of 80% FA and 20% carbonate, mechanically activated for 180 s, and an 8.3 M NaOH solution.
The mixtures containing 20% carbonate were used in the experiments to obtain reliable analysis results of the reaction products after an interaction of the mixtures with sodium hydroxide solution.One gram of the mixture was placed in a beaker, to which 40 mL of the NaOH solution was added.The resulting suspension was stirred using a magnetic stirrer at room temperature (20-22 °C) for 24 h.Subsequently, the solid residue was separated from the liquid phase by filtration, washed with water, and left to dry at room temperature.The solid residues were analyzed using XRD and FTIR spectroscopy.

Mechanical Properties
One of the most important physical and mechanical properties of geopolymers is compressive strength.The compressive strength data of geopolymers derived from (FA +

Mechanical Activation
MA of the (FA + carbonates) blends was carried out using an AGO-2 laboratory planetary mill (Novic, Novosibirsk, Russia) under atmospheric conditions at a centrifugal force of 40 g for 180 s.Steel vials and steel balls with diameters of 7-8 mm were employed as milling bodies.Further details regarding the MA process can be found in [27][28][29].

Reactivity Test
To evaluate the reactivity of carbonates in the blends with respect to the alkaline activator, experiments were conducted using blends consisting of 80% FA and 20% carbonate, mechanically activated for 180 s, and an 8.3 M NaOH solution.
The mixtures containing 20% carbonate were used in the experiments to obtain reliable analysis results of the reaction products after an interaction of the mixtures with sodium hydroxide solution.One gram of the mixture was placed in a beaker, to which 40 mL of the NaOH solution was added.The resulting suspension was stirred using a magnetic stirrer at room temperature (20-22 • C) for 24 h.Subsequently, the solid residue was separated from the liquid phase by filtration, washed with water, and left to dry at room temperature.The solid residues were analyzed using XRD and FTIR spectroscopy.

Mechanical Properties
One of the most important physical and mechanical properties of geopolymers is compressive strength.The compressive strength data of geopolymers derived from (FA + MCO 3 ) (M-Mg, Ca, Sr, Ba) blends, subjected to mechanical activation for 180 s, at the ages of 7, 28, and 180 days [27][28][29] are depicted in Figure 2. The methodology for preparing the geopolymers, including mix design, was outlined in [27][28][29].Briefly, carbonates were incorporated into the FA at varying concentrations: 1%, 3%, 5%, and 10% relative to the total mass of the (FA + carbonate) blend for the preparation of geopolymers.An 8.3 M NaOH solution was used as the alkaline activator.The prepared specimens were cured for 24 h in a closed container at a relative humidity of 95 ± 5% and a temperature of 22 ± 2 • C.After demolding, the specimens were further cured under the same conditions until the testing time.Figure 2 shows the following distinct trends.Calcite (Ca carbonate) as an additive to the FA was significantly more effective compared to other carbonates.With increasing calcite content in the mixture with FA, the compressive strength consistently rose across all curing times.In contrast, the addition of Mg, Sr, and Ba carbonates to FA did not enhance the strength of the geopolymer.However, the strength of geopolymers based on these blends either remained stable or decreased only slightly compared to geopolymers prepared from 100% FA (0% carbonate).
rose across all curing times.In contrast, the addition of Mg, Sr, and Ba carbonates to FA did not enhance the strength of the geopolymer.However, the strength of geopolymers based on these blends either remained stable or decreased only slightly compared to geopolymers prepared from 100% FA (0% carbonate).
The addition of 1-3% MgCO3, SrCO3, and BaCO3 to FA resulted in a decrease in the strength of the geopolymer compared to that of the geopolymer synthesized from 100% FA.As the carbonate content was further increased, the strength continued to rise, approaching that of the geopolymer prepared with 100% FA.This trend was particularly evident for the geopolymers cured for 28 days.The underlying reason for the observed minimum on the strength curves remains unclear and necessitates further investigation.Figure 3 illustrates the strength dependencies of geopolymers synthesized from the (90% FA + 10% MCO3) (M-Mg, Ca, Sr, Ba) blends, mechanically activated for 180 s, at the ages of 7, 28, and 180 days, relative to the radius of the alkaline earth metal cation.It is evident that calcium deviates from the trends observed for the group of alkaline earth metals.The addition of 1-3% MgCO 3 , SrCO 3 , and BaCO 3 to FA resulted in a decrease in the strength of the geopolymer compared to that of the geopolymer synthesized from 100% FA.As the carbonate content was further increased, the strength continued to rise, approaching that of the geopolymer prepared with 100% FA.This trend was particularly evident for the geopolymers cured for 28 days.The underlying reason for the observed minimum on the strength curves remains unclear and necessitates further investigation.
Figure 3 illustrates the strength dependencies of geopolymers synthesized from the (90% FA + 10% MCO 3 ) (M-Mg, Ca, Sr, Ba) blends, mechanically activated for 180 s, at the ages of 7, 28, and 180 days, relative to the radius of the alkaline earth metal cation.It is evident that calcium deviates from the trends observed for the group of alkaline earth metals.Figure 3 illustrates the strength dependencies of geopolymers synthesized from the (90% FA + 10% MCO3) (M-Mg, Ca, Sr, Ba) blends, mechanically activated for 180 s, at the ages of 7, 28, and 180 days, relative to the radius of the alkaline earth metal cation.It is evident that calcium deviates from the trends observed for the group of alkaline earth metals.To investigate the specific effect of CaCO3 addition to FA on the synthesis of geopolymer, the thermodynamic characteristics of reactions involving carbonates and sodium hydroxide solutions were calculated.Additionally, the interaction between mechanically activated mixtures (80% FA + 20% MCO3) (M-Mg, Ca, Sr, Ba) and sodium hydroxide solutions was experimentally examined.To investigate the specific effect of CaCO 3 addition to FA on the synthesis of geopolymer, the thermodynamic characteristics of reactions involving carbonates and sodium hydroxide solutions were calculated.Additionally, the interaction between mechanically activated mixtures (80% FA + 20% MCO 3 ) (M-Mg, Ca, Sr, Ba) and sodium hydroxide solutions was experimentally examined.

Thermodynamic Calculations
To evaluate the reactivity of MCO 3 carbonates (M-Mg, Ca, Sr, Ba) with respect to sodium hydroxide solution, the standard Gibbs energy ∆ r G • (298) of the following reactions was calculated: where M-Mg, Ca, Sr, Ba; m-molality concentration of the solution.
The initial NaOH solution concentration (5 m) was selected to closely approximate the conditions of geopolymer synthesis.In addition, reliable data on activity coefficients are available for this solution [30], which are essential for calculating the Gibbs energy of solution formation.It was assumed that the reaction products consist of a 2.5 m Na 2 CO 3 solution and solid hydroxides M(OH) 2 .The standard Gibbs energy of Na 2 CO 3 solution formation was computed following the methodology outlined in [31], while for the solid phases (MCO 3 and M(OH) 2 ), data were sourced from the IVTANTERMO database [32].The calculated results of ∆ r G • (298) for reactions (1) are presented in Figure 4.
Based on the data depicted in Figure 4, it is evident that thermodynamically, reaction (1) is highly favorable for magnesium carbonate, prohibited for strontium and barium carbonates, while calcium carbonate resides in the transitional zone.It should be noted that thermodynamic calculations were performed for carbonates in the standard state.It is known that as a result of mechanical activation, the Gibbs energy of solids increases due to amorphization, the generation of structural defects, and other factors.In particular, using calorimetric measurements, it was shown that after mechanical treatment of magnesite and calcite in a vibromill, the Gibbs energy of these carbonates increased by 5-8 kJ•mol −1 [33].Thus, the values of ∆ r G • (298) of reactions (1) can be slightly shifted to more negative values, which does not affect the comparative characterization of these reactions.
The initial NaOH solution concentration (5 m) was selected to closely approximate the conditions of geopolymer synthesis.In addition, reliable data on activity coefficients are available for this solution [30], which are essential for calculating the Gibbs energy of solution formation.It was assumed that the reaction products consist of a 2.5 m Na2CO3 solution and solid hydroxides M(OH)2.The standard Gibbs energy of Na2CO3 solution formation was computed following the methodology outlined in [31], while for the solid phases (MCO3 and M(OH)2), data were sourced from the IVTANTERMO database [32].The calculated results of ΔrG o (298) for reactions (1) are presented in Figure 4. Based on the data depicted in Figure 4, it is evident that thermodynamically, reaction (1) is highly favorable for magnesium carbonate, prohibited for strontium and barium carbonates, while calcium carbonate resides in the transitional zone.It should be noted that thermodynamic calculations were performed for carbonates in the standard state.It is known that as a result of mechanical activation, the Gibbs energy of solids increases due to amorphization, the generation of structural defects, and other factors.In particular, using calorimetric measurements, it was shown that after mechanical treatment of magnesite and calcite in a vibromill, the Gibbs energy of these carbonates increased by 5-8 kJmol −1 [33].Thus, the values of ΔrG°(298) of reactions (1) can be slightly shifted to more negative values, which does not affect the comparative characterization of these reactions.
To confirm the reliability of these calculations, we conducted experiments involving the interaction of (80% FA + 20% MCO3) (M-Mg, Ca, Sr, Ba) blends, milled for 180 s, with sodium hydroxide solution.The solid residues were analyzed using XRD analysis and FTIR spectroscopy.To confirm the reliability of these calculations, we conducted experiments involving the interaction of (80% FA + 20% MCO 3 ) (M-Mg, Ca, Sr, Ba) blends, milled for 180 s, with sodium hydroxide solution.The solid residues were analyzed using XRD analysis and FTIR spectroscopy.

XRD and FT-IR Spectroscopy Analysis
Figures 5-8 show XRD patterns of the blends and the corresponding residues after the reactivity test.The outcomes of the test broadly substantiated the thermodynamic assessment of the reactivity of alkaline earth metal carbonates toward the alkaline activator.It is important to note that during the reactivity test, the carbonates in the blend underwent dissolution in a sodium hydroxide solution concurrently with the FA, making the interaction more intricate than described by reaction (1).
FTIR spectroscopy data further support the observations made via XRD measurements.Figure 9 presents the FTIR spectra of the (80% FA + 20% magnesite) blend mechanically activated for 180 s, both before and after reactivity test.The broad band in the 3700-3100 cm −1 region (O-H stretching vibration) in the spectrum of the untreated blend (Figure 9; curve 1) corresponds to the water adsorbed by magnesite and FA from air during MA.The main absorption band at 1081 cm −1 is related to the asymmetric stretching Si-O-T (T = Si, Al) vibrations of the aluminosilicate FA components [34,35].Bands at 1456, 887, and 748 cm −1 correspond to the vibrations of the CO 3 group in magnesite [36].In the FTIR spectrum of the residue (Figure 9; curve 2), compared to that of the blend (Figure 9; curve 1), there is a significant decrease in the intensity of the magnesite bands.In accordance with the XRD data (Figure 5) and thermodynamic calculations (Figure 4), this confirms the high reactivity of magnesium carbonate in the geopolymerization of the (FA + magnesite) blends.
assessment of the reactivity of alkaline earth metal carbonates toward the alkaline activator.It is important to note that during the reactivity test, the carbonates in the blend underwent dissolution in a sodium hydroxide solution concurrently with the FA, making the interaction more intricate than described by reaction (1).
Magnesium carbonate (MgCO3) was nearly entirely converted into brucite (Mg(OH)2) and hydrotalcite (Mg6Al2CO3(OH)16•4H2O) (Figure 5).Calcium carbonate (CaCO3) underwent partial transformation into calcium hydroxide (Figure 6).Conversely, strontium carbonate (Figure 7) and barium carbonate (Figure 8) exhibited no discernible changes after reactivity test according to XRD analysis.assessment of the reactivity of alkaline earth metal carbonates toward the alkaline activator.It is important to note that during the reactivity test, the carbonates in the blend underwent dissolution in a sodium hydroxide solution concurrently with the FA, making the interaction more intricate than described by reaction (1).
Magnesium carbonate (MgCO3) was nearly entirely converted into brucite (Mg(OH)2) and hydrotalcite (Mg6Al2CO3(OH)16•4H2O) (Figure 5).Calcium carbonate (CaCO3) underwent partial transformation into calcium hydroxide (Figure 6).Conversely, strontium carbonate (Figure 7) and barium carbonate (Figure 8) exhibited no discernible changes after reactivity test according to XRD analysis.The bands at 1410 cm −1 and 3694 cm −1 in the FTIR spectrum of the residue (Figure 9; curve 2) can be attributed to carbonate ion vibrations in hydrotalcite [37,38] and hydroxyl vibrations in brucite [39], respectively.This supports the formation of hydrotalcite and brucite during the interaction of the blend with an alkali solution, as revealed by XRD measurements (Figure 5).It should be noted that, according to XRD and FTIR spectroscopy data, in the geopolymers based on the (FA + magnesite) blends, only hydrotalcite was detected as a newly formed crystalline phase [28].The absence of brucite in the geopolymer-ization products was likely due to the significantly lower alkali content in the geopolymer paste compared to that in the suspension during the reactivity test.FTIR spectroscopy data further support the observations made via XRD measurements.Figure 9 presents the FTIR spectra of the (80% FA + 20% magnesite) blend mechanically activated for 180 s, both before and after reactivity test.The broad band in the 3700-3100 cm −1 region (O-H stretching vibration) in the spectrum of the untreated blend (Figure 9; curve 1) corresponds to the water adsorbed by magnesite and FA from air   FTIR spectroscopy data further support the observations made via XRD measurements.Figure 9 presents the FTIR spectra of the (80% FA + 20% magnesite) blend mechanically activated for 180 s, both before and after reactivity test.The broad band in the 3700-3100 cm −1 region (O-H stretching vibration) in the spectrum of the untreated blend (Figure 9; curve 1) corresponds to the water adsorbed by magnesite and FA from air The partial conversion of calcium carbonate to calcium hydroxide, as identified by XRD analysis (Figure 6), is also supported by the FTIR spectroscopy data.Amidst the reduction in the intensity of the calcite peaks at 1431, 876, and 712 cm −1 [36], a distinct sharp peak corresponding to the O-H stretching vibrations in portlandite Ca(OH) 2 appears at 3643 cm −1 [39] in the FTIR spectrum of the residue (Figure 10, curve 2).In the region of 1470-1430 cm −1 , a splitting of the band corresponding to the asymmetric stretching vibrations of the carbonate group is observed.This may be attributed to the superposition of the CO 3 bands from unreacted calcite and those from carbonate ions generated through the chemisorption of atmospheric carbon dioxide onto newly formed calcium hydroxide [40].In geopolymers synthesized using the (90% FA + 10% calcite) blend, the presence of a small amount of Ca(OH) 2 was identified using SEM-EDS.In addition, the partial transformation of calcite to vaterite (CaCO 3 polymorph) was observed by X-ray diffraction [27].
In accordance with the XRD data (Figure 5) and thermodynamic calculations (Figure 4), this confirms the high reactivity of magnesium carbonate in the geopolymerization of the (FA + magnesite) blends.
The bands at 1410 cm −1 and 3694 cm −1 in the FTIR spectrum of the residue (Figure 9; curve 2) can be attributed to carbonate ion vibrations in hydrotalcite [37,38] and hydroxyl in brucite [39], respectively.This supports the formation of hydrotalcite and brucite during the interaction of the blend with an alkali solution, as revealed by XRD measurements (Figure 5).It should be noted that, according to XRD and FTIR spectroscopy data, in the geopolymers based on the (FA + magnesite) blends, only hydrotalcite was detected as a newly formed crystalline phase [28].The absence of brucite in the geopolymerization products was likely due to the significantly lower alkali content in the geopolymer paste compared to that in the suspension during the reactivity test.The partial conversion of calcium carbonate to calcium hydroxide, as identified by XRD analysis (Figure 6), is also supported by the FTIR spectroscopy data.Amidst the reduction in the intensity of the calcite peaks at 1431, 876, and 712 cm −1 [36], a distinct sharp peak corresponding to the O-H stretching vibrations in portlandite Ca(OH)2 appears at 3643 cm −1 [39] in the FTIR spectrum of the residue (Figure 10, curve 2).In the region of 1470-1430 cm −1 , a splitting of the band corresponding to the asymmetric stretching vibrations of the carbonate group is observed.This may be attributed to the superposition of the CO3 bands from unreacted calcite and those from carbonate ions generated through the chemisorption of atmospheric carbon dioxide onto newly formed calcium hydroxide [40].In geopolymers synthesized using the (90% FA + 10% calcite) blend, the presence of a small amount of Ca(OH)2 was identified using SEM-EDS.In addition, the partial transformation of calcite to vaterite (CaCO3 polymorph) was observed by X-ray diffraction [27].The positions and intensities of the CO3 group bands in the FTIR spectra of (80% FA + 20% SrCO3) blend (1465, 860, and 702 cm −1 ), as well as those in the FTIR spectra of (80% FA + 20% BaCO3) blend (1437, 857, and 692 cm −1 ) [36], are the same as those in the FTIR spectra of the corresponding residues after reactivity test (Figures 11 and 12, respectively).This confirms the XRD data and thermodynamic calculations regarding the stability of strontium and barium carbonates with respect to sodium hydroxide solution.The positions and intensities of the CO 3 group bands in the FTIR spectra of (80% FA + 20% SrCO 3 ) blend (1465, 860, and 702 cm −1 ), as well as those in the FTIR spectra of (80% FA + 20% BaCO 3 ) blend (1437, 857, and 692 cm −1 ) [36], are the same as those in the FTIR spectra of the corresponding residues after reactivity test (Figures 11 and 12, respectively).This confirms the XRD data and thermodynamic calculations regarding the stability of strontium and barium carbonates with respect to sodium hydroxide solution.
The results obtained enable us to highlight the factors characterizing the influence of MCO 3 (M-Mg, Ca, Sr, Ba) additives to FA on the strength of composite geopolymers, considering the available literature.In recent years, the enhancement of mechanical properties of alkali-activated materials through the incorporation of Ca/Mg carbonate minerals has been investigated in numerous studies, as reviewed in [26].In particular, three main positive effects have been identified as a result of the addition of calcite and dolomite to low-calcium FA.Besides the common filler effect, due to the presence of fine unreacted carbonate particles, a dilution effect and a chemical effect are also observed.Mixing FA Minerals 2024, 14, 726 10 of 13 with a certain amount of carbonate can be considered a dilution process.This results in an increased proportion of alkaline reagents relative to the aluminosilicates, accelerating the geopolymerization reaction and enhancing the reaction extent of the aluminosilicates.Consequently, a portion of FA can be replaced by carbonates without compromising the performance of the geopolymer.The chemical effect is associated with the partial dissolution of carbonates in an alkaline medium and the formation of new phases.The positions and intensities of the CO3 group bands in the FTIR spectra of (80% FA + 20% SrCO3) blend (1465, 860, and 702 cm −1 ), as well as those in the FTIR spectra of (80% FA + 20% BaCO3) blend (1437, 857, and 692 cm −1 ) [36], are the same as those in the FTIR spectra of the corresponding residues after reactivity test (Figures 11 and 12, respectively).This confirms the XRD data and thermodynamic calculations regarding the stability of strontium and barium carbonates with respect to sodium hydroxide solution.The results obtained enable us to highlight the factors characterizing the influence of MCO3 (M-Mg, Ca, Sr, Ba) additives to FA on the strength of composite geopolymers, considering the available literature.In recent years, the enhancement of mechanical properties of alkali-activated materials through the incorporation of Ca/Mg carbonate minerals has been investigated in numerous studies, as reviewed in [26].In particular, three main positive effects have been identified as a result of the addition of calcite and dolomite to low-calcium FA.Besides the common filler effect, due to the presence of fine unreacted carbonate particles, a dilution effect and a chemical effect are also observed.Mixing FA with a certain amount of carbonate can be considered a dilution process.This results in an increased proportion of alkaline reagents relative to the aluminosilicates, accelerating the geopolymerization reaction and enhancing the reaction extent of the  The filler and dilution effects are evident in all the compositions studied.The primary distinction in the influence of alkaline earth metal carbonates on the properties of geopolymer materials appears to stem from the manifestation of the chemical effect.As demonstrated above, the reactivity of the MCO3 series (M-Mg, Ca, Sr, and Ba) with the alkaline agent varies significantly.Due to the chemical inertness of SrCO3 and BaCO3 towards alkali, their actions are likely limited to the filler and dilution effects.This results in an almost constant strength despite a decrease in FA content in the composition as the proportion of carbonates in the blend increases (Figure 2).
Magnesite exhibits maximum reactivity in accordance with thermodynamic assessments (Figure 4).The interaction of the (FA + MgCO3) blend with sodium hydroxide solution specifically results in the formation of hydrotalcite, Mg6Al2CO3(OH)164H2O.It should be noted that hydrotalcite contains aluminum, which, together with silicon, is a principal component in forming the geopolymer matrix.The exclusion of aluminum from the geopolymer synthesis process can lead to unfavorable outcomes.Additionally, the formation of hydrotalcite consumes an alkaline agent.Calcite occupies an intermedi- The filler and dilution effects are evident in all the compositions studied.The primary distinction in the influence of alkaline earth metal carbonates on the properties of geopolymer materials appears to stem from the manifestation of the chemical effect.As demonstrated above, the reactivity of the MCO 3 series (M-Mg, Ca, Sr, and Ba) with the alkaline agent varies significantly.Due to the chemical inertness of SrCO 3 and BaCO 3 towards alkali, their actions are likely limited to the filler and dilution effects.This results in an almost constant strength despite a decrease in FA content in the composition as the proportion of carbonates in the blend increases (Figure 2).
Magnesite exhibits maximum reactivity in accordance with thermodynamic assessments (Figure 4).The interaction of the (FA + MgCO 3 ) blend with sodium hydroxide solution specifically results in the formation of hydrotalcite, Mg 6 Al 2 CO 3 (OH) 16 •4H 2 O.It should be noted that hydrotalcite contains aluminum, which, together with silicon, is a principal component in forming the geopolymer matrix.The exclusion of aluminum from the geopolymer synthesis process can lead to unfavorable outcomes.Additionally, the formation of hydrotalcite consumes an alkaline agent.Calcite occupies an intermediate position in the series of alkaline earth carbonates, undergoing partial transformation into Ca(OH) 2 and vaterite [27].
It should be noted that carbonate surfaces, as well as the "fresh" surfaces of newly formed phases in mixtures of FA with magnesite or calcite, may have active centers that enhance the rate of condensation of the N-A-S-H gel, the primary cementing phase in geopolymers.The accelerating effect of such centers, particularly those on the surface of calcium carbonate added to aluminosilicate raw materials, has been indicated in the literature [41,42].In the case of the (FA + MgCO 3 ) blend, it is possible that the consumption of aluminum and alkali for the formation of hydrotalcite may be partially offset by the positive effect on the geopolymerization process due to active centers on the surface of the newly formed compound.Consequently, the strength of geopolymers based on FA and magnesite mixtures is only weakly dependent on the proportion of magnesite in the composition (Figure 2).
The moderate reactivity of calcite, along with its transformation products that do not contain the primary elements of geopolymer synthesis (Si and Al), is likely optimal.Additionally, contributing to the chemical effect of CaCO 3 is the fact that, according to literature data [41,43], calcium ions that are dissolved from carbonates or other calciumcontaining sources into an alkaline solution can enhance the release of Si and Al ions from aluminosilicate raw materials, thereby accelerating the formation of the N-A-S-H gel.

Conclusions
In this study, an attempt has been made to investigate the relationship between the strength of geopolymers prepared using mechanically activated mixtures (FA + MCO 3 ) (M-Mg, Ca, Sr, Ba) and the nature of alkaline earth metal carbonates.Calcite (CaCO 3 ) occupies a special position in this series, as its addition to the FA significantly enhanced the geopolymer strength.Blending the FA with Mg, Sr, and Ba carbonates did not improve the geopolymer performance.The addition of alkaline earth carbonates to the FA affected the mechanical properties of the composite geopolymers through filler, dilution, and chemical effects.The filler and dilution effects were observed in all the compositions examined in this study.
The major difference in the influence of MCO 3 (M-Mg, Ca, Sr, Ba) on the performance of geopolymers was attributed to the chemical effect.According to the results of thermodynamic calculations, which were confirmed by experimental data, the reactivity of alkaline earth metal carbonates towards sodium hydroxide solution increased in the order BaCO 3 < SrCO 3 < CaCO 3 < MgCO 3 .Because of the chemical inertness of SrCO 3 and BaCO 3 in an alkaline environment, their influence was limited to the filler and dilution effects.MgCO 3 showed maximum reactivity, forming hydrotalcite (Mg 6 Al 2 CO 3 (OH) 16 •4H 2 O) when mixed with sodium hydroxide.Hydrotalcite formation consumed the alkaline agent and aluminum, which was unfavorable as aluminum is essential for the geopolymer matrix formation.
The unique position of calcium carbonate was determined by its moderate reactivity towards the alkaline agent and its transformation products, which did not contain silicon and aluminum.Additionally, the presence of dissolved calcium ions likely enhanced the release of silicon and aluminum ions from the FA, accelerating the formation of the N-A-S-H gel and contributing to the overall chemical effect of CaCO 3 .

Figure 3 .
Figure 3.The compressive strength of geopolymers synthesized using the (90% FA + 10% MCO3) (M-Mg, Ca, Sr, Ba) blends, milled for 180 s, at the ages of 7 d, 28 d, and 180 d depending on the radius of the alkaline earth metal cation.

Figure 3 .
Figure 3.The compressive strength of geopolymers synthesized using the (90% FA + 10% MCO 3 ) (M-Mg, Ca, Sr, Ba) blends, milled for 180 s, at the ages of 7 d, 28 d, and 180 d depending on the radius of the alkaline earth metal cation.

Figure 4 .
Figure 4.The standard Gibbs energy (∆ r G • (298)) of reactions between alkaline earth metal carbonates and sodium hydroxide solution.