Recycling Ash and Slag Waste from Thermal Power Plants to Produce Foamed Geopolymers

: Ash and slag waste (ASW) from coal combustion creates significant environmental and economic challenges. A promising method of ASW recycling is alkali activation with geopolymer material formation. This study investigates the influence of activating solution components (sodium hydroxide and sodium silicate) on the formation of porous geopolymers using ASW of different ori‑ gins. The sodium hydroxide content of 0–4 wt.% and the sodium silicate content of 17–25 wt.% were studied. An increase in sodium hydroxide resulted in decreased density, but it adversely affected the strength. An increase in sodium silicate led to a compromised porous structure with relatively high density and compressive strength. An optimal composition, S19N3, comprising 3 wt.% of sodium hydroxide and 19 wt.% of sodium silicate obtained porous geopolymers with uniformly distributed 1.4–2 mm pores and a corresponding density of 335 kg/m 3 , a compressive strength of 0.55 MPa, a porosity value of 85.6%, and a thermal conductivity value of 0.075 W/(m · K). A mechanism for porous geopolymer formation was developed, including the interaction of alkaline components with ASW and a foaming agent, foaming, curing, and densification. The mechanism was examined using ASW from the Severodvinsk CHPP‑1. This study allows for the optimization of geopolymer mixtures with various waste sources and the utilization of waste materials in the construction industry.


Introduction
Presently, coal is one of the most economical sources of energy [1].Coal combustion accounts for approximately 40% of global electricity production [2].The combustion of coal results in the formation of ash and slag waste (ASW), which primarily contains unburned carbon (5-25%), fly ash (30-80%), and slag (10-70%), depending on the coal type and combustion process [3].
At present, the Russian Federation deals with approximately 1.6 billion tons of accumulated ASW and an annual rate of ASW production of up to 80 million tons [2,4,5].This accumulation spans more than 25 thousand hectares.Within the Southern Federal District, the biggest coal-based thermal power plant is the Novocherkassk State District Power Plant (NSDPP), which generates 2258 MW.This facility alone consumes around 3.4 million tons annually, resulting in 0.8-1 million tons of ASW.The amassed waste at the NSDPP ash dump surpasses 40 million tons and occupies an area of 250 hectares.
The storage of ASW is a significant environmental and economic problem [6,7].Hazardous substances present in ASW can leach out from ash dumps, polluting the environment and contaminating atmospheric, terrestrial, and ground water and surface water compartments.Such contamination is harmful for human health [8,9].At present, the level of ASW processing in Russia stands at approximately 4-5%.
The utilization of ash and slag waste as a raw material in various industrial sectors offers the opportunity to not only reduce the environmental impact on adjacent landfill Energies 2023, 16 areas but also to generate significant economic benefits.Two main methods are used for ASW utilization.The first approach is metal extraction [10], such as the recovery of alumina for subsequent aluminum production, given that the aluminum oxide content in the ashes of certain coal types can be up to 30%.Still, metal extraction is impractical to waste management as it fails to address the issue of the ash dump footprint size.The second utilization method, recycling ASW for further use, seems to be better as it addresses both sides of the issue.There are five principal ways of utilizing ASW (Figure 1) [11][12][13][14]: -Producing building materials (cement, bricks, blocks, and geopolymers); -Producing road construction materials; -Agriculture (soil stabilizers, fertilizers, etc.); -Producing various fillers; -Producing other materials (sorbents, zeolites, etc.).
compartments.Such contamination is harmful for human health [8,9].At present, the level of ASW processing in Russia stands at approximately 4-5%.
The utilization of ash and slag waste as a raw material in various industrial sectors offers the opportunity to not only reduce the environmental impact on adjacent landfill areas but also to generate significant economic benefits.Two main methods are used for ASW utilization.The first approach is metal extraction [10], such as the recovery of alumina for subsequent aluminum production, given that the aluminum oxide content in the ashes of certain coal types can be up to 30%.Still, metal extraction is impractical to waste management as it fails to address the issue of the ash dump footprint size.The second utilization method, recycling ASW for further use, seems to be better as it addresses both sides of the issue.There are five principal ways of utilizing ASW (Figure 1) [11][12][13][14]: Producing building materials (cement, bricks, blocks, and geopolymers); − Producing road construction materials; − Agriculture (soil stabilizers, fertilizers, etc.); − Producing various fillers; − Producing other materials (sorbents, zeolites, etc.).The specific method of processing ASW into a resulting material depends on many factors.However, several stages are required for every processing technology.The first stage is drying ASW to a constant mass since these waste materials are stored with exposure open air.Further processing involves milling and sieving to reach a required particle size.The resulting powder can be employed in diverse applications.
The primary segment of ASW recycling is its incorporation into building materials.The utilization of ASW to manufacture cement, dry mixes, concrete, and mortars allows for cost reductions of 15 to 30% [15,16].Here, ASW powder is added to other raw materials, and the obtained mixture is used as an ordinary binder.In the case of cement production (or other construction materials with a thermal treatment stage), the mixture can be molded and fired according to the synthesis mode.ASW serves as a versatile material for road construction, and it is applicable to road embankment backfilling, foundations, and all layers of highways.Here, ASW is milled to a required particle size (coarse or fine) and compacted to create a layer of pavement.This approach also leads to cost savings [17,18].Using ASW as a filler in concrete mix is another readily accessible utilization method The specific method of processing ASW into a resulting material depends on many factors.However, several stages are required for every processing technology.The first stage is drying ASW to a constant mass since these waste materials are stored with exposure open air.Further processing involves milling and sieving to reach a required particle size.The resulting powder can be employed in diverse applications.
The primary segment of ASW recycling is its incorporation into building materials.The utilization of ASW to manufacture cement, dry mixes, concrete, and mortars allows for cost reductions of 15 to 30% [15,16].Here, ASW powder is added to other raw materials, and the obtained mixture is used as an ordinary binder.In the case of cement production (or other construction materials with a thermal treatment stage), the mixture can be molded and fired according to the synthesis mode.ASW serves as a versatile material for road construction, and it is applicable to road embankment backfilling, foundations, and all layers of highways.Here, ASW is milled to a required particle size (coarse or fine) and compacted to create a layer of pavement.This approach also leads to cost savings [17,18].Using ASW as a filler in concrete mix is another readily accessible utilization method [19,20].This technology allows for cost reductions in the manufacturing of heavy concrete, benefiting paving, finishing stones, monolithic building construction, etc.It should be noted that utilizing ASW offers several benefits over using conventional building materials.For example, it eliminates the need for extra costs in the extraction and initial processing stages.ASW's proximity to infrastructure also allows for substantial savings in transportation costs.The above-mentioned considerations show that ASW processing is an important research area.
ASW processing also seems to be beneficial to the synthesis of geopolymers.Geopolymer production relies on raw materials containing aluminosilicates like ash, boiler slag, microsilica, rice husk, dehydroxylated clays, volcanic ash, pozzolans, granulated blast furnace slag, and kaolinite, either separately or in various combinations [21][22][23][24].The recycling of waste generated by thermal power plants operating on solid fuels (producing ash, slag, and ash-slag mixtures) is particularly promising, given the existing substantiation accumulation of such waste and its role in environment pollution.
The most promising class of geopolymer materials in contemporary construction is foamed geopolymers [25][26][27][28][29].These materials are utilized in the construction of energyefficient buildings [30], reducing both fossil fuel consumption and carbon dioxide emissions [31].A fundamental distinction in the production of foamed geopolymers versus conventional porous materials is the increased concentration of the alkaline component, commonly referred to as the alkaline activator in the raw mixture.This alkaline activator plays a major role in promoting the synthesis of low-basic phases including hydrosilicate and alkaline aluminosilicate components in hydration products.These phases exhibit superior binding capacities and reduced solubility.
The above-mentioned activators are solutions of sodium and potassium hydroxide and sodium and potassium silicates [32].The preferred activators are sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ), as the more efficient activators, potassium hydroxide (KOH) and potassium silicate (K 2 SiO 3 ), have higher costs [33,34].
Sodium hydroxide and sodium silicate (waterglass) are rarely used separately as activating agents.The industry mostly uses mixtures of these compounds [33].The silicate-tohydroxide proportion in those mixtures determines the reaction system modulus, which is a very important parameter of geopolymer development.Variations of the modulus can significantly alter the polymerization degree, defining both the macro-and microstructures of the produced material [35,36].
Table 1 provides examples of sodium silicate and hydroxide ratios reported in the literature.The sodium hydroxide quantities presented in the table were recalibrated from their dissolved states to the dry powder form, which is a clarification step needed for data consistency, given that the sample composition was computed based on pure NaOH in this study.This table also demonstrates variability in the waterglass/NaOH ratios presented in the existing literature.So, the optimal quantities of these components within the activating solution are still unclear.Fly ash 2.5 20 2.5 [37] Fly ash Not given Not given 1.5-3.0[38] Slag + fly ash Not given Not given 1.5-2.5 [39] Slag + fly ash Not given Not given 1.5-2.5 [40] Fly ash 3.5 17 1.5 [41] Metakaolin + fly ash 3-7 11-26 0.5-2.5 [42] Slag + fly ash Not given Not given 2.5 [43] Metakaolin + fly ash 4-5 37 2.3 [44] Fly ash + sand 1.5-4.5 8-27 1.0-3.0[45] Fly ash + sand 0.5-2 12.5-13.52.0 [46] The process of foamed geopolymer material synthesis depends on the interaction between the foaming agent and activation solution.It is known that variations in the amount of alkaline activator (such as Na 2 SiO 3 and NaOH) have stronger effects on the structure formation than changes in the foaming agent content or temperature.Alkaline hydroxide takes part in aluminum redox reactions, whereas sodium silicate suppresses those reactions [47].It is also advisable to minimize the presence of metallic components within the geopolymer structure, as they increase heat conduction [48].
Considering the above-mentioned factors, the general regularities of "alkaline activator-silicate raw material" are described for dense geopolymers.But there is still no overall model describing the influences of alkaline activator components on geopolymerization stages when synthesizing porous geopolymers.Also, the existing studies do not discuss the optimal content and ratio of alkaline activator components for the production of foamed geopolymers based on ASW.This aim is of particular interest because geopolymer foaming involves additional physical-chemical interactions of mixture components that can affect the resulting structures.Thus, the purpose of this work is to study the influence of the activating solution components (sodium hydroxide and sodium silicate) on the porous structure and properties of porous geopolymers to discover the optimal content and ratio of these components and to describe a mechanism of porous geopolymer structure formation.

Materials
The main raw material employed in foamed geopolymer synthesis was ash and slag waste (ASW) from the Novocherkassk State District Power Plant (NSDPP) and from the Severodvinsk Combined Heat and Power Plant-1 (CHPP-1).ASW was formed during joint disposal of fly ash and boiler slag at the dump of a power plant.The chemical composition of this material is detailed in Table 2.The activating solution components were sodium hydroxide with a purity of 99% (LenReactive, Russia) and sodium silicate with a silicate modulus of 2 and a water content of 52 wt.% (Sil-Ex, Russia).Additionally, the foaming mixture comprised 99% pure aluminum powder of ASD-1 grade with a specific surface area of 148 m 2 /g (GC Metal Energo Holding, Russia).

Synthesis of Foamed Geopolymer Materials
The production of geopolymer materials adhered to the following procedural methodology (as illustrated in Figure 2): initially, a 12 M sodium hydroxide solution was prepared by meticulously blending NaOH powder with deionized water.Subsequently, this resultant solution was combined with sodium silicate, followed by the addition of ASW powder.Ultimately, a foaming agent, namely aluminum powder, was introduced into the mixture.This mixture was thoroughly stirred, cast into molds, and subjected to a curing process at a temperature of 80 • C for 12 h.Table 3 presents the comprehensive component compositions of the raw mixture, with the nomenclature structured according to the following convention: "S" signifies sodium silicate, followed by the respective quantity of sodium silicate, while "N" denotes sodium hydroxide, followed by the corresponding quantity of sodium hydroxide.For example, the sample designated as "S21N3" signifies a composition comprising 21 wt.% of sodium silicate and 3 wt.% of sodium hydroxide.The compositions of the raw mixtures were devised systematically, initially by determining the optimal quantity of sodium hydroxide (as represented by samples S21N0, S21N1, S21N2, S21N3, and S21N4).Subsequently, the optimal quantity of sodium silicate was ascertained, utilizing the previously determined optimal amount of sodium hydroxide (resulting in samples S17N3, S19N3, S23N3, and S25N3).A verification of the proposed mechanism of the porous geopolymer formation and the influence of activating solution components on porous geopolymer formation was conducted by using ASW sourced Table 3 presents the comprehensive component compositions of the raw mixture, with the nomenclature structured according to the following convention: "S" signifies sodium silicate, followed by the respective quantity of sodium silicate, while "N" denotes sodium hydroxide, followed by the corresponding quantity of sodium hydroxide.For example, the sample designated as "S21N3" signifies a composition comprising 21 wt.% of sodium silicate and 3 wt.% of sodium hydroxide.The compositions of the raw mixtures were devised systematically, initially by determining the optimal quantity of sodium hydroxide (as represented by samples S21N0, S21N1, S21N2, S21N3, and S21N4).Subsequently, the optimal quantity of sodium silicate was ascertained, utilizing the previously determined optimal amount of sodium hydroxide (resulting in samples S17N3, S19N3, S23N3, and S25N3).A verification of the proposed mechanism of the porous geopolymer formation and the influence of activating solution Energies 2023, 16, 7535 6 of 19 components on porous geopolymer formation was conducted by using ASW sourced from the Severodvinsk CHPP-1 thermal power plant, situated in a distinct region of Russia, and by employing substantially different coal sources.Five mixture compositions were developed.The "S." index in each mixture number denotes the utilization of ASW sourced from the Severodvinsk CHPP-1: S.S19N3-a composition featuring the previously found optimal sodium silicate and hydroxide content; S.S19N1-a composition with an insufficient sodium hydroxide content; S.S19N5-a composition with an excessive sodium hydroxide content; S.S15N3-a composition with an insufficient sodium silicate content; and S.S25N3-a composition with an excessive sodium silicate content.

Methods
The density of the samples (d), kg/m 3 , was determined as the ratio of the mass to the volume of the sample according to Equation ( 1): where m-sample mass, g; V-sample volume, cm 3 .The porosity (P), %, shows the volume of pores in a porous material, which is defined as the ratio of bulk density d b to the true density d t in the synthesized geopolymer.True density of ash and slag waste was determined using the pycnometric method.True density of ASW from Novocherkassk SDPP was 2332 kg/m 3 , and ASW from Severodvinsk CHPP-1-2034 kg/m 3 .The porosity was calculated according to Equation ( 2): where d b -sample bulk density, kg/m 3 ; d t -sample true density, kg/m 3 .The ultimate compressive strength (R), MPa, was determined using a test press (TP-1-350, TestPress, Misailovo village, Russia) with a force measurement range of 0.1 to 350 kN with a measurement accuracy of ±2% in the range of 0.1 to 7 kN, and with a measurement accuracy of ±1% in the range from 7 to 350 kN.The ultimate compressive strength was calculated according to Equation (3): where P-breaking load, kN; S-sample area, m 2 .The thermal conductivity, W/(m•K), was determined using a thermal conductivity meter (ITP-MG4'100/Zond', SKB StroyPribor, Chelyabinsk, Russia) using the method of stationary heat flow [4].
An analysis of multi-factor regression models and the creation of 3D surface models and 2D contour line maps were conducted using the "Multiple Regression" module within the STATISTICA 10.0.1011 software.The pore size distribution was statistically analyzed based on images of geopolymer material structures using the Nano Measurer 1.2 software.
The phase composition was determined using powder X-ray phase analysis (XRD).The samples were examined using an ARLX'TRA X-ray diffractometer (Thermo Fisher Scientific, Waltham, MA, USA).Geopolymer samples were preliminary milled to a particle size of less than 40 µm.Data interpretation was carried out using the Crystallographica Search-Match Version 3 software package of the ICDD PDF 2 database (International Center for Diffraction Data).
The analysis of microstructure and elemental composition was conducted using polished cubic samples with a side length of 1.5 cm.After placing the samples on the sampleloading platform, the electron microscope chamber was vacuumed to prevent the influence of impurities in the air on the electron trajectory.Then, samples were studied using Quanta 200 scanning electron microscope (FEI Company, Hillsboro, OR, USA) with an EDAX Genesis XVS 30 X-ray microanalysis system (FEI Company, Hillsboro, OR, USA).

Technological Properties of Foamed Geopolymers
In order to determine the optimal quantity of sodium hydroxide to achieve the desired technological properties of foamed geopolymer materials, a range of content levels ranging from 0 wt.% to 4 wt.% was considered.The resultant structures of the geopolymer materials obtained are illustrated in Figure 3.

Technological Properties of Foamed Geopolymers
In order to determine the optimal quantity of sodium hydroxide to achieve the desired technological properties of foamed geopolymer materials, a range of content levels ranging from 0 wt.% to 4 wt.% was considered.The resultant structures of the geopolymer materials obtained are illustrated in Figure 3. Figure 3 shows that the formation of a porous structure is virtually nonexistent when sodium hydroxide is absent, as evidenced by sample S21N0.Relying only on sodium silicate as the activating solution does not lead to a required level of particle dissolution that is essential for the construction of an aluminosilicate framework, primarily due to its inadequate OH − ion content [49].As the alkali content escalates, there is a proportional increase in the pore size.Consequently, a higher alkali concentration within the reaction mixture is correlated with heightened foaming activity in the geopolymer material.An elevation in the Na2O concentration within the geopolymer improves the solubility of the amorphous silicon dioxide and aluminum oxide precursors.Consequently, larger quantities of dissolved Si and Al ions engage in the geopolymer synthesis reaction, leading to excessive synthesis products [32].Sample S21N4, for instance, demonstrates the presence of extensive, irregular pores attributed to an excess of alkali and overly intensive foaming.
The process of pore formation in the porous geopolymer materials is based on the interaction between the alkaline component and aluminum powder.Aluminum powder takes part in a redox reaction within an alkaline solution, accompanied by the evolution of hydrogen, as expressed in Equation (4) [50]: The presence of aluminum in an alkaline solution creates a low-density foamed geopolymer when the viscosity of the mixture allows for pore formation.Table 4 provides a comprehensive overview of the properties exhibited by the resulting geopolymer materials.Figure 3 shows that the formation of a porous structure is virtually nonexistent when sodium hydroxide is absent, as evidenced by sample S21N0.Relying only on sodium silicate as the activating solution does not lead to a required level of particle dissolution that is essential for the construction of an aluminosilicate framework, primarily due to its inadequate OH − ion content [49].As the alkali content escalates, there is a proportional increase in the pore size.Consequently, a higher alkali concentration within the reaction mixture is correlated with heightened foaming activity in the geopolymer material.An elevation in the Na 2 O concentration within the geopolymer improves the solubility of the amorphous silicon dioxide and aluminum oxide precursors.Consequently, larger quantities of dissolved Si and Al ions engage in the geopolymer synthesis reaction, leading to excessive synthesis products [32].Sample S21N4, for instance, demonstrates the presence of extensive, irregular pores attributed to an excess of alkali and overly intensive foaming.
The process of pore formation in the porous geopolymer materials is based on the interaction between the alkaline component and aluminum powder.Aluminum powder takes part in a redox reaction within an alkaline solution, accompanied by the evolution of hydrogen, as expressed in Equation (4) [50]: The presence of aluminum in an alkaline solution creates a low-density foamed geopolymer when the viscosity of the mixture allows for pore formation.Table 4 provides a comprehensive overview of the properties exhibited by the resulting geopolymer materials.Research into the determination of the optimal sodium hydroxide quantity for foamed geopolymer synthesis revealed that the S21N3 sample exhibited the most favorable properties: a medium density of 349 kg/m 3 and a strength of 0.57 MPa.Conversely, samples S21N0, S21N1, and S21N2 displayed higher densities, describing them as less suitable for thermal insulation.Sample S21N4 has a lower density, but it also has strength lower by 35% compared to S21N3.Hence, a sodium hydroxide content of 3 wt.%was selected for the subsequent investigations.
In the synthesis of geopolymers using energy generation waste, the quantity of the activating solution in the reaction mixture should range from 17 wt.% to 25 wt.% (expressed as the ratio of the activating solution to the primary raw material) depending on its chemical composition [51][52][53].Consequently, to explore the impact of the sodium silicate quantity on the technological properties of geopolymer materials, concentrations of 17, 19, 21, 23, and 25 wt.% were chosen.
Elevating the sodium silicate concentration increases the Si/Al ratio and the viscosity of the reaction system, thereby promoting a more uniform pore distribution (as shown in Figure 3) [32].This increased viscosity hinders the pores' coalescence, resulting in a more homogeneous and finely tuned pore structure within the geopolymer matrix [54].The presence of uniformly distributed small pores leads to a microstructure that closely approximates the ideal two-phase state [55].Consequently, samples S19N3 and S21N3 exhibited superior thermal insulation characteristics.In contrast, the geopolymer materials synthesized with lower silicate concentrations displayed non-uniformly distributed macro-pores of substantial diameters (for example, S17N3).Conversely, S23N3 and S25N3 contained excessive sodium silicate, causing extensive fluidity and causing foam collapse.
The conducted research aimed at identifying the optimal quantity of sodium silicate for foamed geopolymer synthesis revealed that the S19N3 sample, containing 19 wt.% of sodium silicate, exhibited the most favorable properties.This sample achieved a moderate density of 335 kg/m 3 and a compressive strength of 0.55 MPa.It is known that foamed geopolymers should exhibit low density values and uniform structures with the highest possible compressive strength [56][57][58].The properties obtained in this study are consistent with the data in the literature.For instance, in [47], porous geopolymers based on fly ash and an aluminum powder foaming agent were synthesized with a density of 630 kg/m 3 and a compressive strength of 0.4 MPa.In [59], porous geopolymers were synthesized using fly ash and blast furnace slag with a complex foaming agent comprising sodium dodecyl sulfate, dodecyl alcohol, carboxy methylcellulose sodium, and anhydrous ethanol.The resulting geopolymers have a density of 351 kg/m 3 and a compressive strength of 0.5 MPa.In [58], metakaolin-based geopolymers with an H 2 O 2 solution foaming agent exhibited a minimum density of 471 kg/m 3 with a compressive strength of 0.4 MPa.The authors Energies 2023, 16, 7535 9 of 19 of [44] synthesized geopolymers based on fly ash with metakaolin additives with an H 2 O 2 foaming agent, and they had the lowest achieved density of 440 kg/m 3 with a compressive strength of 0.26 MPa.Korniejenko et al. [60] synthesized foamed geopolymers with a density of 267 kg/m 3 and a compressive strength of 0.82 MPa.To achieve such properties, they used 36% of perhydrol as a foaming agent and a foam stabilizer-polyglycolic acid.The authors of [61] describe fly ash-based geopolymer samples using a protein foaming agent with a density of 235 kg/m 3 and a compressive strength of 0.51 MPa.Thus, to improve the geopolymer properties, it is promising to not only adjust the activating solution, but also to modify the type and ratio of the foaming agent.It is also promising to add surface-active substances for foam stabilization, which will be the aim of future studies.
Energies 2023, 16, x FOR PEER REVIEW 9 of 19 compressive strength of 0.26 MPa.Korniejenko et al. [60] synthesized foamed geopolymers with a density of 267 kg/m 3 and a compressive strength of 0.82 MPa.To achieve such properties, they used 36% of perhydrol as a foaming agent and a foam stabilizer-polyglycolic acid.The authors of [61] describe fly ash-based geopolymer samples using a protein foaming agent with a density of 235 kg/m 3 and a compressive strength of 0.51 MPa.Thus, to improve the geopolymer properties, it is promising to not only adjust the activating solution, but also to modify the type and ratio of the foaming agent.It is also promising to add surface-active substances for foam stabilization, which will be the aim of future studies.
A statistical analysis of the obtained results facilitated the formulation of relationships between the mechanical properties and the content of alkaline activator components.Figures 4 and 5 depict 3D surfaces and their projections onto a 2D plane, described by Equations ( 5) and ( 6): Density = 6514.2047− 540.9653x − 625.1464y + 13.7518x 2 + 2.9337xy + 87.5817y 2 ; (5) Strength = 19.9396− 1.9111x − 0.5897y + 0.0553x 2 − 0.0972xy + 0.4365y 2 .( 6) where x-sodium silicate, wt.%; y-sodium hydroxide, wt.%.The coefficients of determination (R 2 ) for these dependencies stand at 0.8019 and 0.8080, respectively.This signifies that 80.19% and 80.80% of the data points align with the constructed relationships, which proves the significance of the developed model for predicting the properties of porous geopolymers.
Based on the data presented in Figures 4 and 5, it is evident that an increase in the sodium hydroxide content results in a decrease in both the density and strength, while an The coefficients of determination (R 2 ) for these dependencies stand at 0.8019 and 0.8080, respectively.This signifies that 80.19% and 80.80% of the data points align with the constructed relationships, which proves the significance of the developed model for predicting the properties of porous geopolymers.
Based on the data presented in Figures 4 and 5, it is evident that an increase in the sodium hydroxide content results in a decrease in both the density and strength, while an increase in the sodium silicate content leads to higher values of the density and strength.Thus, a specific sodium silicate and hydroxide ratio that can lower the density while simultaneously increasing the strength should be identified.The modeling results pinpointed the composition of S19N3 as optimal, as the further strengthening of the sample leads to a substantial density increase, which adversely impacts its thermal insulation properties.
The structure of the obtained foamed geopolymer materials primarily consists of gasfilled pores separated by thin walls of geopolymer material.Consequently, the microstructure is characterized by the geopolymer composition of the pore walls, whereas the macrostructure, and subsequently, the material's properties, are defined by the porous structure.It is well established that the type and quantity of pores significantly influence the mechanical properties, particularly the strength.Materials with identical densities and porosity values but distinct porous structures can exhibit strength values that vary by several times.A similar microstructure (the structure of pore walls) has a lesser impact on strength compared to the macrostructure (pore size and distribution).Thus, the uniformity of the pore distribution in porous materials holds significant importance, as it directly influences the material's performance properties.For samples exhibiting a density below 350 kg/m 3 (S21N3, S21N4, and S19N3), an investigation of the pore size distribution was conducted (Figure 6).  Figure 6 illustrates the pore size distribution histogram for samples S21N3, S21N4, and S19N3.An analysis of the figure reveals that the pore size distributions in samples S21N3 and S21N4 are uneven, displaying pores of both small and large sizes.In contrast, sample S19N3 exhibits a more uniform pore size distribution, with the majority (approximately 50%) in the range of 1.4-2 mm.

Phase Composition and Microstructure
For the X-ray diffraction analysis, the following samples were selected: the initial ASW, sample S21N0 (without sodium hydroxide), sample S21N3 (containing the optimal amount of sodium hydroxide), and sample S19N3 (containing the optimal amounts of sodium hydroxide and sodium silicate).An X-ray phase analysis was conducted to investi- Figure 6 illustrates the pore size distribution histogram for samples S21N3, S21N4, and S19N3.An analysis of the figure reveals that the pore size distributions in samples S21N3 and S21N4 are uneven, displaying pores of both small and large sizes.In contrast, sample S19N3 exhibits a more uniform pore size distribution, with the majority (approximately 50%) in the range of 1.4-2 mm.

Phase Composition and Microstructure
For the X-ray diffraction analysis, the following samples were selected: the initial ASW, sample S21N0 (without sodium hydroxide), sample S21N3 (containing the optimal amount of sodium hydroxide), and sample S19N3 (containing the optimal amounts of sodium hydroxide and sodium silicate).An X-ray phase analysis was conducted to investigate the crystalline peaks, as depicted in Figure 7.The obtained results reveal remarkable similarities in the X-ray diffraction patterns of all samples.Consequently, the variations in the activator s quantity do not seem to influence the formation of distinct phases.The X-ray patterns consistently exhibit a "halo" in the 19-35° (2θ) range, signifying the presence of an amorphous phase.Furthermore, all of the investigated samples share a common crystalline phase, primarily high quartz (SiO2), mullite (3Al2O3 2SiO2), and hematite (Fe2O3), which is consistent with other studies [62][63][64].
The semi-quantitative X-ray phase analysis indicates that the compositions are as follows: ASW comprises 62.78% amorphous structure and 27.22% crystalline, S21N0 comprises 70.22% amorphous structure and 29.78% crystalline, S21N3 comprises 71.36% amorphous structure and 28.64% crystalline, and S19N3 comprises 71.06% amorphous structure and 28.94% crystalline.This proves that a significant proportion of the amorphous phase binds particles of the crystalline phase, thereby positively influencing the final technical and operational properties of the geopolymers.Additionally, the presence of uniformly dispersed microcrystals contributes to the enhanced compressive strength of the material.
The microstructure of the synthesized geopolymer materials, observed at magnifications of 50× and 1000×, along with the corresponding EDS spectra, is shown in Figure 8.The obtained results reveal remarkable similarities in the X-ray diffraction patterns of all samples.Consequently, the variations in the activator's quantity do not seem to influence the formation of distinct phases.The X-ray patterns consistently exhibit a "halo" in the 19-35 • (2θ) range, signifying the presence of an amorphous phase.Furthermore, all of the investigated samples share a common crystalline phase, primarily high quartz (SiO 2 ), mullite (3Al 2 O 3 2SiO 2 ), and hematite (Fe 2 O 3 ), which is consistent with other studies [62][63][64].
The semi-quantitative X-ray phase analysis indicates that the compositions are as follows: ASW comprises 62.78% amorphous structure and 27.22% crystalline, S21N0 comprises 70.22% amorphous structure and 29.78% crystalline, S21N3 comprises 71.36% amorphous structure and 28.64% crystalline, and S19N3 comprises 71.06% amorphous structure and 28.94% crystalline.This proves that a significant proportion of the amorphous phase binds particles of the crystalline phase, thereby positively influencing the final technical and operational properties of the geopolymers.Additionally, the presence of uniformly dispersed microcrystals contributes to the enhanced compressive strength of the material.
The microstructure of the synthesized geopolymer materials, observed at magnifications of 50× and 1000×, along with the corresponding EDS spectra, is shown in Figure 8.It can be seen that all three materials consist of spherical particles-hollow aluminosilicate microspheres derived from the fly ash component of ASW.Additionally, particles with irregular, acute-angled shapes are present, attributed to the slag component of ASW.Notably, sample S21N0 exhibits larger particles due to the absence of alkali in its composition, preventing the dissolution of particles during the geopolymerization process.In comparison, sample S21N3 also contains larger particles compared to S19N3, a consequence of the higher concentration of sodium silicate, which accelerates the geopolymerization reaction, limiting particle dissolution, as observed in S19N3.
The elemental compositions of the studied compositions are presented in Table 5.It can be seen that all three materials consist of spherical particles-hollow aluminosilicate microspheres derived from the fly ash component of ASW.Additionally, particles with irregular, acute-angled shapes are present, attributed to the slag component of ASW.Notably, sample S21N0 exhibits larger particles due to the absence of alkali in its composition, preventing the dissolution of particles during the geopolymerization process.In comparison, sample S21N3 also contains larger particles compared to S19N3, a consequence of the higher concentration of sodium silicate, which accelerates the geopolymerization reaction, limiting particle dissolution, as observed in S19N3.
The elemental compositions of the studied compositions are presented in Table 5.
The main elements in all three materials are Na, Al, Si, and O. Their contents are almost the same in all of the samples.There is only a difference in the sodium.A lower amount of sodium in the S21N0 sample is due to the fact that the composition of the raw mixture does not include sodium hydroxide.

Mechanism of Porous Geopolymers Formation
The results obtained in this study allows for the mechanism of porous geopolymer formation to be described (Figure 9).The main elements in all three materials are Na, Al, Si, and O. Their contents are almost the same in all of the samples.There is only a difference in the sodium.A lower amount of sodium in the S21N0 sample is due to the fact that the composition of the raw mixture does not include sodium hydroxide.

Mechanism of Porous Geopolymers Formation
The results obtained in this study allows for the mechanism of porous geopolymer formation to be described (Figure 9).The geopolymerization process comprises several distinct stages, each critical for the development of the desired porous structure.In the initial phase, ASW particles are combined with sodium silicate and a sodium hydroxide solution.Here, sodium hydroxide undergoes dissociation in the solution with Na + and OH − ion formation.The second stage initiates the dissolution of ASW particles under the influence of the alkali.Molecular bonds within the aluminosilicate structure weaken due to the diminishing forces of the intermolecular attraction.Simultaneously, aluminum particles are introduced into the mixture.As the process advances to the third stage, further ASW dissolution takes place, along with the continuous geopolymerization of the dissolved particles.Aluminum powder also reacts with the OH − ions, generating hydrogen gas and initiating the initial pore formation.These pores develop independently, with relatively low probabilities of contact, expansion, or coalescence.At the final stage, the geopolymer mass begins to harden, with the pores increasing in size until the mixture begins to collapse.After this point, new pores cease to form.Concurrently, the geopolymerization reaction progresses to form a continuous three-dimensional structure.At this stage, there is a high likelihood of unreacted ASW particles remaining within the material due to their excess compared to the alkaline activator.Sodium silicate binds these unreacted particles into the final geopolymer structure.
Hence, insufficient sodium hydroxide in the mixture results in numerous unreacted ASW particles that cannot integrate into the geopolymer structure.Furthermore, the reaction of aluminum powder with alkali prevent the formation of a uniform porous structure.The geopolymerization process comprises several distinct stages, each critical for the development of the desired porous structure.In the initial phase, ASW particles are combined with sodium silicate and a sodium hydroxide solution.Here, sodium hydroxide undergoes dissociation in the solution with Na + and OH − ion formation.The second stage initiates the dissolution of ASW particles under the influence of the alkali.Molecular bonds within the aluminosilicate structure weaken due to the diminishing forces of the intermolecular attraction.Simultaneously, aluminum particles are introduced into the mixture.As the process advances to the third stage, further ASW dissolution takes place, along with the continuous geopolymerization of the dissolved particles.Aluminum powder also reacts with the OH − ions, generating hydrogen gas and initiating the initial pore formation.These pores develop independently, with relatively low probabilities of contact, expansion, or coalescence.At the final stage, the geopolymer mass begins to harden, with the pores increasing in size until the mixture begins to collapse.After this point, new pores cease to form.Concurrently, the geopolymerization reaction progresses to form a continuous three-dimensional structure.At this stage, there is a high likelihood of unreacted ASW particles remaining within the material due to their excess compared to the alkaline activator.Sodium silicate binds these unreacted particles into the final geopolymer structure.
Hence, insufficient sodium hydroxide in the mixture results in numerous unreacted ASW particles that cannot integrate into the geopolymer structure.Furthermore, the reaction of aluminum powder with alkali prevent the formation of a uniform porous structure.Conversely, an excess of sodium hydroxide leads to excessive hydrogen production and pore formation.Large pores emerge during the third stage and further expand and merge during the fourth stage, resulting in an uneven porous geopolymer with irregularly shaped pores.Insufficient sodium silicate in the mixture results in unbound ASW particles within the geopolymer structure, making the material brittle and unsuitable for use.Conversely, an excessive sodium silicate presence in the mixture primarily occupies space within the geopolymer binder, with the ASW particles acting as fillers.Additionally, an overly liquid mixture reduces the geopolymer foam's stability.
Consequently, both components of the activating solution play vital roles in the formation of the desired porous structure.Achieving an optimal porous structure necessitates an appropriate quantity of sodium silicate to securely "bind" unreacted ASW particles without over-diluting the suspension or causing foam collapse.Simultaneously, the amount of NaOH should facilitate the generation of the required ASW particle interaction products, avoiding excessive foaming due to the reaction with the aluminum powder.

Evaluation of the Developed Mechanism Applicability for ASW of Other TPP
To validate the proposed mechanism of the porous geopolymer formation, the influence of activating solution components on porous geopolymer formation was conducted by using ASW sourced from the Severodvinsk CHPP-1 thermal power plant, situated in a distinct region of Russia, and by employing substantially different coal sources.Figure 10 shows the internal structures of the obtained geopolymer materials, and Table 6 shows their properties.
Energies 2023, 16, x FOR PEER REVIEW 15 of 19 pore formation.Large pores emerge during the third stage and further expand and merge during the fourth stage, resulting in an uneven porous geopolymer with irregularly shaped pores.Insufficient sodium silicate in the mixture results in unbound ASW particles within the geopolymer structure, making the material brittle and unsuitable for use.Conversely, an excessive sodium silicate presence in the mixture primarily occupies space within the geopolymer binder, with the ASW particles acting as fillers.Additionally, an overly liquid mixture reduces the geopolymer foam s stability.Consequently, both components of the activating solution play vital roles in the formation of the desired porous structure.Achieving an optimal porous structure necessitates an appropriate quantity of sodium silicate to securely "bind" unreacted ASW particles without over-diluting the suspension or causing foam collapse.Simultaneously, the amount of NaOH should facilitate the generation of the required ASW particle interaction products, avoiding excessive foaming due to the reaction with the aluminum powder.

Evaluation of the Developed Mechanism Applicability for ASW of Other TPP
To validate the proposed mechanism of the porous geopolymer formation, the influence of activating solution components on porous geopolymer formation was conducted by using ASW sourced from the Severodvinsk CHPP-1 thermal power plant, situated in a distinct region of Russia, and by employing substantially different coal sources.Figure 10 shows the internal structures of the obtained geopolymer materials, and Table 6 shows their properties.Figure 10 illustrates the critical impact of varying the sodium hydroxide and sodium silicate contents on the properties of the samples.Sample S.S19N1, characterized by an insufficient amount of sodium hydroxide, exhibits a dense, monolithic structure that lacks thermal insulation properties.This results in unreacted ASW particles that fail to integrate into a porous geopolymer framework.Conversely, when there is an excess of sodium hydroxide (S.S19N5), excessive foaming occurs, leading to an uneven pore structure with pores of different sizes.This sample also displays a reduced compressive strength of 0.33 MPa at a density of 349 kg/m 3 .
In the case of insufficient sodium silicate (sample S.S15N3), macropores of significant diameters are heterogeneously distributed due to the failure of the ASW particles to bind with the waterglass.This causes the pores to merge into larger ones, resulting in a significant reduction in the material properties, with a density of 406 kg/m 3 and a compressive strength of 0.62 MPa.On the other hand, excessive sodium silicate (sample S.S25N3) leads to an extensively porous structure.The geopolymer binder occupies a substantial portion of space, and the ASW particles primarily serve as fillers.This sample exhibits a density of 789 kg/m 3 and a compressive strength of 1.88 MPa.
Sample S.S19N3, containing 3 wt.% of sodium hydroxide and 19 wt.% of sodium silicate, demonstrates the most uniform porous structure, with a medium density of 366 kg/m 3 and a compressive strength of 0.58 MPa.Thus, the proposed mechanism (Figure 9) effectively explains the process of porous geopolymer formation and the influence of activating mixture components, regardless of the ASW type used, and it works for a wide range of acidic ASW, regardless of the source of their formation.The use of this mechanism will allow for the optimization of the development of geopolymer compositions when using new types of waste and will expand waste utilization in the production of versatile construction materials.

Conclusions
Alkaline activator components-sodium hydroxide and sodium silicate-play critical roles in the formation of the geopolymer material structure.This study discusses their specific influences on the geopolymerization of ash and slag waste.The distinct roles of each activation mixture component in the structure formation of porous geopolymers are emphasized.Sodium hydroxide plays a critical role in initiating the dissolution of aluminosilicate particles, catalyzing the activation of waste materials and contributing to the formation of a porous structure through the redox reaction with aluminum.Sodium silicate not only accelerates geopolymerization by enhancing the dissolution rate of waste particles, but also binds unreacted waste materials, thereby influencing the final properties of the geopolymers.The balance between these components is vital in achieving the desired structure and properties of porous geopolymers, as demonstrated in this study.The optimal composition for synthesizing porous geopolymers is identified as 3 wt.% of sodium hydroxide and 19 wt.% of sodium silicate.This composition produces materials with favorable density-strength relationships and high thermal insulation properties.
Then, a mechanism that governs the intricate stages of geopolymerization was developed.This mechanism provides a clear framework for understanding how activating solution components interact with waste materials and foaming agents during the formation of porous geopolymers.It accommodates variations in the sodium hydroxide and sodium silicate contents, offering a comprehensive view of their impacts on material properties.The applicability of the developed geopolymerization mechanism was validated through an independent investigation using ASW sourced from Severodvinsk CHPP-1.This expanded study, employing significantly different waste materials and coal sources, confirmed the mechanism's efficacy in predicting and controlling the formation of porous geopolymers.These findings hold promise for the practical application of geopolymer technology in sustainable construction practices, enabling the efficient utilization of diverse waste materials, thereby contributing to environmental preservation and economic sustainability.

Figure 1 .
Figure 1.Areas of application of ash and slag waste.

Figure 1 .
Figure 1.Areas of application of ash and slag waste.

Figure 3 .
Figure 3.The structures of the obtained geopolymer materials with different contents of sodium hydroxide.

Figure 3 .
Figure 3.The structures of the obtained geopolymer materials with different contents of sodium hydroxide.

Figure 4 .
Figure 4. Dependence of density on the contents of sodium silicate and sodium hydroxide: (a) 3D surface; (b) 2D map of level lines.

Figure 4 . 19 Figure 5 .
Figure 4. Dependence of density on the contents of sodium silicate and sodium hydroxide: (a) 3D surface; (b) 2D map of level lines.Energies 2023, 16, x FOR PEER REVIEW 10 of 19

Figure 5 .
Figure 5. Dependence of compressive strength on the contents of sodium silicate and sodium hydroxide: (a) 3D surface; (b) 2D map of level lines.

Author Contributions:
Conceptualization, E.A.Y. and B.M.G.; methodology, V.A.S.; formal analysis, V.M.K.; investigation, L.V.K.; data curation A.I.I.; writing-original draft preparation, A.I.I. and V.M.K.; writing-review and editing, E.A.Y. and B.M.G.; visualization, A.V.R.; supervision, B.M.G.; funding acquisition, E.A.Y.All authors have read and agreed to the published version of the manuscript.Funding: This work was financially supported by a grant from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2022-1111; "Carbon-neutral technologies for recycling large-tonnage fuel energy waste with the production of functional geopolymer materials").The evaluation of the developed mechanism using ASW from Severodvinsk CHPP-1 (Section 3.4) was funded by the Russian Science Foundation (Grant No. 21-19-00203; "Efficient temperature-solidificable eco-geopolymers for road construction in the Arctic zone of the Russian Federation based on waste from the local thermal power plants solid fuel combustion").Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.

Table 1 .
Ratios of sodium silicate and hydroxide in the alkaline activator solution.

Table 4 .
Properties of the resulting geopolymer materials.

Table 6 .
Properties of the resulting geopolymer materials based on ASW from Severodvinsk CHPP-1.