Unlocking the Potential of Biomass Fly Ash: Exploring Its Application in Geopolymeric Materials and a Comparative Case Study of BFA-Based Geopolymeric Concrete against Conventional Concrete

Abstract

The production of conventional cement involves high energy consumption and the release of substantial amounts of carbon dioxide (CO₂), exacerbating climate change. Additionally, the extraction of raw materials, such as limestone and clay, leads to habitat destruction and biodiversity loss. Geopolymer technology offers a promising alternative to conventional cement by utilizing industrial byproducts and significantly reducing carbon emissions. This paper analyzes the utilization of biomass fly ash (BFA) in the formation of geopolymer concrete and compares its carbon and cost impacts to those of conventional concrete. The previous analysis shows great potential for geopolymers to reduce the climate change impact of cement production. The results of this analysis indicate a significant disparity in the computed financial and sustainability costs associated with geopolymers. Researchers have shown that geopolymers may help mitigate the effects of cement manufacturing on the environment. These geopolymers are predicted to reduce green gas emissions by 40–80%. They also show that those advantages can be realized with the best possible feedstock source and the cheapest possible conveyance. Furthermore, our case study on CO₂ emission and cost calculation for BFA-based geopolymer and conventional concrete shows that geopolymer concrete preparation emits 56% less CO₂ than conventional concrete while costing 32.4% less per ton.

1. Introduction

Based on a recent report published by the World Meteorological Organization, the year 2021 has been characterized as a period of unprecedented achievement. The sea level has recently attained a new record high, posing significant challenges for coastal populations and small islands. The levels of ocean heat and acidification are currently unparalleled. As anticipated, the concentration of greenhouse gases continues to increase [1]. Between 2000 and 2010, greenhouse gas emissions had grown by 24%, 3 times as much as the increase in the previous decade [2]. Consumption of fossil fuels, particularly coal in power plants and the iron/steel production industries, significantly impacts CO₂ emissions [3]. Moreover, the cement industry also emits a non-negligible and increasing amount of greenhouse gases. In 2015, cement production accounted for approximately 2.8 billion tons of CO₂, about 8% of global emissions and roughly 4 times more than air transport [4].

Concrete is a composite material composed of sand, water, and cement, which undergoes a process of hardening and bonding the individual sand grains together. It consists of calcium oxide (CaO), silica (Si₂O), and additional binding agents. The process involves subjecting a mixture of pulverized limestone and clay to high temperatures of approximately 1450 °C in kilns. The thermal process causes a chemical transformation of limestone into calcium oxide, contributing to approximately 50% of the carbon dioxide emissions associated with cement production [5]. The emissions resulting from the heating process, which accounts for the remaining 50% of the cement production’s emissions, are primarily caused by the combustion of coal or gas (Figure 1) [6,7].

Due to the increase in the share of renewable energy sources in total energy production, there has been an increasing interest in biomass energy use. Burning biomass, mainly wood (bark, sawdust, leaves, wood chips, cellulose, sludge, etc.), is a way to achieve a higher percentage of coal-free energy sources. However, with more energy to produce, burning biomass causes more ash to accumulate in landfills as a waste product of the combustion process (sending ash to landfills adds to the cost of energy production). Therefore, finding new ways of recycling fly ash is an essential and timely issue. One of the most economical, efficient, and modern ways to eliminate accumulated fly ash is to process it with alkalinized materials known as geopolymer (inorganic polymers) composites [8,9,10,11,12]. Due to their long-term, low-cost, low-CO₂ emissions during production [13], extraordinary thermal and chemical resistance [14], and highly porous structure [15], geopolymers have gained rapid interest and experienced rapid growth over the last 20 years.

The energy and minerals industry produces valuable resources, including fly ash and other byproducts, with significant potential for various applications [16,17,18]. These byproducts have captured significant attention due to their sustainable use options across various sectors. One of the prominent and widely recognized applications of fly ash lies in the realm of cement production [19,20] and geopolymer concrete [9,21,22]. By replacing a portion of cement with fly ash, the resulting concrete exhibits enhanced workability, improved long-term strength, and reduced permeability. Geopolymer concrete not only offers a sustainable alternative to conventional cement materials but also provides an avenue for utilizing large quantities of fly ash that would otherwise be disposed of in landfills [23,24]. Several studies have been conducted to investigate the greenhouse gas emissions caused by concrete and cement, as well as the effect that the addition of fly ash has on this overall amount [6,25,26]. Comparisons between cement and geopolymer were first made in the literature, and the majority of those comparisons were based on the production stage of each material [27]. According to the findings of these studies, geopolymer manufacturing results in greenhouse gas emissions that are anywhere from five to six times lower than those of cement production [28]. This is accomplished by avoiding the significant direct emissions of CO₂ that are produced during cement production and cutting back on part of the energy used in processing [28,29,30].

Although the use of coal-based fly ash in conventional and geopolymer cement has been analyzed recently in a number of studies [31,32,33], and economic and environmental comparisons have been made between the two types of cement, the use of biomass fly ash in concrete has not been addressed. Therefore, this work aims to fill the gap with a case study comparison of the cost and carbon emissions for industrially utilized conventional and biomass fly ash-based geopolymer concrete production with real recipes. The initial section provides a detailed account of biomass fly ash, including its acquisition, composition, practical applications, and associated environmental concerns. The following section presents a comprehensive literature review that assesses the use of biomass in geopolymer. The final sections present a novel case study conducted to evaluate the carbon dioxide emissions and production costs related to biomass utilization in geopolymer. This case study compares these data with the expenses of manufacturing industrial concrete that meets construction industry standards.

2. Description of Fly Ash

Fly ash is a combustion byproduct that is captured by thermal power plants and classified depending on the method of separation. Coarse fly ash is obtained through gravity separation in the boiler’s back passages, while fine fly ash is obtained through electrostatic precipitators [34,35,36]. The physical properties and chemical composition of ash are primarily determined by the type of feedstock burned, the combustion process used, and the origin of the biomass [37,38]. Ash is an alkaline substance rich in calcium, potassium, and other minerals. In addition to nutrients, ash contains a number of toxic substances, organic contaminants, and polyaromatic hydrocarbons [39]. The presence of hazardous components and particles, as well as a high pH value, limit its use. All fly ash can be used as a raw material for the following products: cement and concrete, bricks and blocks, embankment, road and mine backfilling, and geopolymerization [40,41,42].

2.1. Coal Combustion Fly Ash

Coal fly ash is a byproduct of electricity generation in thermal power plants that utilize pulverized coal as a fuel source in their boilers. The temperature is approximately 1500 °C, causing instantaneous combustion of the coal [43]. During fuel combustion, the residues undergo melting and subsequent rapid cooling as they are transported by flue gas. This process leads to the formation of fine particles with sizes ranging from 0.1 to 150 µm [44]. Approximately 80% of the unburned residues are entrained by the flue gas and necessitate removal [45].

Coal fly ash contains SiO₂, Al₂O₃, Fe₂O₃, and CaO, with magnesium, potassium, sodium, titanium, sulfur, and other elements found in trace amounts [46]. The type of coal burned has the greatest influence on the chemical composition of fly ash [47]. The combustion process, the coal supply, the particle form, and the type of unburnable component of the fly ash mass all influence the physicochemical properties of fly ash [48]. Fly ash is divided into two classes: Class C (high calcium content; more than 20%) and Class F (low calcium content; less than 10%, primarily composed of aluminosilicate glass) [49].

2.2. Biomass Combustion Fly Ash

The term “biomass” refers to biodegradable organic matter derived from animals or plants. The most common type of biomass fuel is wood or wood waste, followed by straw from grains and oilseeds. Fungus, bacteria, and cyanobacteria are also considered biomass [50,51,52]. Fly ash is the inorganic fraction of fuel that remains in the boiler after the organic matter in the biomass has been burned, and it contains the majority of the minerals found in the original biomass [53]. Perfect wood combustion produces 6–10% ash (Figure 2).

The type of raw material burned, its origin, the method of wood processing, and the combustion process technology all influence the physical and chemical composition of ash [54]. Over 80% of fly ash particles possess diameters of less than 1 µm.

The average fly ash particle diameter for native wood, chipboard, urban waste wood, and hay is less than 0.25 µm [55]. Biomass mainly comprises fast-growing woody or herbaceous plants, which have the advantages of being easy to sow and having short growing seasons (

Table 1. The categorization of biomass varieties as solid fuel resources [56,57,58].

Fast-Growing Woody Plants	Herbaceous Plants	Aquatic	Animal and Human	Contaminated Biomass
poplar	hemp	macroalgae	meat	fiberboard
willow	amaranth	seaweed	bone	chipboard
alder	sorrel	kelp	meal	waste paper
acacia	sedges	lake weed	poultry litter	plywood
hazel trees	fescues
stems	alfalfa
branches	arundo
foliage	bamboo
bark	beans
sawdust	flax
pellets	corn
lumps	rice

).

2.2.1. Waste Biomass

Biomass mainly includes crop production waste (residues from primary agricultural production and landscape maintenance, waste from orchards and vineyards, corn straw, rapeseed straw, and all other waste and residues from bush clearance); animal production waste (livestock excrements, feed residues—manure, urine, slurry); logging and forestry waste (branches, bark, stumps, roots, trimmings, sawdust, shavings); biodegradable municipal waste (food scraps, paper packaging); biodegradable industrial waste (waste from slaughterhouses and sugar, flour, paper production); and sewage [59,60,61]. Figure 3 depicts the various types of biomass sources that can be used in a biomass power plant.

2.2.2. Dangerous Features of Biomass Ash

Due to the use of a diverse range of fuel (wood, plants, etc.) and sources influenced by growing conditions (weather, soil composition, fertilization methods), biomass fly ash has a higher variability in composition and the amount of contained inorganic matter [62,63]. For instance, heavy metals must be extracted from ash during processing, which raises the manufacturing cost dramatically [64]. As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, and Mo are among the other hazardous elements found in biomass ash [65]. The heavy metal composition of coal and biomass ash is presented in

Table 2. Heavy metal chemical composition of two types of fly ash after combustion processes [58].

Element	Fly Ash from Brown Coal	Fly Ash from Biomass	Unit
Sb	3.56	4.52	mg/kg
As	10.7	0.14	mg/kg
Pb	11.7	3.66	mg/kg
Cd	0.101	0.14	mg/kg
Cr	157	26.9	mg/kg
Co	59	2.5	mg/kg
Cu	131	26.1	mg/kg
Mn	650	48.5	mg/kg
Ni	145	7.82	mg/kg
Hg	0.006	0.0003	mg/kg
Tl	2	0.85	mg/kg
V	165	47.6	mg/kg
F	0.0493	0.0295	%
Cl	0.0044	0.0148	%

.

The risk of heavy metal leakage should not be underestimated because heavy metal solubility varies significantly depending on the chemical composition of the ash and is further influenced by environmental factors [66].

2.3. Current State of Biomass Ash Management

The question of whether biomass ash can be reused as waste or whether it should be approved as a product (for example, fertilizer) remains unresolved. Given the findings of the tests, which show that the elemental makeup of the ash varies greatly from sample to sample, certifying the ash as a product is likely to be difficult [67,68,69]. As a result, if the ash cannot be approved for use as a product, it must be treated as waste and recorded in the waste register. Because of biomass burning, various governments have developed ash rules for agriculture and forestry. Wood ash has long been considered a fertilizer in regions with a high potential for raw wood products (Scandinavia, North America, and German-speaking countries) [70,71].

2.3.1. Use of Biomass Fly Ash in Construction

Biomass fly ash, after being partially treated and made to conform to all technical and regulatory requirements, can be used to make concrete, artificial aggregates, and mine backfill [72,73,74]. Most biomass fly ash has pozzolanic properties; therefore, studies have been carried out on the use of biomass fly ash as an additive in construction mixtures, and it has been shown that it can partially improve the properties of concrete [75,76,77]. At replacement levels of 20–40% in cement, biomass fly ash has favorable characteristics [78]. One study conducted a flexure test on biomass fly ash samples after a fifty-six-day drying period. The results indicated that all ash mixtures, except for a wood mixture, exhibited a 95% confidence interval. However, the increase in the flexure strength of the cement made with biomass fly ash was insufficient [75].

The pozzolanic properties of different feedstocks vary. For example, palm oil ash has been found to enhance the sulfate resistance of concrete [78]. The presence of extremely small particles in palm oil ash minimizes the porosity of concrete and decreases the permeability of chlorides, water, and air [79]. Furthermore, the inclusion of rice husk ash in concrete has been found to improve durability and reduce water adsorption [80]. Rice husk ash, with a particle size in the range of 3.6 μm to 9 μm, exhibits outstanding properties in concrete [81].

Construction materials incorporating biomass fly ash (mixed with binders and other agents) have resistance to fire, freezing, and thermal shock. In a study, Nagrockiene and Daugela [82] divided specimens into seven groups, each containing a different percentage of biomass fly ash: 0%, 5%, 10%, 15%, 20%, 25%, and 30% replaced the cement in the mixture. The test results indicated that incorporating 15% of biomass fly ash into concrete improved its durability and resistance to freeze–thaw cycles, making it suitable for construction applications. Furthermore, fly ash is affordable and can aid in waste disposal [83]. Concrete, mortar, and other curable mixtures containing cement and biomass fly ash can significantly reduce construction costs, for example, in building and highway construction (mixtures can achieve higher compressive strengths) [84], concrete products (production of panels, floors, coverings, as well as road repair) [85], and applications such as acid- and high-temperature-resistant cement composites [86]. It is commonly recognized that fly ash can be reactive and improve the qualities of freshly mixed concrete.

Table 3. Advantages and disadvantages of biomass and coal fly ash in the structure of concrete [48,87,88].

Advantages	Disadvantages
The use of spherical ash particles improves workability.	The use of fly ash is not advisable in low-temperature concrete pours.
Fly ash improves the density of cementitious binder, and the tightness of hardened concrete surface layers inhibits carbonation of the hardened concrete surface.	An excessive amount of fly ash has a significant impact on the water content, rheological properties, and durability of concrete. It often leads to bleeding and a potential decrease in the hardened concrete’s durability, as well as increased permeability when exposed to pressurized water.
Concrete costs less because fly ash is less expensive than cement.	The high chloride content of fly ash can have a negative impact on building structures, such as the danger of corrosion of embedded reinforcing steel.
CO2 emissions have been reduced.	The strength and durability of cement concrete can be affected by the quality of fly ash.
Fly ash concrete shrinks far less than conventional concrete.
Fly ash concrete is resistant to acid and sulfate attacks.

lists the benefits and drawbacks of employing fly ashes in concrete constructions.

2.3.2. Further Use of Biomass Ash

Biomass fly ash can be utilized in the creation of effective filter units for the treatment of air or wastewater [89,90]. Biomass fly ash has also been identified as a viable option for mitigating odors in sewage sludge [91]. This study aimed to investigate the potential utilization of biomass fly ash, a byproduct of biomass combustion, for environmental control purposes. The conversion of toluene from biomass ash was higher compared to coal-only combustion processes. Previous studies have investigated the potential applications of fly ash as an adsorbent, membrane filter, Fenton catalyst, and photocatalyst [92,93]. In one study, biomass fly ash TiO₂ nanoparticles were synthesized through the sol–gel method and employed in the treatment of wastewater from the textile and polyester dyeing industry [94]. The results of the synthetic dye bath treatment indicated that BFA-TiO₂ exhibited a 25% increase in chemical oxygen (COD) demand removal, a 41% decrease in electrical energy per order (EEO), and a 25% reduction in the cost of treatment per kg of COD removed compared to the sample without fly ash. Moreover, biomass fly ash has potential as a raw material in compost preparation [95]. Biowaste in compost typically exhibits a lower pH. Furthermore, during the initial stages of decomposition, there is a potential increase in the population of mesophilic microorganisms, specifically lactic acid bacteria and yeast. This increase is considered undesirable, as it can hinder the efficient biochemical degradation of biowaste. The fly ash produced from burning dendromass is a suitable compost supplement because it can adjust pH levels, enhance nutrient content, and reduce compost odor [92,96].

3. Biomass Fly Ash-Based Geopolymers

3.1. Description of Geopolymer

Geopolymers are, according to the commonly accepted definition, inorganic, amorphous, synthetic aluminosilicate polymers formed from the synthesis of silicon and aluminum and geologically derived minerals. Their chemical composition is similar to that of zeolite but reveals an amorphous microstructure [97,98,99]. The base material used in this context can be either a natural raw material (kaolin, metakaolin, clay, volcanic tuff, laterite) or a waste material, such as fly ash, slag, or inorganic material with pozzolanic properties [100,101].

Geopolymer materials are mechanically durable with high compressive and flexural strength, elasticity, and chemical and fire resistance. They can exhibit compressive strengths higher or similar compared to Portland cement-based concrete [102]. Bakri et al. developed an experimental plan to assess the impact of different ratios of fly ash and aggregate on the compressive strength of concrete [103]. The study compared the use of fly ash-based geopolymer with ordinary Portland cement (OPC). This study utilized various ratios of FA 50%: aggregate (AGG) 50%, FA 40%: AGG 60%, FA 30%: AGG 70%, and FA 20%: AGG 80% in geopolymer concrete. The identical designs have also been employed as control references for OPC concrete. The strength of the material was assessed through compressive strength testing. The findings indicate that the geopolymer made with 30% fly ash and 70% aggregate exhibits superior compressive strength compared to ordinary Portland cement concrete after 1, 7, and 28 days of testing.

The geopolymer matrix appearance is unchanged at exposed temperatures of 1000–1200 °C [104]. Geopolymers are highly resistant to fire and do not emit harmful vapors or smoke. Geopolymers can also potentially be used for producing fire panels or as fire-resistant coatings on metals. The coatings can be designed to maintain temperatures below 550 °C [105]. The geopolymer material has low thermal conductivity, high mechanical strength, excellent resistance to alkaline and acidic environments due to the low calcium content in its chemical structure [106], and even allows the adsorption of toxic chemical wastes [21]. The ability to add different fillers (particles, fibers) [107,108,109] increases not only its performance parameters (strength, mechanical resistance, thermal conductivity) [110] but also its physical aspects [111,112,113,114,115].

3.2. Biomass Fly Ash in Geopolymer Composites

Current research emphasizes the utilization of geopolymer products derived from biowaste materials. These products exhibit superior durability, strength, and fire resistance compared to conventional building materials [116]. The advantages can be further improved by developing the ability to modify the composition of the geopolymer to achieve specific features [117,118].

Table 4. Process of making geopolymers from biomass waste and its areas of application in the latest literature.

Sources of Fly Ash	Geopolymer Preparation Method	Precursor	Application/Goal of Geopolymer	References
Paper waste	A mixture consisting of 15 g of aluminosilicate precursors, comprising 50 wt.% metakaolin and 50 wt.% FA, was subjected to mechanical mixing with 24.38 g of alkaline solution, 4.15 g of water, and 0.75 g of pore-forming agent in order to generate the geopolymer slurry.	Metakaolin	Wastewater treatment	[89]
Paper waste	The SiO2/Al2O3 ratio was 3.1, the Na2O/Al2O3 ratio was 2.0, and the Na2O/SiO2 ratio was 0.6. To investigate the influence of the pore-former on porous geopolymer materials, different quantities of H2O2 were utilized. Sodium silicate was replaced in these compositions by 0.03, 0.15, 0.30, 0.90, and 1.2 wt.% H2O2.	Metakaolin	Board and wall panels	[119]
Co-generation plant (BA)	Here, 75 wt.% BA and 25 wt.% MK were employed in the formulation. The solids were combined for 1 min at 60 rpm in a Kenwood planetary mixer before adding the alkaline activators for 10 min at the same agitation. Stirring was maintained for another 5 min at 95 rpm with the addition of H2O2 as needed.	Metakaolin (MK)	Filtration and separation	[120]
Kraft pulp mill (BFA)	The manufacturing process of GP mortars involves several steps. First, MK and BFA were hand mixed for a duration of 1 min to achieve a consistent blend. Second, sodium hydroxide and silicate were homogenized at a speed of 60 rpm for 5 min. Next, the alkaline solution was mixed with the solid precursors (BFA + MK) in a Hobart-type mixer at a speed of 60 rpm for 9 min. Finally, lime slaker grits were added to the mixture and mixed for an additional 1 min at the same speed to ensure uniformity.	Metakaolin(MK)	Construction and masonry	[73]
Wood biomass (BA)	The alkaline activators were added while still being stirred for 10 min after the solids (BA and MK) had been combined for 1 min at 60 rpm in a Kenwood planetary mixer. The mixture was stirred for 5 more min at 95 rpm.	Metakaolin(MK)	Reducing cost of geopolymer	[121]
Mixed waste from Hauts-de-France (BFA)	NaOH (20 wt.% of the activation solution) and Na2SiO3 (80 wt.%) are the chemicals used to initiate the geopolymerization process. Na2SiO3 was added with the goal of raising the concentration of soluble silicates and the pace of the reaction. A magnetic agitator was used to combine the 2 reagents in a glass container for 6 h before resting the solution in a plastic bottle for 24 h. The alkaline solution was then combined for about 3 min in a mixer with metakaolin and SRS or BFA at a rotating speed of 300 rpm.	Metakaolin (MK) and shooting range soil (SRS)	Immobilization of heavy metal	[122]
Wood biomass (BWA)	Three replacement ratios of FA by BWA were used in the blended biomass wood fly ash–fly ash geopolymer mortars: 10%, 20%, and 30% of the total binder. The activator (Na2SiO3 NaOH)/binder and fine aggregate/binder mass ratios for the geopolymer mortars were fixed at 0.5 and 2.0, respectively.	Fly ash	Economic and environmental benefits	[123]
Mix of pine pruning, forest residues	The solid precursors were combined with the activating solution. The concentration of the sodium hydroxide solution was 8 M, and the ratio of sodium silicate to sodium hydroxide was 1.15, which represents the modulus of the activator. The activator was introduced into the precursors that had been previously combined for a duration of 2 min. Subsequently, the mixture was subjected to agitation for an approximate duration of 5 min using a Proeti planetary mixer.	Metakaolin	Building materials, bricks	[124]
Olive and forest pruning (FBA)	The geopolymers were prepared using five different compositions. These compositions included pure MK, as well as four other compositions referred to as GP1, GP2, GP3, and GP4. GP1 consisted of 50% MK, 25% AIS, and 25% FBA. GP2 consisted of 50% MK, 33% AIS, and 17% FBA. GP3 consisted of 40% MK, 35% AIS, and 25% FBA. GP4 consisted of 40% MK, 25% AIS, and 35% FBA.	Metakaolinaluminum industry slags (AIS)	Partial substitutes for metakaolin and Portland cement	[125]
Burned eucalyptus biomass	The geopolymer mortars were prepared according to a mix design that followed a binder-to-aggregate weight ratio of 1:3. The mixer was supplemented with alkaline activators according to the following procedure: (i) the sodium silicate and NaOH solution were initially homogenized at a rotational speed of 60 rpm for a duration of 5 min; (ii) the alkaline solution was then mixed with the solid materials at the same rotational speed for a period of 10 min; and (iii) the mixture underwent further homogenization and mixing at a rotational speed of 95 rpm for an additional 5 min.	Metakaolin andconstruction and demolition waste	Applications in building, replacing conventional mortars	[126]
Wood biomass	Geopolymers were synthesized by combining a mixture consisting of 2/3 wt.% metakaolin (MK) and 1/3 wt.% biomass FA, which served as an aluminosilicate source. In the present study, various compositions were examined by replacing sodium silicate with different weight percentages (0.03, 0.15, 0.30, 0.60, 0.90, and 1.2 wt.%) of hydrogen peroxide (H2O2). The blending of the mixtures was conducted using a mechanical procedure consisting of the following steps: (i) the sodium silicate and NaOH solution were homogenized at a rotational speed of 60 revolutions per min (rpm) for a duration of 5 min; (ii) the alkaline solution was then mixed with biomass FA and MK at the same rotational speed for a period of 10 min; and (iii) H2O2 was added to the blend in an amount determined by the formulation, followed by an additional mixing period of 2 min at a rotational speed of 95 rpm.	Metakaolin	pH regulators for biogas reactors or wastewater treatment	[127]

summarizes recent research on biomass fly ash based geopolymer production and use.

The immobilization of biomass fly ash is essential for preparing safe concrete for further use [22,128]. Geopolymers play a significant role in environmental protection due to their capacity to immobilize heavy metals. This ability is closely linked to their ion exchange capabilities and extensive surface area development [129]. Metals like cadmium, copper, lead, chromium, zinc, and others can be immobilized within the geopolymer structure. Excessive amounts of zinc or chromium have been found to negatively impact compressive and bending strength [130]. A recent study proved the effective immobilization of toxic heavy metals (such as Cr, Mo, Pb, Sb, Se, and Zn) found in biomass fly ash. This was achieved through the utilization of the geopolymerization/accelerated carbonation technique. The study findings indicate that geopolymerization/carbonation stabilization processes effectively trap various elements, including As, Cr, Mo, Pb, Sb, Se, and Zn, across a broad pH range. The efficiency of this process is significantly influenced by the composition of the metakaolin [122].

Geopolymers can be used for prefabricated building elements, transport structures, and materials that achieve high adhesion to steel, aggregates, and many others [131,132]. Songpiriyakij et al. have demonstrated the bonding strength of geopolymers [133]. Twelve distinct mix proportions of geopolymers were created by adjusting the quantities of the initial binder materials and alkaline concentration. These mixtures were subsequently evaluated for their compressive and bonding strengths. The bonding strengths of the round bar and geopolymer were slightly greater than those of the control concrete, ranging from 1.05 to 1.12 times. The bonding strengths were significantly higher for deformed bars, ranging from 1.03 to 1.60 times. The study also included the presentation of the ratios between bonding strength and compressive strength. The bonding strengths of geopolymer were found to be 1.24–1.81 times higher than those of epoxies when compared to commercial repair materials. Furthermore, geopolymer concrete with the addition of biomass fly ash could be used as an additive for ceramics, chemically resistant exterior and interior cladding, chemically resistant items for industry, a composite item for working with hazardous substances (heavy metals, radioactive substances, etc.), and filler joints for reinforced concrete structures [134,135].

Challenges in utilizing biomass fly ash for geopolymers include the variation in particle sizes, morphology, composition, and reactivity among different fly ash samples [136,137]. The particle differences in biomass fly ash are highly influenced by the production conditions and the composition of the feedstock used in the boiler. It has been observed that the resulting mechanical strengths of geopolymers vary significantly even when using fly ash of apparently similar composition but from different sources, as well as different batches of fly ash from the same source [138,139].

Sharko et al. [140] conducted a recent and comprehensive study on the synthesis of geopolymers utilizing biomass fly ash. Six distinct fly ashes derived from six separate biomass thermal power plants in the Czech Republic were utilized in the study.

Table 5. The chemical composition of biomass fly ash collected from thermal power plants [140].

Thermal Power Plants	Elements (wt.%)
	O	C	Ca	Si	K	Al	S	Mg	Cl	Na	Fe	P	Mn	Zn	Ti
BFA (Loucovice)	40.4	32.9	9.4	6.6	3.6	1.9	1.4	1.0	0.8	-	0.6	-	-	-	-
BFA (Cesky Krumlov)	32.3	50.0	9.8	2.4	1.9	0.9	0.9	0.5	0.5	0.4	0.4	0.2	-	-	-
BFA (Pisek)	32.7	50.7	3.3	3.7	3.6	1.1	1.2	0.9	1.0	0.7	0.4	0.3	0.3	0.1	0.1
BFA (Otin)	39.5	32.5	10.7	5.7	2.9	1.5	1.0	2.1	0.5	0.4	0.9	1.1	1.0	0.2	-
BFA (Mydlovary)	60.3	-	9.4	9.9	8.9	1.2	2.8	1.5	2.3	1.2	0.9	0.5	0.5	0.6	-
BFA (Trhove Sviny)	33.1	43.2	5.4	2.1	7.2	0.9	3.2	1.3	1.4	0.8	0.6	0.4	0.4	-	-

provides information on the chemical composition of the fly ash.

The researchers carried out a study in which geopolymers were synthesized exclusively using biomass fly ash as a precursor material. A scanning electron microscope was used to examine the geopolymer matrices (Figure 4).

Subsequently, these geopolymers were subjected to a mechanical durability test to assess their performance. In addition, a comparison was made between the mechanical test outcomes of geopolymer concrete incorporating metakaolin and conventional concrete (Figure 5). The flexural and compressive strength, as well as impact toughness, exhibit significant variability as a result of variations in chemical composition among different types of biomass fly ash. The experiment demonstrated that the physical properties of geopolymer structures and their durability performances vary depending on the biomass fly ash obtained from different thermal power plants.

To ensure the production of a consistent geopolymer product from a raw material source with varying physicochemical properties, it is necessary to gain a comprehensive understanding of how different synthesis parameters impact the properties of the resulting geopolymer. This understanding will enable precise adjustment of these parameters for the specific product, thereby facilitating potential commercial applications in industries such as construction [141].

The primary determinant of fly ash’s chemical composition is the presence of reactive silicon compounds [142,143]. Silicon creates the primary constituent of the internal structure of the geopolymerization products resulting from the alkalinization process of fly ash [144]. The fly ash’s reactive silicates dissolve under highly alkaline conditions, resulting in the formation of Si-O-Al polymer bonds (Figure 6). Therefore, the presence of abundant reactive silicon compounds leads to the formation of significant quantities of aluminosilicate gel, which contributes to the potential for achieving high strength in the resulting geopolymer material [145,146,147].

The essential characteristics of fly ash that are deemed suitable for the production of geopolymer materials with commendable mechanical properties are as follows: The fly ash should contain a maximum of 5% unburnt material, 10% iron oxide, and 10% CaO [25]. The concentration of reactive silicon should fall within the range of 40 to 50%. The percentage of particles with a size smaller than 45 μm should fall within the range of 80 to 90% [138,148].

3.3. Environmental Impact of Biomass Fly Ash Recovery on Geopolymer Formation

A geopolymer is a replacement for cement that has a significantly lower energy requirement for production and emits a smaller amount of CO₂ greenhouse emissions compared to Portland cement [22,149]. Geopolymer technology offers the benefit of utilizing industrial byproducts, such as kaolin, feldspar, fly ash, slag, palm oil ash, and mining waste, as binders [150]. Geopolymers are a promising area of study in terms of their cost-effectiveness and environmentally friendly nature [151]. The production of geopolymers offers novel technical solutions that enable the utilization of up to 90% of ash, thereby enhancing waste utilization within the circular economy of the country. Furthermore, this procedure has the capability to produce a long-lasting and ecologically sustainable substance [152]. Another significant advantage of porous geopolymer materials is their ability to exhibit low thermal conductivity and high thermal resistance [153,154]. These materials are commonly used in the construction industry as insulation due to their optimal mechanical strength [155,156]. Geopolymers are a viable substitute material in situations where it is crucial to avoid the emission of harmful fumes during combustion due to their non-flammable nature [157,158].

Uses of geopolymers include not just thermal insulation but also pH buffering [159] and wastewater treatment [160]. The feasibility of utilizing biomass fly ash-based geopolymers as lead adsorbents was assessed by Novais et al. [161]. They examined the impact of heavy metal concentration, pH of aqueous solutions, adsorbent quantity, and contact time on the efficiency of lead removal with geopolymers. The results indicate that the novel materials have a lead uptake of up to 35 mg/g, highlighting their potential as effective lead adsorbents. Geopolymer materials have significant potential for enhancing water quality through wastewater treatment. A cost-effective geopolymer was synthesized using solid waste through a process involving acid treatment after geopolymerization [162]. This method effectively removes methylene blue (MB) dye from wastewater. The geopolymer adsorbent demonstrated excellent adsorption performance for a 600 mg/L solution of MB dye (pH = 8) at room temperature. It achieved a maximum adsorption capacity of 115 mg/g and a removal efficiency of 97.8%.

Geopolymer materials do not consistently exhibit promising characteristics. Another study demonstrated their adverse effects. One of the first studies conducted by G. Habert [163] aimed to examine the environmental evaluation of geopolymer-based concrete production through the utilization of the life cycle assessment (LCA) methodology. The researchers substantiated the favorable influence of geopolymer materials on the phenomenon of global warming. However, their investigation also revealed that the utilization of geopolymers is associated with the exacerbation of additional environmental concerns. As an illustration, the researchers documented that the human toxicity of geopolymer-based concrete was 105.4 kg of 1,4-DB eq. (dichlorobenzene equivalent) in contrast to 18.9 kg 1,4-DB eq. for conventional Portland concrete. The ecotoxicity towards freshwater organisms was found to be 2.52 kg 1,4-DB eq. for ordinary Portland concrete, while the corresponding value for geopolymer concrete was more than ten times higher at 27.01 kg 1,4-DB eq. The primary cause of geopolymers’ greater toxicity levels in humans can be attributed to the presence of sodium silicate solution, which is necessary for the geopolymerization process of aluminosilicate source materials. The utilization of fly ash and granulated blast furnace slag geopolymer has been determined to possess a diminished environmental footprint due to its activation process involving small quantities of sodium silicate solution. In order to minimize the utilization of sodium silicate solution, it is imperative to take into account the mix design for geopolymer concrete with a focus on optimizing the Si:Al ratio [164].

4. Comparative Case Studies

4.1. Calculation of Carbon Dioxide Emissions: Conventional Concrete Versus Geopolymer Concrete

Carbon footprints can be categorized into two distinct classifications: direct and indirect.

• The direct (primary) footprint refers to the quantifiable quantity of greenhouse gases that are emitted directly as a result of a particular activity, such as electricity generation, heating, or fuel combustion.
• The indirect (or secondary) footprint refers to the quantity of greenhouse gases that are released throughout the complete life cycle of a product, encompassing its production, usage, and eventual disposal.

The computation was performed for a unit mass of one ton of material under the wet state of the mixture. A local concrete manufacturer in the Czech Republic provided the formula for conventional concrete preparation. However, the geopolymer preparation relied on an internal recipe derived from our previous work [107,165]. The calculation of CO₂ emissions (

Table 6. CO₂ emission values associated with the preparation of 1 ton of conventional concrete.

Material Name Reinforced Concrete	Weight (kg)	Weight Ratio	Density kg/m3	Emission Factor kg CO2 eq/kg
Mixture of sands	303.47	30.3%	1650	0.147
Cement (75 MPa)	298.47	29.8%	3050	1.250
Water	276.51	27.7%	1000	0.00059
Binder mixture	8.74	0.9%	2685	3.210
Steel rods	112.80	11.3%	7850	2.890
Density of concrete			2596.6
Total mixture (kg)	1000.0
Total CO2 (for 1 ton)				771.932
kg CO2 eq/kg				0.772

and

Table 7. CO₂ emission values associated with the preparation of 1 ton of geopolymer concrete.

Material Name Geopolymer	Weight (kg)	Weight Ratio	Density (kg/m3)	Emission Factor (kg CO2 eq/kg)
Metakaolin	292.903	29.3%	1850	0.245
Reagent for alkalinization	287.279	28.7%	1050	0.424
SiO2	31.406	3.1%	319	2.890
Carbon fiber	8.010	0.8%	350	0.051
Sands	94.340	9.4%	1650	0.147
Biomass fly ash	283.019	28.3%	425	0.0
Aluminum	3.043	0.3%	2700	12.790
Density of geopolymer			1140.5
Total mixture (kg)	1000.0
Total CO2 (for 1 ton)				337.530
kg CO2 eq/kg				0.338

) was based on the following assumptions:

• The CO₂ emissions associated with the transportation of primary materials from the seller to the construction site, as well as the on-site application processes such as the use of mixing machines, are equivalent for both concrete and geopolymer materials. Consequently, the significance of transportation costs for the movement of goods will be disregarded.
• The compressive strength of both the conventional and geopolymer concrete mixtures is expected to reach 75 MPa after a curing period of 28 days.
• The calculations have been conducted using a quantity of 1 ton of prepared concrete.

The emission factor for conventional concrete yields a value of 0.772 kg CO₂ per kg of concrete. In contrast, the calculated geopolymer emission factor yields a value of 0.338 kg CO₂ per kg of geopolymer concrete. The geopolymer concrete exhibits a reduction in CO₂ emissions of 56.0%.

To the best of our knowledge, there is currently no existing literature that provides a comparative analysis of CO₂ emissions and cost calculations between biomass fly ash-based geopolymers and conventional concrete. However, as mentioned earlier, a life cycle assessment and comparison between conventional concrete and geopolymer concrete made from slag or coal fly ash was conducted by Habert et al. [163]. The researchers conducted an analysis of the LCA implications and concluded that alkali-activated mix designs produced solely from fly ash or blast furnace slag exhibit lower CO₂ emissions compared to those made with Portland cement.

In another study, Ouellet-Plamondon and Habert [166] found that geopolymeric mixtures exhibit a negligible global warming potential, accounting for only 10% of the emissions associated with cement or concrete composed entirely of OPC. Nevertheless, Davidovits [33] disputed Habert’s calculation, asserting that the data provided for sodium silicate solution relates to the solid glass form, which emits 1.14 kg CO₂ eq. (100%) without dilution in water, rather than the diluted form emitting 0.424 kg CO₂ eq. (37% of solid glass).

In comparison to the LCA studies conducted by Turner [167], Davidovits [33], and Teh [168], the calculation of CO₂ emissions in our case study resulted in a higher value than 4.49% and 49% for Turner et al. and Davidovits, respectively, and a lower value than Teh’s calculation of 2.3%. The OPC findings derived from the current study align with the results of other studies, wherein a range of 0.7–0.9 kg CO₂-eq/kg of OPC has been reported [169,170].

4.2. Calculation of Production Cost: Conventional Concrete versus Geopolymer Concrete

The price of emission allowances has had a notable impact on pricing, leading to an increase in the cost of certain commodities. In October 2018, the price of emission allowances was recorded at 17.30€ per ton. As of July 2023, the quoted price for these allowances had risen to 92.65€ per ton.

The allowance price experienced a significant increase of 435% as depicted in Figure 7, which consequently had an impact on cement prices. The subsequent tables, namely

Table 8. Price calculation for the preparation of 1 ton of conventional concrete.

Material Name Reinforced Concrete	Weight (kg)	Weight Ratio	Price €/kg
Mixture of sands	303.47	30.3%	0.01
Cement (75 Pa)	298.47	29.8%	0.88
Water	276.51	27.7%	1.97
Binder mixture	8.74	0.9%	1.36
Steel rods	112.80	11.3%	2.25
Total mixture (kg)	1000.0
€/t			1076.1

and

Table 9. Price calculation for the preparation of 1 ton of geopolymer concrete.

Material Name Geopolymer	Weight (kg)	Weight Ratio	Price €/kg
Metakaolin	292.903	29.3%	0.40
Reagent for alkalinization	287.279	28.7%	1.95
SiO2	31.406	3.1%	2.25
Carbon fiber	8.010	0.8%	8.1
Sands	94.340	9.4%	0.01
Ash	283.019	28.3%	(−)0.02
Aluminum	3.043	0.3%	1.40
Total mixture (kg)	1000.0
€/t			812.5

, provide a comprehensive overview of the individual components and their corresponding prices.

Table 8 and Table 9 show the cost of preparing one ton of conventional and geopolymer concrete. As of 7 July 2023, 1 ton of conventional concrete costs 32.4% more than 1 ton of geopolymer. Furthermore, geopolymer prices are far less subject to changes in fuel prices and emission limits than conventional concrete.

Thaarrini and Dhivya conducted a cost comparison between conventional and geopolymer concrete [172]. Instead of using fly ash from the biomass combustion industry to prepare geopolymer concrete, they used bottom ash derived from thermal power plants and ground granulated blast furnace slag obtained from the metal industry. Two distinct grades of concrete were prepared, and the results indicate that for the M30 grade of geopolymer concrete, the production cost is slightly higher (1.7%) compared to OPC concrete of the same grade. Conversely, for the M50 grade, the cost of OPC concrete is 11% higher than geopolymer concrete of the same grade.

Verma et al. [173] performed a study in which they calculated the costs of conventional concrete and geopolymer concrete. By incorporating ground granulated blast furnace slag, the cost of the geopolymer concrete was significantly reduced, resulting in a cost reduction of up to 40% compared to OPC at a bulk level. The price of OPC per cubic meter is Rs. 3758, while the price of geopolymer concrete per cubic meter is Rs. 2230.

The examination results reveal significant disparities in the computed financial and environmental costs associated with geopolymers [174,175,176]. These costs can have either positive or negative implications contingent upon factors such as geographical origin, energy source, and transportation method [31].

5. Conclusions

Based on the analysis of available and peer-reviewed reference data, as well as a case study comparing the ecological and financial aspects of conventional concrete and geopolymer concrete, several conclusions can be drawn regarding the role of biomass fly ash in the geopolymeric matrix.

• The composition and quantity of inorganic matter present in biomass fly ash exhibit a higher degree of variability due to the utilization of a wide array of fuels and sources, which are influenced by the prevailing growing conditions. Hence, the chemical and physical composition of fly ash exerts an influence on the ultimate quality of geopolymers.
• Geopolymers derived from biomass fly ash demonstrate enhanced initial mechanical strength, exceptional resistance to acid and sulfate degradation, and reduced shrinkage in comparison to conventional concrete materials.
• Biomass fly ash exhibits potential applications as an adsorbent, membrane filter, Fenton catalyst, and photocatalyst.
• The fly ash should possess a maximum unburnt material content of 5%, iron oxide content of 10%, and CaO content of 10%. The concentration of reactive silicon is typically observed to range between 40% and 50%. The proportion of particles measuring less than 45 μm in size is observed to be between 80% and 90%.
• The impact of reducing carbon dioxide emissions with a geopolymer matrix is a well-established phenomenon. In our case study, we substantiated this claim by utilizing authentic and industrial concrete production recipes. We found the emission factor associated with conventional concrete to be 0.772 kg of carbon dioxide emitted per kilogram of concrete. On the other hand, the emission factor for geopolymer concrete is 0.338 kg CO₂ per kg. The geopolymer concrete demonstrates a decrease in carbon dioxide emissions of 56.0%. The findings of our study are corroborated by the existing literature.

There exists a range of perspectives regarding the financial implications associated with the preparation of geopolymers. While certain sources assert that the initial cost of producing geopolymer concrete is twice as high as that of conventional concrete, it has been established that the cost of geopolymer concrete is also influenced by factors such as the type of biomass fly ash source and the expenses associated with transporting raw materials. Numerous studies have provided evidence supporting the cost-effectiveness of geopolymer concrete, demonstrating that it can be up to 40% more economical than conventional concrete. In the present case study, we found that the cost of 1 ton of conventional concrete exceeds that of 1 ton of geopolymer concrete by 32.4%.