A CONTRIBUTION TO THE STUDY OF LIGHTWEIGHT GEOPOLYMER CONCRETE WITH EXPANDED CLAY: EVALUATION OF ITS THERMAL PERFORMANCE

Purpose: The objective of this work was to evaluate the thermal insulation performance of lightweight geopolymer concretes with expanded clay, considering their implications for energy consumption and construction efficiency. Theoretical framework: The urgent need for sustainable construction practices amid global concerns about climate change and environmental degradation has been increasingly discussed. With the cement industry being a major contributor to CO2 emissions, alternative materials like geopolymers offer a promising solution, once the consumption of concrete tends to grow bigger. The production of geopolymer concrete, known for its strength and low environmental impact, involves combining a precursor rich in aluminosilicates with an alkaline activator, usually sodium hydroxide and sodium silicate. Notably, geopolymer cement can cut up to 64% of greenhouse gas emissions. Expanded clay, as a lightweight aggregate, garners attention for its porous structure and ability to provide thermal and acoustic insulation to concrete. Its application in constructing vertical enclosures, such as concrete walls, enhances thermoacoustic comfort and aids in assembly and transportation on construction sites. Effective thermal insulation, achieved through materials with low thermal conductivity, plays a pivotal role in creating thermally suitable environments, impacting user satisfaction, productivity, and energy conservation. Method and materials: The materials used for concrete production, including metakaolin, sodium hydroxide, sodium silicate, expanded clay, crushed stone,


INTRODUCTION
The escalating evidence of daily climate change and environmental degradation has spurred a global concern, prompting imperative decision-making by governments and industries.This urgency underscores the need for processes that are both low in carbon emissions and resource-conserving, contributing to the development of a more sustainable global economy [1] [2] [3].Within this framework, the cement industry emerges as a sector requiring particular attention, given its responsibility for 7-8% of all anthropogenic CO2 emissions, its consumption of 2-3% of global energy, and its ranking as the third most challenging source to eliminate, following variable load electricity and the iron and steel industry [4] [5].
Significantly, Portland cement plays a pervasive role, especially in the construction sector, serving as an indispensable component of concrete, the most consumed material globally in terms of mass, with an annual estimated consumption of approximately 30 billion tons [4] [6].This is particularly relevant as the anticipated rise in global demand for concrete, driven by the developmental needs of emerging countries grappling with housing and infrastructure deficits, as well as population growth, is becoming more apparent, notably in the case of Brazil [7].
In response to the pressing environmental and social concerns, strategic initiatives to mitigate CO2 emissions from the cement industry require careful evaluation.One promising avenue for achieving this goal involves the utilization of alternative cementitious materials, such as geopolymers, as substitutes for Portland cement in construction.Geopolymers, also known as alkali-activated cement or inorganic polymers, exhibit mechanical strength and durability on par with, or even superior to, Portland cement [8].
Considered high-performance cementitious materials, geopolymer cement comprises a powdered precursor-whether industrial waste or a pozzolanic material-and an alkaline activator [9].According to Alves [10], the production of geopolymers requires approximately 90% less energy than producing Portland cement, with significantly lower carbon dioxide emissions.
Geopolymer concretes are known for their robust mechanical strength, durability, resistance to thermal shocks and heat, and the ability to be used just four hours post-molding for concretes up to 20MPa [11].This rapid transition from a fresh to a hardened state is a pertinent property for the mass production of highly repeatable elements, such as precast vertical sealing panels, also known as concrete walls.
Concrete walls are described by the Brazilian Portland Cement Association [12] as a system that combines structural and sealing functions in a single element and can be executed through on-site casting or prefabrication industries.The use of fast-setting materials, such as geopolymer concrete, has the potential to accelerate the construction process, contributing to meeting deadlines and avoiding idle equipment and workers.
However, regarding the production of panels for concrete walls, a significant drawback is the high specific weight of both conventional and geopolymer concrete.This characteristic hinders the handling and transportation of pieces on the construction site, increases demands on structures, and reduces productivity on-site [13].Therefore, concretes using lightweight aggregates as substitutes for conventional aggregates emerge as viable options globally to address these issues.
In this context, expanded clay stands out as a lightweight aggregate with significant potential.According to Moncada, Silva, and Pacheco [14], among the key advantages of this material are its low density, which can be up to 2.5 times lower than crushed stone, and its chemical inertia, preventing harmful effects on the cement paste.Additionally, Rossignolo [15] emphasizes that implementing expanded clay in concretes can also enhance the thermal and acoustic insulation of the mixture.
Addressing the improvement of the thermal performance of buildings is of paramount importance.Discussions on this topic have gained prominence due to the unalterable nature of climatic variables.According to Lamberts, Dutra, and Pereira [16], studies on the thermal insulation of buildings are crucial due to the increased energy consumption for artificial cooling and heating systems.The use of appropriate insulating materials in building design could render these systems unnecessary, contributing to sustainability and occupant comfort.
Furthermore, when considering materials for building construction, various aspects must be evaluated to ensure occupant safety.Hence, the present research is justified by the need to study alternative materials that enhance certain building characteristics and reduce environmental impacts.Consequently, the development of geopolymer concrete panels enhanced with expanded clay emerges as a possibility for technological and sustainable advancement.
According to Fiengebaum [17], thermal insulation of buildings made with this material is essential for thermal comfort and, most importantly, for reductions in energy consumption.The author highlights that thermal insulation can be achieved in three ways: externally (using insulating coatings and finishes), in double walls (using insulating material between them), or internally (through the incorporation of materials promoting insulation).In this context, the last method requires careful consideration and, therefore, should be thoroughly studied to ensure proper execution.
According to Leite [18], the construction industry is one of the main contributors to environmental degradation and, therefore, requires constant research to make this industry more sustainable.Recognizing the ongoing discussions about the high pollution levels of cement industries worldwide and the ongoing research on new binders, Mazza [19] indicates that the challenge lies in the lack of sufficient information for the widespread use of geopolymers as a replacement for Portland cement.
Moreover, Mazza [19] points out that materials science should continue to seek new technologies and methods for geopolymer cement production at the national level to meet potential future demands for this material.It is also relevant to emphasize that research projects on this topic are crucial since geopolymers were identified as materials with great potential for the construction industry only around 1990.Currently, there is a significant demand from scientists worldwide for behavioral analyses of geopolymers [8].Therefore, the knowledge gap about these materials is a significant obstacle to overcome.
In conclusion, the analysis of lightweight geopolymer concrete behavior aims to present a material that contributes to the reduction of pollutant gas emissions by replacing Portland cement with a less aggressive binder.Additionally, this study seeks to provide lightweight characteristics to the mixture using expanded clay to facilitate the handling and transportation of concrete wall panels and contribute to the thermal insulation of buildings.

THEORETICAL FRAMEWORK
In this section, concepts, materials, and specifications relevant to the understanding of the chosen topic for the development of this study will be outlined.It is emphasized that the creation of a relevant theoretical framework, as well as the research for appropriate bibliographies, is of paramount importance to achieve the study's objectives.

Conventional and alternative concretes
Neville and Brooks [20] provide a broad definition of concrete as a product resulting from the use of a cementing medium.They assert that this mixture can be crafted from various types of binders, with the option of incorporating chemical additives, fibers, polymers, and alternative aggregates.Pedroso [21] highlights that the Federación Iberoamericana de Hormigón Premesclado estimates an annual consumption of around 11 billion tons of concrete, equivalent to an average consumption of 1.9 tons per inhabitant per year, ranking concrete as the second most used material after water.The widespread use of concrete globally is attributed to its versatility in being molded into different shapes and sizes, favorable workability, long service life, and relatively low cost, as pointed out by Mehta and Monteiro [22].
In this context, concrete stands out as a profoundly significant discovery in the realm of global development, heralded as one of the most vital construction materials currently in use [22].Despite the favorable characteristics of conventional concrete, Almeida [23] notes some disadvantages that warrant improvement.Mehta and Monteiro [22] exemplify these limitations citing issues such as the high self-weight of concrete structures, low tensile and cracking resistance, and the need for formwork.
Aiming to overcome certain limitations of the material, Rossignolo [15] notes that concrete has undergone considerable technological evolution in recent decades.This evolution is attributed to advancements in research techniques and equipment, facilitating a deeper understanding of the mixture's behavior when incorporating new materials.These advancements have given rise to "special concretes," including lightweight concrete and geopolymer concrete.
Rossignolo [15] underscores the distinctive feature of lightweight concrete: its reduced specific mass.Mehta and Monteiro [22] elaborate that lightweight concrete, in comparison to conventional concrete, boasts a weight of approximately one-third lower.Moreover, it is important to note that this reduction in specific mass is achieved by introducing air into the mixture, Neville and Brooks [20] identify three distinct methods for this: incorporating lightweight aggregates, using air-entraining additives (referred to as cellular/aerated concrete), or eliminating fines.Rossignolo [15] emphasizes that lightweight concrete with lightweight aggregates is the only variant achieving adequate compressive strength for structural use.It involves partially or completely replacing coarse aggregate with a lightweight and porous aggregate, such as expanded clay [15].
According to NM 35:2008 [24], structural lightweight concretes should have a compressive strength ranging between 17 MPa and 28 MPa and a maximum specific mass between 1680 kg/m³ and 1840 kg/m³.Mehta and Monteiro [22] suggest a specific mass below 1850 kg/m³ and a minimum compressive strength of 17 MPa.Rossignolo [15] proposes that, to be classified as lightweight, concrete must exhibit a specific mass of less than 2000 kg/m³.
Turning to geopolymer concrete, Skaf [25] defines it as a material comprising coarse and fine aggregates, much like Portland cement concrete.These aggregates occupy approximately 80% of the mixture's volume, with the remaining portion filled by a binder known as geopolymer.In this context, Provis [8]  6 solid material resulting from alkaline activation, utilizing a hydroxide and a silicate, of a precursor, typically in powder form and rich in silicon (Si) and aluminum (Al), derived from natural minerals, industrial waste, or another pozzolanic material.
Severo et al. [26] elaborate on alkaline activation as the synthesis of geopolymers, occurring through a hydration reaction of aluminosilicates with alkaline substances.The precursor is defined as a material participating in a chemical reaction giving rise to another compound, presenting a wide range of aluminosilicate inputs.Borges et al. [27] highlight that the geopolymers' geopolymerization stage begins with the "dissolution" stage, referring to the mixing of a solute with a solvent, which occurs upon contact between the precursor and the alkaline solution, resulting in a synthetic rock.Substituting a geopolymer for Portland cement in concrete tends to reduce CO2 emissions from the cement industry and cut up to 64% of greenhouse gases [27].
Regarding the production of geopolymer concrete, Pouhet and Cyr [28] propose preparing the activating solution 24 hours before molding the concrete.This involves initially dissolving sodium hydroxide in water, subsequently adding it to sodium silicate, allowing the necessary reactions to occur and the mixture to cool.After this period, the authors recommend the initial insertion of coarse aggregates into the mixer, followed by metakaolin and part of the mixing water.Once these materials are mixed, the activating solution and the remaining calculated water are added, with the mixer running until a homogeneous material is achieved.
Furthermore, the curing of the geopolymer paste, unlike conventional binders, lacks an optimally defined method.Thus, various alternatives exist for executing this step.Komnitsas [29] suggests that curing can be performed quickly and effectively at room temperature or, in some cases, at higher temperatures for a short duration.Heah et al. [30] and Pouhet and Cyr [28], for example, present different perspectives on curing, with considerations of temperature and time.
Concerning the production of the geopolymer paste, Liew et al. [31] stress that the precursor/activating solution ratio obtained significantly affects the properties of the final concrete.This includes modifying workability, geopolymerization reactions, and overall strength.The authors highlight that excessively high values impair workability, while very low values interfere with geopolymerization.Additionally, the ratios between silicate/hydroxide for the mixture also influence the behavior of the manufactured concrete.However, given the relatively short period of study for geopolymers in civil construction, there is currently no established method for determining these ratios, necessitating tests to establish appropriate values.

Geopolymer cement
Broader investigations into geopolymers emerged from research conducted by Joseph Davidovits around 1972 when he commenced studies on materials capable of withstanding high temperatures.Pereira et al. [32] elucidate that geopolymers, concerning the spatial organization of the material, are composed of chains or rings of Si 4+ and Al 3+ with coordination number IV with oxygens.The chemical composition is similar to zeolitesmicroporous crystalline materials that release water vapor when heatedbut with a structure ranging from amorphous to semi-crystalline.Geopolymers can also be called polysialates, a considerable molecular chain consisting of aluminum, oxygen, and silicon.The term "sialate" is understood as an abbreviation for silico-oxide-aluminate [25].
According to Caballero [9], the raw materials used for the production of geopolymer cement are based on two components: a precursor, rich in aluminosilicates and high amorphous content, and an alkaline activator with a high pH value.Komnitsas [29] elucidates that numerous waste and natural materials possess properties suitable for use in the production of geopolymer materials such as concrete, construction components, insulating coatings, and fireresistant materials.The same author cites wastes from metallurgy, construction, demolition, and mining, such as fly ashes and slags, and natural clays, such as metakaolin.Also, it is important to highlight that all of these materials are produced in large quantities worldwide.
For the alkaline activator, the most commonly used materials are classified as simple and compound.The former refers to the use of only an alkaline base, sodium hydroxide, or potassium hydroxide.The latter refers to the combined use of an alkaline base and a silicate.According to Pereira [32], there is still no in-depth understanding of the role of each component of the activator in the mixture; however, the author attributes the role of the binder to the silicate and the role of dissolving the precursor to the hydroxide.
Severo et al. [26] suggest that studies on mixtures with metakaolin as a precursor and sodium silicate with sodium hydroxide combined as an alkaline activator show a higher degree of reaction, emphasizing that, to date, the research conducted demonstrates that geopolymer cement has great potential to become an alternative material to Portland cement.In this scenario, this research focuses primarily on metakaolin, sodium hydroxide, and silicate.
According to Caballero [9], metakaolin contains essential elements for the production of geopolymers, including SiO2 and Al2O3 around 52% and 40%, respectively.Provis [8] suggests that the calcination method of kaolin is relatively simple, with the possibility of occurrence in medium-sized laboratories, after calcining kaolin at a temperature between 750 °C and 800 °C.Santos [33] highlights that Brazil is among the top five world markets for metakaolin, growing at about 20% per year.
As stated by Mazza [19], sodium hydroxide -NaOHis the most economical, widely used, and readily available hydroxide to employ in the alkaline activation process of a geopolymer.Provis and Van Deventer [34] emphasize that the disadvantage of using NaOH as a sole activator lies in the corrosive nature of the substance, necessitating the use of customized equipment for large-scale geopolymer production.The suggestion to minimize corrosion is the application of silicates as activators or a combination of a silicate with a hydroxide.
In accordance with Severo et al. [26], sodium silicate -Na2SiO3when used as an alkaline activator, promotes an accelerated geopolymer reaction, as silica reacts with free alumina in the solution, thus generating a product with greater mechanical strength.Brito [35] defines sodium silicate as the most common soluble silicate, also known as "liquid glass." The material received this designation around the end of the 19th century by Von Fuchs when studying the solubility process of alkali glasses in large amounts of water [35].Brito [35] mentions that engineer Adolf Kleinlogel suggests the practice of painting concrete surfaces with sodium silicates, called "silicification of concrete," reducing the permeability of surfaces to water.From this, its use in geopolymer cement can produce concrete with fewer pores, but further studies are required on this matter.
Finally, Shadnia, Zhang, and Li [36] mention that the use of geopolymers in civil construction has been the subject of studies regarding the thermal insulation of buildings.This happens because geopolymer cements tend to exhibit refractory characteristics due to their composition.Cao et al. [37] report that in recent years, studies have been conducted to assess the reduction of energy consumption by artificial temperature change systems and the impact of using geopolymers on the internal and external temperatures of building surfaces.

Lightweight aggregate: expanded clay
According to Bauer [38] an aggregate is defined as a particulate, incohesive material with virtually no chemical activity, consisting of a mixture of particles covering an extensive range of sizes.Mehta and Monteiro [22] classify aggregates with a specific gravity of less than 1120 kg/m³ as lightweight and suitable for use in the production of lightweight concrete.They emphasize that the lightness of aggregates comes from their cellular or highly porous structure.
As stated by Ambrozewicz [39], expanded clay is an aggregate produced from pyroexpansive clays, in furnaces where temperatures exceed 1000 °C.The particle size ranges from 4.8 mm to 25 mm and is widely used in the production of lightweight concrete.Moravia et al. [40] describe that raw materials are thermally treated until incipient fusion to expand the material.In this process, gases are generated and trapped inside the structure, maintaining its porosity after cooling.
Among the main advantages that the use of expanded clay in lightweight concrete can provide, Rossignolo [15] highlights the thermal and acoustic insulation made possible by the porous structure of the material.Commercially, different particle sizes are identified as follows: designation 2215 and 1506 for expanded clays with dimensions equivalent to coarse aggregate and 0500 for expanded clays equivalent to sand.

Vertical sealing: concrete walls
Based on Franco [41], vertical enclosures are classified as a subsystem with the primary functions of defining and limiting buildings, controlling external agents, and facilitating the proper progress of activities for which the spaces were designed.The author emphasizes that vertical enclosures serve as interfaces with other building subsystems, such as horizontal enclosures, installations, structures, etc. Examples of vertical enclosures include masonry and concrete walls.
The concrete wall system, as per the Brazilian Portland Cement Association [12], is suitable for large-scale production because it allows for the rapid construction of a high number of units, resembling an industrial production system.The concept of industrialization applies to this system, as there is large-scale, repetitive production of identical products.
This wall system requires qualified professionals for execution, as the final product must be capable of serving both structural and enclosure functions simultaneously.El Debs [42] emphasizes that there are different methods for executing concrete walls: pre-fabrication or onsite molding, both of which involve stages related to project conception, planning, manufacturing, and completion of the pieces.
In the case of pre-fabrication, according to NBR 9062:2017 [43], it involves elements molded previously outside the definitive location in the structure.In the pre-fabrication system, the manufactured piece is, according to NBR 9062:2017 [43], a pre-fabricated element produced industrially, in permanent facilities of a company intended for this purpose.
El Debs [42] highlights the advantages of using these products in terms of quality and off-site execution, avoiding potential construction defects.Moravia et.al. [40] notes that lightweight concrete, when used as a vertical enclosure element in the form of panels or blocks, slabs, or structural walls, in addition to assisting in the assembly and transportation of the pieces on the construction site, it also improves the thermoacoustic comfort of environments.

Habitability: thermal insulation
Parmeggiani [44] characterizes the term habitability as the conditions of a building and how individuals interact with it.According to NBR 15575-1 [45], the parameters that help verify these conditions include accessibility, water and air tightness, functionality, health, hygiene, air quality, tactile and anthropodynamic comfort, and especially thermal insulation.9 The knowledge of a space's habitability translates to its quality for habitation, considering aspects related to building performance and user comfort.
In this sense, building designs must be developed to provide environments suitable for their occupants, considering that climate variables cannot be altered [46].Consequently, the introduction of thermal energy into the environment occurs through radiation or convection on the external face of the building, and within the structure, heat flow occurs through conduction, with the heat passing back into the internal environment through radiation [46].
Sousa [47] explains that heat exchange through radiation occurs through electromagnetic waves, thus not requiring a propagation material.Solar radiation is characterized as the primary means of thermal energy transfer.On the other hand, convection is clarified by Frota and Schiffer [48] as the heat exchange between two bodies, one of them being solid and the other a fluid.Finally, for the same authors, conduction is defined as the heat exchange between two bodies in contact or parts of the same element that have different temperatures.
Bezerra [46] also emphasizes that factors such as wall thickness, material density, and thermal conductivity directly influence the intensity of the transmitted heat flux.Lamberts, Dutra and Pereira [16] suggest that materials with low thermal conductivity and reduced density due to confined air inside, making heat transfer more difficult, are suitable as insulators.Figure 1 illustrates the heat transfer mechanisms in action in a wall exposed to the environment.According to Lamberts, Dutra and Pereira [16], approximately 20% of electricity consumption in buildings is allocated to artificial cooling systems.The author mentions that this value is expected to increase shortly as individuals' purchasing power grows and due to buildings not being adequately adapted to their climatic conditions.
Mascarô and Mascarô [49] characterize thermal insulation as the resistance the materials composing facades, floors, and ceilings offer to heat exchange between the external and internal environments.Frota and Schiffer [48] explain that in the past, thermal insulation was directly related to the thickness of materials, and greater mass was believed to provide better insulation.However, technological advances now allow for adapting existing materials to project conditions and local climatic needs, and thickness is not a determining factor.
Furthermore, the appropriate selection of materials for buildings is crucial for effective thermal insulation [49].This choice depends on the location of the construction and its specificities.For example, there are climate differences between a European country and Brazil, performance  where the priority is to retain heat inside buildings due to cold in the former, while in the latter, the main objective is to prevent excessive heat from entering buildings [16].
Finally, for Lamberts, Dutra and Pereira [16], the importance of studies on the comfort and thermal performance of materials used in buildings is based on three pillars.The first refers to user satisfaction in thermally suitable environments.The second relates to productivity, as discomfort caused by heat or cold can impact productivity.The third pillar refers to energy conservation, as artificially conditioned environments have become common, even though the use of cooling or heating systems can be avoided.

METHODOLOGY
The precursor powder for the geopolymer cement used in this study was Metakaolin HP Ultra with high reactivity, supplied by the company Metacaulim do Brasil Ltda., marketed for use in conventional concretes and mortars with Portland cement.Concerning its physicochemical characterization, the specific surface area was obtained using the BET method by nitrogen gas adsorption on the Micromeritics ASAP2020N equipment; particle size distribution was measured using the Anton PAAR equipment under dispersion in isopropyl alcohol; density was indicated by the producer; the chemical composition was analyzed using a Shimadzu equipment, model XRF1800, with sequential X-ray fluorescence spectrometer; Xray diffraction (XRD) was determined using the Phillips equipment, model X'Pert MDP.The pozzolanic activity index was determined following the standard NBR 5752:2014 [50].The results obtained can be observed in Table 1 and Figure 2.   The X-ray diffraction (XRD) analysis conducted on the metakaolin reveals a highly amorphous structure with a halo within the 2θ angles of 15° to 35°.The primary crystalline phases identified are anatase (ICSD: 202242) and halloysite (ICSD: 26716).These characteristics highlight the material's high reactivity potential, essential for use in alkali activation.Regarding the sodium hydroxide and the sodium silicate used as alkali activators, the characterization was provided by the manufacturers and can be observed in Tables 2 and 3, respectively.For the fine aggregate, a commercially identified quartz sand labeled as 'medium' was used.For its characterization, the samples were dried in an oven at approximately 100°C until a constant mass was achieved to remove humidity.In order to obtain its specific mass NBR 16916:2021 [51] was used and for loose unit mass the NBR 16972:2021 [52] was employed.Granulometry was conducted to determine the fineness modulus and maximum diameter, as outlined in NBR 17054:2022 [53].The results obtained can be observed in Table 4.  12 For the conventional coarse aggregate, a commercially identified as "gravel number zero" was used and for the lightweight aggregate a commercially identified as "expanded clay number 1506", supplied by Cinexpan, was used.The same drying process adopted for the sand was executed for the conventional coarse aggregate characterization.Subsequently, tests indicated for specific mass and water absorption by NBR 16917:2021 [54], compacted unit mass by NBR 16917:2021 [54], and particle size distribution to provide the maximum diameter by NBR 17054:2022 [53] were performed.The results obtained can be observed in Table 5.The data related to expanded clay, observed in Table 6, were provided by the manufacturer.In determining the concrete mix proportions, the initial step involved defining the quantities of both the precursor and alkaline activator for the geopolymer cement.In accordance with Skaf [25], the analysis and adaptation of previously proposed mix proportions are recommended due to the absence of methods and regulations for geopolymer dosage calculation.From this perspective, the chosen mix for the continuation of the study and development of the cement was based on the results reported by Albidah et al. [55].This decision was made once the authors utilized metakaolin as a precursor, and sodium hydroxide and sodium silicate as alkaline activators, which exhibit properties similar to the materials evaluated in this article.
Furthermore, Albidah et al. [55] assessed 04 groups containing 04 mix proportions each, with varying sodium hydroxide/sodium silicate ratios from a reference mix, denominated M3, established as the main mix.Thus, among the 16 proportions presented in the study, M3 yielded the best compressive strength data, reaching a value close to 60 MPa at 28 days.Consequently, the proportions of this mix were replicated for the geopolymer cement, totalizing an activation percentage (AP) of 15% and a solution modulus (MS) of 1,25.The AP data means that for every 100 g of metakaolin, 15 g of Na2O is added to the mixture.The MS data represents the molar ratio between SiO2/Na2O in the activator, with SiO2 supplied by the sodium silicate and Na2O supplied by both the hydroxide and the silicate.
Once the cement was dosed for the study, the concrete dosage methodology followed the prescriptions proposed by the Brazilian Portland Cement Association (ABCP).This choice is justified by its recognized effectiveness and alignment with the specific conditions of the country's construction environment.This method, supported by established technical guidelines, provides a robust approach for the initial calculation of dosage, considering factors such as the water/binder ratio (w/b) and the desired compression strength goals.

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The ABCP methodology offers a solid foundation for the quality control of concrete, ensuring not only adequate strength but also the necessary workability for the effective molding of test specimens.Its selection reflects a commitment to adopting proven practices and adapting them, when necessary, to meet the specific demands of the study at hand, resulting in a reliable dosing process in compliance with Brazilian regulations in the field.In this scenario, an initial manual dosage calculation was performed, considering ordinary Portland cement, 120 mm for slump, and 30 MPa for compressive strength at 28 days to obtain the quantities of cement, sand, and gravel.
Subsequently, the quantity of cement obtained was adjusted for the geopolymer cement, considering metakaolin, sodium hydroxide, and the solid part of the sodium silicate.To add lightness to the mixes, it was established the replacement of 30% and 70% of the volume of the crushed stone with expanded clay.In this way, a total of 3 mixes were identified as GP, GP30%, and GP70%, with their material quantities per m³ described in Table 7, aiming for a w/b ratio established at 0.60 to achieve sufficient workability for proper molding of the specimens.For molding the test specimens, as suggested by Pouhet and Cyr [28], the activating solution was first prepared and left to rest for 24 hours to allow the necessary reactions to take place.In this scenario, for the preparation of the solution, sodium hydroxide was gradually added to the water intended for its dilution.It's important to note that this mixture was done slowly and carefully since high amounts of heat are released in the exothermic reactions of diluting the flakes, and protective gear such as gloves, masks, and goggles were used as the formed material is highly aggressive.
The sodium hydroxide was left to cool until it reached room temperature.Next, sodium silicate was added to the solution, and it was mixed until a homogeneous texture was obtained.Subsequently, the activating solution was covered with plastic bags to prevent accidents and avoid contact with air and ambient humidity, remaining at rest for 24 hours until the molding of the geopolymer concrete.
It's worth noting that for the production of geopolymer concrete with expanded clay, the clay was immersed in water for 24 hours for saturation.This was done because, according to studies conducted by Schimanowski, Oliveira, and Lopes [56], the porosity of unsaturated clay removes the kneading water necessary for cement reactions and can impair workability.Thus, the sand and the crushed stone, previously dried in an oven for 24 hours, and the metakaolin were weighed and separated for concrete molding.Furthermore, it's emphasized that the clay was weighed before and after saturation to consider this data in obtaining the appropriate water/binder ratio for the molded concrete.
Regarding the actual mixtures, for the reference geopolymer concrete (GP), the mixer was initially moistened, and the crushed stone was added.Then, the metakaolin was introduced, followed by the activating solution.Finally, the sand was added to the mixture, along with the remaining mixing water.Moreover, to assess the reduction in specific mass provided by the incorporation of expanded clay in the systems in the fresh state, the procedure outlined in NBR 9833:2009 [57]

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As for molding the test specimens, for the thermal insulation test, rectangular plates with dimensions of 20 x 40 x 12 cm were fabricated in molds to simulate a small panel of a concrete wall.The compaction of the plates was performed using a vibrating table for 35 seconds.Finally, for a better finish, the plate was smoothed with the help of a beveled ruler to achieve the smoothest surface possible.Figure 3 (A) illustrates the molding process, while Figure 3 (B) shows the compaction method used and Figure 3 (C) displays the final finish.Once molded, the specimens of the geopolymer concrete underwent a thermal curing process in an oven at approximately 50°C ± 10°C for 24 hours and, thereafter, they were kept wrapped in plastic at room temperature until the age of 28 days for testing.This procedure was adopted to ensure the effective occurrence of the binder reactions.
The test for the analysis of heat transmission through the concrete does not have a specific standard to follow; therefore, it was conducted as recommended by Sousa [47].For this test, a cubic-shaped thermal chamber was used, with one of its faces open.The chamber's interior is lined with insulating material and contains a heat source, specifically an infrared lamp.Additionally, there is equipment on its side that allows for the control of the lamp temperature, set at approximately 100 °C, the maximum achievable value.Three thermocouples were used for the test, with two attached with silicone to the concrete slab to be evaluated and one to measure the room temperature.
The thermocouple named "T1" was positioned on the surface exposed to the heat source, simulating the external side of a concrete wall.The thermocouple "T2" was placed on the surface opposite the lamp, simulating the internal side of a building.Finally, thermocouple "T3" measured the ambient temperature during the test.Subsequently, the remaining opening on the open face of the chamber was closed with insulating material to prevent heat loss during the test.Figure 4 illustrates the step-by-step process described above, as well as the chamber used.Once the equipment was turned on in a room with controlled temperature and humidity, the values recorded by the thermocouples were measured for over 12 hours, at 30-minute intervals, as suggested by Sousa [47].From the acquisition of experimental data, considering the study conducted by Oliveira et al. [58], it is noted that the period designated for temperature collection is not always sufficient for the stabilization of values.Additionally, experimental errors may occur during the test execution.
To adequately analyze the insulation, the decision was made to perform a curve fitting to obtain the equation representing the material's behavior and, consequently, provide the final stabilization temperatures of the plate surfaces.To address this, the Modified Grid Search Method was utilized, given the nature of the experiment, which posed an Inverse Problem.In an Inverse Problem, the challenge is to identify the underlying causes of a phenomenon by analyzing its effects and results, as emphasized by Avi [59].
The chosen method involves defining intervals for each parameter within the function and subsequently subdividing these intervals into N partitions to obtain the best combination [60].In this context, the MatLab software was used to apply the computational method based on the code used by Avi [59] in his dissertation.

RESULTS AND DISCUSSIONS
The results obtained from the thermal insulation test of the concrete panels for each calculated mixture were analyzed.As previously mentioned, a curve-fitting approach was chosen for the experimental data since, in tests involving temperature, the specimen under analysis tends to stabilize its temperature after a certain time of exposure to the heat source.However, as this stabilization was not observed for the faces of the tested panels, obtaining a mathematical curve that provides the stabilization values allows for a better understanding of the insulation provided.Thus, the experimental values obtained for the produced panels can be observed in Table 8. 16 Once the experimental data was obtained, the process of curve fitting into logarithmic equations with three parameters was performed.In the graphs presented subsequently, the purple points represent the temperatures of the side exposed to the heat source, simulating exposure to the sun, and the purple curve represents the equation fitted to these data.Meanwhile, the green points indicate the temperatures measured on the opposite side of the panels, simulating the inner side of the exposed wall, and the green curve symbolizes the equation fitted to these temperatures.
Furthermore, it is important to note that the R² value indicated near the equation refers to the representativeness of the curve, demonstrating how well the curve fits the experimental data.The closer the value is to 1, the more efficient the application of the method.In this scenario, Figure 5, Figure 6, and Figure 7 show, respectively, the graphs plotted for GP, GP30% and GP70%.After conducting the test and subsequently adjusting the mathematical curves, it was possible to observe the logarithmic behavior commonly encountered in experiments involving heat exchange.The curve exhibited a pronounced growth until approximately 400 minutes into the test, and thereafter, temperature variations were smaller.However, within the data collection interval (0 to 720 minutes), temperature constancy for the plate was not observed.In this perspective, the function derived through the applied numerical method enables extrapolation of the data, thus determining the temperature reached by the concrete at any desired time.
It is important to highlight that all the performed curve fittings resulted in R² values equal to or higher than 0,95 indicating an adequate representativity and suggesting the analysis of the thermal insulation of the panels through the adjusted equations is viable.In this context, by applying the numerical method, it is determined that the stabilization temperature and, consequently, the maximum value to be reached by the exposed surface to the heat source, is 76,12 °C, which is 1,28% higher than the value obtained in the test, equal to 77,1 °C.
This difference can be justified by the possible occurrence of experimental errors inherent in laboratory tests, such as changes in weather conditions throughout the day of the test or even inaccuracies in the thermocouples, which influenced the measured temperatures.Alternatively, it may be due to the adjusted curve not being entirely representative, reaching an R² value of 0,95 out of a maximum of 1,0.Despite this, the decision was made to analyze the studied concretes based on the adjusted temperatures since the numerical method was appropriately applied, and the differences in the data can be considered small.On the other hand, the opposite face reached 43,1 °C, thus not achieving the stabilization proposed by the equation at the value of 44,3 °C as expected.
For the GP30% mix, the same situation observed in the GP mix occurred on the surface exposed to the heat source, reaching 80,8 °C in the test, 1,97% higher than the value established by the adjustment.Regarding the opposite face, the surface presented a temperature of 42,3 °C and a stabilization value in the fitted equation of 43,94 °C.
Finally, for the GP70% plate, the exposed face reached a temperature of 81,6 °C in the test, very close to the stabilization value proposed by the adjustment, which is 81,91 °C.As for the opposite surface, the test temperature achieved of 41,0 °C and a stabilization value of 41,67 °C.
Regarding thermal insulation, considering the adjusted data, it was found that the GP, GP30%, and GP70% plates insulated, respectively, 31.82°C, 35.30°C, and 40.24 °C.Thus, the effect of expanded clay is evident, increasing thermal insulation as the amount of the lightweight aggregate in the systems increases, confirming the insulating characteristic of the material indicated by the studied references.Finally, regarding the incorporation of expanded 18 clay in the systems concerning the reduction of specific mass, the results obtained can be observed in Table 9.When analyzing the specific mass data for the concretes, the weight reduction provided by the use of expanded clay instead of crushed stone becomes evident.When replacing 30% of crushed stone with expanded clay, there is an approximate 18,3% reduction in specific mass.Moreover, when substituting 70%, the reduction increases to approximately 27,7%.
According to NM 35:2008 and Mehta and Monteiro [22], only the GP70% mix proportion can be considered a lightweight concrete.On the other hand, in Rossignolo's [15] definitions, both compositions, the 30% and 70% mixes, can be categorized as lightweight.

CONCLUSIONS
The main objective of this research was to analyze the feasibility of using lightweight geopolymer concretes, produced with the use of expanded clay as an alternative aggregate, concerning thermal insulation and its mass reduction.Regarding the thermal insulation test, it was observed that the incorporation of expanded clay in the geopolymer concrete increased the insulation of the manufactured panels, confirming their insulating properties.In this context, the greater the amount of expanded clay used in concrete production, the higher its insulation tends to be, resulting in a better performance of the GP70% when compared to GP30% and GP.
Furthermore, the results obtained in this research significantly demonstrate the effectiveness of using expanded clay as an alternative aggregate in specific mass for both concrete mixes.The incorporation of this lightweight material proved to be a viable strategy for producing lighter concretes offering notable benefits in terms of energy efficiency and thermal insulation.These findings encourage the consideration and adoption of this innovative approach in the development of construction materials, aligning with the principles of sustainable construction and seeking more efficient and ecologically responsible solutions.

Figure 1 .
Figure 1.Heat transmission in a wall exposed to the environment.Source: Adapted from Frota and Schiffer [48].

Figure 4 .
Figure 4. Test where A) thermocouples T1 and T2 and B) Closed face of the chamber.Source: Authors (2023).

Table 6 .
Expanded clay characterization

Table 8 .
Resumed experimental data of the thermal insulation test contribution to the study of lightweight geopolymer concrete with expanded clay: evaluation of its thermal performance A ___________________________________________________________________________ Rev. Gest.Soc.Ambient.| Miami | v.18.n.4 | p.1-22 | e04555 | 2024.

Table 9 .
Results of specific mass