Geopolymers based on Metakaolin reinforced with Carbon Nanotubes

Geopolymers based on Metakaolin reinforced with Carbon Nanotubes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Geopolymers based on Metakaolin reinforced with Carbon Nanotubes Daniele Brandão, Paulo H. R. Borges, Peter Ludvig, Michelle Colão This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2909512/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Geopolymers offer an alternative to Portland cement, boasting high mechanical strength, as well as chemical and thermal durability. It is an inorganic polymer that has environmental advantages in its production, in addition to being considered a high-performance material. This study seeks to evaluate the mechanical properties of geopolymeric mortars reinforced with Carbon Nanotubes (CNT) as well as the method of incorporation into the mortar. CNTs are promising materials as reinforcements in geopolymeric matrices for the development of construction materials with specific applications aimed at better performance. The studied matrix results from the alkaline activation of metakaolin (MK) with sodium silicate and sodium hydroxide. CNTs were added in percentages of 0.1%, 0.3%, and 0.5% by mass of metakaolin. The evaluated properties for the resulting composite were compressive strength and flexural strength. Scanning Electron Microscopy (SEM) and Infrared Spectroscopy (FTIR) tests were carried out to evaluate the dispersion of CNTs in MK (before activation). The results show that when well distributed in the matrix, CNTs promote an increase in compressive and flexural strength of the samples. Dispersion Geopolymer Carbon Nanotubes Compressive Strength Flexural Strength. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. INTRODUCTION Portland cement is widely recognized as one of the most important materials in the construction industry. However, its manufacturing process is associated with high energy consumption and carbon dioxide (CO2) emissions that contribute to global warming. In light of the urgent need to reduce emissions and promote sustainability in construction, it is crucial to identify alternative binders that can replace Portland cement. As the demand for construction materials continues to grow, finding an alternative binding material to replace Portland cement and promote sustainability in construction becomes increasingly important. To reduce its consumption, given the significant exploitation of natural resources and the emissions during its production, studies have been carried out to explore new materials that may serve as alternatives to Portland cement. Among these materials, geopolymers have emerged as a promising new construction technology with the potential to facilitate green building practices. [ 1 ] Developing materials through alkaline activation has become of great interest in current research; thus, geopolymers emerge as a promising substitute due to their significant benefits. The main advantages of using geopolymers over Portland cement can be cited as better profitability, better chemical stability, corrosion resistance, high initial strength, and high CO 2 reduction capacity, as their production does not result in significant emissions, making them less harmful to the environment [ 2 ]. Geopolymers are part of a class of materials with great technological potential; they are inorganic polymers produced through chemical reactions in highly alkaline media, presenting good chemical stability, corrosion resistance, and high initial strength [ 3 ]. Producing new materials through polymerization reactions represents a technological breakthrough, with the opening of new applications and procedures, where techniques that previously required high temperatures are not as necessary to obtain ceramic materials [ 5 ]. Geopolymers can be synthesized by mixing an alkaline solution (usually a soluble silicate) and one or more solid aluminosilicates, with the geopolymers forming at ambient or slightly elevated temperatures [ 6 ]. Any pozzolanic compound or source of silica and alumina that dissolves in an alkaline solution can act as a precursor source for a geopolymer. A chemical activator, usually consisting of hydroxide and silicate solutions, is necessary to initiate the geopolymerization reaction [ 7 ]. The presence of soluble silicate complements the amount of SiO 2 needed for the formation of the geopolymers. Although aluminosilicate compositions contain SiO 2 , the amount is not sufficient to obtain geopolymers with the best mechanical properties and durability. Moreover, the presence of soluble silica accelerates the reactions, favoring the polymerization process and contributing more Si to obtain a product with higher strength at early ages [ 8 – 10 ]. Due to the type of bonding that occurs in its structure, the geopolymer can present defects in the form of pores and voids, making the structure fragile and prone to fracture. This brittle character is quite similar to other ceramic materials. In order to achieve improvement in properties and structural applications, reinforcements such as different types of fibers have been investigated and added to increase strength and control crack propagation [ 3 , 11 ]. Carbon nanotubes (CNTs) synthesized and observed by physicist Iijima in 1991 are considered one of the most promising materials due to their excellent properties and unique carbon structure [ 12 , 13 ]. They consist of long sheets of graphene rolled up and can be classified according to the number of sheets, single-walled carbon nanotubes (SWCNTs) having a single sheet and multi-walled carbon nanotubes (MWCNTs) with two or more sheets. These materials are important due to their wide range of scientific and technological applications, which depend on their properties that vary according to their structure, diameter, length, purity, and structural defects [ 13 ]. CNTs have been considered as a potential reinforcement in composites because they have mechanical, electrical, chemical, and thermal properties superior to traditional fibers [ 11 , 14 ]. The properties of CNTs are determined according to their morphology and dimensions. Currently, they are the materials with the highest known hardness and strength, which are a consequence of their perfect structural arrangement and the strength of the bonds between their atoms, making them one of the most beneficial nanomaterials for reinforcement. Their unique mechanical, electrical, and chemical properties make them an attractive candidate for reinforcing composite materials. This is due to their flexible nature, which does not damage their structure when subjected to large stresses [ 15 ]. With significant mechanical advantages, CNTs can be used as reinforcement in cementitious matrices, as they can provide significant improvements in mechanical properties and crack propagation resistance [ 16 ]. Due to the strong bond between their molecules, achieving the desired level of dispersion of CNTs becomes difficult, as they tend to agglomerate and form bundles in the mixtures. Insufficient dispersion has been a key factor in reducing the performance of cementitious composites containing CNTs; since ineffective dispersion leads to the formation of various defects in the composite, limiting its mechanical properties. On the other hand, better dispersion can result in a larger interfacial contact area between CNTs and the matrix, as well as more uniformly distributed stresses, requiring a pre-treatment in liquid dispersions [ 16 – 18 ]. Thus, techniques such as carboxylation process and the use of dispersing agents in combination with sonication have been employed to improve the bonding between CNT and cement. In geopolymer, the addition of CNT to the alkaline solution of sodium hydroxide and sodium silicate is not satisfactory as agglomeration occurs, whereas the use of superplasticizers together with sonication shows effective results for the dispersion of CNT in matrices [ 19 , 20 ]. Therefore, the objective of this study is to analyze the effect of CNT addition on the mechanical properties of geopolymer produced with different nanocomposite additions, as well as to evaluate the efficiency of the dispersion method used for the incorporation of the material. 2. MATERIALS AND METHODS 2.1 Materials Metakaolin was used as a source of silica and alumina to prepare the geopolymers, and its chemical composition (oxide percentages) is given in Table 1 . The activating solution used was composed of solutions of 14 M sodium hydroxide and a sodium silicate solution with a chemical composition of 31.79% SiO 2 , 15% Na 2 O, and 53.21% H 2 O. The fine aggregate used was Brazilian normal sand, produced by IPT - Instituto de Pesquisa Tecnológicas, according to ABNT 7214/82, with its characteristics presented in Table 2 . For the production of geopolymers, 50% of fine sand (material retained on sieve 50) and 50% of medium-fine sand (material retained on sieve 30) were weighed and pre-mixed. The MWCNTs used were provided by the Laboratory of Nanomaterials of the Department of Physics - ICEX (UFMG). These are multi-walled carbon nanotubes (MWCNTs), type HP-2627, obtained by the chemical vapor deposition method, with a mass of 50,000 mg and a purity of over 93%. The concentrations of MWCNTs chosen for addition to the geopolymers are 0.1, 0.3, and 0.5% by mass of metakaolin (m/m). Table 1 Chemical Composition of Metakaolin Oxides Quantity (%) Al 2 O 3 53.436 SiO 2 45.731 Fe 2 O 3 0.448 K 2 O 0.116 SO 3 0.060 CaO 0.058 TiO 2 0.010 Table 2 Granulometric Characteristics of Normal Sand Sieve ABNT Aperture (mm) Accumulated retained material (%) Limits- NBR-7214/82 8 2.4 0 0 10 2 5 5 ± 5 16 1.2 30 25 ± 5 30 0.6 52 50 ± 5 50 0.3 74 75 ± 5 100 0.15 98 97 ± 3 2.2 CNT dispersion To disperse the CNT in the metakaolin, absolute ethanol (99.0%) was used as the dispersant medium. The CNT contents (0.1%, 0.3%, and 0.5%) were weighed and added to ethanol, then subjected to ultrasonic bath for 1 hour at a frequency of 60 Hz. Next, the metakaolin was added to the system (CNT + ethanol) and subjected to ultrasonic treatment for 30 minutes, followed by magnetic stirring for 1 hour. Finally, the mixture was allowed to settle until complete decantation of the metakaolin, and the upper phase (ethanol) was removed. The MWCNT-containing metakaolin was then placed in an oven at a controlled temperature of 80°C until complete evaporation of the ethanol. Figure 1 shows the sequence for adding CNT to metakaolin. 2.3 Geopolymer production The definition of molar ratios in geopolymers are activation parameters that must be followed for the production of materials with adequate mechanical strength and durability. Based on the molar ratios established in previous studies [ 21 ] and once the chemical composition of the raw materials has been determined, the quantity of each constituent in the geopolymerization process can be calculated. It was determined that the composition of the geopolymeric matrix should comply with the following ratios: Solution/Solids: 1.30; Na 2 SiO 3 / NaOH (by mass) in the alkaline solution: 2.0 and the fine aggregate, which is inert like in Portland cement matrices, was decided to be maintained in this work at a 2:1 mass ratio between fine aggregate and binder (metakaolin). Four geopolymer formulations were produced, reference (0% CNT); 0.1% CNT; 0.3% CNT; and 0.5% CNT. For the production of geopolymer with the aid of a mechanical stirrer, the metakaolin was incorporated in small quantities into the activating solution until complete. The paste was mixed continuously and then the sand was added with constant stirring until complete homogenization of the mortar. Test specimens were molded in a silicone mold with cylindrical shape in the dimensions (ø 20 x 55) mm and prismatic test specimens with dimensions (40x40x160) mm. After molding, the forms were vibrated, and then several clusters were visible as spots and dark points corresponding to carbon nanotubes on the surface of the geopolymer samples (Fig. 2 ). The material was cured for 24 hours at a temperature of 60°C and another 24 hours at room temperature. 2.4 Characterization of Dispersion Quality After dispersing the CNTs in metakaolin, scanning electron microscopy (SEM) images and Fourier-transform infrared spectroscopy (FTIR) analysis were performed to evaluate the quality of dispersion. 2.5 Mechanical tests After 48 hours of curing, compression tests were carried out on the cylindrical specimens, with 6 specimens being tested for each formulation. The results were expressed as the mean values and their standard deviation. Flexural strength tests were also carried out after 48 hours of curing, with 3 specimens being tested for each formulation, and the results were presented as the mean values and their standard deviation. 3. RESULTS 3.1 Infrared Spectroscopy Infrared Spectroscopy (FTIR) is a simple and rapid instrumental technique that is used to demonstrate the presence of various functional groups based on the vibrations and rotations of the bonds between atoms of a molecule, with these vibrations being specific to each type of chemical bond providing the peaks and frequency bands of stretching [ 22 ][ 23 ] [ 24 ]. The FTIR analysis was used to analyze the behavior of the metakaolin after the dispersion process using absolute ethyl alcohol, aiming to evaluate if the use of ethyl alcohol as a dispersant for the CNTs would cause any alteration in the characteristics of the metakaolin. Figure 3 shows the spectrum obtained from metakaolin samples with dispersed CNTs and a reference sample. The typical spectrum of metakaolin shows bands between 3450 cm − 1 to 1650 cm − 1 , coming from the OH group present in the adsorbed water from the atmosphere, a stretching band at 1088 cm-1 from the Si-O bond, a vibration band at 810 cm − 1 from the Si-O-Al bond, and a vibration band at 450 cm − 1 for the Al-O bond [ 25 ] [ 26 ]. Figure 3 presents the FTIR analysis of the samples with dispersed CNTs using absolute ethyl alcohol, where the spectra of pure metakaolin (0.0% CNT), metakaolin with the addition of 0.1% CNT, metakaolin with the addition of 0.3% CNT, and metakaolin with the addition of 0.5% CNT show the same characteristics. In all samples, bands between 3450 cm − 1 to 1650 cm − 1 (1), a stretching band at 1088 cm − 1 (2), a vibration band at 810 cm − 1 (3), and a vibration band at 450 cm − 1 (4), as mentioned above, are observed, which are typical bands of metakaolin. Based on the spectral characteristics of metakaolin, it is concluded that the dispersion method used does not cause any changes in it, indicating that the use of ethyl alcohol as a dispersing medium has been effective. 3.2 Scanning electron microscopy (SEM) One of the greatest challenges in using CNTs as an additive in materials is their difficult dispersion. Finding a method that enables their incorporation is of great importance, as better dispersion has a direct influence on the efficiency of CNTs and the incidence of defects in the structure. After incorporating the CNTs into the metakaolin, images were taken with the aim of evaluating the effectiveness of the method used. The images of metakaolin with added CNT obtained by scanning electron microscopy (SEM) are shown in Figs. 4 to 6 , respectively for contents of 0.1, 0.3, and 0.5% of CNT, with the grouped images taking into consideration the content of CNT dispersed in the metakaolin. Analyzing the images obtained in the SEM assay, it is noted that the nanocomposite is present in all points of the sample, presenting a good distribution in the metakaolin. However, Figs. 5 and 6 present a greater quantity of CNT agglomerates, indicating a weak dispersion in the samples with the addition of 0.3 and 0.5% of CNT. This fact can be attributed to an insufficient sonication time for the alcohol + CNT system, so that as the amount of CNT to be incorporated into the sample increases, the difficulty of dispersion also increases. 3.2.1 Evidence of reinforcement in Geopolymer Figure 7 displays SEM images of a CNT reinforced geopolymer sample taken immediately after the compressive strength test, where the fracture is fully traversed by a series of CNTs. Analysis of the images leads to the conclusion that the CNTs are well distributed in the sample, as they are isolated on the fracture surface and without the formation of clusters, indicating that the dispersion method used was appropriate. It can also be observed that the CNTs were not torn out after the compression test, suggesting good connectivity between the CNTs and the matrix [ 3 ]. 3.3 Compression Strength Test of Geopolymers with CNTs Figure 8 presents the results obtained in the compressive strength test of the geopolymer specimens. The analysis of the results showed that the addition of CNT to the geopolymer contributes to an increase in the compressive strength in relation to the reference geopolymer. It is also important to note that the best compressive strength results were obtained for the formulation with the lowest CNT content, 0.1%. Figure 8 also indicates that, for the 0.1% CNT concentration, the significant increase in compressive strength (~ 40%) accompanied by a smaller standard deviation presents a distribution of values closer to the mean found, while in samples with higher concentrations, the results obtained tend to have greater variation. It can be said that the addition of CNT has a favorable effect on compressive strength, with better performance directly linked to the better dispersion factor of CNT in metakaolin and the amount of nanocomposites added. CNT agglomeration may be the main influencing factor in the reduction of mechanical properties, where higher CNT contents may require a longer dispersion time. This situation is also observed in Portland cement matrices. According to the authors, high levels of CNT (greater than 0.5% CNT) cause poor distribution and severe agglomeration, causing points of matrix fragility. [ 3 ] [ 17 ] [ 27 ] [ 28 ]. 3.4 Flexural Strength Test of Geopolymers with CNT The results obtained from the bending test on rectangular specimens (40x40x160) mm for the reference geopolymer and the geopolymer with varying additions of CNT (0.1%, 0.3%, and 0.5%) are presented in Fig. 9 . Upon analysis, all samples exhibited an average increase in flexural strength compared to the reference, with an approximate 10% increase observed for CNT contents of 0.1% and 0.3%, and a 15% increase observed for samples with the addition of 0.5% CNT. However, in comparison to the results obtained in the Compressive Strength Test, the increase observed is not as significant. This may be attributed to the agglomeration of CNT on the surface of the sample, as depicted in Fig. 2 . Achieving effective reinforcement necessitates uniform dispersion, which is the key factor. A higher degree of homogeneity in the dispersion of CNTs enables them to act more effectively as reinforcement for geopolymers [ 18 ][ 20 ][ 28 ]. 4. CONCLUSION It was evidenced that the dispersion method used to add CNT to metakaolin was satisfactory, and the use of ethanol as a dispersing medium was interesting, as it does not alter the properties of metakaolin, does not react with it, and after use, its recovery is almost complete. The compression tests demonstrated that the incorporation of CNT in the geopolymer contributed beneficially to the compressive strength. Composites with 0.1% CNT presented better results, with an increase of approximately 40% in strength compared to the reference, indicating that better results are directly linked to a better dispersion and incorporation of the material. As for the flexural behavior, the best result was observed for composites with higher CNT content (0.5%), with a 15% increase in strength compared to the reference. 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Preparation of macroporous ceramic from metakaolinite-based geopolymer by calcination , Ceramics International, Volume 41, 2015, Pages 11177-11183. https://doi.org/10.1016/j.ceramint.2015.05.067. ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 5739:2007 - Concreto - Ensaios de compressão de corpos-de-prova cilíndricos Rio de Janeiro ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 12142:2010 - Concreto : Determinação da resistência à tração na flexão de corpos de prova prismáticos Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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R. Borges","email":"","orcid":"","institution":"Federal Center for Technological Education of Minas Gerais","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Paulo","middleName":"H. R.","lastName":"Borges","suffix":""},{"id":198985418,"identity":"2689f0e0-3ec3-4a73-a5a2-daee4c06a5f0","order_by":2,"name":"Peter Ludvig","email":"","orcid":"","institution":"Federal Center for Technological Education of Minas Gerais","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Peter","middleName":"","lastName":"Ludvig","suffix":""},{"id":198985419,"identity":"2b95740f-7eb7-445a-a3e3-cd33c940062e","order_by":3,"name":"Michelle Colão","email":"","orcid":"","institution":"Federal University of Rio de Janeiro","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Michelle","middleName":"","lastName":"Colão","suffix":""}],"badges":[],"createdAt":"2023-05-09 00:44:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2909512/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2909512/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":37040164,"identity":"7673a413-a527-423a-9f27-c653a08535ce","added_by":"auto","created_at":"2023-05-15 14:35:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":392411,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDispersion Sequence of CNT in Metakaolin\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/1fe0b34bc30679628d7e7b93.png"},{"id":37038231,"identity":"d83b8ed2-7b62-4618-bf70-6513f09d2a86","added_by":"auto","created_at":"2023-05-15 14:19:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":447369,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCNT points on the surface of the samples.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/2ea996199dd480423ebaeb88.png"},{"id":37039310,"identity":"99e75191-6eff-49cd-a02c-7ec0bd74bfcf","added_by":"auto","created_at":"2023-05-15 14:27:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":94369,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eInfrared spectroscopy analysis of the Metakaolin samples.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/0b6f4ea8212839c9cddc30d6.png"},{"id":37036928,"identity":"cd258fd1-ac57-4c7c-a880-b030dd719317","added_by":"auto","created_at":"2023-05-15 14:11:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":431148,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMetakaolin with 0.1% CNT.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/112f65de2d8c78d3ddbc037b.png"},{"id":37036920,"identity":"0096d31c-c937-4161-89f9-bf15f37bd7ca","added_by":"auto","created_at":"2023-05-15 14:11:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":265825,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMetakaolin with 0.3% CNT.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/2d449f1c6d68edcc14c667a4.png"},{"id":37038232,"identity":"d6134a1c-3f48-4f3c-b235-d01671593cac","added_by":"auto","created_at":"2023-05-15 14:19:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":268393,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMetakaolin with 0.5% CNT.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/7e7ed847c76c58137335f879.png"},{"id":37036923,"identity":"00379f77-a925-4660-8370-2e695b07a35e","added_by":"auto","created_at":"2023-05-15 14:11:22","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":89365,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMicrocrack with CNT.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/573653f14d05c11afcdaf6ef.png"},{"id":37036927,"identity":"59365e3e-329f-44d7-b1b5-c2326af49cc0","added_by":"auto","created_at":"2023-05-15 14:11:22","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":14882,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCompressive strength of geopolymer specimens with adition of CNT\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/7c7bc433f1c246197c2f5f24.png"},{"id":37036922,"identity":"50f6a55e-a48d-4441-a49f-59dc9402770e","added_by":"auto","created_at":"2023-05-15 14:11:22","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":12985,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFlexural strength of geopolymer specimens with adition of CNT\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/b8d9e66e7179e2c9e88a07df.png"},{"id":60822064,"identity":"44034551-d91b-420f-b469-adfa5253c021","added_by":"auto","created_at":"2024-07-22 13:14:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4130485,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2909512/v1/4ae34935-248f-4648-8b1a-b510bdc7b524.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Geopolymers based on Metakaolin reinforced with Carbon Nanotubes","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003ePortland cement is widely recognized as one of the most important materials in the construction industry. However, its manufacturing process is associated with high energy consumption and carbon dioxide (CO2) emissions that contribute to global warming. In light of the urgent need to reduce emissions and promote sustainability in construction, it is crucial to identify alternative binders that can replace Portland cement. As the demand for construction materials continues to grow, finding an alternative binding material to replace Portland cement and promote sustainability in construction becomes increasingly important. To reduce its consumption, given the significant exploitation of natural resources and the emissions during its production, studies have been carried out to explore new materials that may serve as alternatives to Portland cement. Among these materials, geopolymers have emerged as a promising new construction technology with the potential to facilitate green building practices. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eDeveloping materials through alkaline activation has become of great interest in current research; thus, geopolymers emerge as a promising substitute due to their significant benefits. The main advantages of using geopolymers over Portland cement can be cited as better profitability, better chemical stability, corrosion resistance, high initial strength, and high CO\u003csub\u003e2\u003c/sub\u003e reduction capacity, as their production does not result in significant emissions, making them less harmful to the environment [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGeopolymers are part of a class of materials with great technological potential; they are inorganic polymers produced through chemical reactions in highly alkaline media, presenting good chemical stability, corrosion resistance, and high initial strength [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Producing new materials through polymerization reactions represents a technological breakthrough, with the opening of new applications and procedures, where techniques that previously required high temperatures are not as necessary to obtain ceramic materials [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGeopolymers can be synthesized by mixing an alkaline solution (usually a soluble silicate) and one or more solid aluminosilicates, with the geopolymers forming at ambient or slightly elevated temperatures [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Any pozzolanic compound or source of silica and alumina that dissolves in an alkaline solution can act as a precursor source for a geopolymer. A chemical activator, usually consisting of hydroxide and silicate solutions, is necessary to initiate the geopolymerization reaction [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The presence of soluble silicate complements the amount of SiO\u003csub\u003e2\u003c/sub\u003e needed for the formation of the geopolymers. Although aluminosilicate compositions contain SiO\u003csub\u003e2\u003c/sub\u003e, the amount is not sufficient to obtain geopolymers with the best mechanical properties and durability. Moreover, the presence of soluble silica accelerates the reactions, favoring the polymerization process and contributing more Si to obtain a product with higher strength at early ages [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDue to the type of bonding that occurs in its structure, the geopolymer can present defects in the form of pores and voids, making the structure fragile and prone to fracture. This brittle character is quite similar to other ceramic materials. In order to achieve improvement in properties and structural applications, reinforcements such as different types of fibers have been investigated and added to increase strength and control crack propagation [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCarbon nanotubes (CNTs) synthesized and observed by physicist Iijima in 1991 are considered one of the most promising materials due to their excellent properties and unique carbon structure [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. They consist of long sheets of graphene rolled up and can be classified according to the number of sheets, single-walled carbon nanotubes (SWCNTs) having a single sheet and multi-walled carbon nanotubes (MWCNTs) with two or more sheets. These materials are important due to their wide range of scientific and technological applications, which depend on their properties that vary according to their structure, diameter, length, purity, and structural defects [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. CNTs have been considered as a potential reinforcement in composites because they have mechanical, electrical, chemical, and thermal properties superior to traditional fibers [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe properties of CNTs are determined according to their morphology and dimensions. Currently, they are the materials with the highest known hardness and strength, which are a consequence of their perfect structural arrangement and the strength of the bonds between their atoms, making them one of the most beneficial nanomaterials for reinforcement. Their unique mechanical, electrical, and chemical properties make them an attractive candidate for reinforcing composite materials. This is due to their flexible nature, which does not damage their structure when subjected to large stresses [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. With significant mechanical advantages, CNTs can be used as reinforcement in cementitious matrices, as they can provide significant improvements in mechanical properties and crack propagation resistance [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDue to the strong bond between their molecules, achieving the desired level of dispersion of CNTs becomes difficult, as they tend to agglomerate and form bundles in the mixtures. Insufficient dispersion has been a key factor in reducing the performance of cementitious composites containing CNTs; since ineffective dispersion leads to the formation of various defects in the composite, limiting its mechanical properties. On the other hand, better dispersion can result in a larger interfacial contact area between CNTs and the matrix, as well as more uniformly distributed stresses, requiring a pre-treatment in liquid dispersions [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThus, techniques such as carboxylation process and the use of dispersing agents in combination with sonication have been employed to improve the bonding between CNT and cement. In geopolymer, the addition of CNT to the alkaline solution of sodium hydroxide and sodium silicate is not satisfactory as agglomeration occurs, whereas the use of superplasticizers together with sonication shows effective results for the dispersion of CNT in matrices [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Therefore, the objective of this study is to analyze the effect of CNT addition on the mechanical properties of geopolymer produced with different nanocomposite additions, as well as to evaluate the efficiency of the dispersion method used for the incorporation of the material.\u003c/p\u003e"},{"header":"2. MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eMetakaolin was used as a source of silica and alumina to prepare the geopolymers, and its chemical composition (oxide percentages) is given in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The activating solution used was composed of solutions of 14 M sodium hydroxide and a sodium silicate solution with a chemical composition of 31.79% SiO\u003csub\u003e2\u003c/sub\u003e, 15% Na\u003csub\u003e2\u003c/sub\u003eO, and 53.21% H\u003csub\u003e2\u003c/sub\u003eO. The fine aggregate used was Brazilian normal sand, produced by IPT - Instituto de Pesquisa Tecnol\u0026oacute;gicas, according to ABNT 7214/82, with its characteristics presented in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. For the production of geopolymers, 50% of fine sand (material retained on sieve 50) and 50% of medium-fine sand (material retained on sieve 30) were weighed and pre-mixed. The MWCNTs used were provided by the Laboratory of Nanomaterials of the Department of Physics - ICEX (UFMG). These are multi-walled carbon nanotubes (MWCNTs), type HP-2627, obtained by the chemical vapor deposition method, with a mass of 50,000 mg and a purity of over 93%. The concentrations of MWCNTs chosen for addition to the geopolymers are 0.1, 0.3, and 0.5% by mass of metakaolin (m/m).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical Composition of Metakaolin\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOxides\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eQuantity (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e53.436\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45.731\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.448\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.060\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.058\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.010\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eGranulometric Characteristics of Normal Sand\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSieve ABNT\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAperture (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAccumulated retained material (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLimits- NBR-7214/82\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e75\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e97\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 CNT dispersion\u003c/h2\u003e \u003cp\u003eTo disperse the CNT in the metakaolin, absolute ethanol (99.0%) was used as the dispersant medium. The CNT contents (0.1%, 0.3%, and 0.5%) were weighed and added to ethanol, then subjected to ultrasonic bath for 1 hour at a frequency of 60 Hz. Next, the metakaolin was added to the system (CNT\u0026thinsp;+\u0026thinsp;ethanol) and subjected to ultrasonic treatment for 30 minutes, followed by magnetic stirring for 1 hour. Finally, the mixture was allowed to settle until complete decantation of the metakaolin, and the upper phase (ethanol) was removed. The MWCNT-containing metakaolin was then placed in an oven at a controlled temperature of 80\u0026deg;C until complete evaporation of the ethanol. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the sequence for adding CNT to metakaolin.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Geopolymer production\u003c/h2\u003e \u003cp\u003eThe definition of molar ratios in geopolymers are activation parameters that must be followed for the production of materials with adequate mechanical strength and durability. Based on the molar ratios established in previous studies [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] and once the chemical composition of the raw materials has been determined, the quantity of each constituent in the geopolymerization process can be calculated. It was determined that the composition of the geopolymeric matrix should comply with the following ratios: Solution/Solids: 1.30; Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e / NaOH (by mass) in the alkaline solution: 2.0 and the fine aggregate, which is inert like in Portland cement matrices, was decided to be maintained in this work at a 2:1 mass ratio between fine aggregate and binder (metakaolin). Four geopolymer formulations were produced, reference (0% CNT); 0.1% CNT; 0.3% CNT; and 0.5% CNT. For the production of geopolymer with the aid of a mechanical stirrer, the metakaolin was incorporated in small quantities into the activating solution until complete. The paste was mixed continuously and then the sand was added with constant stirring until complete homogenization of the mortar. Test specimens were molded in a silicone mold with cylindrical shape in the dimensions (\u0026oslash; 20 x 55) mm and prismatic test specimens with dimensions (40x40x160) mm. After molding, the forms were vibrated, and then several clusters were visible as spots and dark points corresponding to carbon nanotubes on the surface of the geopolymer samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The material was cured for 24 hours at a temperature of 60\u0026deg;C and another 24 hours at room temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Characterization of Dispersion Quality\u003c/h2\u003e \u003cp\u003eAfter dispersing the CNTs in metakaolin, scanning electron microscopy (SEM) images and Fourier-transform infrared spectroscopy (FTIR) analysis were performed to evaluate the quality of dispersion.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Mechanical tests\u003c/h2\u003e \u003cp\u003eAfter 48 hours of curing, compression tests were carried out on the cylindrical specimens, with 6 specimens being tested for each formulation. The results were expressed as the mean values and their standard deviation. Flexural strength tests were also carried out after 48 hours of curing, with 3 specimens being tested for each formulation, and the results were presented as the mean values and their standard deviation.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULTS","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Infrared Spectroscopy\u003c/h2\u003e \u003cp\u003eInfrared Spectroscopy (FTIR) is a simple and rapid instrumental technique that is used to demonstrate the presence of various functional groups based on the vibrations and rotations of the bonds between atoms of a molecule, with these vibrations being specific to each type of chemical bond providing the peaks and frequency bands of stretching [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e][\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The FTIR analysis was used to analyze the behavior of the metakaolin after the dispersion process using absolute ethyl alcohol, aiming to evaluate if the use of ethyl alcohol as a dispersant for the CNTs would cause any alteration in the characteristics of the metakaolin.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the spectrum obtained from metakaolin samples with dispersed CNTs and a reference sample. The typical spectrum of metakaolin shows bands between 3450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, coming from the OH group present in the adsorbed water from the atmosphere, a stretching band at 1088 cm-1 from the Si-O bond, a vibration band at 810 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e from the Si-O-Al bond, and a vibration band at 450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the Al-O bond [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the FTIR analysis of the samples with dispersed CNTs using absolute ethyl alcohol, where the spectra of pure metakaolin (0.0% CNT), metakaolin with the addition of 0.1% CNT, metakaolin with the addition of 0.3% CNT, and metakaolin with the addition of 0.5% CNT show the same characteristics. In all samples, bands between 3450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (1), a stretching band at 1088 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (2), a vibration band at 810 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (3), and a vibration band at 450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (4), as mentioned above, are observed, which are typical bands of metakaolin. Based on the spectral characteristics of metakaolin, it is concluded that the dispersion method used does not cause any changes in it, indicating that the use of ethyl alcohol as a dispersing medium has been effective.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Scanning electron microscopy (SEM)\u003c/h2\u003e \u003cp\u003eOne of the greatest challenges in using CNTs as an additive in materials is their difficult dispersion. Finding a method that enables their incorporation is of great importance, as better dispersion has a direct influence on the efficiency of CNTs and the incidence of defects in the structure. After incorporating the CNTs into the metakaolin, images were taken with the aim of evaluating the effectiveness of the method used.\u003c/p\u003e \u003cp\u003eThe images of metakaolin with added CNT obtained by scanning electron microscopy (SEM) are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e to \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, respectively for contents of 0.1, 0.3, and 0.5% of CNT, with the grouped images taking into consideration the content of CNT dispersed in the metakaolin. Analyzing the images obtained in the SEM assay, it is noted that the nanocomposite is present in all points of the sample, presenting a good distribution in the metakaolin. However, Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e present a greater quantity of CNT agglomerates, indicating a weak dispersion in the samples with the addition of 0.3 and 0.5% of CNT. This fact can be attributed to an insufficient sonication time for the alcohol\u0026thinsp;+\u0026thinsp;CNT system, so that as the amount of CNT to be incorporated into the sample increases, the difficulty of dispersion also increases.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Evidence of reinforcement in Geopolymer\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e displays SEM images of a CNT reinforced geopolymer sample taken immediately after the compressive strength test, where the fracture is fully traversed by a series of CNTs. Analysis of the images leads to the conclusion that the CNTs are well distributed in the sample, as they are isolated on the fracture surface and without the formation of clusters, indicating that the dispersion method used was appropriate. It can also be observed that the CNTs were not torn out after the compression test, suggesting good connectivity between the CNTs and the matrix [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Compression Strength Test of Geopolymers with CNTs\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the results obtained in the compressive strength test of the geopolymer specimens. The analysis of the results showed that the addition of CNT to the geopolymer contributes to an increase in the compressive strength in relation to the reference geopolymer. It is also important to note that the best compressive strength results were obtained for the formulation with the lowest CNT content, 0.1%. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e also indicates that, for the 0.1% CNT concentration, the significant increase in compressive strength (~\u0026thinsp;40%) accompanied by a smaller standard deviation presents a distribution of values closer to the mean found, while in samples with higher concentrations, the results obtained tend to have greater variation. It can be said that the addition of CNT has a favorable effect on compressive strength, with better performance directly linked to the better dispersion factor of CNT in metakaolin and the amount of nanocomposites added. CNT agglomeration may be the main influencing factor in the reduction of mechanical properties, where higher CNT contents may require a longer dispersion time. This situation is also observed in Portland cement matrices. According to the authors, high levels of CNT (greater than 0.5% CNT) cause poor distribution and severe agglomeration, causing points of matrix fragility. [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Flexural Strength Test of Geopolymers with CNT\u003c/h2\u003e \u003cp\u003eThe results obtained from the bending test on rectangular specimens (40x40x160) mm for the reference geopolymer and the geopolymer with varying additions of CNT (0.1%, 0.3%, and 0.5%) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Upon analysis, all samples exhibited an average increase in flexural strength compared to the reference, with an approximate 10% increase observed for CNT contents of 0.1% and 0.3%, and a 15% increase observed for samples with the addition of 0.5% CNT. However, in comparison to the results obtained in the Compressive Strength Test, the increase observed is not as significant. This may be attributed to the agglomeration of CNT on the surface of the sample, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Achieving effective reinforcement necessitates uniform dispersion, which is the key factor. A higher degree of homogeneity in the dispersion of CNTs enables them to act more effectively as reinforcement for geopolymers [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e][\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e][\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. CONCLUSION","content":"\u003cp\u003eIt was evidenced that the dispersion method used to add CNT to metakaolin was satisfactory, and the use of ethanol as a dispersing medium was interesting, as it does not alter the properties of metakaolin, does not react with it, and after use, its recovery is almost complete. The compression tests demonstrated that the incorporation of CNT in the geopolymer contributed beneficially to the compressive strength. Composites with 0.1% CNT presented better results, with an increase of approximately 40% in strength compared to the reference, indicating that better results are directly linked to a better dispersion and incorporation of the material. As for the flexural behavior, the best result was observed for composites with higher CNT content (0.5%), with a 15% increase in strength compared to the reference. For the other contents, the improvement in results reached 12% for both cases. Thus, even though the properties of CNT do not allow for good distribution and homogeneity, its incorporation in the geopolymer led to an improvement in mechanical properties, especially in the case of compressive strength.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHASSAN, A., Arif, M. \u0026amp; Shariq, M. \u003cstrong\u003eEffect of curing condition on the mechanical properties of fly ash-based geopolymer concrete\u003c/strong\u003e. \u003cem\u003eSN Appl. 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The improvement of strength and microstructural properties of fly ash-based geopolymer by adding elemental aluminum powder\u003c/strong\u003e. \u003cem\u003eJ Mater Cycles Waste Manag\u003c/em\u003e \u003cstrong\u003e25\u003c/strong\u003e, 157\u0026ndash;170 (2023). https://doi.org/10.1007/s10163-022-01520-8\u003c/li\u003e\n\u003cli\u003eSAAFI, Mohamed; ANDREW, Kelly; TANG, Pik Leung; MCGHON, David; TAYLOR, Steven; RAHMAN, Mahubur; YANG, Shangtong; ZHOU, Xiangming. \u003cstrong\u003eMultifunctional properties of carbon nanotube/ fly ash geopolymeric nanocomposites. \u003c/strong\u003eConstruction and Building Materials, n 49, 46-55.2013. https://doi.org/10.1016/j.conbuildmat.2013.08.007\u003c/li\u003e\n\u003cli\u003eSAAFI, Mohamed; TANGB, Leung; FUNGA, Jason; RAHMANA, Mahbubur; LIGGATC, John. \u003cstrong\u003eEnhanced properties of graphene/fly ash geopolymeric composite cement. \u003c/strong\u003eCement and Concrete Research, n 67, 292-299. 2015. 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It is an inorganic polymer that has environmental advantages in its production, in addition to being considered a high-performance material. This study seeks to evaluate the mechanical properties of geopolymeric mortars reinforced with Carbon Nanotubes (CNT) as well as the method of incorporation into the mortar. CNTs are promising materials as reinforcements in geopolymeric matrices for the development of construction materials with specific applications aimed at better performance. The studied matrix results from the alkaline activation of metakaolin (MK) with sodium silicate and sodium hydroxide. CNTs were added in percentages of 0.1%, 0.3%, and 0.5% by mass of metakaolin. The evaluated properties for the resulting composite were compressive strength and flexural strength. Scanning Electron Microscopy (SEM) and Infrared Spectroscopy (FTIR) tests were carried out to evaluate the dispersion of CNTs in MK (before activation). The results show that when well distributed in the matrix, CNTs promote an increase in compressive and flexural strength of the samples.\u003c/p\u003e","manuscriptTitle":"Geopolymers based on Metakaolin reinforced with Carbon Nanotubes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-05-15 14:11:17","doi":"10.21203/rs.3.rs-2909512/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3990d9b4-ce64-4d21-ba16-25cf1783ea92","owner":[],"postedDate":"May 15th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-22T13:06:43+00:00","versionOfRecord":[],"versionCreatedAt":"2023-05-15 14:11:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2909512","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2909512","identity":"rs-2909512","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}