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The thermal behavior of a model MK based K‐geopolymer (Si/Al = 1.38 and K/Al = 0.68; obtained by alkaline activation of a very pure metakaolin) was investigated between room temperature and 1400°C in order to evaluate its potentiality for high‐temperature applications. The purpose of our study was to monitor the behavior of a geopolymer during a te...
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The thermal behavior of a model MK‐based K‐geopolymer was investigated between room temperature and 1400°C in order to evaluate its potentiality for high‐temperature applications. The purpose of our study was to monitor the behavior of a geopolymer during a temperature rise in order to better understand its variations with respect to temperature. T...
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... Using 2% NK results in an enhancement for the matrix morphology and filling most of the formed pores where it acts as nucleation sites for accumulation [48,49] of the geopolymer constituents leading to the formation of three dimensional network (Fig. 8b). Increasing NK to 3% leads to formation of 6 Bulk density of alkali activated specimens enhanced with mixed ratio of nano materials, a nano-glass and nano-kaolin, b nano-silica fume and nano-kaolin agglomerates from NK and N-glass forming wide voids between the matrix binder and so hinder the formation of the geopolymer network as indicated from the micrograph (Fig. 8C) [51,52]. ...
Influence of various types of nano powder on the physico-mechanical properties of geopolymer materials has been studied, in addition to studying their firing stability up to 1000 °C. Alumino–silicate materials used are kaolin, fired kaolin and lime stone. Materials prepared at water/binder ratios of 0.40; whereas the used equal volume of activator 5 M sodium hydroxide with liquid sodium silicate. Nano-kaolin admixed with Nano-powder as a partial replacement from metakaolin material. The control mixes incorporating either 7% Nano-glass or 5% Nano-silica fume. Nano-kaolin was partially replaced Nano-glass powder and Nano-silica fume. It is possible to use the mixes containing limestone and nano materials to solve the problem of using heat curing, thermal energy consumption and pollution by reducing the MK used for MK-geopolymer cement. Results indicated an enhancement in the physico-mechanical properties of mix incorporating 1: 6% and 2:3% of NK:N-glass and NK:N-silica fume, respectively. Firing of hardened geopolymer resulted in high thermal resistance up to 700 °C then exposed to decrease up to 1000 °C. However, no micro-cracks were noticed up to 800 °C for all samples as recorded by visual examination of the fired samples, while micro-cracks were recorded for hardened composites at 1000 °C.
Graphical Abstract
... Additionally, the formulation of geopolymer systems with a low molar ratio of SiO 2 /Al 2 O 3 [35] and the careful selection of the alkaline cation (Na + , K + , or Cs + ) present in the liquid reagents are important parameters to consider when designing binders with thermal stability [16][17][18]40,42,[47][48][49]. While the presence of alkaline oxides in refractory ceramics is typically controlled to maximize their refractoriness and minimize the formation of a liquid phase within the resulting structure under working conditions, some researchers have observed the crystallization of refractory phases from the geopolymeric matrix when heating these materials to high temperatures. ...
... While the presence of alkaline oxides in refractory ceramics is typically controlled to maximize their refractoriness and minimize the formation of a liquid phase within the resulting structure under working conditions, some researchers have observed the crystallization of refractory phases from the geopolymeric matrix when heating these materials to high temperatures. Examples of such crystallization include nepheline (NaAlSiO 4 ) [20,[36][37][38][39], leucite (KAlSi 2 O 6 ) [35,[40][41][42][43][44], and polucite (CsAlSi 2 O 3 ) [45,46]. ...
The development of cement‐free refractories has gained attention in recent years. Geopolymers (GP) are cold‐setting binders that may be applied in the manufacturing of such ceramics. However, a systemic comparison between calcium aluminate cement (CAC) and GP‐bonded formulations is still necessary to understand the strengths and limitations of these novel binders. In this study, three GP were designed and characterized, and the most promising composition was incorporated into an alumina refractory. A corresponding CAC‐containing formulation was also evaluated as a reference material. The physico‐mechanical properties of the prepared ceramics were analyzed after heat treatments ranging from 40–1250°C. Among the designed GP, the sodium‐containing binder showed the best performance, although its addition to alumina refractory resulted in cured samples with low mechanical strength. However, with the progression of liquid formation, sintering, and densification during firing at 1100–1250°C, the GP‐bonded refractory displayed enhanced flexural strength, surpassing the performance of the CAC‐containing composition.
... Marble is a metamorphic rock that forms when limestone is subjected to extreme heat and pressure. Where, Marble occurs under such conditions because the calcite in the limestone recrystallizes, resulting in a denser rock with about the same composition as the limestone [45]. ...
Influence of nano-glass powder on physico-mechanical properties of geopolymer composite materials has been studied, in addition to studying their firing stability against high temperature from 500 up to 1000 °C. For increasing mechanical, physical as well as firing stability for metakaolin composites; Nano-glass powder was used. Alumino-silicate materials are kaolin, fired kaolin (metakaolin) and limestone all pass 75 µm. Materials prepared at water/binder ratios in a range of 0.40; whereas the used activator 5 M sodium hydroxide with equal volume of liquid sodium silicate. Nano-glass powder added as a partial replacement for metakaolin material in the ratio of 1 up to 9%. Results indicated that compressive strengths of geopolymer mixes incorporating Nano glass powder were obviously higher than those of control one and increased up to 7% followed by slight decrease of on using 9%. Firing of hardened geopolymer results in high thermal resistance up to 500 °C then exposed to decrease up to 800 then increased again at 1000 °C for specimens incorporated 7 and 9% nano-glass powder. However, no micro-cracks were noticed up to 800 °C for all samples as recorded by visual examination of the fired samples.
... When concrete structures containing Portland cement are exposed to elevated temperatures, this leads to cracking, spelling and loss of strength for the concrete structure, due to the loss of calcium silicate hydrate (CSH) water at 105 • C onwards, the decomposition of calcium hydroxide (Ca(OH) 2 at 400-550 • C forming lime (CaO) and free water vapor; [7][8][9]. In contrast, in metakaolin-based activated alkaline geopolymers, the amorphous sodium aluminate silicate hydrates (N-A -S -H) structure has been reported to be stable at elevated temperatures up to 900 • C [10][11][12]; It crystallizes into nepheline at higher temperatures, which in some cases leads to improved physico-mechanical properties, as a result of closing and condensing the microstructure, [13,14]. In several researches the stability of alkaline activated geopolymer under high temperatures has been confirmed, e.g. ...
... These results are interpreted to the thermal decomposition of geopolymer hydration products with the appearance of new phases. Marble and nepheline are formed upon interaction of aluminosilicates residues from MK and free sodium cations from the activator at high temperature reactions, due to the transformation of amorphous N-A -S -H/N-(C)-A -S -H gels [12]. The disappearance of some phases and increasing the internal thermal stresses generated around pores that generate micro-cracks, and consequent thermal expansion of the specimens. ...
... The nepheline phase is formed as a result of the transformation of amorphous (N-A-S-H / N-(C) -A-S-H) gels, at higher temperatures; a reaction may occur between the aluminum silicate residue from metakaolin and the free sodium cations from the activator. [12]. The disappearance of some phases led to lower compressive strength of NSF5 with exposure the temperature up to 1000 ℃. ...
Thermal Stability of geopolymer at high temperatures has attracted great interest from several researchers. The use of nanosilica fumes gets more attention Because of its great influence on the mechanical properties and thermal stability of geopolymer composites. Therefore, this work reviewed the effect of Nano-silica fume on thermal stability and mechanical behaviors of geopolymers at high temperatures. The main geopolymer mixes made up of metakaolin having various nano-silica fume ratios in addition to kaolin and lime stone. The activators used are Na2SiO3 and NaOH (5 M) in the ratio of (1:1). The geopolymer mix control has been composed of metakaolin (MK), kaolin (K) and lime stone (LS) in the ratio of (2:1:1). Nano-silica fume (NSF) was added in the ratios from 0 to 7 wt% from the metakaolin. To realization investigate the effect of Nano-silica fume on MK geopolymer binder, classical tests of bulk density and strength at various ages. The results were analyzed by using advanced devices. The thermo-physical, micro-structural and mechanical properties of the MK geopolymers binder after the exposure to elevated temperatures of 500, 700, 800 and 1000 °C have been investigated. The results indicated that the NSF has a positive effect on the physicomechanical properties of MK-geopolymer. The inclusion of 5 % NSF to MK geopolymer binder enhanced the properties in terms of strength and microstructure. The mechanical strength of MK-geopolymer mixtures incorporating NSF increased with temperature up to 700 °C and increases with increasing NSF up to 5 %. The inclusion of 5 % NSF enhanced the physico-mechanical properties and microstructure before and after exposure to high temperature.
... Geo-polymers have distinguished physical, and chemical properties that reflected on the mechanical behavior such as high compressive strength and highly durability (Atiş et al., 2015;Muhammad et al., 2018), resist corrosive, and abrasion (Yuan et al., 2020), low water permeability, acid attack resistance (Mehta and Siddique, 2017), stability against fire, good electric insulator, thermally stable (Gomes et al., 2020). Some classes of geopolymers are chemical-resistats and biologically compatible (Catauro et al., Production of Geopolymer Materials from Solid Wastes of Water Treatment Plant and Alum Industry 2014), and some studies proved its ability to resist radiation penetration, and hence it can be used for radioactive nuclides capsulation (Soonthornwiphat et al., 2020). ...
In this study, the geopolymer concrete was produced from two by-product wastes as an alternative for environmentally green construction and building materials without using Ordinary Portland Cement (OPC). Water treatment sludge (WTS) was taken from Sheikhzaied, Marg and Obour Water Treatment Plants at Cairo, Egypt. Meta-Kaolin (MK) and De-Aluminated Meta-Kaolin (DaMK) wastes were collected from Aluminum Sulfate Co. of Egypt. The mineralogical and chemical compositions of the WTS, MK and DaMk were done using X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) analyses. Sodium hydroxide (NaOH) solution was used as an alkaline activator. The effect of the various influential factors on the geopolymer compressive strengths for different mixtures of WTS, DaMK and MK based was investigated. The 40% DaMK/MK mixtures were achieved the highest compressive strength (50 MPa). Moreover, the WTS and DaMK wastes that traditionally disposed into landfills or drainage canals can be sustainably used in developing and producing cement-free geopolymers with economic and environmental significance. The immobilization behaviors of Cd2+, Pb2+ and Hg2+ ions in a geopolymer based on the WTS and DaMK solid wastes from the Alum industry were investigated in this study. WTS/DaMK based geopolymer mortar has been tested for leaching to study its immobilization behavior of selected heavy metals under the optimum condition. The Cd2+, Pb2+ and Hg2+ ions were used and effectively immobilized in this study. The heavy metals with concentrations of 100, 200 and 300 ppm were used in the geopolymer matrix giving about 98% of immobilization efficiency. The study showed that Pb2+ has the best immobilization efficiency followed by Hg2+ and Cd2+ at high concentrations.
... Upon heating of portland cement-based materials, calcium silicate hydrate (C-S-H) starts to lose water from 105 • C onwards, while portlandite (Ca(OH) 2 ) decomposes at 400-550 • C forming lime (CaO) and water vapor [20,21]; this leads to porosity, cracking, spalling and loss of strength [20,22]. In contrast, in alkali-activated cements based on MK (AAC-MK, geopolymers) [23], the XRD-amorphous 3D network N-A-S-H structure has been reported as stable up to 700-900 • C [24][25][26]; at higher temperatures, it crystallizes into nepheline [24,27], which usually is accompanied by a densification of the microstructure, improving in some cases the mechanical properties [28]. The phase stability of alkali activated cements (AACs) under high temperatures has been confirmed in several reports by observations of XRD diffractograms, e.g. ...
... Duxon et al. [26,43] indicated after studies by FTIR that the viscous flow and collapse of the highly distributed pore network in the N-A-S-H contributed to densification of the gel in AAC-MK exposed to temperatures close to that of crystallization. On the other hand, studies on AAC based on MK [44] and fly ash/slag [25] reported variations in the 27 Al and 29 Si MAS NMR spectra after treating the specimens at temperatures below that of crystallization (400-900 • C); this involved shifting of the signals towards lower frequencies at 400 or 800 • C and shifting towards higher frequencies at higher temperatures. Such variations were interpreted as reorganization processes of the gel structure upon thermally induced reactions. ...
... After the HTR600, the resonance of both samples shifted by about 3 ppm towards lower frequencies and increased their full width at half height (FWHH) in about 10 ppm. Similar displacement towards a more shielded chemical shift was reported by Gomez et al. [25] and Park et al. [44] after comparable thermal exposures of AAC-MK and AAC of fly ash/slag, respectively, and was attributed to a condensation of the amorphous aluminosilicate framework. It is possible that during the heating, the Al-O bonds from the remnant MK were broken and the Al atoms were attracted towards water molecules to form Al-OH, which can then reconnect with Si-OH species forming Si-O-Al bonds. ...
Alkali-activated limestone/metakaolin (AAC-LSMK) pastes of Na2O/Al2O3 = 0.6 and 1.08 and LS/MK = 20/80 and 60/40 were exposed to 300, 600 and 900 °C, and the structural changes were studied. The exposure to 300 °C reduced 21–25% the weight of samples without affecting the structure and strength (∼50 MPa) of formulations with high LS/MK, and/or low Na2O/Al2O3. The exposure to 600 °C extended the reactions/dissolution of MK while 1–17% of the N-A-S-H/N-(C)-A-S-H was depolymerized due to a water attack on the stretched Si–O-T bonds, with bridging oxygens transformed into non-bridging oxygen, while limestone favored the formation of C-(A)-S-H; the residual strength was of 30 MPa. The exposure to 900 °C led to the formation of nepheline and decarbonization of the CaCO3, which significantly reduced the strength to residual values of up to 20 MPa. These findings bring new insights on the performance of the AAC-LSMK and the thermally induced structural changes, while highlighting their potential high-temperature applications.
... In the first part of the present project, 33,34 we presented a detailed description of the thermal variation, up to 1400°C, of a pure model metakaolin-based K-geopolymer. These two previous papers indicated the thermal stability of the geopolymer matrix up to a temperature of 700°C for its mesoporous properties, 34 and 900°C for its amorphous features. ...
... These two previous papers indicated the thermal stability of the geopolymer matrix up to a temperature of 700°C for its mesoporous properties, 34 and 900°C for its amorphous features. 33 Above these temperatures, the crystallization process begins and the mineral matrix densifies. These results made it possible to have a better understanding of the structure of a model geopolymer, and of its behavior during a rise in temperature. ...
... The samples used in this paper were based on the model K-geopolymers thoroughly investigated in the previous part of this project. 33,34 As a summary, the model geopolymer was obtained by the potassium silicate activation of a metakaolin of high purity -non-commercial product -supplied by Imerys (AGS quarrying, F-17270 Clérac, France). Alkali-activated materials in the present study were obtained by substituting x weight percent (wt %) of metakaolin by Callovo-Oxfordian argillite (COx) preheated at 750°C for 4 hours (with x = 0, J o u r n a l P r e -p r o o f 33, 67 and 100; i.e. from pure model K-geopolymer to purely K-activated COx argillite material). ...
Pure model geopolymers present a promising potential for high-temperature binders for structural or coating applications. However, their well-known chemical stability, which can reach 1400°C in the case of K-activated geopolymer, is accompanied by deleterious dimensional behavior, with significant shrinkage on heating. The work in the present paper focuses on the impact of tailored calcined argillite added to geomaterial formulations for fireproofing applications. Calcined argillite acts both as a filler and a reagent, with a supplementary delayed reaction activated by humidity and heat. K-activation of a mix of one-third metakaolin with two-thirds calcined argillite enables the shrinkage rate to be limited, and is accompanied by improved self-healing during viscous creep (allowing to preserve mechanical resistance at high temperature). This composite formulation preserves chemical stability, improves physical behavior, and maintains interesting mesoporosity for hydric properties. These different aspects are beneficial for a high temperature application.
... In the first part of this study, the thermal stability of the mineral matrix of a potassium-based geopolymer was highlighted up to a temperature of 900°C, before crystalizing into kalsilite and leucite. 30 In order to complete our investigations on the refractory potential of such a geopolymer, we are focusing in the present paper on the behavior of the porous part of the material when subjected to a temperature increase: the pore dimensions in volume and radius, and also its ability for rehydration. It is well accepted that the polycondensation of silicate and aluminate species, at the origin of the formation mechanism of geopolymers, enables a porous material to be obtained with an alkaline residual solution contained in the pores. ...
... The samples used in this paper were exactly those previously prepared and characterized in part 1 of this study. 30 As a summary, the geopolymer was obtained by the potassium silicate activation of a metakaolin of high purity-noncommercial product-supplied by Imerys (AGS quarrying, F-17270 Clérac). The samples were called T RH with T the heat treatment temperature after geopolymer preparation (25,100,200,300,450, 600, 700, 800, 900, 1000, 1100, 1250, or 1400) and relative humidity (RH) the storing conditions after heat treatment ("95" for 95% RH, "30" for 30% RH or "bak" for the baked powders), making a total of 37 samples. ...
... As previously observed for the R BJH average pore size, there is only very little change in the specific surface area up to 800°C, with a value of around S BET = 80 m 2 /g. The only significate variation is observed between the 25 30 and 100 30 samples (S BET = 112 m 2 /g), with potentially erroneous calculated values for the as-prepared 25 30 geopolymer due to an incomplete evacuation of water from the porous network. Indeed, in order not to misrepresent the porous characteristics of this untreated sample, degassing was performed at room temperature. ...
The thermal behavior of a model MK‐based K‐geopolymer was investigated between room temperature and 1400°C in order to evaluate its potentiality for high‐temperature applications. The purpose of our study was to monitor the behavior of a geopolymer during a temperature rise in order to better understand its variations with respect to temperature. The works from the present paper focus only changes in the porous network; it follows a first part devoted to variations in the mineral matrix. The results obtained here show that the geopolymer material preserves its porous integrity up to 800°C, while maintaining the reversibility of water exchanges corresponding to about 25 weight percent. Together with the results of part 1, the findings of this study allow us to affirm that geopolymer materials are only very little affected by temperatures up to 800°C, or even 900°C (keeping its mesoporous amorphous structure).
Alternative binders represent a promising avenue for the development of ultra-low and cement-free refractories, significantly reducing CO2 emissions and the energy consumption associated with these products. Geopolymers (GP) are sustainable binders that can be utilized in the manufacture of these ceramics. However, a comprehensive comparison of systems containing calcium aluminate cement (CAC) or GP is necessary to identify the advantages and limitations of these new materials. In this study, the synthesis, characterization, and application of geopolymers in alumina-based castables were evaluated, aiming to enable the partial or complete replacement of CAC in these refractories. Initially, metakaolin (MK)-based geopolymer systems were formulated using liquid reagents (LR) composed of NaOH and different sources of amorphous silica (sodium silicate, silica fume, or colloidal silica suspension). Subsequently, the most promising LR was chosen, adjusting the content of NaOH and/or KOH to enhance the thermal stability of the geopolymers. Furthermore, the effect of incorporating these new binders into the properties of the refractories was investigated between 40-1400°C through various tests, including flowability, physico-thermo-mechanical measurements, and XRD, ATR-FTIR, and/or SEM analyses. Geopolymers prepared with colloidal silica suspension (CS) exhibited the best characteristics for use in the development of innovative refractories. Alumina-based castables with 8 wt.% MK and LR containing NaOH-CS (8MK-Na) and 6 wt.% MK and LR containing KOH-CS (6MK-K) showed optimized behavior, where the generation of a liquid phase during sintering induced densification of the samples between 1100-1250°C. These transformations resulted in microstructures with strong interfacial bonds between the glassy phase and alumina grains. Additionally, the refractory containing a combination of 2.7 wt.% CAC and 1.3 wt.% MK exhibited 1.18% shrinkage, an elastic modulus of 135 GPa, mechanical strength of 39.1 MPa, and high thermal shock resistance. These advancements resulted in the development of refractories with remarkable performance, surpassing the reference material.
