Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Novel thermal insulating and lightweight composites from metakaolin geopolymer and polystyrene particles

In this work, new foamed thermal insulation geopolymer composite based on polystyrene particles (PP) and metakaolin was developed. Compressive strength, flexural strength, high temperature resistance and microstructure were evaluated. The experimental results show that compressivestrengthand flexural strength of the thermal insulation geopolymer composite decrease with increasing polystyrene particle content. However, it still exhibits considerable and sufficient strength. The dry density and thermal conductivityalso decrease as polystyrene particle content increases due to the contribution of polystyrene particles with low density. The floatation of the thermal insulation geopolymer composite on water surface indicates the relatively low density and a good quadratic function relationship can be found between thermal conductivity and dry density. Furthermore, the dense interfacial transition zone indicates the high compressive strength and flexural strength of thermal insulation geopolymer composites. The cumulative intrusion volume corresponding to the porosity decreases and the critical pore diametersshift to lower values with addition of polystyrene particles. Geopolymer composites gain strength after exposure around 400 °C, and it suffers dramatic strength loss after 800 °C temperature exposure especially for the 100% polystyrene particles addition specimen.     

Characterisation and properties of geopolymer composites. Part 2: Role of cordierite-mullite reinforcement

Our previous research paper on geopolymer-mullite composites showed promising results on compressive strength and fire resistance. However, no improvement in thermal shock resistance was observed in the afore mentioned study. In this study, further attempts to improve thermal shock resistance of the geopolymer were explored. The research was performed by compositing a fly ash-based geopolymer with cordierite-mullite at 20, 40 and 60 wt% replacement. X-ray diffraction (XRD) of the cured geopolymer composite specimens showed the existence of cordierite, mullite, quartz, cancrinite and lazurite. It was found that compressive strength and strength retention after thermal exposure at 400 °C were improved in the geopolymer composite specimens, especially those with 20–40 wt% replacement. Upon further heating to 600 °C, all geopolymer specimens showed insignificant differences in compressive strength. Fire resistance was found to improve with increasing proportion of replacement contents.     

Mechanical and microstructural evolutions of fly ash/slag-based geopolymer at high temperatures: Effect of curing conditions

Designing a building material with excellent heat resistance is crucial for protection against catastrophic fires. Geopolymer materials have been investigated as they offer better heat resistance than traditional cement owing to their ceramic-like properties. Curing temperature and conditions are crucial factors that determine the properties of geopolymers, but their impacts on the heat resistance of geopolymers remain unclear. This study produced geopolymers from fly ash and ground granulated blast furnace slag by using sodium silicate and sodium hydroxide solutions as alkaline solutions. To examine the effect of curing conditions on the high-temperature performance of geopolymer, four different curing conditions, namely, heat curing (70 °C for 24 h), ambient curing (20 °C), water curing, and the combination of heat and water curing (70 °C for 24 h followed by water curing), were applied. At 28 d, the specimens were subjected to high temperatures (500 °C, 750 °C, and 950 °C), and their mechanical and microstructural evolutions were studied. The results revealed that the curing condition significantly affects the properties of the unexposed geopolymer; the effect on its high-temperature performance is insignificant. Furthermore, all the specimens could maintain adequate compressive strength after exposure to the maximum temperature of 950 °C, promising the use of geopolymer for structural applications.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.     

Effect of transient creep on compressive strength of geopolymer concrete for elevated temperature exposure

The strength and transient creep of geopolymer and ordinary Portland cement (OPC)-based material (paste and concrete) were compared at elevated temperatures up to 550 °C. The strength properties were determined using an unstressed hot strength test and unstressed residual strength test for paste and concrete, respectively. At 550 °C, compared with the original strength, the strength of geopolymer was increased by 192% while the strength of OPC paste showed little change. However, after exposure to 550 °C, the residual strength percentage of both geopolymer and OPC concretes was similar. Transient creep data show that geopolymer had little change in transitional thermal creep (TTc) between 250 and 550 °C while OPC paste developed significant TTc in this temperature range. In comparison with OPC concrete, a higher strength loss of geopolymer concrete is thus believed to be due to the absence of TTc to accommodate nonuniform deformation during thermal exposure.     

Effects of elevated temperature on pumice based geopolymer composites

Abstract

Aluminosilicate type materials can be activated in alkaline environment and can produce geopolymer cements with low environmental impacts. Geopolymers are believed to provide good fire resistance so the effects of elevated temperatures on mechanical and microstructural properties of pumice based geopolymer were investigated in this study. Pumice based geopolymer was exposed to elevated temperatures of 100, 200, 300, 400, 500, 600, 700 and 800°C for 3?h. The residual strength of these specimens were determined after cooling at room temperature as well as ultrasonic pulse velocity, and the density of pumice based geopolymer pastes before and after exposing to high temperature was determined. Microstructures of these samples were investigated by Fourier transform infrared for all temperatures and SEM analyses for samples that were exposed to 200, 400, 600 and 800°C. Specimens, which were initially grey, turned whitish accompanied by the appearance of cracks as temperatures increased to 600 and 800°C. Consequently, compressive strength losses in geopolymer paste were increased with increasing temperature level. On the other hand, compressive strength of geopolymer paste was less affected by high temperature in comparison with the ordinary Portland cement. As a result of this study, it is concluded that pumice based geopolymer is useful in compressive strength losses exposed to elevated temperatures.     

High-porosity geopolymer membrane supports by peroxide route with the addition of egg white as surfactant

Metakaolin-based geopolymer membrane supports were synthesized by gelcasting using hydrogen peroxide with the addition of albumen powder as surfactant. A geopolymer slurry was prepared using metakaolin and an alkaline medium at room temperature, the obtained viscous paste was expanded by means of the decomposition of peroxide in combination with protein-assisted foaming, and the geopolymerization was conducted in a closed environment at 75 °C. The combination of peroxide and albumen protein enabled the production of geopolymer membrane supports with a total porosity of ~74.29%, open porosity as high as ~65.25%, and possessing a compressive strength of ~4.47 MPa. Moreover, factors that influence the compressive strength, the porosity, and the pore size distribution were investigated. The results showed that the sizes of cell could be controlled by adding different content of peroxide and protein, and by heat-treating at different temperatures.     

Assessment of recycling use of GFRP powder as replacement of fly ash in geopolymer paste and concrete at ambient and high temperatures

Environmental issues caused by glass fiber reinforced polymer (GFRP) waste have attracted much attention. The development of cost-effective recycling and reuse methods for GFRP composite wastes is therefore essential. In this study, the formulation of the GFRP waste powder replacement was set at 20–40 wt%. The geopolymer was formed by mixing GFRP powder, fly ash (FA), steel slag (SS) and ordinary Portland cement (OPC) with a sodium-based alkali activator. The effects of GFRP powder content, activator concentration, liquid to solid (L/S) ratio, and activator solution modulus on the physico-mechanical properties of geopolymer mixtures were identified. Based on the 28-day compressive strength, the optimal combination of the geopolymer mixture was determined to be 30 wt% GFRP powder content, an activator concentration of 85%, L/S of 0.65, and an activator solution modulus of 1.3. The ratios of compressive strength to flexural strength of the GFRP powder/FA-based geopolymers were considerably lower than those of the FA/steel slag-based geopolymers, which indicates that the incorporation of GFRP powder improved the geopolymer brittleness. The incorporation of 30% GFRP powder in geopolymer concrete to replace FA can enhance the compressive and flexural strengths of geopolymer concrete by 28%. After exposure to 600 °C, the flexural strength loss for geopolymer concretes containing 30 wt% GFRP powder was less than that of specimens without GFRP powder. After exposure to 900 °C, the compressive strength and flexural strength losses of geopolymer concretes containing 30 wt% GFRP powder were similar to those of specimens without GFRP powder. The developed GFRP powder/FA-based geopolymers exhibited comparable or superior physico-mechanical properties to those of the FA-based geopolymers, and thus offer a high application potential as building construction material.     

Fully reacted high strength geopolymer made with diatomite as a fumed silica alternative

Geopolymers are formed by mixing of aluminosilicate sources with alkaline meta-silicate solution at room temperature. In the current study, diatomite of Turkish origin was fully utilized as a fumed silica alternative for the preparation of geopolymer, having a typical formula of K₂O•Al₂O₃•4SiO₂•11H₂O. From XRD of this sample, a broad peak centered at 28° 2θ indicated the well-known formation of amorphous geopolymer, as well as a fully reacted microstructure of geopolymer as seen by scanning electron microscopy. Additionally, geopolymer having the same formula was made by using fumed silica, in order to compare with geopolymers prepared from diatomite. The Weibull modulus was calculated from four-point bending and compressive strength testing of both geopolymer composites. The use of diatomite as a fumed silica substitute in geopolymer production resulted in a very close flexure strength 9.2 (± 4.2 MPa) when compared to geopolymer made from fumed silica 10.2 (± 3.3 MPa). There was a significantly higher compressive strength 71 (± 13.9 MPa) and Weibull modulus (5.4), than comparable properties of geopolymer made from fumed silica, which had a compressive strength 54 (± 25.8 MPa) and Weibull modulus of 2.0. The discrepancy was attributed to some self-reinforcement of the geopolymer matrix due to unreacted diatomite.     

Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35 MPa which significantly increased 37% when the 10 wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30 wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.     

Fire resistance of geopolymer concrete produced from Elazığ ferrochrome slag

This paper presents the effect of elevated temperatures up to 700 °C on compressive strength and water absorption of two alkali‐activated aluminosilicate composites (one of them is river sand aggregate geopolymer concrete; the other one is crushed sand aggregate geopolymer concrete) and ordinary Portland cement based concretes. To obtain binding geopolymer material, Elaz?? ferrochrome slag was ground as fine as cement, and then it was alkali activated with chemical (NaOH and Na₂SiO₃). Geopolymer concrete samples were produced by mixing this binding geopolymer material with aggregates. At each target temperature, concrete samples were exposed to fire for the duration of 1 h. Fire resistance and water absorption of geopolymer and ordinary Portland cement concrete samples were determined experimentally. Experimental results indicated that compressive strength of geopolymer concrete samples increased at 100 °C and 300 °C temperatures when compared with unexposed samples. In geopolymer concrete samples, the highest compressive strength was obtained from river aggregates ones at 300 °C with 37.06 MPa. Water absorption of geopolymer concrete samples increased at 700 °C temperature when compared with unexposed samples. However, a slight decrease in water absorption of concrete samples was observed up to 300 °C when compared with unexposed samples. SEM and X‐ray diffraction tests were also carried out to investigate microstructure and mineralogical changes during thermal exposure. Copyright © 2016 John Wiley & Sons, Ltd.     

Sustainable activation of pumice with partially variable substitutions of metakaolin and/or fumed silica

Sustainable alkali activation of pumice from Turkish origin was studied by a partial replacement of metakaolin and/or fumed silica additives. Following the characterization of as-received pumice by X-ray fluorescence spectroscopy, x-ray diffraction, and nuclear magnetic resonance spectroscopy, a series of powder mixtures were prepared by introducing metakaolin and/or fumed silica (8, 14, and 20 M) into 1 M of the pumice. The mixtures were then dissolved in 11 M NaOH or sodium silicate solutions. The slurries were poured into polyacetal molds to obtain geopolymer samples for mechanical testing and cured in a constant 50°C temperature in a humidity oven for 48 h and then left for 1 week to undergo additional curing at ambient temperature. The microstructural, mechanical, and thermal properties of the final geopolymer samples were determined by XRD, scanning electron microscopy, Weibull analysis of 3-point flexural and compressive tests and thermal conductivity measurements. Results showed that all the Weibull values were best for 14 M of metakaolin and/or fumed silica. The metakaolin-added pumice yielded higher compressive strengths of (53.78 ± 33.30 MPa) than fumed silica (10.87 ± 4.04 MPa) and fumed silica plus metakaolin (41.22 ± 5.16 MPa). Thermal conductivities (0.19–0.46 Wm^–1K^–1) were also comparable to the thermal conductivity of metakaolin-based geopolymers.     

Effect of nano silica and fine silica sand on compressive strength of sodium and potassium activators synthesised fly ash geopolymer at elevated temperatures

Environment friendly geopolymer is a new binder which gained increased popularity due to its better mechanical properties, durability, chemical resistance, and fire resistance. This paper presents the effect of nano silica and fine silica sand on residual compressive strength of sodium and potassium based activators synthesised fly ash geopolymer at elevated temperatures. Six different series of both sodium and potassium activators synthesised geopolymer were cast using partial replacement of fly ash with 1%, 2%, and 4% nano silica and 5%, 10%, and 20% fine silica sand. The samples were heated at 200°C, 400°C, 600°C, and 800°C at a heating rate 5°C per minute, and the residual compressive strength, volumetric shrinkage, mass loss, and cracking behaviour of each series of samples are also measured in this paper. Results show that, among 3 different NS contents, the 2% nano silica by wt. exhibited the highest residual compressive strength at all temperatures in both sodium and potassium‐based activators synthetised geopolymer. The measured mass loss and volumetric shrinkage are also lowest in both geopolymers containing 2% nano silica among all nano silica contents. Results also show that although the unexposed compressive strength of potassium‐based geopolymer containing nano silica is lower than its sodium‐based counterpart, the rate of increase of residual compressive strength exposed to elevated temperatures up to 400°C of potassium‐based geopolymer containing nano silica is much higher. It is also observed that the measured residual compressive strengths of potassium based geopolymer containing nano silica exposed at all temperatures up to 800°C are higher than unexposed compressive strength, which was not the case in its sodium‐based counterpart. However, in the case of geopolymer containing fine silica sand, an opposite phenomenon is observed, and 10% fine silica sand is found to be the optimum content with some deviations. Quantitative X‐ray diffraction analysis also shows higher amorphous content in both geopolymers containing nano silica at elevated temperatures than those containing fine silica sand.     

Mechanical properties of geopolymer concrete exposed to dynamic compression under elevated temperatures

Through adoption of a self-designed high temperature SHPB apparatus herein, an experimental study is made on the mechanical properties of geopolymer concrete (GC) exposed to dynamic compression under elevated temperatures. As the results have turned out, the weight loss is remarkable within temperature ranges from room temperature to 200 °C as well as from 600 °C to 800 °C. The dynamic compressive strength of GC grows higher at 200 °C than at room temperature, but suffers a dramatic drop at 800 °C. The critical strain is higher at elevated temperature than that at room temperature. At 200 °C and 600 °C, respectively, its energy absorption property is superior to that at room temperature. However, at 400 °C and 800 °C, respectively, it is inferior to that at room temperature. The strain rate effect of the dynamic increase factor (DIF) obtained from test data can reflect the inherent nature of GC. The DIF assumes a linear relationship with the logarithm of strain rate.     

Bone ash reinforced geopolymer composites

Potassium-based, geopolymer composites were made with BASF^® metakaolin and Mymensingh clay-derived metakaolin from Bangladesh. Since the natural Mymensingh clay contained 40 wt.% quartz, this same amount of quartz particulates was added to the BASF^® metakaolin to make a synthetic analog of the natural calcined clay. By analogy with bone china, bone ash or calcined hydroxyapatite (5CaO•3P₂O₅ or “HA”) particles, having a Ca: P ratio of 3.3:1, were added to make the three types of geopolymer-based composites described above. For less refractory particulate additions, dicalcium phosphate (DCP) (2CaO•P₂O₅ or “DCP”) particles, having a Ca: P ratio of 2:1, were also added to another set of geopolymers. The ambient temperature compressive and flexural strengths were measured for all of the geopolymer composites. The HA or DCP reinforced geopolymer composites were fabricated and heat-treated to 1150°C/1 h, after which they were converted to their mineralogical analogs. Their mechanical properties of compressive and 3-point flexural strengths were again measured. Flexural strengths of 22.42 ± 11.0 MPa and 31.97 ± 8.3 MPa were measured in 1 × 1 × 10 cm³ heat-treated geopolymer bars reinforced with 10 wt.% of DCP and in geopolymer reinforced with 10 wt.% DCP +40 wt.% quartz additions, respectively. Significant improvements to ambient temperature properties were observed due to the self-healing effect of the flowing amorphous DCP, whose presence was verified by SEM. The geopolymer samples exhibited reduced water absorption (WA) (on a percentage dry weight basis) of within 0.03-0.5% after being heated at 1100℃/1 h and 1125℃/1 h, as compared with those at room temperature, which varied between 2.56% and 7.89%.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

Mechanical and chemical properties of metakaolin-based geopolymer exposed to elevated temperature

A method is presented to fabricate metakaolin-based geopolymers that are structurally and mechanically stable up to 600°C. The chemical environment of the geopolymers is characterized using thermogravimetric analysis and Fourier-transform infrared spectroscopy. Residual free water turned into steam and caused damage to the geopolymer when exposed to elevated temperatures. The curing temperature was increased from 80 to 120°C to remove water during the curing process. A correlation was drawn between the amount of Si-O-Al linkage formed and the position of fingerprint peaks in infrared spectra, providing a tool to evaluate the level of geopolymerization. Flexural and tensile properties of geopolymers fabricated using the optimized method were measured for no heat treatment and for exposure to elevated temperatures of 200, 400, and 600°C. The flexural strength was measured to be 10.80 ± 2.99 MPa at room temperature, 10.36 ± 0.64 MPa at 400°C, and 8.04 ± 1.60 MPa at 600°C. The flexural modulus is reported to be 13.09 ± 3.40 GPa at room temperature and 11.03 ± 0.53 GPa at 600°C. The flexural toughness decreased with increasing temperature. The tensile properties of the geopolymer were measured with direct tensile tests paired with an extensometer. The tensile strength decreased from 4.16 ± 2.08 MPa at room temperature to 3.13 ± 0.97 MPa at 400°C, and 2.75 ± 0.86 MPa at 600°C. The Young's modulus decreased from 45.38 ± 30.30 GPa at room temperature to 26.88 ± 6.65 GPa at 600°C. Both flexural and tensile tests have shown that the metakaolin-based geopolymers cured at 120°C is mechanically stable at temperatures up to 600°C.     

Synthesis,Thermal Properties and Electrical Conductivity of Na-Sialate Geopolymer

This work aims to study the thermal behavior of basic-geopolymers derived from metakaolin (clay). The geopolymers were characterized by different techniques: thermal analysis (DTA, TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy. Some physicochemical properties of the products were also determined: the phases obtained after geopolymer heat treatment and their electrical properties. The results obtained after drying and heat treatment showed that the products kept their initial shapes, but revealed variable colors depending on the temperatures at which they were treated. The products obtained are amorphous between 300 up to 600 °C with peaks relating to the presence of nanocrystallites of muscovites and zeolite, thus at 900 °C it is quite amorphous but only contains nanocrystallites of muscovites. From the temperature of 950 °C, we notice that the geopolymer has been transformed into a crystalline compound predominated by the Nepheline (NaAlSiO₄) with the presence of a crystalline phase by minor peaks of Muscovite, this crystalline character has been increased at 1100 °C to obtain a whole phase crystalline of a Nepheline. The treatment of this geopolymer for one hour at 1200 °C shows an amorphous phase again corresponding to corundum (α-Al₂O₃). This indicates that the dissolution of the grains by the liquid phase induces the conversion of the material structure from sialate [–Si–O–Al–O] to sialate siloxo [–Si–O–Al–O–Si–O–] and the formation of a new crystalline phase (α-Al₂O₃). This development of sialate to sialate-siloxo was confirmed by IR spectroscopy. As mentioned above, from 300 to 900 °C, Na-sialate geopolymer exhibits the same disorder structure of nepheline. The crystal structure of nepheline is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built by Regularly alternating tetrahedral AlO₄ and SiO₄. Stacking the layer’s parallel to the c axis gives a three-dimensional network containing channels occupied by Na cations. This topology favors easy movement of Na⁺ ions throughout the structure. For this reason, ionic migration in nepheline is widely reported. The refinement of Na-Sialate geopolymer at room temperature gives bulk high ionic conductivity of about 5 × 10^?5 S cm^?1 and this is due to the probable joint contribution of H⁺ and Na⁺ ions. Above 200 °C, Na⁺ seems to remain the only charge carrier with a low activation energy of about Ea?=?0.26 eV. At higher temperatures, the characteristic frequencies become so close that it is impossible to distinguish the contributions. A total resistance comprising both grain and grain boundaries contribution is then determined.

     

Mechanical characterization of lightweight engineered geopolymer composites exposed to elevated temperatures

In this study, the effect of different factors, such as PVA fibers (2% by total volume) and precursor type (slag, fly ash, or a combination of both), on the behavior of green lightweight engineered geopolymer composites (LEGC) and lightweight engineered cementitious composites (LECC) after exposure to temperatures up to 800 °C for 1 h is investigated. Expanded glass granules were used as lightweight aggregate instead of silica sand to reduce the spalling tendency and density of the composite. The flowability, density, color change, mass loss, spalling resistance, residual mechanical properties (compressive strength, stress-strain diagram, tensile stress-strain diagram, load-deflection response, failure mode), and microstructural analysis (by scanning electron microscopy) were investigated before and after exposure to thermal deterioration. The findings pointed out that the dry density, compressive strength, fiber bridging stress, strain capacity, maximum load, and maximum deflection of the developed mixtures before exposure to fire deterioration were in the range of 1703–1883 kg/m³, 16.66–64.11 MPa, 2.66–4.97 MPa, 2.40–3.33%, 1573–4824 N, and 2.92–5.53 mm respectively. It's worth mentioning that the substitution of 50% slag in the lightweight EGC mixture demonstrated the optimal tensile strain capacity and deformation capacity and further enhanced both ultimate tensile strength and flexural strength of fly ash-based EGC (FA-EGC) mixtures. After heat exposure, both LEGC and LECC composites demonstrated strain hardening behavior and deflection hardening behavior up to 300 °C of heat treatment, while after exposure to a temperature of 300 °C and above, both deflection hardening behavior and strain hardening behavior are dramatically damaged. This is attributable to the melting of the PVA fibers. Also, the microstructural analysis showed that incorporating fly ash into lightweight EGC mixtures can effectively reduce the melting point of PVA fibers and further improve the fire resistance of EGC mixtures.