Temperature stability of a pure metakaolin based K-geopolymer: Part 1. Variations in the amorphous mineral network

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 temperature rise in order to better understand its variations with respect to temperature. The works from the present paper focus only variations in the internal structure of the mineral matrix. The results presented here show that the amorphous mineral matrix is preserved up to 900°C. The results also show that there is a densification of the internal structure of tetrahedral network during heating, due to changes in the Q³ environments in fully-connected Q⁴ for both silicates and aluminates. Thus, our work provides a new more precise vision of the 3D geopolymeric mineral matrix for which the silicoaluminous network is not exclusively composed of Q⁴ entities, contrary to what is frequently encountered in the literature before.     

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.     

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.     

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.     

Properties of Geopolymer Composites Reinforced with Basalt Chopped Strand Mat or Woven Fabric

Geopolymers or polysialates are inorganic polymeric, ceramic‐like materials composed of alumina, silica, and alkali metal oxides that can be made without any thermal treatment. Additions of reinforcing phases vastly improve the mechanical properties and high‐temperature stability of the geopolymer. The processing and mechanical properties of both chopped strand mat as well as 2‐D woven fabric‐reinforced potassium geopolymer composites have been evaluated. Hand lay‐up and hydraulic press processing methods were used to produce composite panels. The room‐temperature tensile and flexural strength of chopped strand mat composites was 21.0 ± 3.1 and 31.7 ± 4.4 MPa, respectively, while those of basalt weave‐reinforced geopolymer composites reached 40.0 ± 5.9 and 45.2 ± 9.3 MPa, respectively. Composite microstructures were examined using optical microscopy as well as scanning electron microscopy (SEM). Mass, volume, and porosity fractions were also determined. The effect of high‐temperature treatments at 25°C, 300°C, 600°C, and 800°C were analyzed. Finally, Weibull statistical analysis was performed, which showed an increase in reliability when a reinforcement phase was added to K‐geopolymer.     

Effect of temperature on the mechanical behaviour of an oxide/oxide composite

Oxide/oxide composites are being considered in the aerospace industry for the design of next-generation exhaust components. This study aims to determine the mechanical behaviour of a composite consisting in Nextel?610 fibres fabric (8 HSW) embedded in a porous alumina matrix. The mechanical properties were studied in tension and four-point bending up to 1300 °C. Up to 800 °C, the material behaviour is elastic and exhibits a few damage with only a low effect of the temperature. Increasing the temperature leads to the progressive apparition of a viscous behaviour up to 1000 °C and a superplastic behaviour beyond 1200 °C.     

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.     

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.     

One‐Part Geopolymers Based on Thermally Treated Red Mud/NaOH Blends

In this study, one‐part “just add water” geopolymer binders are synthesized through the alkali‐thermal activation of the red mud which is relatively rich in both alumina and calcium. Calcination of the red mud with sodium hydroxide pellets at 800°C leads to decomposition of the original silicate and aluminosilicate phases present in the red mud, which promotes the formation of new compounds with hydraulic character, including a partially ordered peralkaline aluminosilicate phase and the calcium‐rich phases C₃A and α‐C₂S. The hydration of the “one‐part geopolymer” leads to the formation of zeolites and a disordered binder gel as the main reaction products, and the consequent development of compressive strengths of up to 10 MPa after 7 d of curing. These results demonstrate that red mud is an effective precursor to produce one‐part geopolymer binders, via thermal and alkali‐activation processes.     

Structural and chemical properties of thermally treated geopolymer samples

This article presents the results of the compositional, structural and morphological study of geopolymers synthesized from metakaolin and an alkali activator. The study involved the investigation of the structural and chemical properties of the geopolymer, in addition to thermally treated geopolymers up to 600 and 900 °C. The precursor of the geopolymer, and the geopolymer samples before and after the thermal treatment, were investigated by Fourier transformation infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and SEM analysis. The corrected average value of the ratio of silicon and aluminum in the geopolymer samples (Si_GP:Al) is about 1.46, which suggests that the obtained geopolymer samples represent a mixture of roughly equal amounts of sialate and sialate-siloxo units. Annealing the geopolymer samples at 600 °C decreases the amount of Si-ONa bonds and induces the cross-linking of polymer changes. At the same time, other sodium containing alumino-silicate phases are created. The thermal treatment at 900 °C leads to a considerable reduction of oxygen and particularly sodium, followed by significant morphological changes i.e. formation of a complex porous structure. Additionally, a new semicrystaline phase appears. Both XRD and XPS results imply that this new phase may be nepheline and it is plausible that this phase begins to nucleate at temperatures below 900 °C.     

Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater

This work aims to verify the feasibility of utilizing iron ore tailing (IOT) in porous geopolymer and intends to broaden the application of porous geopolymer in heavy metal removal aspect. Porous geopolymer was prepared using fly ash as resource material, which was partially replaced by IOT at level of 30%, by weight, with H₂O₂ as foaming agent and removal efficiency, adsorption affecting factors, adsorption isotherms and thermodynamics of Cu²⁺ by the developed porous geopolymer were investigated.The experimental results uncover that the porous amorphous geopolymer was successful synthesized with total porosity of 74.6%. The transformation of fly ash and IOT into foaming geopolymer leads to the formation of porous structure encouraging Cu²⁺ sorption. Batch sorption tests were carried out and geopolymer dosage, Cu²⁺ initial concentration, pH, contact time and temperature were the main concern. Both Langmuir and Freundlich models could explain the adsorption of Cu²⁺ on the porous geopolymer due to the high fitting coefficients. The uptake capacity reaches the highest value of 113.41 mg/g at 40 °C with pH value of 6.0. The thermodynamic parameters ΔHº, ΔSº and ΔGº suggests the spontaneous nature of Cu²⁺ adsorption on porous geopolymer and the endothermic behavior of sorption process.     

High-temperature electrical properties of polymer-derived ceramic SiBCN thin films fabricated by direct writing

The in situ temperature monitoring of hot components in harsh environments remains a challenging task. In this study, SiBCN thin-film resistance grids with thicknesses of 1.8 μm were fabricated on alumina substrates via direct writing. Owing to their dense microscopic morphology and extremely high graphitisation level, the produced SiBCN films exhibited large high-temperature oxidation resistance and electrical conductivity. The resistance–temperature, stability, and repeatability characteristics of these films were examined in an aerobic environment at temperatures up to 800 °C. The obtained results revealed that the thermistor resistance decreased monotonously with increasing temperature from room temperature to 800 °C. The SiBCN film resistance variations observed during repeated temperature cycling in the regions of 505–620 °C and 610–720 °C were 0.09% and 1.7%, respectively. The high cyclability and stability of the SiBCN thin film thermistor suggested its potential applicability for the in situ temperature monitoring of hot components in harsh environments.     

Mixed-ionic and electronic conduction and stability of YSZ-graphene composites

Composites of 8 mol% Yttria-stabilized Zirconia (YSZ) containing 0, 7, 10 and 14 vol.% of graphene nano-platelets (GNP) were fully densified by Spark Plasma Sintering. The effect of GNP on the electrical performance of the composites was analyzed by impedance spectroscopy as a function of temperature (150–800 °C) and oxygen partial pressure (0.21–10⁻²⁰ atm). Results show that below GNP percolation threshold (7.1 vol.%), the electrical behavior is dominated by the matrix oxygen-ion conductivity. Above the threshold, the conductivity is predominantly electronic provided by the GNP network. The total conductivity of composites was used as an indicator of GNP stability in different atmospheres. YSZ/GNP composites remain stable in inert conditions up to 600 °C, and in reducing conditions up to 800 °C, making them good alternatives to perovskite-based materials used for electrochemical applications.     

Flexural behavior of hybrid PVA fibers reinforced ferrocement panels at elevated temperatures

Fire safety should consider not only the performance of the structure after the fire but also the behavior during the fire. The structural fire reliability performance of hybrid PVA fiber reinforced ferrocement (HFF) panels is experimentally determined based on its flexural characteristics and damage during the exposure to elevated temperatures. The residual compressive strength of 60 cubs was also tested after exposed to temperatures. In addition, 30 HFF panels were tested to evaluate their structural capacity by conducting an in‐situ binding test during the heating of up to 200°C, 400°C, 600°C, and 800°C, and compared with control samples tested at ambient (24°C) temperature condition. The main parameters investigated were the specimen thickness and the effect of using mineral admixtures (fly ash and silica fume) in the mortar mixtures. The results show a strength decline of both flexural and compressive strengths as temperature increases. The bending capacity at 800°C is reduced to about 90% of the ambient capacity only. In between the 2 temperatures, the reduction rate is found to be almost linear. A theoretical prediction of the moment capacity reduction shows a good agreement with the test results.     

Properties and characterization of alumina platelet reinforced geopolymer composites

Geopolymers are porous, amorphous, alkali-aluminosilicate hydrate materials formed at room temperature via a solution process. Geopolymer based on metakaolin had a relatively homogeneous microstructure that offered consistent behavior but suffered from dehydration cracking and large densification shrinkages when heated. It was found that by reinforcing a metakaolin geopolymer of composition (K₂O·Al₂O₃·4SiO₂·11H₂O) with 50 µm diameter alumina platelets, dehydration cracking could be prevented, and shrinkage could be reduced by an order of magnitude. Samples were reinforced with 30, 50, and 70 wt% of alumina platelets. Although the properties of the 30 and 50 wt% conditions were better than those of unreinforced geopolymer, those samples still showed warping, cracking, and strength losses on heating. The 70 wt% samples did not warp or crack when heated to temperatures of up to 1500°C. The room-temperature 4-point flexural strength of these samples remained at around 20 MPa regardless of heat treatments. The in situ measured flexural strength increased to almost 40 MPa at 600°C, and remained higher than 20 MPa until 1200°C. Samples subjected to propane-torch thermal shock heating and subsequent quenching did not crack or fragment. Dilatometry, X-ray diffraction, and scanning electron microscopy were used for additional characterization. Given these properties, this material showed promise as a castable refractory.     

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.     

Preparation of one-part geopolymeric precursors using vanadium tailing by thermal activation

The main purpose of this study is to develop a user-friendly one-part geopolymer using vanadium tailing (VT). Geopolymeric precursor consists of activated VT and metakaolin that can react directly with water to form geopolymers. The roasting temperature plays an important role in the VT activation, which affects the compressive strength of the final geopolymer. The geopolymer with accepted compressive strength, that is 29.0 MPa after 7 days curing in ambient condition, can be prepared using VT after thermal activation at appropriate temperature (400-600°C). As the roasting temperature is increased to 700°C, the VT is molten and sintered and the ability of providing alkaline and Si⁴⁺ is drastically weakened, which results in a poor compressive strength geopolymer.     

From bulk to porous structures: Tailoring monoclinic SrAl2Si2O8 ceramic by geopolymer precursor technique

In this paper, monoclinic SrAl₂Si₂O₈ ceramics with porous structures were prepared based on ion-exchanged geopolymer precursor technique. Micron-level pores with a homogeneous pore-size distribution were introduced into the inorganic framework using foaming agents. The results demonstrated that the apparent density, pore-size distribution and specific surface area of porous geopolymer precursors can be well-engineered via tailoring the category and concentration of the foaming agent. After being treated at 900°C, hexagonal SrAl₂Si₂O₈ first-crystallized from the amorphous geopolymer matrix and then gradually converted into monoclinic SrAl₂Si₂O₈ between 1100°C and 1200°C. The resulting monoclinic SrAl₂Si₂O₈ ceramics maintained the porous structures during high-temperature treatments and exhibited high porosity, specific surface area, and compressive strength. The aforementioned strategy not only achieves monoclinic SrAl₂Si₂O₈ ceramics with well-defined and robust microstructures, but also provides an alternative route to prepare other porous ceramics, with potential applications in fields of high-temperature filters, adsorbents, and heterogeneous catalysis.     

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.     

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.