Damage evaluation and deformation behavior of mine tailing-based Geopolymer under uniaxial cyclic compression

Geopolymerized mine tailings (MTs), as an alternative to reuse the mine wastes, can be used for construction materials (e.g., geopolymer concrete and bricks) depending on their mechanical properties. Their strength values, which can range from a couple of MPa to tens of MPa, are significant evidence for their application in the construction industry. In practice, geopolymers activated with different NaOH molarities can significantly affect the mechanical properties of MTs. The mechanical behavior of geopolymers under monotonic loading also has been widely investigated. However, the potential hazard of the exposure of geopolymer concrete/bricks to cyclic loading has received limited attention. This paper presents a study we conducted on geopolymers made by activation of MTs under cyclic loading to understand their crack and damage behaviors, including the influence of factors such as NaOH molarity and loading patterns. The influence of NaOH molarity on the elastic and plastic strains of the geopolymer specimen at different cycles was explored. A series of unconfined compression tests of cubic specimens with different NaOH molarities as well as microscopic investigations and observations via XRD, FTIR, and SEM were carried out in this study. The Young's modulus of the geopolymer was found to increase followed by a decrease with the cycles for all the selected NaOH molarities. The geopolymers activated with lowest NaOH molarity were first to start damage and activated with the highest NaOH molarity were the last to damage. The damage variable was shown to increase rapidly at the initial cycles and then gradually approached the maximum value.     

Preparation and characterization of Cf/Pollucite composites through geopolymer precursors

Continuous carbon-fibre-reinforced Cs-geopolymer composite (C_f/CsGP) were prepared, and its in-situ conversion was investigated during high-temperature treatments. The effect of treatment temperature on the thermal evolution process and mechanical properties of the resulting products were systematically evaluated. The results indicated that the crystallization temperature of C_f/CsGP composite was considerably delayed because the amorphous structure of carbon fibres was not conducive as a nucleation substrate for pollucite derived from the CsGP matrix. Moreover, the integrity of the corresponding resulting products derived from the C_f/CsGP composite were damaged due to thermal shrinkage that occurred during the high-temperature treatment process. When treatment temperature was ≤1200^oC, the mechanical properties of the corresponding products exhibited an upward trend, which was ascribed to the improvement of the densification degree of the resulting composite and well interface-bonding state between carbon fibres and pollucite. However, the mechanical properties of the resulting composites decreased with the treatment temperature continued increased from 1200 to 1400^oC. This phenomenon was attributed to the impairment of fibre properties caused by interfacial reactions.     

Mechanical and microstructural characterization of geopolymers synthesized from FCC waste catalyst and silica fume

A FCC waste catalyst-based geopolymer was synthesized from FCC waste catalyst and silica fume, which were used as the main silicon-aluminum raw material and correction material, respectively. Meanwhile, NaOH and water glass composite were used as alkaline activator in the preparation process. Herein, the effects of silicon correction materials, alkaline activator modulus, and silica fume content on the compressive strength performance of prepared geopolymers were discussed. The microstructure was comprehensively analyzed by X-ray diffraction, fourier infrared spectroscopy, nuclear magnetic resonance spectroscopy and scanning electron microscope. The results showed that the prepared geopolymer has good early property when the silica fume content is 50% and the water glass modulus is 1.2. The 3d compressive strength of the obtained sample reaches 23.77 MPa. Microstructure and geopolymerization process analysis indicate that the FCC waste catalyst and silica fume have a good synergistic effect, which confirms the feasibility of preparing the geopolymer by using these industrial waste materials.     

Pressureless sintering of binderless tungsten carbide

The aim of this study was to produce dense, single phase polycrystals. The research was carried out on the submicron tungsten carbide powder without additives, with either a carbon or tungsten additive and on the powder with both additives. The primary task of carbon was to reduce surface oxide impurities which passivate WC grains; tungsten in turn bounds free carbon in the WC. The authors manufactured fine-grained, dense (96–98% T.D.) and single-phase WC polycrystals, using the technique of pressureless sintering at the temperature not exceeding 2000 °C. A positive effect of carbon addition on tungsten carbide sinterability was observed, whereby a dense, fine-grained polycrystals can be obtained at 1900 °C. It was also observed that a significant excess of temperature of sintering process resulted in a strong abnormal grain growth of WC grains.     

Leaching behavior of fly ash-waste glass and fly ash-slag-waste glass-based geopolymers

The leaching behavior of contaminants in the fly ash/waste glass or fly ash/blast furnace slag/waste glass-based geopolymers was investigated through surface area analysis and leaching tests. Spent fluorescent lamps classified as hazardous waste due to their specific contaminant contents were used as a source of waste glass. The results from semi-dynamic leaching tests showed that the mobility of contaminants increases as the amount of waste glass added to the synthesis mixture increases. These results are consistent with those obtained from the surface area analysis. However, the comparative analysis of mobility of contaminants between types of activated mixtures highlighted a different trend of mobility of contaminants to that of pore size distributions. Therefore, the leaching behavior of contaminants suggests that their immobilization in the activated mixtures might occur by both physical and chemical mechanisms. Also, it was emphasized that waste glass is not compliant with respect to its leachable Hg content, and that Hg and Pb exhibit high mobility in the geopolymers with high amount of waste glass added to the synthesis mixtures.     

Factors affecting the suitability of fly ash as source material for geopolymers

The suitability of fly ash stock piles for geopolymer manufacturing was studied. The results of chemical analyses, X-ray diffraction (XRD) and particle size distribution (PSD) of five sources of fly ash obtained from coal-fired power generating plants in the US are presented. Geopolymer paste and concrete specimens were prepared from each stock pile. The specimens were subjected to an array of chemical and mechanical tests including XRD, RAMAN spectroscopy, setting time and compressive strength. A correlation study was undertaken comparing the fly ash precursor chemical and crystallographic compositions as well as particle size distribution, with the mechanical and chemical characteristics of the resulting geopolymer. Factors inherent to the fly ash stockpile such as particle size distribution, degree of vitrification and location of the glass diffraction maximum were found to play an important role in the fresh and hardened properties of the resulting geopolymer.     

The interface between natural siliceous aggregates and geopolymers

The reaction products as well as the formation mechanisms of alkali-activated binders, or geopolymers, have been studied intensively. However, the interface between mineral aggregates, such as sand and/or natural rocks, and geopolymers has not been studied. This paper reports the microstructure and the bonding strength (Mode I bending) of the interface between natural siliceous aggregates and fly ash-based geopolymers. It was found that when the activating solution that contained no or little soluble silicates, the compressive strengths of the geopolymeric binders, mortars and concretes were significantly weaker than those activated with high dosages of soluble silicates. The presence of soluble silicates in the initial activating solution was also effective in improving the interfacial bonding strengths between rock aggregates and geopolymeric mortars. No apparent interfacial transition zone (ITZ) could be identified near the aggregates if the systems were free from chloride contamination. Chloride (KCl) was found to decrease the interfacial bonding strength between the aggregates and the binders probably by causing gel crystallisation near the aggregate surfaces, which resulted in debonding.     

Predicting compressive strength of different geopolymers by artificial neural networks

In the present study, six different models based on artificial neural networks have been developed to predict the compressive strength of different types of geopolymers. The differences between the models were in the number of neurons in hidden layers and in the method of finalizing the models. Seven independent input parameters that cover the curing time, Ca(OH)₂ content, the amount of superplasticizer, NaOH concentration, mold type, geopolymer type and H₂O/Na₂O molar ratio were considered. For each set of these input variables, the compressive strength of geopolymers was obtained. A total number of 399 input-target pairs were collected from the literature, randomly divided into 279, 60 and 60 data and were trained, validated and tested, respectively. The best performance model was obtained through a network with two hidden layers and absolute fraction of variance of 0.9916, the absolute percentage error of 2.2102 and the root mean square error of 1.4867 in training phase. Additionally, the entire trained, validated and tested network showed a strong potential for predicting the compressive strength of geopolymers with a reasonable performance in the considered range.     

Preparation and performance of nanosized tungsten carbides for electrocatalysis

The principle of the intermittent microwave heating (IMH) method and the details on the working procedure for prepare nanosized materials were presented along with the comparison to the traditional continuous microwave heating (CMH) method. The nanosized tungsten carbides were synthesized as an example by this novel method. It produced WC with the average particle size of 21.4 nm at the procedure of 15 s-on and 15 s-off for 20 times, however, the particle size increased to 35.7 nm by CMH method for 5 min. The pure WC was obtained by post-treating the sample in NaOH solution, which gave the better performance as support.The nanosized WC was used as support for the Pt nanoparticles (Pt-WC/C(IMH)) for alcohol oxidation and oxygen reduction. It was proved that the Pt-WC/C(IMH) electrocatalysts gave the better performance than that prepared by CMH method (Pt-WC/C(CMH)) or Pt/C electrocatalysts in terms of the activity and CO-tolerance. The intermittent microwave heating method is easier to scale-up for mass production of the nanosized tungsten carbides and other nanosized materials as well.     

Waste stabilization/solidification of an electric arc furnace dust using fly ash-based geopolymers

The stabilization/solidification (S/S) of a carbon steel electric arc furnace (EAF) dust containing hazardous metals such as Pb, Cd, Cr or Zn using geopolymerization technology is described in this paper. Different reagents such as sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, kaolinite, metakaolinite and blast furnace slag have been used. Mixtures of EAF waste with these geopolymeric materials and class F fly ash have been processed for studying the potential of geopolymers as waste immobilizing agents. Compressive strength tests and leaching tests for determining the efficiency of heavy metal immobilisation have been carried out. Comparison of fly ash-based geopolymer systems with classic Portland cement stabilization methods has also been accomplished. Compressive strength values far better than those achieved by hydraulic S/S methods were easily obtained by geopolymer solids at 28 days. Regarding leachability, the geopolymer S/S solids also manifested in general a better behaviour, showing very promising results.     

Geopolymerization kinetics of fly ash based geopolymers using JMAK model

Geopolymers are versatile materials possessing excellent mechanical properties and resistance against aggressive environments, these materials present a benefit of improving simultaneously both the environmental and engineering performance as compared to classical conventional materials. This paper determines the geopolymerization kinetics of fly ash based geopolymers using Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The experiments were designed using Taguchi method by varying four factors (Si/Al ratio, Na/Al ratio, W/S ratio, and curing temperature). The degree of reaction of fly ash (α) was used as a measure of the changes occurring during geopolymerization reaction. The characterization of the cured geopolymers was also carried out. The values of n were in the range of 0.0931–0.2321 while the values of k were in the range of 0.366–0.671. According to the JMAK model results, geopolymerization of fly ash based geopolymers is a one dimensional diffusion controlled reaction and its growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction, and further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The asymmetric stretching band of Si-O-T shifted to 992 cm⁻¹ and increased in intensity indicating the formation of geopolymer. Microstructural analysis showed the heterogeneous nature of geopolymers consisting of geopolymer, unreacted fly ash, and different types of needle like structures while one sample showed plate like morphology consistent with the JMAK model results. The geopolymer was found to be an amorphous material with only few peaks due to unreacted crystalline fly ash.     

Evolution of interfacial microstructures between tungsten wires/flakes or graphite flakes and Al2O3/ZrO2 eutectic ceramics via a novel combustion synthesis centrifugal casting

The Al₂O₃/ZrO₂ eutectic ceramics with adding tungsten wires/flakes and graphite flakes were successfully prepared via a novel Al/Zr(NO₃)₄ combustion synthesis centrifugal casting under low-pressure, and the interfacial bonding between them and the Al₂O₃/ZrO₂ eutectic was studied via using the characterization methods of Energy Dispersive Spectrometry (EDS), X-ray diffraction (XRD), Hardness and Raman. The results show that there was a thin chemical reaction layer at the interface between the tungsten wires or flakes and the Al₂O₃/ZrO₂ eutectic system, which increased slightly with the increase of the reaction temperature from 3000 K to 3600 K, indicating that the interface was well combined. When the adiabatic temperature was 3600 K, the reaction layer thickness can reach ∼9.2 μm with the composition of WO₃/Al₂Zr. However, when graphite flakes were added to the combustion system, they can be oxidized by Al₂O₃ or ZrO₂ to form CO or CO₂, and the gas generation leaded to poor interface bonding between them and Al₂O₃/ZrO₂ eutectic system. These studies provided some theoretical guidance for the future addition of tungsten fibers and carbon fibers to Al₂O₃/ZrO₂ eutectic ceramics to improve the fracture toughness and strength of the materials. Moreover, it has guiding significance for preparing various fibers or woven fibers reinforced composite materials with large size, high density and excellent mechanical properties under ultra-high temperature.     

Metakaolin-based geopolymers: Efflorescence and its effect on microstructure and mechanical properties

Efflorescence in geopolymers results from mobility of excess alkali and consequent crystallization of alkali carbonates. Efflorescence potential of various geopolymers has been reported previously but the knowledge regarding the effect of efflorescence on the microstructure and mechanical properties of geopolymers remains limited. In this work, metakaolin-based geopolymers were exposed to air, partially immersed in water, and fully immersed, to simulate different processes involved in efflorescence formation. The mechanical properties were assessed by compressive, splitting tensile and flexural strengths, and linear deformation. The microstructural features were investigated by SEM, synchrotron XRD, multinuclear MAS NMR, MIP and synchrotron X-ray microtomography. Extensive efflorescence resulted in a reduction of mechanical strength and changes in the nanostructure and microstructure, which is different from observations for Portland cement-based materials, where efflorescence is usually regarded as a surface or aesthetic problem. The understanding of the relationship between efflorescence formation, the synthesis and exposure conditions provides important insight into the manufacturing and application conditions of geopolymer related materials.     

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between T_g and T_x, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of T_g + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.     

Synthesis of tungsten carbide nanopowders by direct carbonization of tungsten oxide and carbon: Effects of tungsten oxide source on phase structure and morphology evolution

In the paper, WC nanopowders are successfully prepared by carbothermal reduction method, and the effect of tungsten oxide source on the phase structure evolution and products properties of the as-synthesized WC nanopowders has been investigated. Four tungsten oxide powders are chosen as tungsten oxide sources, e.g., rods-like WO₃ , WO₃ nanopartiles, WO₃ micro-particles and WO_2.9 micro-particles. Compared with other three tungsten oxide sources, the WO₃ micro-particles possesses small particle size, less agglomerates and good dispersity and the uniform tungsten oxide-carbon mixture after ball milling can be easily obtained. The appropriate tungsten oxide source can result in lower processing temperature (≤1200 °C) and shorter holding time (≤3 h). Single-phase WC powders with average particle size of 100 nm and uniform particle distribution can be achieved by micro-particle-like WO₃ at 1100 °C for 3 h. The as-prepared WC products by other three tungsten oxide sources exhibit problems of more aggregates, non-uniform particle size and large particle size (250 nm), respectively. In addition, the method can provide a facile, low-cost, efficient, and industrially feasible pathway for large scale preparation of WC nanopowders.     

Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures

This paper presents the results of a study on the effect of elevated temperatures on geopolymers manufactured using metakaolin and fly ash of various mixture proportions. Both types of geopolymers (metakaolin and fly ash) were synthesized with sodium silicate and potassium hydroxide solutions.

The strength of the fly ash-based geopolymer increased after exposure to elevated temperatures (800 °C). However, the strength of the corresponding metakaolin-based geopolymer decreased after similar exposure. Both types of geopolymers were subjected to thermogravimetric, scanning electron microscopy and mercury intrusion porosimetry tests. The paper concludes that the fly ash-based geopolymers have large numbers of small pores which facilitate the escape of moisture when heated, thus causing minimal damage to the geopolymer matrix. On the other hand, metakaolin geopolymers do not possess such pore distribution structures. The strength increase in fly ash geopolymers is also partly attributed to the sintering reactions of un-reacted fly ash particles.     

Thermoelectric properties of geopolymers with the addition of nano-silicon carbide (SiC) powder

This paper investigated the effects of nano-silicon carbide (SiC) powder on the thermoelectric properties of both fly ash and metakaolin based geopolymer. The influences of different parameters, i.e., SiC dosage, alkali concentration and curing temperature, were investigated. Results showed that the addition of SiC powder effectively increased the Seebeck coefficient of geopolymers because of the quantum confinement effect. A higher alkali concentration is capable of increasing the Seebeck coefficient of both fly ash and metakaolin based geopolymer, while curing temperature has an insignificant influence on the Seebeck effect of metakaolin based geopolymer.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

New high temperature dielectrics: Bi-free tungsten bronze ceramics with stable permittivity over a very wide temperature range

High relative permittivity, ε_r, over a very wide temperature range, ?65 °C to 325 °C, is presented for ceramics designed to be compatible with base metal electrode multilayer capacitor manufacturing processes. We report a ≥ 300 °C potential Class II capacitor material, free from Bi or Pb ions, developed by doping Sr₂NaNb₅O₁₅ with Ca²⁺, Y³⁺ and Zr⁴⁺ ions, according to the formulation Sr_2?2zCa_zY_zNaNb_5-zZr_zO₁₅. For sample composition z = 0.025, ε_r values are 1565 ± 15 % (1 kHz) from ?65 °C to 325 °C. At a slightly higher level of doping, z = 0.05, ε_r values are 1310 ± 10 % from ?65 °C to 300 °C. Values of the dielectric loss tangent, tanδ are ≤ 0.025 from ?60 °C to 290 °C, for z = 0.025, with tanδ increasing to 0.035 at 325 °C. Microstructural analyses exclude core-shell mechanisms being responsible for the flattening of the ε_r –T response.     

Effect of fiber factor on the workability and mechanical properties of polyethylene fiber-reinforced high toughness geopolymers

In this work, we studied the influence of fiber factor (F) on the workability, mechanical properties, and internal defects of polyethylene (PE) fiber-reinforced high toughness geopolymers, and two specific fiber parameters, the critical fiber factor (F_c = 150) and dense fiber factor (F_d = 600), were proposed. Subsequently, the workability and hardening properties of the 26 PE fiber-reinforced geopolymer (FRG) groups were divided into three grades. When F was lower than F_c, the fibers had an insignificant effect on the workability and hardening properties of the material. However, when F was higher than F_d, many fibers agglomerated and deteriorated the workability and mechanical properties. Thus, when F was between F_c and F_d, satisfactory workability and excellent mechanical properties were simultaneously obtained. Furthermore, with similar fiber stiffness coefficients, the relative yield stress of FRG was similar to the fiber-reinforced Portland cement-based composites. Moreover, the flexural and compressive strength values of the PE FRG were both higher than the PE fiber-reinforced Portland cement-based composites. As a result, this work provides relevant conclusions for the theoretical reference and design of FRG with satisfactory workability and mechanical behaviors for concrete engineering.