Nepheline Formation Research Articles

Physico-mechanical and microstructural evolution of sintered pressed geopolymer: Dual effects of aging period and sintering temperature

Ceramics are highly valued for their exceptional thermal resistance and ability to withstand high temperatures. This study investigated the production of ceramic-like pressed geopolymer, focusing on the effects of aging period and sintering temperature. The results showed that the 7-day pressed geopolymer achieved the highest compressive (134.7 MPa) and flexural (34.9 MPa) strengths after sintering at 1000°C, with a density retention of 93.7 %, a mass loss of 7.9 %, and a thermal shrinkage of 1.4 %. Microstructural analysis at 1000°C showed a dense ceramic-like structure with nepheline formation. Phase analysis of the 7-day pressed geopolymer revealed a decrease in the amorphous phase as temperature increased, with nepheline formed at 800°C (26.0 %) and maximized at 1000°C (61.0 %). This work offers an optimal aging period and sintering temperature to maximize the mechanical strength and nepheline crystal formation, making them perfect for fire-resistant panels and precast construction products.

Formation of nepheline in the heating process of mould fluxes and its effect on sintering behaviour

This study explores the formation mechanism of nepheline (NaAlSiO4) in the 500–900 °C heating process of mould flux and the influence of Al-containing materials on the nepheline formation using thermogravimetric-differential scanning calorimetry, scanning electron microscope and X-ray diffraction. The results show that the formation behaviour of nepheline depends on the type of Al2O3-containing materials. At 500 °C, flint clay (Al2SiO5) forms nepheline by solid-state reaction with sodium carbonate (Na2CO3). During the solid-state reaction process, F− improves the kinetic conditions by promoting the diffusion of Na+ into the flint clay particle. F- is derived from the tiny NaF formed by the reaction of cryolite (Na3AlF6) with sodium carbonate. Unlike flint clay, bauxite (Al2O3) reacts with liquid phase at 700 °C to form nepheline. Therefore, the low-temperature sintering of mold flux bauxite-containing is weaker than the flux containing cryolite and flint clay. At 500 °C, the sintered rate of bauxite-containing flux is 22.55%, and the sintered rate of flux containing cryolite and flint clay is 39.50%. With the increase in temperature, the sintered rate of bauxite-containing flux increases, which is gradually close to that of flux containing cryolite and flint clay. Until 900 °C, the sintered rate of bauxite-containing flux (63.45%) is the same as that of flux containing cryolite and flint clay (62.15%).

Geopolymers: A viable binder option for ultra-low-cement and cement-free refractory castables?

Alternative binders offer an avenue for advancing ultra-low-cement and cement-free castables while reducing carbon emissions and energy consumption. In this study, geopolymer binders partially or fully replaced calcium aluminate cement (CAC) in high-alumina castables. The formulations underwent diverse testing, including flowability, physico-thermo-mechanical measurements, and XRD, ATR-FTIR, and SEM analyses. The findings highlighted the feasibility of utilizing geopolymers to craft innovative refractories. The formulation comprising 2.7 wt% CAC and 1.3 wt% geopolymer (AT-3 C-1 G*) exhibited the most successful performance. At 1400 °C, these samples displayed shrinkage of 1.18%, elastic modulus of 135 GPa, flexural strength of 39.1 MPa, and high thermal shock resistance (∆T∼1000 °C). The interaction between CAC and geopolymer inhibited cement hydration while enhancing the geopolymerization process. Moreover, firing the samples at 800–1400 °C led to liquid phase and nepheline formation from the geopolymeric gel, resulting in viscous sintering and improved ceramic densification. These developments produced refractories with remarkable overall performance, surpassing the reference material.

Compositional dependence of crystallization and chemical durability in alkali aluminoborosilicate glasses

This study aims to understand the impact of composition on crystallization and chemical durability in alkali aluminoborosilicate based model nuclear waste glasses designed in the peralkaline, metaluminous and peraluminous regimes. The glasses have been thermally treated using the canister centerline cooling (CCC) schedule. The chemical durability of both parent and CCC-treated glasses has been assessed by product consistency test (PCT-B) for 120 days. The peraluminous glasses exhibit the highest dissolution rates, followed by peralkaline and metaluminous glasses. In general, increasing B2O3 content in glasses tends to suppress nepheline formation, thus, decreasing the negative impact of nepheline on durability of the final waste form. However, higher B2O3 content itself may result in detrimental impact on the durability of the final waste form. The thermal history has been shown to have a significant impact on the durability of the glasses.

Observation of nepheline crystallization in the Na2O-B2O3-Al2O3-SiO2 system for the vitrification of zeolites absorbing Cs

ABSTRACT For the vitrification of zeolites used for absorbing radioactive cesium, the composition of the glass in which nepheline forms at the cooling rates of 0.5, 1, 2.5°C min−1 were clarified for the Na2O-B2O3-Al2O3-SiO2 quaternary system to avoid crystallization during cooling of the melt. The trend of the nepheline formation composition was similar to that suggested in previous studies, i.e. the nepheline discriminator (N.D.) < 0.62 and the optical basicity (O.B.) >0.575, especially at the higher B2O3 content, while the criteria was eased at a lower B2O3 content at the cooling rate of 2.5℃ min−1. The increase in the B2O3 content of the glasses seemed to reduce the effect of the cooling rate on the crystallization. The results obtained by adding 1 wt% Cs2O in the quaternary system shows that the effect of Cs2O would be negligible as long as its content in the formed glass would not exceed 1 wt%. In contrast to the quaternary system, the Cs2O-Na2O-Al2O3-SiO2 system showed the different formation of the crystals. The fine granulous crystals were dispersed in the glass matrix and not in the region contacting the crucible as observed in the Na2O-B2O3-Al2O3-SiO2 system. Moreover, the number of non-crystal samples was greater than that predicted by the criteria, N.D. <0.62 and O.B. >0.575.

The upper devonian magmatic complexes of the South-East Belarus

For the first time, the authors determined lateral-temporal series of magmatic complexes of the Late Frasnian-Early Famennian Pripyat-Dnieper Magmatic Area according to the results of petrological and geochemical study of rocks of the Pripyat-Dnieper Magmatic Area (north-western part of the Pripyat-Dnieper-Donetsk Magmatic Province) in the southwest of the East European Platform using the principles of structural-material analysis and taking into account previous studies. The series consists of four complexes, formed during stages of magmatic activity, separated by time intervals: Zhlobin Complex (Rechitsa Time, the beginning of the Late Frasnian), Uvarovichi complex (Late Voronezh Time, the middle of the Late Frasnian), Pripyat Complex (Skolodin (Skolodin-Chernin?) Time, the end of the Late Frasnian) and Loev complex (Yelets (Yelets- Petrikov?) Time, Early Famennian). The rocks of the Zhlobin Complex belong to the alkaline-ultramafic (carbonatite-kimberlite-nephelinite) formation; Uvarovichi Complex - to the alkaline-mafic formation (basaltoids and phonolites); Pripyat Complex - to the alkaline-mafic-salic formation (trachyandesites); Loev Complex - to the alkaline-ultramafic (nepheline) formation.

A data‐driven approach for predicting nepheline crystallization in high‐level waste glasses

AbstractHigh‐level waste (HLW) glasses with high alumina content are prone to nepheline crystallization during the slow canister cooling that is experienced during large‐scale production. Because of its detrimental effects on glass durability, nepheline (NaAlSiO4) precipitation must be avoided; however, developing robust, predictive models for nepheline crystallization behavior in compositionally complex HLW glasses is difficult. Using overly conservative constraints to predict nepheline formation can limit the waste loading to lower than the achievable capacity. In this study, a robust data‐driven model using five compositional features has been developed to predict nepheline formation. A new descriptor is introduced called the “difference based on correlation”, which has higher accuracy compared to previous descriptors and also has more balanced false positive and false negative rates. The analysis of the model and the data show an overlap, instead of a distinct compositional boundary, between glasses that form and do not form nepheline. As a result, the model's predictive accuracy is not the same throughout the feature space and instead is dependent on the location of the glass composition in the dimensionally reduced feature space.

Chemistry of Nepheline Formation in the Batch Mixture in Ceramic Production

Under the conditions of preliminary treatment during the preparation to annealing of a mixture of Tuva clays with the dearsenation cake of the Khovu-Aksy dump sludge, the material was found to undergo a chemical process that forms nepheline. Based on the calculated Gibbs energies of the reactions, it was concluded that the reaction occurs at the minimum number of components in the system: silicate Na4Mg2Si3O10–kaolinite (kaolinite + orthoclase).

Developing a space-filling mixture experiment design when the components are subject to linear and nonlinear constraints

This article presents a case study of developing a space-filling design (SFD) for a constrained mixture experiment when the experimental region is specified by single-component constraints (SCCs), linear multiple-component constraints (LMCCs), and nonlinear multiple-component constraints (NMCCs). Traditional methods and software for designing constrained mixture experiments with SCCs and LMCCs (using either optimal design or SFD approaches) are not directly applicable because of the NMCCs. A SFD algorithm in the JMP® software was modified to accommodate the NMCCs; the modification is described in this article. The case study involves high-level waste (HLW) glass that is subject to the formation of nepheline crystals as the glass cools. This can significantly reduce the durability of HLW glass (which is undesirable). The goal of the study was to develop a SFD for the HLW glass compositional region where nepheline may form, and generate data for modeling nepheline formation as a function of HLW glass composition. The HLW glass composition region was specified in terms of eight components with SCCs, two LMCCs, and two NMCCs. The NMCCs were based on a nonlinear logistic regression model for a binary nepheline response that was developed from previous data. This article discusses the HLW glass example, the constraints specifying the experimental composition region, and how an existing algorithm for generating SFDs was modified to accommodate the NMCCs. The methodology discussed in this article can be applied to any example in which the experimental region is specified by one or more nonlinear constraints in addition to linear constraints on mixture components and/or non-mixture variables.

Nepheline crystallization behavior in simulated high-level waste glasses

The U.S. Department of Energy immobilizes legacy nuclear high-level waste (HLW) in borosilicate glass. HLW glass with elevated Na2O and Al2O3 concentrations have a demonstrated propensity for precipitating nepheline (NaAlSiO4) during cooling, which adversely affects the waste form performance. Currently, nepheline is suppressed by controlling the HLW glass composition, but this can limit the waste loading and result in longer mission life and cost. Correlation between composition and thermal history experienced during processing with time-temperature relationships that enable nepheline crystal growth would facilitate a more realistic prediction of nepheline crystallization in HLW glass and allow for a wider range of compositions to be accessed. In this work, crystallization behavior in simulated HLW glass was investigated to identify key attributes that lead to nepheline formation, with results indicating that the rate of diffusion for crystal growth may be controlled by glass structure connectivity.

Thermodynamic assessment of the Na2O-Al2O3-SiO2-B2O3 pseudo-binary and -ternary systems

Thermodynamic assessments of the pseudo-binary and -ternary systems formed by B2O3 with Na2O, Al2O3, and SiO2 were conducted according to the CALPHAD methodology. The compound energy formalism and two-sublattice partially ionic liquid models were used to thermodynamically represent the solid solutions malinkoite and mullite and the liquid phase of each system, respectively. A comprehensive literature review of available phase equilibria and thermodynamic experimental data as well as a detailed discussion of the modeling approaches implemented to optimize each system is provided. Assessment results are then presented and discussed, and the future path forward is outlined. The addition of B2O3 to the Na2O-Al2O3-SiO2 system contributes to the development of a thermodynamic database that will ultimately predict the equilibrium behavior of nepheline formation in high-level radioactive waste glass.

Thermodynamic assessment of the pseudoternary Na2O–Al2O3–SiO2 system

AbstractVitrified high‐level radioactive waste that contains high concentrations of Na2O and Al2O3, such as the waste stored at the Hanford site, can cause nepheline to precipitate in the glass upon cooling in the canisters. Nepheline formation removes oxides such as Al2O3 and SiO2 from the host glass, which can reduce its chemical durability. Uncertainty in the extent of precipitated nepheline necessitates operating at an enhanced waste loading margin, which increases operational costs by extending the vitrification mission as well as increasing waste storage requirements. A thermodynamic evaluation of the Na2O–Al2O3–SiO2 system that forms nepheline was conducted by utilizing the compound energy formalism and ionic liquid model to represent the solid solution and liquid phases, respectively. These were optimized with experimental data and used to extrapolate phase boundaries into regions of temperature and composition where measurements are unavailable. The intent is to import the determined Gibbs energies into a phase field model to more accurately predict nepheline phase formation and morphology evolution in waste glasses to allow for the design of formulations with maximum loading.

Formation of one-part-mixing geopolymers and geopolymer ceramics from geopolymer powder

Geopolymer powder prepared through pre-curing and pulverization showed great potential to produce one-part-mixing geopolymers as well as high flexural strength geopolymer ceramics. The one-part-mixing geopolymers were prepared by mixing geopolymer powder with water while the geopolymer ceramics were prepared by powder metallurgy and sintering. The one-part-mixing geopolymers achieved a compressive strength of 10MPa after 28days with formation of geopolymer precipitates in conjunction with zeolite phases. Despite the lower strength, they remained stable and did not disintegrate when immersed in water. Besides, the geopolymer ceramics exhibited high flexural strength (90MPa) after sintering at 1200°C as result of nepheline formation.

The study of the thermal behavior of solid mixtures of metakaolin and sodium hydroxide by isoconversional model-free analyzes

Interactions in solid mixtures of 6Al2Si2O7: 12NaOH and 6Al2Si2O7: 12NaOH: 2Al2O3 under thermal treatment were studied. Ultrasonic pre-treatment of suspensions with a frequency of 22 kHz was applied. X-ray phase analysis, scanning electron microscopy, and synchronous thermal analysis were used in this research. It was shown that after evaporation of the suspensions, the hydrated LTA zeolite and sodium hydroaluminate are synthesized. During thermal treatment up to 500 °C, the adsorption water is removed first and then the dehydration of the zeolite and hydroaluminate occurs. Calcination at temperatures above 500° C leads to the synthesis of Na6Al4Si4O17 and Na8Al4Si4O18 by the interaction of metakaolin and zeolite with sodium hydroxide. At temperatures above 700 °C, the formation of mullite and nepheline occurs in the temperature range of 500–800 °C. The kinetic parameters have been calculated using Friedman analysis (differential method), Kissinger–Akahira–Sunose and Ozawa-Flynn-Wall analyzes (integral methods). It was shown that all analyzes give similar dependences of the apparent activation energy vs the conversion extent and the values of E are in the range of 70–350 kJ mol−1. It was established that ultrasonic pre-treatment allows to reduce the values of the apparent activation energy. It was discovered that the Al2O3 excess over the stoichiometry of the LTA zeolite synthesis is an inhibitor of the mullite and nepheline formation reactions.

Nepheline formation in chondrite parent bodies: Verification through experiments

Nepheline is present as fine grains mainly in refractory inclusions and chondrules in CV and CO carbonaceous chondrites. The nepheline has been formed primarily by replacement of melilite and plagioclase in refractory inclusions and plagioclase and glass in chondrules. The nepheline formation is thought to have occurred during aqueous alteration and thermal metamorphism in the meteorite parent bodies. To verify this hypothesis, we performed the following experiments.Hydrothermal experiments of gehlenite (Al-rich melilite) and plagioclase (An48) were carried out at 200°C and ∼15bar for 168h using solutions of pH 0, 7, 13, and 14 with a uniform Na concentration. In the gehlenite experiments, various amounts of SiO2 were added. The results revealed that a Na zeolite, analcime, was produced from 10/3 and 10/6 mixtures of gehlenite/SiO2 at pH 7, 13, and 14, and from a 10/10 mixture of gehlenite/SiO2 and plagioclase at pH 13 and 14. In particular, at pH 14, in addition to analcime, significant amounts of two other zeolites, fabriesite and hydroxycancrinite, were produced from the 10/6 mixture of gehlenite/SiO2, and fabriesite from plagioclase.Isothermal heating experiments for 24h showed that fabriesite, hydroxycancrinite, and analcime transform to nepheline at 600–650, 550–600, and 750–800°C, respectively. Differential thermal analysis of these zeolites revealed that fabriesite and hydroxycancrinite exhibit exothermic peaks, which correspond to transformation to nepheline, and that the temperatures of those peaks decrease steadily with decreasing heating rate. Kinetic analysis using these data revealed that fabriesite transforms to nepheline at <600°C if heated for durations >102yr and hydroxycancrinite transforms to nepheline at <430°C if heated for durations >1yr. Analcime heated non-isothermally at a rate of 1°C/min transformed to nepheline at temperature higher than that determined by the isothermal experiments, suggesting that its transformation temperature also decreases if it is heated for a much longer duration. From these experiments and analyses, we conclude that fabriesite, hydroxycancrinite, and possibly analcime are capable of transforming to nepheline by heating in meteorite parent bodies.From our results, we propose that the nepheline in refractory inclusions and chondrules in meteorites formed by a two-stage alteration process: (1) formation of the Na zeolites from melilite, plagioclase, and glass by hydrothermal alteration at low temperature (probably <300°C) in the presence of small amounts of Na-rich and alkaline aqueous solutions, and (2) transformation of the Na zeolites to nepheline due to heating at higher temperature (300–700°C) for very long durations after the solutions were lost.

Effect of ultrasound on the thermal behavior of the mixtures for the LTA zeolite synthesis based on metakaolin

The effect of ultrasonic treatment on the thermal behavior of the mixtures from metakaolin, sodium hydroxide and alumina designed for LTA zeolites synthesis was studied. X-ray diffraction analysis, infrared spectroscopy, scanning electron microscopy and synchronous thermal analysis have been used. It was shown that after evaporation of the suspension, LTA zeolite (24 mass%) is contained in the samples. It was established that the new phase (sodium aluminum silicate) is formed at a calcination temperature of about 600 °C. It was demonstrated that at a calcination temperature over 800 °C, nepheline is synthesized. The reaction of nepheline formation has been described by the topochemical equation of four-dimensional nucleation/nucleus growth according to Avrami/Erofeev. Using the Ozawa–Flynn–Wall analysis for non-isothermal data, the values of the activation energy and the pre-exponential factor have been calculated. It is shown that after the ultrasonic treatment the activation energy of the nepheline synthesis reaction has smaller values than in the sample without pretreatment. These phenomena have been explained by differences between the structural parameters of the particles (dimension of the coherent scattering region, the value of microdeformations).

Submixture model to predict nepheline precipitation in waste glasses

AbstractHigh‐alumina high‐level waste (HLW) glasses are prone to nepheline precipitation during canister‐centerline cooling (CCC). If sufficient nepheline forms, the chemical durability of the glass will be significantly impacted. Overly conservative constraints have been developed and used to avoid the deleterious effects of nepheline formation in U.S. HLW glasses. The constraints used have been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A 90‐glass study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. The CCC crystallinity data from these glasses were combined with 657 glasses found in the literature. The trends showed significant effects of Na2O, Al2O3, SiO2, B2O3, CaO, Li2O, and potentially K2O on the propensity for nepheline formation. A pseudo‐ternary submixture model was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo‐ternary with axes of SiO2 + 1.98B2O3, Na2O + 0.653Li2O + 0.158CaO, and Al2O3 was found to divide glasses that precipitate nepheline during CCC from those that do not. Application of this constraint is anticipated to increase the loading of Hanford high‐alumina HLWs in glass by roughly one‐third.

Nepheline Crystallization in High-Alumina High-Level Waste Glass

ABSTRACTThe understanding of the crystallization of aluminosilicate phases in nuclear waste glasses is a major challenge for nuclear waste vitrification. Robust studies on the compositional dependence of nepheline formation have focused on large compositional spaces with hundreds of glass compositions. However, there are clear benefits to obtaining complete descriptions of the conditions under which crystallization occurs for specific glasses, adding to the understanding of nucleation and growth kinetics and interfacial conditions. The focus of this work was the investigation of the microstructure and composition of one simulant high-level nuclear waste glass crystallized under isothermal and continuous cooling schedules. It was observed that conditions of low undercooling, nepheline was the most abundant aluminosilicate phase. Further undercooling led to the formation of additional phases such as calcium phosphate. Nepheline composition was independent of thermal history.

He and Au ion radiation damage in sodalite, Na4Al3Si3O12Cl

Sodalite, a candidate ceramic for the immobilisation of pyroprocessing nuclear waste, showed no observable lattice dilatation in grazing incidence X-ray diffraction when irradiated with up to 1017 5MeVHe ions/cm2. However micro-Raman scattering showed considerable spectral broadening characteristic of radiation damage near the end of the ∼22μm He range. Partial amorphism plus nepheline formation was observed in grazing incidence X-ray diffraction when sodalite was irradiated by 101612MeVAu ions/cm2. Nepheline appeared less susceptible to 12MeV Au ion damage than sodalite, with ∼25% less amorphous fraction at 1016ions/cm2.

Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study

We experimentally investigated the phase rela- tions of a peralkaline phonolitic dyke rock associated with the Ilimaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilimaussaq com- plex. This dyke rock is therefore perfectly suited for per- forming phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate for- mation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950-750 C. H2O contents ranging from anhy- drous to H2O saturated (*5 wt% H2O) and varying fO2 (*DlogFMQ - 3t o?1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystalliza- tion of alkali feldspar, nepheline, hedenbergite-rich clino- pyroxene, fayalite-rich olivine and minor amounts of ulvospinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H2O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO2 and aH2O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO2 and aH2O produce residual melts with relatively low FeO*, high SiO2 and a relatively constant alkalinity index. We show that variations of aH2O and fO2 lead to contrasting trends regarding the liquid lines of descent of iron-rich silica- undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na2Fe5TiSi6O20), coexisting with the most evolved melts at 750 C. Contrary to what might be expected, experiments with high aH2O and interlinked high fO2 exhibit higher liquidus T's compared with exper- iments performed at low aH2O and fO2 for experiments where magnetite is liquidus phase. This is because ulvospinel-poor magnetite crystallizes at higher fO2 and has a higher melting point than ulvospinel-rich magnetite, which is favored at lower fO2.