Language：English  

DOI： 10.2139/ssrn.4849308

Language：English  

DOI： 10.2139/ssrn.4772600

Language：English   Publisher：Elsevier BV  

M-S-H gels were synthesised via reaction of Mg(OH)<inf>2</inf> with silica fume, cured at 35 °C for up to 112 days, and their chemical and nanostructural evolution was examined. M-S-H gels with structural similarity to the thermodynamically stable serpentine-group mineral lizardite were formed. Quantification of ²⁵Mg and ²⁹Si MAS and ¹H[sbnd]²⁹Si CPMAS NMR, electron microscopy, and thermogravimetric data showed dissolution of brucite and silica fume, and M-S-H formation, all occurred linearly with time up to 56 days. Data showed strong correlation with the Avrami-Erofeyey nucleation kinetic model, indicating M-S-H formation was governed by nucleation reactions. After 112 days, two distinct M-S-H gels were formed: a Si-rich M-S-H gel with molar Mg/Si = 0.55(±0.2), and a Mg-rich M-S-H gel with molar Mg/Si = 0.80(±0.5). Nanostructural rearrangement of M-S-H continues up to 112 days, with increased crosslinking and polymerisation. This new insight is important for application of M-S-H binders in both construction and radioactive/toxic waste immobilisation.

DOI： 10.1016/j.cemconres.2023.107295

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Language：English   Publisher：International Congress on the Chemistry of Cement (ICCC)  

Language：English   Publisher：International Congress on the Chemistry of Cement (ICCC)  

Language：English  

Language：English   Publisher：Institute of Materials, Minerals \& Mining  

Language：English  

DOI： 10.2139/ssrn.4500612

Language：English   Publisher：Elsevier BV  

The thermal treatment of limestone (mainly CaCO<inf>3</inf>) to produce lime (CaO) is a major contributor to CO<inf>2</inf> emissions and the literature on decarbonising the lime industry is scarce. Subsequent hydration of lime would lead to the synthesis of slaked/hydrated lime Ca(OH)<inf>2</inf>; the production of a tonne of Ca(OH)<inf>2</inf> emits ∼1.2 tonnes of CO<inf>2</inf> arising mainly from the process chemistry and fossil fuel combustion. Carbon Capture & Storage (CCS) technologies are currently believed to have the highest potential to mitigate these CO<inf>2</inf> emissions, assuming that the thermal calcination of CaCO<inf>3</inf> is unavoidable. Despite intensive research efforts and development, CCS technologies cannot be industrially applied yet due to their limited efficiency and the associated capital and operational costs. In this review, the current state of the lime industry and its processing configurations is visualised. This is followed by a detailed description of the current status of the relevant CCS technologies (including their CO<inf>2</inf> avoidance costs) and eco-efficient alternative fuels. This work then gives voice to two novel and potentially more sustainable decarbonisation routes that do not involve the thermal calcination of CaCO<inf>3</inf>, one of which involves simultaneous mineralisation leading to permanent storage of CO<inf>2</inf>. These technologies are particularly interesting especially if high temperature lime kilns, as we know them, are phased out to meet climate goals. It is revealed that the energy shift to green electricity might lead to a no-carbon lime industry and subsequent carbon neutrality (or negativity) of other hard-to-abate sectors.

DOI： 10.1016/j.rser.2022.112765

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Language：English   Publisher：American Chemical Society (ACS)  

Decarbonizing calcium carbonate (CaCO3) is a crucial step for a wide range of major industrial processes and materials, including Portland cement (PC) production. Apart from the carbon footprint linked to fuel combustion, the process CO2 embodied within CaCO3 represents the main concern for the sustainability of production. Our recent works demonstrated that it is possible to avoid both the fuel and process CO2 by reacting CaCO3 with aqueous NaOH and obtain Ca(OH)2 and Na2CO3·xH2O (x = 0 and 1). This present study provides a further understanding of the process by testing different raw calcareous sources. A high decarbonization (80%) of CaCO3 was achieved for silica-rich chalk, whereas a lower extent was obtained (50%) for limestone. To understand the difference in their reaction behavior, the effect of impurities was studied. The effects of the major impurities (Si, Al, and Fe) were found to be marginal, which is advantageous to process industrial grade materials, while the morphology of the raw materials presents a significant impact. The applicability of our decarbonization technology was also demonstrated on magnesite (MgCO3).

DOI： 10.1021/acssuschemeng.2c02913

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PubMed

Language：English   Publisher：Royal Society of Chemistry (RSC)  

The decarbonisation of CaCO<inf>3</inf> is essential for the production of lime (Ca(OH)<inf>2</inf> and CaO), which is a commodity required in several large industries and the main precursor for cement production. CaCO<inf>3</inf> is usually decarbonised at high temperatures, generating gaseous CO<inf>2</inf> which will require post-process capture to minimise its release into the environment. We have developed a new process that can decarbonise CaCO<inf>3</inf> under ambient conditions, while sequestering the CO<inf>2</inf> as Na<inf>2</inf>CO<inf>3</inf>·H<inf>2</inf>O or Na<inf>2</inf>CO<inf>3</inf> in the same stage. Here, the effects of increasing stirring rates and residence times on reaction efficiency of the key reaction occurring between CaCO<inf>3</inf> and NaOH solution are studied. It is shown that the reaction is enhanced at lower stirring rates and longer residence times up to 300 seconds of contact between the reactants. The mass balance performed for Ca and CO<inf>2</inf> revealed that up to the 95% of the process CO<inf>2</inf> embodied in CaCO<inf>3</inf> was sequestered, with maximum capture rate assessed at nn moles CO<inf>2</inf> captured per second of reaction progress. A deeper insight into the precipitation of Na<inf>2</inf>CO<inf>3</inf>·H<inf>2</inf>O or Na<inf>2</inf>CO<inf>3</inf> under different reaction conditions was gained, and SEM-EDX analysis enabled the observation of the reaction front by detection of Na migrating towards inner regions of partially-reacted limestone chalk particles.

DOI： 10.1039/d2cp01412b

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PubMed

Language：English   Publisher：Elsevier BV  

Cementation of aqueous radioactive waste contaminated with a significant ⁹⁰Sr is challenging due to the potential radiolysis of water contents. Utilization of calcium aluminate cement (CAC) modified with sodium polyphosphate (CAP) is interesting as its water content can be reduced by thermal treatment. The present study investigated solidification and stabilization of Sr²⁺ and Cl⁻ ions in CAC and CAP with or without thermal treatment. A leaching test showed a superior Sr²⁺ ion stabilization in CAP: apparent diffusion coefficient of Sr²⁺ was smaller than in the CAC by 5 orders of magnitude. CAC cured at 20 °C had the best stabilization for Cl⁻ ions among the samples tested. Friedel's salt formed in CAC may have contributed to the immobilization of Cl⁻ ions. The stabilization of Cl⁻ ions by CAP was significantly improved by the thermal treatment likely because of the improvement in the microstructure previously reported.

DOI： 10.1016/j.cemconres.2022.106758

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Language：English   Publisher：Société française d’énergie nucléaire (SFEN)  

Language：English  

DOI： 10.21203/rs.3.rs-1597303/v1

Language：English   Publisher：The Royal Society of Chemistry  

The CO<inf>2</inf> released upon calcination of limestone accounts for the largest portion of the emissions from the cement, lime, and slaked lime manufacturing industries. Our previous works highlighted the possibility for a no-combustion decarbonisation of CaCO<inf>3</inf> through reaction with NaOH solutions to produce Ca(OH)<inf>2</inf> at ambient conditions, while sequestrating the process CO<inf>2</inf> in a stable mineral Na<inf>2</inf>CO<inf>3</inf>·H<inf>2</inf>O/Na<inf>2</inf>CO<inf>3</inf>. In this study, the effect of temperature was assessed within the range of 45-80 °C, suggesting that the process is robust and only slightly sensitive to temperature fluctuations. The proportioning of the precipitated phases Na<inf>2</inf>CO<inf>3</inf>·H<inf>2</inf>O/Na<inf>2</inf>CO<inf>3</inf> was also assessed at increasing NaOH molalities and temperatures, with the activity of water playing a crucial role in phase equilibrium. The activation energy (E<inf>a</inf>) of different CaCO<inf>3</inf> : NaOH : H<inf>2</inf>O systems was assessed between 7.8 kJ·mol⁻¹ and 32.1 kJ·mol⁻¹, which is much lower than the conventional calcination route. A preliminary energy balance revealed that the chemical decarbonisation route might be ∼4 times less intensive with respect to the thermal one. The present work offers a further understanding of the effect of temperature on the process with the potential to minimise the emissions from several energy-intensive manufacturing processes, and correctly assess eventual industrial applicability.

DOI： 10.1039/d2ra05827h

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Language：English   Publisher：Japan Concrete Institute  

Large quantities of aqueous secondary waste are generated from the processing of contaminated water after the nuclear accident in Japan. Cementation of these wastes is challenging because their significant radioactivity may cause the radiolysis of water contents in cement, posing a risk of hydrogen gas generation. The application of calcium aluminate cement modified with phosphates (CAP), as an alternative cementing system, is interesting because this system is based on acid-base reaction, and its water content can be reduced by mild heating once the system is mixed. The present study focused in the use of Secar 71, a calcium aluminate cement with a high alumina and low silica compositions, and its effects of on the production of CAP system at elevated temperatures. The modification of Secar 71 with phosphates was successful, and the reduction of water content by about 35% was achieved in the CAP system containing SrCl<inf>2</inf> after curing the system at 80°C for 7 days. The micro cracks, typically observed in the CAP system cured at lower temperature, was significantly reduced by curing at 80°C. The obtained results show a potential of Secar 71 to prepare CAP for cementation of aqueous secondary wastes.

DOI： 10.3151/jact.19.1296

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Language：English   Publisher：European Nuclear Society  

Language：English   Publisher：Frontiers Media SA  

Alite–ye’elimite–ferrite (AYF) cement is a more sustainable alternative to Portland cement (PC) that may offer improved mechanical, rheological, and chemical performance. Using traditional raw materials and conventional clinker processing conditions, alite (C<inf>3</inf>S) and ye’elimite (C<inf>4</inf>A<inf>3</inf>$), the major phases in PC and calcium sulfoaluminate (CSA) cements, respectively, cannot be coproduced. The typical formation temperature in the kiln for alite is >1350°C, but ye’elimite normally breaks down above 1300°C. However, with careful composition control and in the presence of fluoride, alite can be mineralized and formed at lower temperatures, thus enabling the production of AYF clinkers in a single stage. In this study, the production of AYF cement clinkers with different chemical compositions is attempted at 1250°C. The sensitivity of the fluoride content is initially assessed with a fixed target clinker composition to determine the optimal requirements. The effect of altering the target ferrite (C<inf>4</inf>AF) and alite (C<inf>3</inf>S) contents is also assessed followed by the effect of altering the target C<inf>4</inf>AF and C<inf>4</inf>A<inf>3</inf>$ contents. It is shown that AYF clinkers can be produced in a single stage through the careful control of the fluoride content in the mix; however, the formation/persistence of belite and mayenite could not be avoided under the conditions tested. It is also shown that ∼10 wt% ferrite in the target composition provides sufficient AYF clinker burnability and the amount of fluoride needs to be controlled to avoid stabilization of mayenite.

DOI： 10.3389/fbuil.2021.698830

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Language：English   Publisher：Elsevier BV  

The cement industry is carbon-intensive, and the valorisation of industrial side-streams/residuals for use as alternative raw materials can enable the cement industry to reduce its carbon footprint as well as promote resource efficiency. Apart from key clinker ingredients such as CaO, Al<inf>2</inf>O<inf>3</inf>, and SiO<inf>2</inf>, industrial residues can also contain MgO, CaCl<inf>2</inf>, and SO<inf>3</inf>. Therefore, this study investigates the formation of cement clinker assemblages in the system CaO-SiO<inf>2</inf>-Al<inf>2</inf>O<inf>3</inf>-SO<inf>3</inf>-CaCl<inf>2</inf>-MgO at temperatures ranging between 1100 and 1300 °C. The production of a clinker composed mainly of alinite and ye'elimite is first attempted; it is found that these phases cannot be simultaneously produced. Ternesite is also not compatible with alinite under the conditions studied. Wadalite is compatible with both ye'elimite and ternesite, while ternesite is also compatible with chlormayenite at 1150 °C. Additionally, the low-temperature formation of alite was also observed with the presence of CaCl<inf>2</inf> in the raw-material mix.

DOI： 10.1016/j.cemconres.2021.106418

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Language：English   Publisher：Elsevier  

The valorisation of cement kiln bypass dust (CBPD) is explored for sustainable production of cementitious clinker phases. CBPD can replace the calcareous component in cement manufacture and allow for a reduction in CO<inf>2</inf> emissions, as the calcium component in CBPD is mostly decarbonised. CBPD was heated with/out the addition of alumina or silica at temperatures of 900 – 1100 °C to produce clinker phases. Belite and mayenite formed at temperatures as low as 900 °C, while alite formation was achieved at 1200 °C, which is significantly lower than is conventional (~1450 °C). Alite is thermodynamically stable only above 1250 °C; thus, the reduced temperature is potentially owing to the presence of chloride found in the CBPD (as KCl) which was also simultaneously removed through pyro processing at 1100 – 1200 °C. The investigation also confirmed that the formation of belite is enhanced in the presence of molten KCl.

DOI： 10.1016/j.conbuildmat.2020.120420

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Language：English  

Language：English   Publisher：University of Sheffield  

Language：English   Publisher：University of Sheffield  

Language：English   Publisher：University of Sheffield  

Language：English  

Language：English   Publisher：Elsevier BV  

Blast furnace slag from the steel industry is commercially utilized as a cement replacement material without major processing requirements; however, there are many unutilized steel production slags which differ considerably from the blast furnace slag in chemical and physical properties. In this study, calcium sulfoaluminate belite (CSAB) cement clinkers were produced using generally unutilized metallurgical industry residues: AOD (Argon Oxygen Decarburisation) slag from stainless steel production, Fe slag from zinc production, and fayalitic slag from nickel production. CSAB clinker with a target composition of ye'elimite-belite-ferrite was produced by firing raw materials at 1300 °C. The phase composition of the produced clinkers was identified using quantitative XRD analyses, and the chemical composition of the clinker phases produced was established using FESEM-EDS and mechanical properties were tested through compressive strength test. It is demonstrated that these metallurgical residues can be used successfully as alternative raw materials for the production of CSAB cement that can be used for special applications. In addition, it is shown that the available quantities of these side-streams are enough for significant replacement of virgin raw materials used in cement production.

DOI： 10.1016/j.scitotenv.2019.136208

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PubMed

Language：English   Publisher：Elsevier BV  

This study investigated characteristics of a calcium aluminate cement modified with a phosphate (CAP) by changing an amount and concentration of mixing solution with sodium polyphosphate. When the amount of mixing solution was increased with a constant amount of sodium polyphosphate, an enhanced consumption of monocalcium aluminate was observed compared with gehlenite in calcium aluminate cement. Formation of gibbsite, Al(OH)<inf>3</inf>, was also increased as a hydration product in the CAP and a reduction of water in the amorphous gel phase. When the amount of mixing solution was increased with a constant concentration of sodium polyphosphate, the enhanced consumption of monocalcium aluminate was not observed. Neither gibbsite nor any other crystalline hydration products were identified in this series. In addition, unreacted sodium polyphosphate remained in the system. The increased formation of gibbsite and the possible reduction of water from the amorphous gel phase appears to contribute to the improvement of the microstructure in the products.

DOI： 10.1016/j.cemconres.2019.105951

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Language：English  

Language：English   Publisher：University of Bath  

Language：English  

Language：English   Publisher：Trans Tech Publications  

Cements have been used to encapsulate low and intermediate level radioactive wastes. Here, phosphate-modified calcium aluminate (CAP) cement is explored as an encapsulant for strontium radioanuclide-containing wastes. Electron microscopy indicates strontium chloride, used in place of strontium radionuclides, increases porosity in CAP possibly due to increased viscosity of CAP cement during mixing. X-ray diffraction analysis detects formation of halite phase suggesting strontium chloride reacts with cement to form sodium chloride not usually detected in CAP systems as well as formation of an amorphous phase in CAP cement when thermally treated at 90°C.

DOI： 10.4028/www.scientific.net/kem.803.341

Scopus

Language：English   Publisher：Elsevier  

Cementation of the secondary aqueous wastes from TEPCO Fukushima Daiichi Nuclear Power Plant is challenging due to the significant strontium content and radioactivity, leading to a potential risk of hydrogen gas generation via radiolysis of water content. The present study investigates the reduction of water content in calcium aluminate cement (CAC) with/out phosphate modification by a heat-treatment during the solidification. The reduction of water in the CAC was found restricted by the rapid formation of crystalline hydration phases, whereas the phosphate-modified system allowed the gradual reduction of water, achieving the reduction of 60% water content at 95 °C. Curing at 60–95 °C also eliminated the significant cracks found at 35 °C in the phosphate system. The possible difference in the amorphous products, NaCaPO<inf>4</inf>∙nH<inf>2</inf>O type at 35 °C and Ca(HPO<inf>4</inf>)∙xH<inf>2</inf>O type at 60–95 °C, may have contributed to the improvement in the microstructure together with the change in the pore size distribution.

DOI： 10.1016/j.cemconres.2018.04.019

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Language：English   Publisher：Royal Society of Chemistry  

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)<inf>2</inf> was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na<inf>2</inf>O/slag ratios. Particles of Mg(OH)<inf>2</inf> do not chemically react within these cements. Instead, Mg(OH)<inf>2</inf> acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)<inf>2</inf> replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)<inf>2</inf> still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)<inf>2</inf> cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)<inf>2</inf>-bearing wastes such as Magnox sludge.

DOI： 10.1039/c8ra03717e

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PubMed

Language：English  

Language：English   Publisher：American Chemical Society  

The mineralogy and structure of cements in the system Mg(OH)<inf>2</inf>-NaAlO<inf>2</inf>-SiO<inf>2</inf>-H<inf>2</inf>O are investigated, with a view toward potential application in the immobilization of Mg(OH)<inf>2</inf>-rich Magnox sludges resulting from historic United Kingdom nuclear operations. The reaction process leading to the formation of these aluminosilicate binders is strongly exothermic, initially forming zeolite NaA (LTA structure), which is metastable in low SiO<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> binders, slowly evolving into the more stable sodalite and faujasite framework types. Notable chemical reaction of Mg(OH)<inf>2</inf> was only identified in the formulation with SiO<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> = 1.3 (the lowest molar ratio among those tested) after extended curing times. In this case, some of the Mg(OH)<inf>2</inf> reacted to form an Mg-Al-OH layered double hydroxide. These results demonstrate that encapsulation of Magnox sludge waste streams could be carried out in these alternative binders but that the binders would encapsulate rather than chemically incorporate the Mg(OH)<inf>2</inf> into the wasteform unless low SiO<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> ratios are used.

DOI： 10.1021/acs.iecr.7b04201

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Language：English   Publisher：Whittles Publishing  

Language：English   Publisher：Elsevier  

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. The addition of small amounts of germanium into these systems can significantly improve oxidation resistance. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, in particular the effect of introducing germanium on the formation of the Nb<inf>5</inf>Si<inf>3</inf> intermetallic. Limited data is available in the literature. To provide coherent information on the effect of germanium on the phase equilibrium in the Nb-Si system, a comprehensive thermodynamic description of the Ge-Nb-Si system has been developed in the current paper using the CALPHAD method. Initially the Ge-Nb phase diagram was reassessed using the CALPHAD method to take into account recent ab initio data. To supplement limited information on the ternary system in the literature between 800 and 1820 °C, the pseudo binary between Nb<inf>5</inf>Si<inf>3</inf> and Nb<inf>5</inf>Ge<inf>3</inf> was studied experimentally between 1200 and 1500 °C. Experimental and modelling results indicate that the W<inf>5</inf>Si<inf>3</inf> prototype of Nb<inf>5</inf>Si<inf>3</inf> can be stabilised to lower temperatures on the addition of germanium. Ge contents in excess of 12.4 at. % at 1200 °C in stoichiometric Nb<inf>5</inf>(Ge,Si)<inf>3</inf> stabilise the W<inf>5</inf>Si<inf>3</inf> prototype. In non-stoichiometric Nb<inf>5</inf>(Ge,Si)<inf>3</inf>, where Nb < 62.5 at. %, lower amounts of Ge are required to stabilised the W<inf>5</inf>Si<inf>3</inf> prototype. The liquidus projection suggests a ternary eutectic with Nb<inf>5</inf>(Ge,Si)<inf>3</inf>, Nb<inf>ss</inf> and Nb<inf>3</inf>Si can form in Nb-Si rich alloys during solidification.

DOI： 10.1016/j.jallcom.2017.04.279

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Language：English  

Language：English  

Language：English  

Language：English   Publisher：2017 International Congress on Advances in Nuclear Power Plants Icapp 2017 A New Paradigm in Nuclear Power Safety Proceedings  

A solidification technique with minimized water content is being developed using phosphate cements for the safe storage of secondary radioactive wastes in the Fukushima Daiichi Nuclear Power Plant. Conventional cement systems become solidified via hydration reactions, and need a certain water content. Phosphate cement systems, however, become solidified via an acid-base reaction, and so they only require water mainly for reasons of workability. A reduced water content of phosphate cement systems is beneficial for the immobilization of the radioactive wastes from mitigating the potential to generate hydrogen gas by the radiolysis of water by radioactive wastes. The current study investigated the water content and mineralogy of calcium aluminate cement (CAC) and phosphate-modified CAC (CAP) cured in open systems at 60, 90 and 120 °C and in a closed system at 20 °C as a reference case. Water contents in both the CAC and the CAP were seen to decrease as curing progressed. For ≥ 90 °C, the CAP contained less water than CAC. Free water in CAC converted to structural water by heat treatment, but this was not the case for CAP. An orthophosphate hydrate salt, a precursor phase of hydroxyapatite, was found in CAP when cured at 20 and 60 °C, and a mixture of the orthophosphate hydrate salt and hydroxyapatite, Ca<inf>10</inf>(PO<inf>4</inf>)<inf>6</inf>(OH)<inf>2</inf>, were formed in the CAP when cured at 90 °C. Phosphate products in CAP cured at 120 °C appears to consist of a different phosphate phase compared with the CAP cured at 20, 60 and 90 °C.

Scopus

Language：English   Publisher：Elsevier  

The gamma irradiation resistance of early age Ba(OH)<inf>2</inf>-Na<inf>2</inf>SO<inf>4</inf>-slag cementitious grouts, formulated for the immobilisation of sulfate bearing nuclear waste, was assessed. The observable crystalline phases were not modified upon heating (50 °C) or upon gamma irradiation up to a total dose of 2.9 MGy over 256 h, but the compressive strengths of the irradiated samples increased significantly. ²⁷Al and ²⁹Si MAS NMR spectroscopy showed that the main binding phase, a calcium aluminosilicate hydrate (C-A-S-H) type gel, had a more ordered and polymerised structure upon heating and irradiation compared to that identified in reference samples. This is associated with a higher degree of reaction of the slag. Samples formulated with the waste simulant Na<inf>2</inf>SO<inf>4</inf>, but without Ba(OH)<inf>2</inf>, became porous and cracked upon heating and irradiation, but still retained their compressive strength. The Ba(OH)<inf>2</inf>-Na<inf>2</inf>SO<inf>4</inf>-slag grouts evaluated in this work withstand gamma irradiation without showing identifiable damage, and are thus a technically feasible solution for immobilisation of sulfate-bearing nuclear wastes.

DOI： 10.1016/j.jnucmat.2016.10.032

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Language：English   Publisher：Royal Society of Chemistry  

The utilisation of Mg(OH)<inf>2</inf> to capture exhaust CO<inf>2</inf> has been hindered by the limited availability of brucite, the Mg(OH)<inf>2</inf> mineral in natural deposits. Our previous study demonstrated that Mg(OH)<inf>2</inf> can be obtained from dunite, an ultramafic rock composed of Mg silicate minerals, in highly concentrated NaOH aqueous systems. However, the large quantity of NaOH consumed was considered an obstacle for the implementation of the technology. In the present study, Mg(OH)<inf>2</inf> was extracted from dunite reacted in solid systems with NaOH assisted with H<inf>2</inf>O. The consumption of NaOH was reduced by 97% with respect to the NaOH aqueous systems, maintaining a comparable yield of Mg(OH)<inf>2</inf> extraction, i.e. 64.8-66%. The capture of CO<inf>2</inf> from a CO<inf>2</inf>-N<inf>2</inf> gas mixture was tested at ambient conditions using a Mg(OH)<inf>2</inf> aqueous slurry. Mg(OH)<inf>2</inf> almost fully dissolved and reacted with dissolved CO<inf>2</inf> by forming Mg(HCO<inf>3</inf>)<inf>2</inf> which remained in equilibrium storing the CO<inf>2</inf> in the aqueous solution. The CO<inf>2</inf> balance of the process was assessed from the emissions derived from the power consumption for NaOH production and Mg(OH)<inf>2</inf> extraction together with the CO<inf>2</inf> captured by Mg(OH)<inf>2</inf> derived from dunite. The process resulted as carbon neutral when dunite is reacted at 250 °C for durations of 1 and 3 hours and CO<inf>2</inf> is captured as Mg(HCO<inf>3</inf>)<inf>2</inf>.

DOI： 10.1039/c5fd00047e

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PubMed

Language：English   Publisher：Royal Society of Chemistry  

A cementitious system for the immobilisation of magnesium rich Magnox sludge was produced by blending an Mg(OH)<inf>2</inf> slurry with silica fume and an inorganic phosphate dispersant. The Mg(OH)<inf>2</inf> was fully consumed after 28 days of curing, producing a disordered magnesium silicate hydrate (M-S-H) with cementitious properties. The structural characterisation of this M-S-H phase by ²⁹Si and ²⁵Mg MAS NMR showed clearly that it has strong nanostructural similarities to a disordered form of lizardite, and does not take on the talc-like structure as has been proposed in the past for M-S-H gels. The addition of sodium hexametaphosphate (NaPO<inf>3</inf>)<inf>6</inf> as a dispersant enabled the material to be produced at a much lower water/solids ratio, while still maintaining the fluidity which is essential in practical applications, and producing a solid monolith. Significant retardation of M-S-H formation was observed with larger additions of phosphate, however the use of 1 wt% (NaPO<inf>3</inf>)<inf>6</inf> was beneficial in increasing fluidity without a deleterious effect on M-S-H formation. This work has demonstrated the feasibility of using M-S-H as binder to structurally immobilise Magnox sludge, enabling the conversion of a waste into a cementitious binder with potentially very high waste loadings, and providing the first detailed nanostructural description of the material thus formed.

DOI： 10.1039/c5dt00877h

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PubMed

Language：English   Publisher：Cambridge University Press  

Irradiation is one of the characteristic conditions that nuclear wasteforms must withstand to assure integrity during their service life. This study investigates gamma irradiation resistance of an early age slag cement-based grout, which is of interest for the nuclear industry as it is internationally used for encapsulation of low and intermediate level radioactive wastes. The slag cement-based grout withstands a gamma irradiation dose of 4.77 MGy over 256 h without reduction in its compressive strength; however, some cracking of irradiated samples was identified. The high strength retention is associated with the fact that the main hydration product forming in this binder, a calcium aluminum silicate hydrate (C-A-S-H) type gel, remains unmodified upon irradiation. Comparison with a heat-treated sample was carried out to identify potential effects of the temperature rise during irradiation exposure. The results suggested that formation of cracks is a combined effect of radiolysis and heating upon irradiation exposure.

DOI： 10.1557/jmr.2014.404

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Language：English  

Language：English   Publisher：5th International Conference on Accelerated Carbonation for Environmental and Material Engineering 2015  

Mg(OH)<inf>2</inf> can capture CO<inf>2</inf> by forming Mg carbonate minerals, but its limited availability in natural deposits has hindered this application. In our previous study, we demonstrated that Mg(OH)<inf>2</inf> can be obtained from dunite, an ultramafic rock composed of Mg silicate minerals in highly concentrated NaOH aqueous systems at 180 °C. However, the large quantity of NaOH consumed could be an obstacle for the implementation of the technology. The present study shows that the reduction of NaOH consumption is achievable via the solid state reaction of Mg silicate minerals with NaOH for the extraction of Mg(OH)<inf>2</inf>. The results demonstrated that NaOH consumption was reduced by 97 % with respect to the NaOH aqueous system, maintaining a comparable yield of Mg(OH)<inf>2</inf> extraction, i. e. 55-62 %. The addition of H<inf>2</inf>O in small amount was also found beneficial and improved the extraction of Mg(OH)<inf>2</inf>.

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Language：English   Publisher：Life Cycle of Structural Systems Design Assessment Maintenance and Management Proceedings of the 4th International Symposium on Life Cycle Civil Engineering Ialcce 2014  

Long-term integrity is an important issue required for cementitious materials. It is essential to understand the long-term behavior of the materials to assure the sufficient performance. The lifetime should be over thousands of years in extreme cases, such as use in the nuclear waste encapsulation and disposal facilities. In this study, an electrical acceleration technique for degradation of cementitious materials under exposure to water has been developed. The effect of electrical leaching action on the property of hydrated cement samples, which are simulated as nuclear waste forms containing Cs and Sr, has been investigated. The results showed that a higher current density resulted in a faster leaching process on behavior of CSNO<inf>3</inf> and calcium aluminate phase, and that the described leaching acceleration technique could simulate a natural progression in the leaching phenomena depending on the time spent in the device at a constant current density.

DOI： 10.1201/b17618-246

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Language：English  

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Language：English  

Language：English   Publisher：Elsevier  

Soluble sulfate ions in nuclear waste can have detrimental effects on cementitious wasteforms and disposal facilities based on Portland cement. As an alternative, Ba(OH)<inf>2</inf>-Na<inf>2</inf>SO<inf>4</inf>-blast furnace slag composites are studied for immobilisation of sulfate-bearing nuclear wastes. Calcium aluminosilicate hydrate (C-A-S-H) with some barium substitution is the main binder phase, with barium also present in the low solubility salts BaSO<inf>4</inf> and BaCO<inf>3</inf>, along with Ba-substituted calcium sulfoaluminate hydrates, and a hydrotalcite-type layered double hydroxide. This reaction product assemblage indicates that Ba(OH)<inf>2</inf> and Na <inf>2</inf>SO<inf>4</inf> act as alkaline activators and control the reaction of the slag in addition to forming insoluble BaSO<inf>4</inf>, and this restricts sulfate availability for further reaction as long as sufficient Ba(OH)<inf>2</inf> is added. An increased content of Ba(OH)<inf>2</inf> promotes a higher degree of reaction, and the formation of a highly cross-linked C-A-S-H gel. These Ba(OH)<inf>2</inf>-Na<inf>2</inf>SO<inf>4</inf>-blast furnace slag composite binders could be effective in the immobilisation of sulfate-bearing nuclear wastes. © 2014 Elsevier Ltd.

DOI： 10.1016/j.cemconres.2014.07.006

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Language：English   Publisher：SAGE Publications  

In this study, binders within the system Mg(OH)<inf>2</inf>-NaAlO<inf>2</inf>-SiO<inf>2</inf>-H<inf>2</inf>O were produced and characterised for the potential immobilisation of radioactive Mg(OH)2 sludges derived from the UK Magnox nuclear programme. The formation of zeolites promoted hardening within 24 h, with the structures and compositions of the zeolites dependent on the contents of SiO<inf>2</inf> in the systems. Limited consumption of Mg(OH)<inf>2</inf> was identified in the binders cured for up to 90 days, except for a hydrotalcite type layered double hydroxide in binders formulated with low SiO<inf>2</inf> contents. This study has elucidated that binders within this system can effectively encapsulate high contents of Mg(OH)<inf>2</inf>; however, the reaction of Mg(OH)<inf>2</inf> to form binding phases is limited by the content of SiO<inf>2</inf> in the system, and occurs primarily at advanced times of curing.

DOI： 10.1179/1743676114y.0000000198

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Language：English   Publisher：Taylor \& Francis  

Cementing systems based on calcium aluminate cement (CAC) have been studied as an alternative cement matrix for the encapsulation of intermediate level wastes (ILWs) arising from the nuclear industry. Calcium aluminate cement based systems have great potential for the incorporation of aluminium containing ILW owing to the near neutral internal pH of these binders. However, CAC based binders usually undergo phase conversion from the metastable hydration phases C3AH6 and/or C<inf>2</inf>AH<inf>8</inf> to the stable C<inf>3</inf>AH<inf>6</inf>, resulting in strength regression and dimensional instability. The present study investigates the feasibility of CAC modification to prevent this detrimental process of phase conversion. Two different types of sodium phosphates are used to modify a CAC system, and the setting behaviour and the mineralogy of the binder products were studied. It is shown that it is possible to avoid the conventional phase conversion of CAC hydrates due to the formation of an amorphous aluminate phase in place of the metastable hydrates, leading to the production of a phase assemblage, which is stable for at least 180 days.

DOI： 10.1179/1743676114Y.0000000147

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Language：English  

Language：English  

Language：English   Publisher：Taylor \& Francis  

The leaching behaviour and physicochemical properties of cementitious nuclear wasteforms containing caesium and strontium waste simulants has been investigated. The cement wasteform consisted of a 9: 1 blend of blast furnace slag and ordinary Portland cement. Both non-loaded samples and samples that were waste loaded with 3 wt-% caesium and strontium added as nitrates have been studied. The cement hydration phases in the samples were identified, and the porosity and microstructure were analysed before leaching. The samples were leached for up to 6 months and the leached elements quantified. In the waste loaded cements, portlandite was not formed, and the monosulphate AF<inf>m</inf> phase appeared to be altered by the incorporation of the Sr(NO<inf>3</inf>)<inf>2</inf>. Incorporation of Cs and Sr also resulted in the increase in the leach rate of Ca²⁺.

DOI： 10.1179/1743676114Y.0000000146

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Language：English  

Language：English   Publisher：Journal of Hazardous Materials  

The BaSO<inf>4</inf> scales obtained from piping decontamination from oil and gas industries are most often classified as low level radioactive waste. These wastes could be immobilised by stable cement matrix to provide higher safety of handling, transportation, storage and disposal. However, the information available for the effects of the basic formulation such as waste loading on the fundamental properties is still limited. The present study investigated the effect of BaSO<inf>4</inf> loading and water content on the properties of OPC-BaSO<inf>4</inf> systems containing fine BaSO<inf>4</inf> powder and coarse granules. The BaSO<inf>4</inf> with different particle size had a marked effect on the compressive strength due to their different effects on hydration products formed. Introduction of fine BaSO<inf>4</inf> powder resulted in an increased formation of CaCO<inf>3</inf> in the system, which significantly contributed to the compressive strength of the products. Amount of water was important to control the CaCO<inf>3</inf> formation, and water to cement ratio of 0.53 was found to be a good level to maintain a low porosity of the products both for fine BaSO<inf>4</inf> powder and coarse BaSO<inf>4</inf> granule. BaSO<inf>4</inf> loading of up to 60wt% has been achieved satisfying the minimum compressive strength of 5MPa required for the radioactive wasteforms. © 2013 Elsevier B.V.

DOI： 10.1016/j.jhazmat.2013.06.048

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Language：English   Publisher：GDCh - Gesellschaft Deutscher Chemiker e.V.  

Language：English  

Language：English  

Language：English   Publisher：Cement and Concrete Research  

Corrosion of aluminium metal in ordinary Portland cement (OPC) based pastes produces hydrogen gas and expansive reaction products causing problems for the encapsulation of aluminium containing nuclear wastes. Although corrosion of aluminium in cements has been long known, the extent of aluminium corrosion in the cement matrices and effects of such reaction on the cement phases are not well established. The present study investigates the corrosion reaction of aluminium in OPC, OPC-blast furnace slag (BFS) and calcium aluminate cement (CAC) based systems. The total amount of aluminium able to corrode in an OPC and 4:1 BFS:OPC system was determined, and the correlation between the amount of calcium hydroxide in the system and the reaction of aluminium obtained. It was also shown that a CAC-based system could offer a potential matrix to incorporate aluminium metal with a further reduction of pH by introduction of phosphate, producing a calcium phosphate cement. © 2013 Published by Elsevier Ltd.

DOI： 10.1016/j.cemconres.2013.03.016

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Language：English   Publisher：Maney Publishing  

The present study is part of a wider investigation to develop an alternative cementing system for the encapsulation of problematic low and intermediate level radioactive waste. It has been suggested that alternative cementing systems, with lower internal pH than conventional Portland cement based composite cements, may reduce the corrosion of some reactive metals and may be beneficial for the long term durability of wasteforms. A potential alternative is an acid-base cementing system, based on mixing calcium aluminate cement (CAC) with acidic phosphate solutions. Although these systems have been studied previously, there has been no systematic investigation to identify phosphates for producing suitable matrixes for application in radioactive waste encapsulation. In the current study, monophosphate modified CAC formulations did not set or develop significant strength, whereas polyphosphate modified CAC formulations exhibited rapid setting and strength development. It is proposed that polyphosphate modified systems form amorphous reaction products, which act as binders between the partially and unreacted CAC particles, and were responsible for high strength development. Thermogravimetric analysis and scanning electron microscopy results suggest that this binding matrix consists of amorphous calcium phosphate and alumina gel. The results presented in this investigation suggest that polyphosphate modified CAC has potential as an alternative cementing system for radioactive waste encapsulation. © 2013 Institute of Materials.

DOI： 10.1179/1743676112Y.0000000033

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Language：English   Publisher：Minerals Engineering  

The sequestration of CO<inf>2</inf> by carbonating natural minerals has a great potential for secure reduction of net CO<inf>2</inf> emissions. Feedstock Mg-silicate minerals are usually converted into Mg rich solutions or Mg(OH) <inf>2</inf> before the carbonation process, due to the slow reaction kinetic of direct carbonation. The present work studied the alkaline digestion of Mg-silicate minerals into Mg(OH)<inf>2</inf> for CO<inf>2</inf> sequestration. Powdered dunite containing ∼73 ± 2 wt% of forsterite (Mg <inf>2</inf>SiO<inf>4</inf>) was dissolved using highly concentrated NaOH aqueous systems at 90 and 180 °C with varied NaOH concentration and duration of reaction. Thermal analysis and Rietveld Refinement Quantitative Phase Analysis (QPA) confirmed that an effective digestion of dunite was possible at 180°C achieving 80 wt% of Mg(OH)<inf>2</inf>. It was found that NaOH concentration in solution, temperature and duration of reaction significantly influence the progress of digestion. © 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.mineng.2013.10.015

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Language：English   Publisher：Minerals Engineering  

Carbonation of ordinary Portland cement occurs naturally. This process is, however, not sufficient for the application of CO<inf>2</inf> sequestration due to the very slow kinetics of the diffusion-controlled process. The present study shows that the carbonation can be enhanced in the hardened cement systems blended with blast furnace slag or pulverised fuel ash under the condition tested. Both blended systems, with a very high replacement level of Portland cement with slag or fly ash, indicated nearly two times of CO<inf>2</inf> intake compared to the pure Portland cement system in the particle form, and the system with blast furnace slag gained significantly more CO<inf>2</inf> than other samples in the powdered form. The carbonation of the slag containing particles appeared to result in a significant release of H<inf>2</inf>O, which may have caused the coarse pore structure due to the shrinkage of C-S-H. This seems to be a very attractive system to capture CO<inf>2</inf> as the sufficient level of porosity and free water could be maintained in the system. The lower Ca²⁺ concentration in the system appeared to favour the formation of vaterite as the carbonation product. © 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.mineng.2013.12.004

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Language：English  

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Language：English   Publisher：The Electrochemical Society  

DOI： 10.1149/ma2010-02/37/2194

Language：English   Publisher：The Electrochemical Society  

DOI： 10.1149/ma2010-02/37/2186

Language：English   Publisher：Ecs Transactions  

Corrosion behavior of stainless steel types AISI 316L, 316Ti and 321 was studied at 750 °C in NaCl-KCl equimolar melts. Iron, chromium and manganese species constitute the major corrosion products. The following mechanism of stainless steel corrosion in molten chlorides was proposed: 1) chemical interaction between the alloy and the salt intensified by the formation of microgalvanic pairs; 2) formation of chromium and molybdenum carbide-containing phases in steel as a result of heating to 750 °C; 3) additional formation of galvanic pairs between the grains of austenitic alloys and the carbide phases at the grain boundaries resulting in enhanced intergranular corrosion. ©The Electrochemical Society.

DOI： 10.1149/1.3484790

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Language：English   Publisher：Ecs Transactions  

High-temperature spectroelectrochemestry was applied to study corrosion of various types of stainless steel in molten salts. The electronic absorption spectra of products of anodic dissolution of stainless steel major components (iron, chromium, nickel, molybdenum, manganese, titanium) were measured in NaCl-KCl melt at 750 °C The effectiveness and limitations of applying spectroscopic method for studying alloys corrosion was demonstrated on example of anodic dissolution of AISI 316L, 316Ti and 321 austenitic steels. The major corrosion products of steels are iron, manganese and chromium species. Prolongation of anodic dissolution leads to increasing chromium-to-iron ratio in the melt. Titanium in steels forms very stable carbonitride species that aren't dissolved during anodic oxidation. © The Electrochemical Society.

DOI： 10.1149/1.3484785

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Language：English   Publisher：Inorganic Chemistry  

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO<inf>2</inf>Cl <inf>4</inf>]^2-. Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO<inf>2</inf>Cl<inf>4</inf>]^2-. The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts. © 2008 American Chemical Society.

DOI： 10.1021/ic701415z

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Language：English   Publisher：Materials Research Society Symposium Proceedings  

The possibility of a simple heating process of POM to obtain tungsten bronze was investigated for nuclear waste immobilisation via DTA/TG and high temperature XRD. Heating process up to 900 °C caused the decomposition of structure for both systems. Cooling process seemed to have little effect on the final product for the K<inf>11</inf>[Nd(PW<inf>11</inf>O<inf>39</inf>) <inf>2</inf>]·xH<inf>2</inf>O, whereas the cooling profile showed a significant effect on the K<inf>13</inf>[Nd(SiW<inf>11</inf>O<inf>39</inf>) <inf>2</inf>]·xH<inf>2</inf>O. Nd formed two types of tungsten bronzes, namely Nd<inf>2</inf>WO<inf>6</inf> and Nd<inf>4</inf>W<inf>3</inf>O <inf>15</inf> in K<inf>11</inf>[Nd(PW<inf>11</inf>O<inf>39</inf>) <inf>2</inf>]·xH<inf>2</inf>O and K<inf>13</inf>[Nd(SiW <inf>11</inf>O<inf>39</inf>)<inf>2</inf>]·xH<inf>2</inf>O, respectively. © 2008 Materials Research Society.

DOI： 10.1557/proc-1107-347

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Language：English   Publisher：Materials Research Society Symposium Proceedings  

The reaction of CO<inf>2</inf> gas with OPC, OPC-BFS and OPC-PFA composite cement systems were studied using XRD, SEM and TG to investigate the applicability of these materials to immobilise carbon arising from graphite waste. XRD results suggested that calcite formed in OPC system after the carbonation reaction, whereas calcite and vaterite were observed in OPC-BFS and OPC-PFA systems. In OPC system, nearly half of Ca(OH)<inf>2</inf> was consumed to form CaCO<inf>3</inf>. In OPC-BFS and OPC-PFA systems, the amount of CaCO<inf>3</inf> formed, corresponded to the consumption of greater than 100% of Ca(OH)<inf>2</inf> initially present, suggesting that other hydration products e.g. C-S-H were also consumed, either directly or indirectly during the carbonation process. The OPC-BFS system became more porous after carbonation. OPC-PFA system indicated a high efficiency on the conversion of Ca in the system into CaCO<inf>3</inf>. © 2008 Materials Research Society.

DOI： 10.1557/proc-1107-143

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Language：English  

Language：English   Publisher：Proceedings Electrochemical Society  

The various chloro complexes of the transition metals U, Y, Zr, Ru, Tc and Re in high-temperature chloride melts (both liquid and quenched) have been investigated using EXAFS spectroscopy. All the oxidation states of uranium (III, IV, V and VI) are stable in molten LiCl. U(III) and U(IV) form UCl <inf>6</inf>^x- ions and U(V) and U(VI) form UO<inf>2</inf>Cl <inf>4</inf>^x- ions. U(III) and U(IV) are the most stable oxidation states in LiCl-BeCl<inf>2</inf> eutectic melt; even under a Cl<inf>2</inf> atmosphere (but in the absence of oxygen) U(VI) species are converted into U(IV){can't make sense of this sentence: do you really mean this, +6 reduced to +4 by chlorine?}. U(III) and U(IV) are present in the melt as hexachloro complexes. Yttrium(III) forms YCl<inf>6</inf>^3- ions and there is evidence that these units are joined together into chains by sharing two Cl ligands between every two Y atoms. Zr in LiCl melt forms Zr(II) and Zr(IV) ions, both six-coordinated. Ruthenium in LiCl-KCl and NaClKCl between 450 and 750 °C, and in quenched samples forms sixcoordinated complexes with Ru-Cl distances essentially independent of temperature and melt composition. Only quenched melt samples of technetium and rhenium have so far were investigated. Tc can form Tc(IV) and Tc(VIII) ions, TcCl<inf>6</inf>^2- and TcO <inf>4</inf>^-, and rhenium under the same conditions always yielded the Re(IV) species, ReCl<inf>6</inf>^2-.

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Language：English   Publisher：ACS Symposium Series  

We report the results of recent research that we have undertaken to increase our understanding of key actinide and fission product speciation in a range of ionic melts. These results will be used to develop novel electrochemical methods of separation of uranium and plutonium from irradiated nuclear fuel. Our studies in high temperature alkali metal melts (including LiCl and the eutectics LiCl-KCl and CsCl-NaCl) have focussed on in-situ speciation of U, Tc and Ru using both Electronic Absorption Spectroscopy (EAS) and X-ray Absorption Spectroscopy (XAS). The XAS studies have included Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge Structure (XANES) measurements. We report what could be unusual uranium speciation in high temperature melts and evaluate the likelihood of Ru or Tc volatilisation during plant operation. Our studies in lower temperature melts, commonly known as ionic liquids, have focussed on salts containing tertiary alkyl group 15 cations and the bis(trifluoromethylsulphonyl)imide anion, melts which we know to have exceptionally wide electrochemical windows. We report Ln, Th, U and Np speciation (XAS, EAS and vibrational spectroscopy) and electrochemistry in these melts. © 2006 American Chemical Society.

DOI： 10.1021/bk-2006-0933.ch014

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Language：English   Publisher：Materials Research Society Symposium Proceedings  

Mechanical properties of YSZ doped with Np oxide were studied to investigate the sufficiency to be a waste form for immobilisation of highly concentrated TRU. The study was conducted focusing on Vickers (H<inf>V</inf>) and Knoop (H<inf>K</inf>) hardness, Young's modulus (E) and fracture toughness (K<inf>IC</inf>). The results showed that YSZ is harder and more resistant to elastic deformation and crack development than such waste forms as borosilicate glass and synroc. The effects of porosity and Np content on H<inf>V</inf>, H<inf>K</inf>, E and K<inf>IC</inf> are also discussed. © 2006 Materials Research Society.

DOI： 10.1557/proc-932-52.1

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Language：English  

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Language：English   Publisher：Materials Research Society Symposium Proceedings  

Solidification of molten ceramics during molten slate processing of High-Level Waste (HLW) was studied to investigate a possibility of a simpler and flexible process for immobilisation of HLW. Simulated HLW was heated together with different reducing agents (TiN with AIN or TiO<inf>2</inf>) either at 1673 K or 1873K for 1 to 3 hours under Ar atmosphere. Cooling rates of 5 K/min and 10 K/min did not result in any obvious differences in the products obtained under the standard condition used in the present study. The composition and quantity of the reducing agent had a significant effect on the melting and solidifying behaviour of the simulated HLW. The results suggest the possible further studies to improve the solidification process and the final product. © 2006 Materials Research Society.

DOI： 10.1557/proc-932-51.1

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Language：English  

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Language：English  

Language：English  

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Language：English   Publisher：Inorganic Chemistry  

In situ EXAFS spectroscopic studies of uranium compounds in high temperature alkali chloride melts indicate the presence of oligomeric species. An investigation into UCl<inf>3</inf> and UCl<inf>4</inf> dissolved in LiCl reveals long range ordering of uranium atoms in the molten state which is not maintained on quenching. Studies of uranium dioxide dissolved in LiCl-KCl eutectic with HCl exhibit long range ordering in both molten and quenched states, and the EXAFS data can be modeled using multiple coordination shells.

DOI： 10.1021/ic048617v

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Language：English  

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Language：English   Publisher：Journal of Nuclear Materials  

A thermodynamic modelling was performed for PuO<inf>2</inf>-UO <inf>2</inf>-ZrO<inf>2</inf> system based on the data available in the literature to evaluate stability of the fluorite structure phase in PuO <inf>2</inf>-UO<inf>2</inf>-ZrO<inf>2</inf> system. The assessment of the heat capacity and phase diagram suggested that the deviation from the ideality in the PuO<inf>2</inf>-UO<inf>2</inf> system is negligibly small. The calculated pseudo-ternary phase diagrams implied that the fluorite structure phase of PuO<inf>2</inf>-ZrO<inf>2</inf> subsystem is more stable than that of UO <inf>2</inf>-ZrO<inf>2</inf> subsystem in the lower temperature range and that the latter is more stable than the former in the higher temperature range. The effect of ZrO<inf>2</inf> dissolution to PuO<inf>2</inf> was larger both on the enthalpy and entropy of the fluorite structure phase than that to UO <inf>2</inf> in the low ZrO<inf>2</inf> composition range. On the other hand, UO<inf>2</inf> had a larger effect on the stability of the fluorite structure phase in the high ZrO<inf>2</inf> composition range. © 2004 Published by Elsevier B.V.

DOI： 10.1016/j.jnucmat.2004.04.322

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Language：English   Publisher：Wiley  

DOI： 10.1002/chin.200426021

Language：English   Publisher：Journal of Alloys and Compounds  

Thermodynamic modelling was demonstrated for ZrO<inf>2</inf>-ThO <inf>2</inf> system based on the data for the ZrO<inf>2</inf>-UO<inf>2</inf> and ZrO<inf>2</inf>-CeO<inf>2</inf> systems in the literature. The calculated phase diagram for the ZrO<inf>2</inf>-ThO<inf>2</inf> system showed a good agreement with the experimental data in the literature. Using the data obtained, together with the data for ZrO<inf>2</inf>-UO<inf>2</inf>, ZrO<inf>2</inf>-PuO <inf>2</inf> and ZrO<inf>2</inf>-CeO<inf>2</inf> systems, the stability of the fluorite structure phases in the ZrO<inf>2</inf>-MO<inf>2</inf> (M=Th, U, Pu, Ce) systems was also studied with respect to the partial molar quantities. The effect of the ZrO<inf>2</inf> on the enthalpy in the MO<inf>2</inf> rich region increased as the cation size of the matrices decreased. The solubility limits of the ZrO<inf>2</inf> in the fluorite structure phase in this region also increased as the cation size of the matrix decreased. The effect of MO <inf>2</inf> on the partial molar Gibbs energy in the ZrO<inf>2</inf> rich region increased as the cation size of the MO<inf>2</inf> increased. It implies that the dissolution of the larger cation stabilises the fluorite structure of the ZrO<inf>2</inf> more. © 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2003.09.010

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Language：English  

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Language：English   Publisher：Journal of Nuclear Materials  

The thermodynamic modelling has been performed for the O-Pu-U ternary system. In order to calculate the O-Pu-U ternary phase diagram, the interaction parameters of the excess Gibbs energies of the liquid and fluorite structure phase in the O-Pu-U system have been determined. The thermodynamic data for the liquid phase have been reassessed from the associated model in literatures to the ionic model using a least-square method. The thermodynamic data for the fluorite structure phase have been assessed using the oxygen potential data both in the hyper- and hypo-stoichiometric regions. The calculated ternary phase diagram reproduces the general features of the ternary phase diagram. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2004.01.011

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Language：English  

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Language：English   Publisher：Wiley  

DOI： 10.1002/chin.200328014

Language：English   Publisher：Journal of Alloys and Compounds  

Thermodynamic modelling was performed to assess the phase relation in the O-Pu-Zr system. Firstly a preliminary thermodynamic modelling was attempted for the O-Pu system with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to phase diagram, oxygen potential and heat capacity. Secondly a possible set of phase diagrams for the O-Pu-Zr system was calculated using obtained data for the O-Pu system together with those for the O-Zr and Pu-Zr sub-systems available in the literature. Thermodynamic parameters assessed for the PuO<inf>2</inf>-ZrO<inf>2</inf> pseudobinary system gave a phase diagram with a feature very similar to those for CeO<inf>2</inf>-ZrO<inf>2</inf> and UO<inf>2</inf>-ZrO<inf>2</inf> systems. O-Pu-Zr ternary phase diagrams were also calculated in the temperature range of 773-1773 K, and phase relation in the system was discussed. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0925-8388(02)01340-3

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Language：English   Publisher：Journal of Chemical Thermodynamics  

Thermodynamic modelling was performed for O-Th and O-Np systems to assess the phase stability of the MO<inf>2-x</inf> phase together with the data in the literature for O-Pu, O-U, and O-Zr systems. Firstly, thermodynamic modelling was demonstrated for O-Th and O-Np systems with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to the phase diagrams of the both systems and the oxygen potential in the ThO<inf>2-x</inf> and NpO<inf>2-x</inf> phases. Secondly the phase stability of the MO<inf>2-x</inf> phase was assessed using the obtained sets of data together with those for O-Pu, O-U, and O-Zr. The phase stability of the MO<inf>2-x</inf> phase was discussed with respect to the deviation from the ideal solution in oxygen potential and Gibbs free energy.

DOI： 10.1016/S0021-9614(03)00002-8

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Language：English   Publisher：Progress in Nuclear Energy  

The phase relationship between ZrO<inf>2</inf> and PuO<inf>2</inf> was examined in a low PuO<inf>2</inf> content region, from 3.1 to 11.2 mol% PuO<inf>2</inf>, at temperatures between 1273 K and 1473 K, by high temperature X-ray diffractometry. The measurements were carried out in air. At 1273 K, the samples in this composition range consisted of two phases, monoclinic and cubic. Another phase, tetragonal, was observed at 1373 K. The low temperature monoclinic phase disappeared at 1473 K. It was confirmed that the monoclinic phase disappears around 1463 K; the disappearance temperature does not depend on the composition of the sample. It was, thus, inferred that there should be a eutectoid line in the phase diagram. Though the eutectoid point is not clear, the PuO<inf>2</inf> content at the point should be less than 3.1 mol%.

DOI： 10.1016/S0149-1970(00)00107-4

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Language：English  

DOI： 10.1557/proc-608-443

Language：English  

DOI： 10.1557/proc-608-449

Language：English  

DOI： 10.1557/proc-608-393

Language：English   Publisher：Materials Research Society Symposium Proceedings  

We studied the phase stability and melting temperature of alumina-doped alkaline earth and rare earth -Ti-O system with the composition variation. Melting of the mixture of Nd<inf>2</inf>O<inf>3</inf>, CeO<inf>2</inf>, SrO, TiO<inf>2</inf> and Al<inf>2</inf>O<inf>3</inf> at 1673 K in 1 hour produced RE<inf>2</inf>Ti<inf>3</inf>O<inf>9</inf> phase, and the chemical formula of the oxide was assumed to be (Ce,Nd,Sr)<inf>2</inf>(Ti,Al)<inf>3</inf>O<inf>9</inf>. A Differential Scanning Calorimetry (DSC) measurement showed that the melting temperature of this compound was 1646 K. Al and Sr was considered to contribute to the reduced melting temperature of the oxides in the system. The hardness comparing with ceramic wastes and the leach rate superior to glass waste forms were confirmed.

Scopus

Language：English   Publisher：Materials Research Society Symposium Proceedings  

Calcined simulated high level liquid waste (HLLW) with a desired amount of TiN and AlN mixture was heat-treated at 1673-1873 K. It was revealed that the mixture of TiN and AlN (the atomic ratio of Al to Ti is 1:9) caused the melting of the specimen at 1673 K and the separation of the elements into two groups: alloy phase and oxide phase. The analysis of the oxide phase showed that the compounds in it could be divided into four phases, and that all fission product elements formed the complex oxides with Ti and Al. It is considered that Al and Zr dissolution in each phase contribute to the melting of the oxide phase at 1673K. A 30-days Soxhlet leach test showed that the chemical durability of the oxide phase as a waste form was superior to that of glass waste form[1]. © 2000 Materials Research Society.

Scopus

Language：English   Publisher：Materials Research Society Symposium Proceedings  

Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol%) were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability.

Scopus

Language：English   Publisher：Wiley  

DOI： 10.1002/chin.200041003

Language：English  

Language：English   Publisher：Journal of the American Ceramic Society  

Phase stability and effects of sintering atmosphere on the crystalline structure of Np-doped yttria-stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce-doped YSZ. Different sintering atmospheres for Ce-doped YSZ led to differences in phase formation through reduction of the dopant from Ce⁴⁺ to Ce³⁺. On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite-structure phase regardless of sintering atmosphere. Yttria-stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.

DOI： 10.1111/j.1151-2916.2000.tb01202.x

Web of Science

Scopus

Language：English  

Language：English  

Language：English  

Language：English   Publisher：Technology Reports of the Osaka University  

A fundamental behavior of Se and Te in the super high temperature method was studied. By this method, HLLW is to be reprocessed to obtain a metal and an oxide phases. The metal phase which mainly consists of platinum group metals, Mo, Fe, Cr and Ni is to be recovered, and the oxide phase which consists of alkaline earth elements, rare earth elements, Zr, actinides and Ti is to be disposed as an oxide waste form. Through TG-DTA technique, it was revealed that SeO<inf>2</inf>, Se, TeO<inf>2</inf> and Te evaporated completely under the condition of this method (1873 K) without any additives. The reaction behavior of Se, Te and components of the metal phase with and without TiN were studied. By XRD and EPMA analysis, it was confirmed that Se and Te existed in both metal and oxide phases of the products forming alloys and compounds, respectively. It was revealed that Se formed Pd-Se and Mo-Se alloys, and Te formed Pd-Te, Fe-Te, Mo-Ru-Te and Mo-Rh-Te alloys with components of metal phase, and that Se formed Ti-Se compounds, and Te formed Ti-Te-O compound with Ti which was the main element of the oxide matrix.

Scopus

Language：English  

Language：English   Publisher：Journal of Nuclear Science and Technology  

An experimental study made on the reducing reactions and the formation of alloys taking place in oxide mixtures representing high-level liquid waste (HLLW) generated by the PUREX process, and in particular, on the effect brought on the metal melting temperature by the presence of corrosion products mainly Fe, Cr, Ni deriving from the vessels used in the process. It proved that heat treatment at 1,873K melted the metal phases produced from all the starting mixtures taken up. The metal phases obtained from starting mixtures of corrosion products or their oxides proved to contain alloys of Cr-Fe-Ni, Fe-Ni and Cr-Ni, and in addition Ru-Fe alloy and Mo, Pd metals when the starting mixture further included metals and molybdenum oxides, which also are found in the HLLW from PUREX process. The same six alloys and metals cited above were formed in the metal phases obtained from simulated calcined HLLW used as starting mixture. The melting of Ru and Mo at 1,873K far below their melting points was attributed to the dissolution of Fe into Ru and of Fe or Ni into Mo. © 1996 Taylor and Francis Group, LLC.

DOI： 10.1080/18811248.1996.9732040

Web of Science

Scopus

Language：English   Publisher：Informa UK Limited  

An experimental study made on the reducing reactions and the formation of alloys taking place in oxide mixtures representing high-level liquid waste (HLLW) generated by the PUREX process, and in particular, on the effect brought on the metal melting temperature by the presence of corrosion products mainly Fe, Cr, Ni deriving from the vessels used in the process. It proved that heat treatment at 1,873 K melted the metal phases produced from all the starting mixtures taken up. The metal phases obtained from starting mixtures of corrosion products or their oxides proved to contain alloys of Cr-Fe-Ni, Fe-Ni and Cr-Ni, and in addition Ru-Fe alloy and Mo, Pd metals when the starting mixture further included platinum metals and molybdenum oxides, which also are found in the HLLW from PUREX process. The same six alloys and metals cited above were formed in the metal phases obtained from simulated calcined HLLW used as starting mixture. The melting of Ru and Mo at 1,873 K far below their melting points was attributed to the dissolution of Fe into Ru and of Fe or Ni into Mo.

DOI： 10.3327/jnst.33.973

Scopus

DOI： 10.1016/b978-1-84569-626-9.50018-3

DOI： 10.1039/bk9780854046782-00608

DOI： 10.1039/bk9780854046782-00485

DOI： 10.1039/bk9780854046782-00134

DOI： 10.1039/bk9780854046782-00794