Investigation of microwave-assisted concrete recycling using single-particle testing

Microwave heating stands as a strong candidate for selective liberation of multiphase materials like concrete. It takes advantage of the differences in thermal, dielectric and mechanical properties of each of the components to create stress gradients that can lead to grain boundary fracture and embrittlement. The work and results reported are concerned with selective liberation of concrete’s raw constituents for recycling by combination of microwave heating and comminution. A single particle testing approach is presented for detailed analysis of the process. Concrete particles 10 mm in size are treated individually in a single mode cavity microwave (2.45 GHz, 2 kW) test apparatus. The microwave induced effects are quantified by single particle impact testing on a fast Hopkinson bar. Analysis of impact traces reveals a thorough embrittlement of concrete particles from microwave treatment and fragment analysis confirms the potential of microwaves for selective liberation of the raw constituents of concrete. These results validate that microwaves and comminution can be combined to liberate concrete’s raw constituents

Chasing the link between processability and texture in multiphase materials

Using concrete recycling process design as an illustrative example, this paper supports the concept that efficient comminution and separation process design should be based on understanding, and then exploiting, known links between textural properties and processing performance criteria

Recycling-Oriented Investigation of Local Porosity Changes in Microwave Heated-Concrete

Large quantities of concrete waste are being produced continuously throughout the world, of which only a fraction are downcycled as construction backfill or as road-base. Seeking total concrete recyclability; this work concerns the development of microwave-based solutions for the separation of individual constituents of concrete. By focusing on the interaction between microwaves and concrete at the microscopic level, the paper makes important connections between local changes in the microwave-heated concrete texture and macroscopic changes in mechanical properties. Through analysis of the concrete texture using SEM imaging, it is found that the microwave heating of concrete causes fracture porosity. The size and shape of fracture porosity can be correlated with recycling performance indicators namely aggregate liberation, concrete strength and product fineness.. In particular, the work finds that only a short exposure to microwaves promotes the formation of a primary fracture network responsible for selective liberation of aggregates. Longer exposure to microwave heating creates a secondary network of smaller fractures that spreads throughout the cement phase, which is directly associated with the changes in mechanical strength of concrete and product fineness. The work introduces the concept of textural versus physical liberation, and shows that while microwave heating creates a high selective textural liberation of aggregate particles, the comminution of microwave-heated concrete may not necessarily yield high physical liberation. The work concludes that the key to designing a microwave-based process for concrete recycling resides in finding comminution and separation technologies that can best harvest the benefits of the textural and mechanical changes produced by microwave heating

Assessment of a microwave-assisted recycling process for the recovery of high-quality aggregates from concrete waste

This study presents an innovative method for concrete waste up-cycling based on concrete weakening through microwave heating before impact crushing. Two series of tests were conducted in order to assess the influence of the aggregate properties (size distribution, mineralogical nature) and the influence of the operating conditions of the microwave heating pretreatment on concrete fragmentation; and thus to evaluate the feasibility and the robustness of this process. Experiments were carried out on lab-made, cylindrical concrete specimens and on no-slump concrete waste with a multimode cavity microwave equipment (2.45 GHz, 6 kW) and an impact crusher. Results showed that microwave heating always induced an embrittlement of concrete samples which resulted in lower fracture energy, higher fragmentation of samples and higher liberation of aggregates (i.e. free of cement paste). A microwave-assisted comminution process is therefore an effective recycling technique for the recovery of high-quality aggregates from concrete waste

Études sur la texture du béton pour le développement d’un processus de recyclage du béton en utilisant un chauffage par micro-ondes

This thesis is concerned with the development of efficient concrete recycling technology. Concrete is the most used manufactured material on the planet and as a consequence uses more natural resources than any other industry and is responsible for 7% of the world’s carbon dioxide production, 50% of that originating from the decarbonisation of limestone. Given the magnitude of materials used and the waste produced the recycling of concrete would be a major environmental boon and should be made a priority. The main obstacle to the development and implementation of a concrete recycling process is the necessity of separating concrete components, aggregate and cement paste before recycling. Microwave heating stands as an ideal candidate due to its capacity to selectively heat different phases in a multiphase material, thus producing phase boundary fracture and increasing the liberation of the component phases. The effects of microwave heating on concrete samples were investigated with the intent of concluding on the possibility of deriving a microwave-based recycling process for concrete. The effects were measured using standard macroscopic techniques and found a strong correlation between heating, increased porosity and decreased mechanical strength for concrete. In order to better understand the changes measured at the macroscopic scale, a dedicated image analysis technique was developed using electron microscopy to investigate local microscopic changes in concrete texture. Local investigation of the changes of heat treated concrete identified the presence of fracture porosity, which has significant explanatory power for observed changes in concrete properties most relevant to recycling. Textural analysis of concrete subjected to microwave heating showed the growth of two different networks of fractures throughout the cement paste matrix, whose development is associated with the microwave settings. These textural fracture properties correspond directly to the observed changes in mechanical properties as well as the observed liberation of aggregate particles. Textural analysis shows that phase boundary fracture growth occurs rapidly in the early stages of microwave treatment and that the absolute value of textural liberation is significantly higher than that of physical liberation. This highlights the importance of choosing an appropriate comminution method to make efficient use of phase boundary fracture. Moreover, the work established causal relationships between variations in fracture porosity and changes in properties of concrete most relevant to recycling, revealing the possibility of designing a concrete waste beneficiation process through manipulating concrete texture. Since microwave heating was found to be able to manipulate the form taken by the fracture porosity, this work concludes that microwave heating of concrete is a promising technology for designing a concrete recycling scheme. Moreover, through demonstration of causal links between textural properties and processing performance criteria, this work opens the possibility of an alternative approach for analysing and designing comminution process for minerals.L’objet de cette thèse porte sur le développement de technologies efficaces pour le recyclage du béton. Le béton est le plus utilisé des matériaux conçus par l’homme, et par voie de conséquence, sa fabrication consomme plus de ressources naturelles que n’importe quelle autre industrie. Le béton est responsable de 7% des émissions anthropiques de CO2, la moitie provenant de la décarbonatation du calcaire. Compte-tenu des quantités de matières utilisées et de déchets produits, le recyclage du béton est un enjeu environnemental majeur, et une priorité grandissante. Le principal défi au développement et à la mise en oeuvre d’un procède de recyclage du béton concerne la difficulté à séparer ses constituants, les graviers et la pâte de ciment. Le chauffage microonde est un candidat idéal pour parvenir à ce résultat, de par sa capacité à chauffer sélectivement les différentes phases d’un matériau multiphasique, induisant ainsi des fractures aux interfaces entre phases et conduisant à leur libération. Les effets du chauffage microonde sur le béton ont donc été étudies dans cette thèse avec l’objectif de conclure quant au bien-fondé de développer un procède de recyclage qui intègre les microondes. Les effets ont été mesures par des analyses classiques d’échelle macroscopique qui ont mises en évidence une corrélation significative entre le chauffage, l’augmentation de porosité et la diminution des propriétés mécaniques du béton. Pour mieux comprendre les changements mesures à une échelle macroscopique, une technique d’analyse locale d’images de la texture du béton, base sur la microscopie électronique, a été développée. L’analyse locale des changes induits par le chauffage du béton a révélé la présence d’une porosité de fractures, qui a permis d’expliquer les changements observes des propriétés du béton les plus pertinentes vis-à-vis du recyclage. L’analyse de la texture du béton soumis au chauffage par microondes a mis en évidence la formation de deux réseaux de fractures dans la pâte de ciment, dont le développement est dicte par les conditions du chauffage microonde. Les propriétés texturales de ces réseaux de fractures ont été corrélées directement aux variations des propriétés macroscopiques du béton, ainsi qu’à la libération des agrégats. L’analyse texturale a montré que la croissance de la fracture à l’interface entre les agrégats et la pâte de ciment survient durant les premiers instants du chauffage, et que la libération des agrégats dans le béton, appelée libération texturale, est significativement plus grande que la libération mesurée après fracture du béton par impact. Cette observation a démontré l’importance de choisir un mode de fragmentation capable de tirer parti de la fracture sélective des interfaces agrégat-ciment. De plus, le travail réalise a établi qu’il existe des relations de causalité entre les variations de la porosité de fractures présente dans le béton et les propriétés du béton les plus significatives pour son recyclage, revelant ainsi la possibilite de concevoir un procede de valorisation des dechets de béton en manipulant la texture du béton. En etablissant que le chauffage microonde permet de modifier la porosite de fractures du béton, ce travail conclut que le chauffage du béton par microondes est une technologie prometteuse pour concevoir un procede de recyclage du béton. Au-dela du seul objectif du recyclage du béton, la demonstration de l’existence de relations de causalite entre les proprietes de texture du béton et les criteres de performance associes a sa fragmentation ouvre des perspectives nouvelles pour analyser et concevoir des procedes de broyage des minerais

Investigation of fracture porosity as the basis for developing a concrete recycling process using microwave heating

L’objet de cette thèse porte sur le développement de technologies efficaces pour le recyclage du béton. Le béton est le plus utilisé des matériaux conçus par l’homme, et par voie de conséquence, sa fabrication consomme plus de ressources naturelles que n’importe quelle autre industrie. Le béton est responsable de 7% des émissions anthropiques de CO2, la moitie provenant de la décarbonatation du calcaire. Compte-tenu des quantités de matières utilisées et de déchets produits, le recyclage du béton est un enjeu environnemental majeur, et une priorité grandissante. Le principal défi au développement et à la mise en oeuvre d’un procède de recyclage du béton concerne la difficulté à séparer ses constituants, les graviers et la pâte de ciment. Le chauffage microonde est un candidat idéal pour parvenir à ce résultat, de par sa capacité à chauffer sélectivement les différentes phases d’un matériau multiphasique, induisant ainsi des fractures aux interfaces entre phases et conduisant à leur libération. Les effets du chauffage microonde sur le béton ont donc été étudies dans cette thèse avec l’objectif de conclure quant au bien-fondé de développer un procède de recyclage qui intègre les microondes. Les effets ont été mesures par des analyses classiques d’échelle macroscopique qui ont mises en évidence une corrélation significative entre le chauffage, l’augmentation de porosité et la diminution des propriétés mécaniques du béton. Pour mieux comprendre les changements mesures à une échelle macroscopique, une technique d’analyse locale d’images de la texture du béton, base sur la microscopie électronique, a été développée. L’analyse locale des changes induits par le chauffage du béton a révélé la présence d’une porosité de fractures, qui a permis d’expliquer les changements observes des propriétés du béton les plus pertinentes vis-à-vis du recyclage. L’analyse de la texture du béton soumis au chauffage par microondes a mis en évidence la formation de deux réseaux de fractures dans la pâte de ciment, dont le développement est dicte par les conditions du chauffage microonde. Les propriétés texturales de ces réseaux de fractures ont été corrélées directement aux variations des propriétés macroscopiques du béton, ainsi qu’à la libération des agrégats. L’analyse texturale a montré que la croissance de la fracture à l’interface entre les agrégats et la pâte de ciment survient durant les premiers instants du chauffage, et que la libération des agrégats dans le béton, appelée libération texturale, est significativement plus grande que la libération mesurée après fracture du béton par impact. Cette observation a démontré l’importance de choisir un mode de fragmentation capable de tirer parti de la fracture sélective des interfaces agrégat-ciment. De plus, le travail réalise a établi qu’il existe des relations de causalité entre les variations de la porosité de fractures présente dans le béton et les propriétés du béton les plus significatives pour son recyclage, revelant ainsi la possibilite de concevoir un procede de valorisation des dechets de béton en manipulant la texture du béton. En etablissant que le chauffage microonde permet de modifier la porosite de fractures du béton, ce travail conclut que le chauffage du béton par microondes est une technologie prometteuse pour concevoir un procede de recyclage du béton. Au-dela du seul objectif du recyclage du béton, la demonstration de l’existence de relations de causalite entre les proprietes de texture du béton et les criteres de performance associes a sa fragmentation ouvre des perspectives nouvelles pour analyser et concevoir des procedes de broyage des minerais.This thesis is concerned with the development of efficient concrete recycling technology. Concrete is the most used manufactured material on the planet and as a consequence uses more natural resources than any other industry and is responsible for 7% of the world’s carbon dioxide production, 50% of that originating from the decarbonisation of limestone. Given the magnitude of materials used and the waste produced the recycling of concrete would be a major environmental boon and should be made a priority. The main obstacle to the development and implementation of a concrete recycling process is the necessity of separating concrete components, aggregate and cement paste before recycling. Microwave heating stands as an ideal candidate due to its capacity to selectively heat different phases in a multiphase material, thus producing phase boundary fracture and increasing the liberation of the component phases. The effects of microwave heating on concrete samples were investigated with the intent of concluding on the possibility of deriving a microwave-based recycling process for concrete. The effects were measured using standard macroscopic techniques and found a strong correlation between heating, increased porosity and decreased mechanical strength for concrete. In order to better understand the changes measured at the macroscopic scale, a dedicated image analysis technique was developed using electron microscopy to investigate local microscopic changes in concrete texture. Local investigation of the changes of heat treated concrete identified the presence of fracture porosity, which has significant explanatory power for observed changes in concrete properties most relevant to recycling. Textural analysis of concrete subjected to microwave heating showed the growth of two different networks of fractures throughout the cement paste matrix, whose development is associated with the microwave settings. These textural fracture properties correspond directly to the observed changes in mechanical properties as well as the observed liberation of aggregate particles. Textural analysis shows that phase boundary fracture growth occurs rapidly in the early stages of microwave treatment and that the absolute value of textural liberation is significantly higher than that of physical liberation. This highlights the importance of choosing an appropriate comminution method to make efficient use of phase boundary fracture. Moreover, the work established causal relationships between variations in fracture porosity and changes in properties of concrete most relevant to recycling, revealing the possibility of designing a concrete waste beneficiation process through manipulating concrete texture. Since microwave heating was found to be able to manipulate the form taken by the fracture porosity, this work concludes that microwave heating of concrete is a promising technology for designing a concrete recycling scheme. Moreover, through demonstration of causal links between textural properties and processing performance criteria, this work opens the possibility of an alternative approach for analysing and designing comminution process for minerals

Towards complete recycling of concrete using microwave heating

Recycling of concrete into high quality products requires the separation of the cement matrix, sand and aggregate particles. Microwave heating stands as a strong candidate for selective liberation of these constituents, either directly or as a pre-treatment to a comminution step, as it takes advantage of the many contrasts in properties of the phases: thermal, mechanical, electrical and dielectric. Rapid microwave heating generates stresses within the concrete, which may lead to crack formation at phase boundaries, yielding liberated constituents. This work is a contribution to demonstrating the potential of microwave treatment for the selective liberation of concrete components

Investigation of fracture porosity as the basis for developing a concrete recycling process using microwave heating

L objet de cette thèse porte sur le développement de technologies efficaces pour le recyclage du béton. Le béton est le plus utilisé des matériaux conçus par l homme, et par voie de conséquence, sa fabrication consomme plus de ressources naturelles que n importe quelle autre industrie. Le béton est responsable de 7% des émissions anthropiques de CO2, la moitié provenant de la décarbonatation du calcaire. Compte-tenu des quantités de matières utilisées et de déchets produits, le recyclage du béton est un enjeu environnemental majeur et une priorité grandissante. Le principal défi au développement et à la mise en oeuvre d un procédé de recyclage du béton concerne la difficulté à séparer ses constituants, les graviers et la pâte de ciment. Le chauffage microonde est un candidat idéal pour parvenir à ce résultat, de par sa capacité à chauffer sélectivement les différentes phases d un matériau multiphasique, induisant ainsi des fractures aux interfaces entre phases et conduisant à leur libération. Les effets du chauffage microonde sur le béton ont donc été étudiés dans cette thèse avec l objectif de conclure quant au bien-fondé de développer un procédé de recyclage qui intègre les microondes. Les effets ont été mesurés par des analyses classiques d échelle macroscopique qui ont mis en évidence une corrélation significative entre le chauffage, l augmentation de porosité et la diminution des propriétés mécaniques du béton. Pour mieux comprendre les changements mesurés à une échelle macroscopique, une technique d analyse locale d images de la texture du béton, basé sur la microscopie électronique, a été développée. L analyse locale des changes induits par le chauffage du béton a révélé la présence d une porosité de fractures, qui a permis d expliquer les changements observés des propriétés du béton les plus pertinentes vis-à-vis du recyclage. L analyse de la texture du béton soumis au chauffage par microondes a mis en évidence la formation de deux réseaux de fractures dans la pâte de ciment, dont le développement est dicté par les conditions du chauffage microonde. Les propriétés texturales de ces réseaux de fractures ont été corrélées directement aux variations des propriétés macroscopiques du béton, ainsi qu à la libération des agrégats. L analyse texturale a montré que la croissance de la fracture à l interface entre les agrégats et la pâte de ciment survient durant les premiers instants du chauffage, et que la libération des agrégats dans le béton, appelée libération texturale, est significativement plus grande que la libération mesurée après fracture du béton par impact. Cette observation a démontré l importance de choisir un mode de fragmentation capable de tirer parti de la fracture sélective des interfaces agrégat-ciment. De plus, le travail réalisé a établi qu il existe des relations de causalité entre les variations de la porosité de fractures présente dans le béton et les propriétés du béton les plus significatives pour son recyclage, révelant ainsi la possibilité de concevoir un procédé de valorisation des déchets de béton en manipulant la texture du béton. En établissant que le chauffage microonde permet de modifier la porosité de fractures du béton, ce travail conclut que le chauffage du béton par microondes est une technologie prometteuse pour concevoir un procédé de recyclage du béton. Au-delà du seul objectif du recyclage du béton, la démonstration de l existence de relations de causalité entre les propriétés de texture du béton et les critères de performance associés à sa fragmentation ouvre des perspectives nouvelles pour analyser et concevoir des procédés de broyage des minerais.This thesis is concerned with the development of efficient concrete recycling technology. Concrete is the most used manufactured material on the planet and as a consequence uses more natural resources than any other industry and is responsible for 7% of the world s carbon dioxide production, 50% of that originating from the decarbonisation of limestone. Given the magnitude of materials used and the waste produced the recycling of concrete would be a major environmental boon and should be made a priority. The main obstacle to the development and implementation of a concrete recycling process is the necessity of separating concrete components, aggregate and cement paste before recycling. Microwave heating stands as an ideal candidate due to its capacity to selectively heat different phases in a multiphase material, thus producing phase boundary fracture and increasing the liberation of the component phases. The effects of microwave heating on concrete samples were investigated with the intent of concluding on the possibility of deriving a microwave-based recycling process for concrete. The effects were measured using standard macroscopic techniques and found a strong correlation between heating, increased porosity and decreased mechanical strength for concrete. In order to better understand the changes measured at the macroscopic scale, a dedicated image analysis technique was developed using electron microscopy to investigate local microscopic changes in concrete texture. Local investigation of the changes of heat treated concrete identified the presence of fracture porosity, which has significant explanatory power for observed changes in concrete properties most relevant to recycling. Textural analysis of concrete subjected to microwave heating showed the growth of two different networks of fractures throughout the cement paste matrix, whose development is associated with the microwave settings. These textural fracture properties correspond directly to the observed changes in mechanical properties as well as the observed liberation of aggregate particles. Textural analysis shows that phase boundary fracture growth occurs rapidly in the early stages of microwave treatment and that the absolute value of textural liberation is significantly higher than that of physical liberation. This highlights the importance of choosing an appropriate comminution method to make efficient use of phase boundary fracture. Moreover, the work established causal relationships between variations in fracture porosity and changes in properties of concrete most relevant to recycling, revealing the possibility of designing a concrete waste beneficiation process through manipulating concrete texture. Since microwave heating was found to be able to manipulate the form taken by the fracture porosity, this work concludes that microwave heating of concrete is a promising technology for designing a concrete recycling scheme. Moreover, through demonstration of causal links between textural properties and processing performance criteria, this work opens the possibility of an alternative approach for analysing and designing comminution process for minerals.TOULOUSE-INP (315552154) / SudocSudocFranceF

Introducing the concept ofmechanical texture in comminution: The case of concrete recycling

International audienceModern comminution research and development are mainly product driven rather than material driven. An opinion that is gaining acceptance throughout the comminution community is that it is desirable for the comminution field to evolve toward material driven process design. To this end, this paper introduces the concept of mechanical texture, which corresponds to those textural properties of materials that have a direct bearing on their mechanical and fracture properties, which in turn should be the primary target for comminution process research and equipment design. The paper shows that mass specific fracture energy Ecs is a fracture parameter that is highly sensitive to variations in material texture, leading to selecting Ecs as a sound mechanical texture index. The paper then shows that, in the case of concrete, a set of specific features of the fracture porosity that can be measured inside concrete texture correlate highly with Ecs, thereby defining mechanical texture for concrete comminution. The demonstration that it is possible to establish a direct link between textural properties of concrete andmacroscopic properties relevant to comminution shows that material driven comminution process modeling and design are possible and should be encouraged

CoCr F75 scaffolds produced by additive manufacturing: Influence of chemical etching on powder removal and mechanical performance

Additive manufacturing techniques such as Selective Laser Melting (SLM) allow carefully controlled production of complex porous structures such as scaffolds. These advanced structures can offer many interesting advantages over conventionally produced products in terms of biological response and patient specific design. The surface finish of AM parts is often poor because of the layer wise nature of the process and adhering particles. Loosening of these particles after implantation should be avoided, as this could put the patient's health at risk. In this study the use of hydrochloric acid and hydrogen peroxide mixtures for surface treatment of cobalt-chromium F75 scaffolds produced by SLM is investigated. A 27% HCl and 8% H2O2 etchant proved effective in removing adhering particles while retaining the quasi-static and fatigue performance of the scaffoldspublisher: Elsevier articletitle: CoCr F75 scaffolds produced by additive manufacturing: Influence of chemical etching on powder removal and mechanical performance journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2017.02.005 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.status: publishe