A principal limitação do uso de MgO em refratários é a facilidade com que reage com água formando hidróxido de magnésio (Mg(OH)2) que, devido à sua menor densidade, causa tensões destrutivas nesses materiais. Para outras aplicações, no entanto, a reação de hidroxilação do MgO é necessária, como em produção de agentes antichamas, em compósitos poliméricos e na correção de pH de solos. Observações empíricas na literatura demonstraram que diferentes fontes de MgO possuem reatividades e sensibilidades à hidroxilação distintas. Este estudo analisou o impacto de variáveis externas (por exemplo, a liberação de calor que ocorre durante a reação ou o volume das amostras) que ainda não foi completamente compreendido. O impacto auto-catalítico da temperatura reacional e da exotermia da reação foi avaliado. Por meio de medidas de temperatura in situ e de grau de hidroxilação termogravimétrico, também foram estudados os impactos do volume das amostras testadas e da concentração de sólidos nas suspensões, por meio de medidas de temperatura in situ e termogravimetria. Analisou-se também as principais diferenças estruturais entre duas principais fontes de MgO (sínter de MgO e magnésia cáustica): morfologia de partículas, densidade e área superficial específica. Em seguida, os mecanismos de hidroxilação em suspensões aquosas e seus efeitos foram avaliados por meio de testes de hidroxilação seguidos de termogravimetria, difração de raios-X, medidas de condutividade iônica, densidade, área superficial específica e microscopia eletrônica, e relacionado com as características físico-químicas e morfológicas das respectivas fontes de MgO. Pôde-se constatar que diferenças significativas entre a temperatura nominal do meio reacional e no interior da amostra podem afetar a cinética de hidroxilação do material. O volume e a concentração de sólidos variáveis também podem acentuar consideravelmente os efeitos da exotermia e gerar gradientes de hidroxilação. Também se verificou que a morfologia e a quantidade do Mg(OH)2 formado mudam significativamente dependendo do precursor e em função das condições de tempo-temperatura. / The use of MgO in refractories is restrict due to the easy reaction with water forming magnesium hydroxide (Mg(OH)2). Its lower density causes compressive stresses that can crack their structure. On the other hand, for applications such as the production of flame retardant agents for polymer composites and pH correcting of contaminated soil, this reaction is necessary. Empirical observations in the literature have shown that different sources of MgO have district levels of chemical reactiveness. The present study analyzed the main structural differences between the two main sources of MgO (magnesia sinter and caustic magnesia): particle morphology, density and specific surface area. The mechanisms of hydroxylation of these raw materials in aqueous suspensions and their effects were followed by hydroxylation tests, X-ray diffraction, ionic conductivity, density, specific surface area and scanning electron microscopy. They were associated with the physical characteristics morphological, chemical of these MgO sources. The impact of external variables (e.g., heat release during the reaction or the sample volume), that was not yet completely understood, was also evaluated through temperature measurements carried out in situ and hydroxylation degree accessed by thermogravimetry. The effects of samples volume and solid concentration in aqueous suspension were also investigated. The results showed that differences between the ambient temperature and reaction inside sample temperature can affect the kinetics of hydroxylation of the material. The samples volume and solids concentration can also enhance significantly the effects of heat release and generate gradients of hydroxylation. It was also found out that the morphology and the amount of Mg(OH)2 formed can change depending on the precursor and on the time-temperature conditions.

A principal limitação do uso de MgO em refratários é a facilidade com que reage com água formando hidróxido de magnésio (Mg(OH)2) que, devido à sua menor densidade, causa tensões destrutivas nesses materiais. Para outras aplicações, no entanto, a reação de hidroxilação do MgO é necessária, como em produção de agentes antichamas, em compósitos poliméricos e na correção de pH de solos. Observações empíricas na literatura demonstraram que diferentes fontes de MgO possuem reatividades e sensibilidades à hidroxilação distintas. Este estudo analisou o impacto de variáveis externas (por exemplo, a liberação de calor que ocorre durante a reação ou o volume das amostras) que ainda não foi completamente compreendido. O impacto auto-catalítico da temperatura reacional e da exotermia da reação foi avaliado. Por meio de medidas de temperatura in situ e de grau de hidroxilação termogravimétrico, também foram estudados os impactos do volume das amostras testadas e da concentração de sólidos nas suspensões, por meio de medidas de temperatura in situ e termogravimetria. Analisou-se também as principais diferenças estruturais entre duas principais fontes de MgO (sínter de MgO e magnésia cáustica): morfologia de partículas, densidade e área superficial específica. Em seguida, os mecanismos de hidroxilação em suspensões aquosas e seus efeitos foram avaliados por meio de testes de hidroxilação seguidos de termogravimetria, difração de raios-X, medidas de condutividade iônica, densidade, área superficial específica e microscopia eletrônica, e relacionado com as características físico-químicas e morfológicas das respectivas fontes de MgO. Pôde-se constatar que diferenças significativas entre a temperatura nominal do meio reacional e no interior da amostra podem afetar a cinética de hidroxilação do material. O volume e a concentração de sólidos variáveis também podem acentuar consideravelmente os efeitos da exotermia e gerar gradientes de hidroxilação. Também se verificou que a morfologia e a quantidade do Mg(OH)2 formado mudam significativamente dependendo do precursor e em função das condições de tempo-temperatura. / The use of MgO in refractories is restrict due to the easy reaction with water forming magnesium hydroxide (Mg(OH)2). Its lower density causes compressive stresses that can crack their structure. On the other hand, for applications such as the production of flame retardant agents for polymer composites and pH correcting of contaminated soil, this reaction is necessary. Empirical observations in the literature have shown that different sources of MgO have district levels of chemical reactiveness. The present study analyzed the main structural differences between the two main sources of MgO (magnesia sinter and caustic magnesia): particle morphology, density and specific surface area. The mechanisms of hydroxylation of these raw materials in aqueous suspensions and their effects were followed by hydroxylation tests, X-ray diffraction, ionic conductivity, density, specific surface area and scanning electron microscopy. They were associated with the physical characteristics morphological, chemical of these MgO sources. The impact of external variables (e.g., heat release during the reaction or the sample volume), that was not yet completely understood, was also evaluated through temperature measurements carried out in situ and hydroxylation degree accessed by thermogravimetry. The effects of samples volume and solid concentration in aqueous suspension were also investigated. The results showed that differences between the ambient temperature and reaction inside sample temperature can affect the kinetics of hydroxylation of the material. The samples volume and solids concentration can also enhance significantly the effects of heat release and generate gradients of hydroxylation. It was also found out that the morphology and the amount of Mg(OH)2 formed can change depending on the precursor and on the time-temperature conditions.

Στην συγκεκριμένη διατριβή διερευνήθηκαν οι συνθήκες γένεσης του μικροκρυσταλλικού μαγνησίτη στις περιοχές Βάβδου και Γερακινής-Ορμύλιας της Χαλκιδικής. Η πετρογραφική μελέτη του μαγνησίτη και των ξενιστών του αποκάλυψαν δύο δομές δημιουργίας του μαγνησίτη: in situ εξαλλοίωση μέσα σε μανδυακά πετρώματα ξενιστές και ex situ αποθέσεις μέσα στους μανδυακούς σχηματισμούς. Επιπλέον, πιθανά υπάρχει στενή σχέση μεταξύ της κοιτασματογένεσης του μαγνησίτη και των περιβαλλόντων πυριτιωμένων πετρωμάτων (λιστβανίτες). Επίσης, οι γεωχημικές παρατηρήσεις συγχρόνως με τις ορυκτοχημικές αναλύσεις, δίνουν νέα στοιχεία όσον αφορά στην γένεση του κοιτάσματος. Παράλληλα, έγινε εργαστηριακή παραγωγή καυστικής μαγνησίας και παράχθησαν, με προσθήκη αυτής συγχρόνως με χρήση ερυθράς ιλύς και τέφρας πυθμένα, νέα κεραμικά υψηλού πορώδους. Τα αποτελέσματα υπήρξαν ιδιαίτερα ικανοποιητικά, καθώς παρατηρήθηκε ότι με προσθήκη μαγνησίας σε μικρό ποσοστό βελτιώνονται οι φυσικομηχανικές ιδιότητές τους. Κεραμικά υψηλού πορώδους είναι κατάλληλα για χρήση σε φίλτρα και σε απορροφητικά υλικά. Τέλος, προτείνεται νέος τρόπος αξιοποίησης στείρων υλικών μαγνησίτη από τις εξορυκτικές διεργασίες, καθώς αποδεικνύεται ότι ο μαγνησίτης έχει πολύ θετικά αποτελέσματα με προσθήκη του σε οξινισμένα ύδατα, δίνοντας νέες προοπτικές αξιοποίησης του σε περιβαλλοντικές εφαρμογές. Το πειραματικό σκέλος της δοκιμής σε οξινισμένα νερά, πραγματοποιήθηκε σε σύγκριση με πετρώματα που χρησιμοποιούνται για την αντιμετώπιση όξινων απορροών μεταλλείων. / In the present thesis, the microcrystalline magnesite from Central Chalkidiki (Vavdos and Gerakini areas) are investigated in order to clarify the conditions of its genesis. Petrographic study of the magnesite and its host rocks revealed two types of formation: a) in situ alteration in the mantle host rocks, due to metasomatic process, and b) ex situ deposit within mantle formations, related to hydrothermal fluids. Furthermore, petrographic observations suggest a close association between the genesis of the microcrystalline magnesite with the surrounding high silica-carbonate rocks, also termed as listwaenites. Moreover, scanning electron microscopy (S.E.M.) observations along with geochemical analysis provide new evidence for the formation of the magnesite. Caustic magnesia produced in the laboratory was mixed with red mud and bottom ash from industrial wastes, in order to make new, high porosity ceramics. It was observed that the addition of small amounts of caustic magnesia improved the physicomechanical properties of the briquettes. High porosity ceramics are suitable for their use in filters and absorbent materials for environmental applications. Finally, a new way for utilizing waste materials from magnesite mining processes in the disposal of acidified waters is proposed. Addition of magnesite improved the alkalinity of high acidic waters, offering new perspectives for environmental applications. The experimental results from magnesite were compared with similar results from other rocks used to treat acid mine drainage.