A carregar...
Publicação
Caracterização Mineralógica e Mecânica de Rochas Graníticas Expostas a Elevadas Temperaturas
| Nome: | Descrição: | Tamanho: | Formato: | |
|---|---|---|---|---|
| 5.06 MB | Adobe PDF |
Autores
Resumo(s)
As rochas graníticas são frequentemente utilizadas na ornamentação dos edifícios (exterior ou interior), em agregados e outras aplicações, o que justifica a necessidade de haver uma caracterização mineralógica, física e mecânica que garanta que são preenchidos os requisitos de cada uma das aplicações. No caso de Portugal esta rocha é utilizada com mais frequência no interior centro e norte devido à sua presença geológica, o que faz com tenha sido utilizada na construção, desempenhando não só uma função ornamental, mas também estrutural em numerosas vilas e aldeias. Nos últimos anos, mais propriamente entre 2017 e 2019, ocorreram no país vários incêndios que provocaram alguns danos no edificado das vilas e aldeias e, por isso, pretende-se com este trabalho estudar as alterações (mineralógicas e mecânicas) de um granito quando sujeito a temperaturas elevadas, para se perceber que tipo de influência a temperatura tem na estrutura da rocha. Estudaram-se duas amostras do mesmo granito, com diferentes estados de alteração, granito fresco (GFTE) e granito alterado (GATE), proveniente da pedreira de Açores (Celorico da Beira) da Mota-Engil. Foram preparados trinta e dois provetes para estudo das alterações produzidas no granito quando exposto a temperaturas elevadas, nomeadamente a 400ºC (recriando um incêndio de pequena porporções), a 500ºC e a 600ºC com o objectivo de verificar o efeito da transição do quartzo-α para β (573ºC±1ºC) nas características físicas e mecânicas da rocha. Fizeram-se, também, ensaios à temperatura ambiente para as duas amostras de granito para obter valores de referência. Efectuaram-se ensaios de determinação da porosidade aberta, da massa volúmica, de capilaridade, de compressão uniaxial, o ensaio brasileiro e a velocidade de propagação das ondas P e S. O estudo petrográfico focou-se na caracterização macro e microscópica das amostras antes e após aquecimento, realizando-se também análises por difracção de raio-X. Recorreu-se à microssonda electrónica para identificar as alterações na composição dos cristais de biotite e os produtos de preenchimento formados nas fracturas. Os resultados dos ensaios efectuados permitiram concluir que a rocha começa a perder resistência após o aquecimento a 400ºC, mas é aos 600ºC que se verifica a queda abruta nas características mecânicas (resistência à compressão e à tracção), aumentando a porosidade e a capilaridade. Entre os 400ºC e os 500ºC não se observam alterações significativas. Considera-se que as alterações mais evidentes entre os 500ºC e os 600ºC são causadas pela transformação de quartzo α para β e à inversão do mesmo durante o arrefecimento da rocha. A expansão e posterior contracção térmica do quartzo provocam a criação de fracturas (intra e intergranulares). Neste granito com abundantes megacristais de feldspato verificou-se que estes são também fortemente afectados pelo aumento de temperatura.
Granitic rocks have various applications in our society, from ornamental stone to aggregates, and therefore the study of the characteristics and mineralogy is needed to comply with the requirements for each application. In Portugal granitic rock is used with more frequency in the northern and center interior, due to its geological abundance. So, most towns and villages have their buildings made from granites. In the last few years, from 2017 to 2019, there had been big wildfires that affected most of the countryside of Portugal, damaging some local houses and buildings in the villages and towns. The objective of this work is to study the characteristics (mineralogical, physical and mechanical) of a selected granite when it is exposed to high temperatures. The selected granite is subdivided in two samples, fresh granite (GFTE) and weathered granite (GATE), collected in the Mota-Engil Quarry in Açores (Celorico da Beira). Thirty-two specimens were prepared in order to study the effect of the exposition to high temperatures, the lower of 400ºC, to recreate a small fire, and then 500ºC and 600ºC. Besides the recreation of a wildfire, it aimed at analysing the influence of the transformation of quartz-α to β (573ºC±1ºC) on the physical and mechanical characteristics of the rock. Specimens were also tested at room temperature as reference. The laboratory performed focused on the measurement of open porosity, density, water absorption, uniaxial compression strength, tensile strength and ultrasound velocity. The petrographic study included the macro and microscopic observation, X-ray fluorescence and X-ray diffraction. Electron probe microanalysis was carried out to study the variation of the chemical composition of the biotite crystals due to heating and to identify the products filling the cracks. The results acquired from the various test permitted to conclude the granite starts to lose its mechanic characteristic since the first temperature tested (400ºC), but it was at 600ºC degrees that the highest lost in mechanical characteristic occurred, main due to the transformation of quartz-α to β and its inversion during the cooling. The thermal expansion and contraction of quartz during each transformation originates new fractures or intensifies the pre-existing ones. It was also concluded that the abundant megacrysts of feldspar are also affected by the exposition to high temperature.
Granitic rocks have various applications in our society, from ornamental stone to aggregates, and therefore the study of the characteristics and mineralogy is needed to comply with the requirements for each application. In Portugal granitic rock is used with more frequency in the northern and center interior, due to its geological abundance. So, most towns and villages have their buildings made from granites. In the last few years, from 2017 to 2019, there had been big wildfires that affected most of the countryside of Portugal, damaging some local houses and buildings in the villages and towns. The objective of this work is to study the characteristics (mineralogical, physical and mechanical) of a selected granite when it is exposed to high temperatures. The selected granite is subdivided in two samples, fresh granite (GFTE) and weathered granite (GATE), collected in the Mota-Engil Quarry in Açores (Celorico da Beira). Thirty-two specimens were prepared in order to study the effect of the exposition to high temperatures, the lower of 400ºC, to recreate a small fire, and then 500ºC and 600ºC. Besides the recreation of a wildfire, it aimed at analysing the influence of the transformation of quartz-α to β (573ºC±1ºC) on the physical and mechanical characteristics of the rock. Specimens were also tested at room temperature as reference. The laboratory performed focused on the measurement of open porosity, density, water absorption, uniaxial compression strength, tensile strength and ultrasound velocity. The petrographic study included the macro and microscopic observation, X-ray fluorescence and X-ray diffraction. Electron probe microanalysis was carried out to study the variation of the chemical composition of the biotite crystals due to heating and to identify the products filling the cracks. The results acquired from the various test permitted to conclude the granite starts to lose its mechanic characteristic since the first temperature tested (400ºC), but it was at 600ºC degrees that the highest lost in mechanical characteristic occurred, main due to the transformation of quartz-α to β and its inversion during the cooling. The thermal expansion and contraction of quartz during each transformation originates new fractures or intensifies the pre-existing ones. It was also concluded that the abundant megacrysts of feldspar are also affected by the exposition to high temperature.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospecção Mineral) Universidade de Lisboa, Faculdade de Ciências, 2020
Palavras-chave
Granito Temperatura Resistência Mecânica Composição Mineralógica Relatórios de estágio de mestrado - 2020