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Reacções álcalis-carbonato em betão: caracterização mineralógica e química
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Resumo(s)
A reacção alcális-carbonato (ACR) é uma das reacções alcális-agregado (AAR) mais sujeitas a
debate mundialmente, correspondendo a uma reacção expansiva em betão envolvendo rochas
carbonatadas dolomíticas impuras como agregado. As pedreiras de Kingston e Spratt (Canadá) foram
ambas identificadas, desde a década de 50, como produtoras de agregado altamente reactivo quando
incorporado em betão. O agregado de Spratt apresenta grandes quantidades de sílica reactiva dispersa
na sua matriz, originando a reacção alcális-sílica (ASR), enquanto que o agregado de Kingston não
aparenta originar produtos de reacção que expliquem a expansão observada.
O presente estudo envolve duas aproximações ao problema: o estudo de agregados de rocha; a
análise de danos e o estudo de prismas de betão ensaiados segundo o procedimento normativo CSA
A23.2-14A (2014). Para a petrografia dos agregados de rocha, os grãos individuais foram separados em
fracções de acordo com a cor e texturas observadas, levando à produção de superfícies polidas, assim
como a produção das mesmas para cada prisma de betão disponível. Usando microscopia óptica,
microscopia electrónica de varrimento e microssonda electrónica foram obtidas as composições
químicas das diferentes mineralogias dos agregados de rocha, a composição de produtos de reacção
identificados e a sua possível relação com a mineralogia identificada.
No agregado de Kingston observou-se a presença de um processo metassomático, de baixa
temperatura, que levou à silicificação e feldspatização da rocha carbonatada, assim como ao
desenvolvimento da matriz dolomítica acompanhada de ilite e celestite. Não foram identificados
produtos de reacção danosos nos prismas de betão com agregado de Kingston, embora a reacção entre
o quartzo, feldspatos, filossilicatos, sulfatos (celestite) e sulfuretos (pirite) observados possa muito
possivelmente levar à formação de reacções previamente conhecidas como danosas em betão, como é o
caso da ASR ou da formação retardada de ettringite (DEF), levantando mais questões no que toca à
ACR.
Alkali-carbonated reaction (ACR) is one of the most discussed and studied alkali-aggregate reactions (AAR) world-wide. It involves impure carbonate rocks containing dolomite, leading to an expansive reaction inside concrete. Spratt and Kingston quarries have been identified some decades ago as the origin of strongly alkali reactive aggregates. Spratt aggregate contains free dispersed silica, and it experiences most probably an alkali-silica reaction (ASR) whilst the Kingston aggregate is classified as alkali-carbonate reactive, although no deterious products have been identified in deteriorated concrete. In the present study two approaches were used, giving emphasis on the petrographic characterization: the study of the rock aggregates; the analysis of concrete prisms manufactured for laboratory expansion tests, after performing the CSA A23.2-14A (2014) test. For the petrographic study of these rock aggregates, the crushed grains were split accordingly to their colour and texture under binocular microscope and thin sections were produced from each fraction, as well from the concrete prism available for the study. Optical microscope, scanning electron microscope and electron probe micro-analysis were used to obtain the mineralogical composition of the aggregates, chemical composition of the reaction products and possible relationship with the aggregate minerology. From the petrographic analysis of the Kingston aggregates, it was identified a low temperature metassomatic process, leading to the silicification and feldspatization of the carbonate rock, as well as the development of the dolomitic matrix accompanied with illite and celestite formation. No reaction products were observed in the concrete manufactured with the Kingston aggregate, although the presence of quartz, feldspars, sulphates (celestite) and sulphides (framboidal pyrite) in contact with the typical pore fluids in concrete, could lead to some previously known harmful reactions, such as ASR or delayed ettringite formation, leaving new questions regarding ACR.
Alkali-carbonated reaction (ACR) is one of the most discussed and studied alkali-aggregate reactions (AAR) world-wide. It involves impure carbonate rocks containing dolomite, leading to an expansive reaction inside concrete. Spratt and Kingston quarries have been identified some decades ago as the origin of strongly alkali reactive aggregates. Spratt aggregate contains free dispersed silica, and it experiences most probably an alkali-silica reaction (ASR) whilst the Kingston aggregate is classified as alkali-carbonate reactive, although no deterious products have been identified in deteriorated concrete. In the present study two approaches were used, giving emphasis on the petrographic characterization: the study of the rock aggregates; the analysis of concrete prisms manufactured for laboratory expansion tests, after performing the CSA A23.2-14A (2014) test. For the petrographic study of these rock aggregates, the crushed grains were split accordingly to their colour and texture under binocular microscope and thin sections were produced from each fraction, as well from the concrete prism available for the study. Optical microscope, scanning electron microscope and electron probe micro-analysis were used to obtain the mineralogical composition of the aggregates, chemical composition of the reaction products and possible relationship with the aggregate minerology. From the petrographic analysis of the Kingston aggregates, it was identified a low temperature metassomatic process, leading to the silicification and feldspatization of the carbonate rock, as well as the development of the dolomitic matrix accompanied with illite and celestite formation. No reaction products were observed in the concrete manufactured with the Kingston aggregate, although the presence of quartz, feldspars, sulphates (celestite) and sulphides (framboidal pyrite) in contact with the typical pore fluids in concrete, could lead to some previously known harmful reactions, such as ASR or delayed ettringite formation, leaving new questions regarding ACR.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospecção Mineral) Universidade de Lisboa, Faculdade de Ciências, 2022
Palavras-chave
ACR Petrografia MEV Agregado de rocha Prisma de betão Relatórios de estágio de mestrado - 2022