Comportamiento de presas de gravedad de hormigón con problemas de ataque interno por sulfatos

Araújo, Georgia Serafim; Chinchón, Servando; Aguado, Antonio

doi:10.1590/s1983-41952008000100005

Cited by 4 publications

(7 citation statements)

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“…A degradação do concreto por íons sulfato ocorre a partir de sua reação química com os compostos hidratados do cimento os quais ficam, normalmente, disponibilizados nos poros [1,6,[17][18][19][20]. Basicamente, três reações estão envolvidas: (i) a combinação de gipsita do cimento e dos aluminatos hidratados de cálcio com o monossulfato hidratado, formando a etringita (CaO 3 .Al 2 O 3 .…”

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“…Além disto, os óxidos formados podem provocar uma expansão localizada nesta região, com liberação de íons sulfato e redução do pH local, gerando condições favoráveis para a deterioração da pasta de cimento [6][7][8][9][10][23][24].…”

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“…A presença do sulfato, em meio cimentício, tende a aumentar a ocorrência de pressões internas que podem levar à expansão devido à formação dos compostos descritos no caso do ataque externo, como a etringita e a gipsita [6,22], promovendo um aumento de volume da ordem de 172 cm³/mol de sulfeto, valor superior ao observado na etapa de oxidação da pirita [21]. Na Figura 7 é apresentada imagem da barra de argamassa com 10% de adição de pirita onde as manchas oriundas da oxidação do mineral são evidentes.…”

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“…Com o diagnóstico precoce da litologia e das suas possíveis contaminações mineralógicas e químicas, pode-se antever a produção de concretos com um mínimo de interferência na sua vida útil de projeto, uma vez que, devido a uma caracterização inadequada, muitas manifestações patológicas só são verificadas no decorrer do tempo de operação da grande obra [5][6][7][8][9][10], como são os casos das reações álcali agregado ou ataque interno por sulfatos, devido a contaminação do agregado por sulfetos minerais.…”

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Oxidação da pirita e seus efeitos em argamassas de cimento Portland sujeitas ao ataque por sulfatos de origem interna

Pereira¹,

Portella²,

Bragança³

et al. 2016

Matéria (Rio J.)

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RESUMOO ataque por sulfatos de origem interna é resultante da reação química entre os íons Fe 2+ e SO 4 2-, provenientes da oxidação de agregados sulfetados, e os compostos da pasta cimentícia, e tende a promover a degradação do concreto devido formação de produtos expansivos que geram fissuração e desagregação do material. Neste trabalho, a morfologia da pirita, presente em alguns tipos de agregado, foi avaliada. O mineral foi utilizado em substituição parcial à areia (10%, em massa) na dosagem de argamassas, para estudo das modificações microestruturais e da variação dimensional linear sob envelhecimento natural durante 42 dias. A caracterização microestrutural foi executada em equipamento FEG/SEM com sonda analítica de EDS. Verificouse, na superfície da pirita oxidada, a formação de óxidos de ferro, os quais afetaram a interface entre a pasta de cimento e o agregado nos materiais produzidos com a pirita. Quanto a variação dimensional, a argamassa apresentou apenas contrações durante o período de estudo. Verificou-se a ocorrência de fissuração e a predominância na formação de cristais aciculares de etringita aos 42 dias enquanto aos 90 dias a presença de gipsita predomina. Óxidos de ferro oriundos da oxidação da pirita são evidentes nas argamassas aos 90 dias.Palavras-chave: Pirita, oxidação, sulfatos, ataque interno por sulfatos, argamassa. ABSTRACTThe internal sulfates attack (ISA) is resultant of the chemical reaction between the ions Fe 2+ and SO 4 2-, by products of sulfide aggregates oxidation, and the composites of the cement paste. These actions promote the degradation of concrete due to the formation of expansive products that generates cracking and dissolution of the material. In this study, the morphology of the pyrite, present in some kinds of aggregate, was evaluated. In parallel, the mineral was utilized in partial replacement for sand (10%, in mass) in mortar dosage, for study of microstructural modifications and linear dimensional variation under natural aging during 42 days. The microstructural characterization was executed in a FEG/SEM equipment with EDS analytical probe. On the surface of the oxidized pyrite were formed iron oxides, which affected the interface between the cement paste and the aggregate in the produced materials with the pyrite. As the dimensional variation, the mortar presented only contractions during the period of study. It was verified the occurrence of cracking and the predominance of ettringite acicular crystals formation at 42 days, while at 90 days the presence of gypsum predominates. Iron oxides from the oxidation of the pyrite were evident in mortars at 90 days.

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Oxidação da pirita e seus efeitos em argamassas de cimento Portland sujeitas ao ataque por sulfatos de origem interna

Pereira¹,

Portella²,

Bragança³

et al. 2016

Matéria (Rio J.)

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“…It can be concluded that it is difficult to avoid the use of these aggregates in the concrete. This is even more difficult in places where the availability of another source of material is economically impracticable or non-existent as in the case of concrete dams where large volumes of aggregates are needed [1,2].…”

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Long-term behavior of mortars produced with sulfide-bearing aggregates

et al. 2019

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The variety of sulfides (inorganic compounds formed by combining sulfur with a metal element) presented in earth crust is not small, which makes it difficult to avoid the use of sulfide-bearing aggregates in the concrete, especially in places where another source of material is non-existent or economically impracticable, as is the case of dams. Within this scenario, this work presents an evaluation of the long-term behavior of mortar produced with aggregates containing pyrite and pyrrhotite. For the study, rock samples from the Irapé Hydroelectric Power Plant area in Minas Gerais were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost 1300 days, subjected to three different exposed conditions: 1-temperature of 23° ± 2°C and relative humidity of 95 to 100%; 2-submerged in calcium hydroxide solution diluted in water during almost 850 days and then kept in water up to 1300 days, both at room temperature; 3-submerged in calcium hydroxide solution diluted in water at 50° C during almost 850 days and then kept in water (50° C) up to 1300 days. The expansion caused by the iron sulfide oxidation as well as the content of each sulfide were evaluated over time. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content over time. The results showed that the calcium hydroxide solution speeds up the expansion just like the temperature. It was also observed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not only by the total amount of sulfur.

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Physicochemical evaluation of oxidation of diabase aggregates with sulfide minerals: influence of particle size, pH of the medium and pyrite concentration

et al. 2016

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Large constructions, such as concrete dams, commonly use aggregates extracted from rock masses near the implantation sites, which may contain sulfide minerals in its composition. These aggregates, inside the concrete, may oxidize, forming sulfate ions (SO 2À 4 ), which can cause the internal sulfate attack. Therefore, in order to study aggregates for concrete dosages of these structures, aimed to evaluate on this paper the oxidation process of aggregate containing sulfide minerals in amounts of up to 5.0 %, by way of studies into the effects of particle size and pH of the medium (Study 1) and the pyrite content (Study 2). In Study 1, diabase aggregates containing pyrite (FeS 2 ) were separated into three particle sizes (sand, coarse aggregate 0 and coarse aggregate 2). Samples were analyzed in two groups and immersed in solutions of distilled water and calcium hydroxide, with pH and electric potential measurements. In Study 2, diabase sand from the same origin of Study 1, but in smaller particle size, and natural aggregates (sand) with the addition of pyrite in four levels (0, 0.5, 1.0 and 5.0 %) were immersed in distilled water, with measurements of pH, electric conductivity and total dissolved solids. The results of the tests, under constant oxygenation, showed that the reduced grain size and alkaline medium accelerate the oxidation reactions, in contrast to aggregates immersed in neutral solutions.Comparing the results by ANOVA test indicates the presence of around 0.5 % of sulfide mineral in the diabase aggregates, with better results from monitoring of electric conductivity and dissolved solids in contrast to pH readings.

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Comportamiento de presas de gravedad de hormigón con problemas de ataque interno por sulfatos

Cited by 4 publications

References 7 publications

Oxidação da pirita e seus efeitos em argamassas de cimento Portland sujeitas ao ataque por sulfatos de origem interna

Oxidação da pirita e seus efeitos em argamassas de cimento Portland sujeitas ao ataque por sulfatos de origem interna

Long-term behavior of mortars produced with sulfide-bearing aggregates

Physicochemical evaluation of oxidation of diabase aggregates with sulfide minerals: influence of particle size, pH of the medium and pyrite concentration

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