Evaluation of pyrite and pyrrhotite in concretes

Abstract

ResumoIt is well known that aggregate characteristics can intensively interfere in concrete behavior especially when sulfides are presented in the aggregates. The lack of consensus to content limit value of these deleterious sulfur compounds in concrete structures for dams has motivated several investigations worldwide. Within this scenario, this work presents a methodology to evaluate the presence of pyrite and pyrrhotite in concretes produced with aggregates containing sulfides. For the study, rock samples from the Irapé hydroelectric power plant area in Minas Gerais (Brazil) 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 one year, subjected to three different exposed conditions: temperature of 23° ± 2°C and relative humidity of 95 to 100%; calcium hydroxide solution diluted in water kept at two different temperatures: room temperature and 50° C. 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. The results showed 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 solely by the total amount of sulfur.Keywords: sulfide aggregates, concrete, behavior, dams.As características dos agregados podem interferir intensamente no comportamento do concreto, especialmente quando sulfetos, como pirita e pirrotita, estão presentes nos agregados. A falta de consenso para o valor limite do teor desses compostos de enxofre em estruturas de concreto de barragens tem motivado várias investigações em todo o mundo. Dentro desse cenário, este trabalho apresenta uma metodologia para a avaliação da presença desses compostos de enxofre em concretos fabricados com agregados contendo sulfetos. Para o estudo, foram utilizadas amostras de rochas da área de usina hidrelétrica do Irapé, em Minas Gerais (Brasil). Esta usina foi construída em um sítio geológico onde a rocha apresentava níveis de sulfeto de pelo menos 3%. Essas amostras de rochas foram moídas e depois utilizadas como agregados em argamassas submetidas durante aproximadamente um ano a três diferentes condições de exposição: temperatura de 23° ± 2ºC e umidade relativa de 95 a 100%; solução de hidróxido de cálcio diluída em água mantida em duas temperaturas diferentes: temperatura ambiente e 50°C ± 2ºC. A presença e quantidade de pirrotita foram obtidas a partir de um processo de lixiviação do material (agregado ou argamassa) numa solução de ácido clorídrico para dissolução desta, o que permitiu a avaliação do teor de pirita, sulfeto esse não dissolvido pelo ácido clorídrico. A quantidade de pirrotita foi obtida pela diferença entre o valor inicial de enxofre total da amost...

Concrete damage due to oxidation of pyrrhotite-bearing aggregate: a review

“…Marcelino et al [52] presented an acid leaching procedure to evaluate the amount of pyrrhotite in sulfide-bearing concrete samples based on the methodology developed by Lorenzen [53]. First, an aggregate sample was ground to a particle size lower than 0.15 mm.…”

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

“…To have good concrete workability during batching, different aggregate size compositions were investigated [12]. Aggregates were first crushed and sieved to achieve the size distribution according to the requirements of NBR 15577part 4 (ABNT, 2008) [13].…”

“…The total sulfur content was then determined through direct combustion (with infrared detection) using the equipment LECO SC 632 (Figure 2). For the evaluation of the content of the different iron sulfides, a leaching process was employed [12]. For the selective dissolution of the pyrrhotite, 1g of the smashed mortar sample was blended with a 100ml hydrochloric acid solution (HCl), in the proportion of (1:1) in an Erlenmeyer flask, and agitated for an hour at 60ºC.…”

Development of a Sulfide-Bearing Aggregate Oxidation Test in Concrete

“…The screening method developed by Marcelino et al (2016) discriminates between pyrite and pyrrhotite content in aggregate or mortar after curing. Initially, the qualitative assessment of the aggregate is done through RAMAN spectroscopy.…”

“…The interaction between pyrite and pyrrhotite content in sulfide bearing aggregate should also be investigated by chemically quantifying the amount of each mineral in the aggregate and then analysing their expansion under SMBT conditions. An adaptation following the procedure developed by Marcelino, et al (2016) can be used to quantify the amount of pyrite and pyrrhotite in each aggregate and could be used to find a correlation between pyrite-pyrrhotite content versus expansion.…”