Microstructure of concrete subjected to elevated temperatures: physico-chemical changes and analysis techniquesMicroestrutura do concreto submetido a altas temperaturas: alterações físico-químicas e técnicas de análise Abstract ResumoThe exposure of concrete to high temperatures, such as in a fire, leads to physical and chemical changes, which may cause deterioration of mechanical properties, cracking and spalling. This paper presents a critical review of microstructural changes in concrete exposed to high temperature. The transformations developed in the cement paste, aggregates and interfacial transition zone were studied, as well as the experimental techniques of microanalysis presented in recent related researches. Lastly, a critical analysis of experimental results from literature was performed. It was verified that microstructural changes are related to concrete properties and the heating process. The experimental techniques has a potential use for assessment of thermally damaged concrete, however, these techniques must be applied simultaneously and specific methods must be established.Keywords: microstructure, concrete, concrete structures, high temperatures.A exposição a altas temperaturas, como as de um incêndio, promove alterações físicas e químicas no concreto, provocando deterioração das propriedades mecânicas nas estruturas, fissuração e desplacamento. O presente trabalho consiste em uma revisão crítica das alterações microestruturais que incidem no concreto submetido a altas temperaturas. As transformações desenvolvidas na pasta de cimento hidratada, nos agregados e na zona de transição foram estudadas, bem como as técnicas experimentais de microanálise utilizadas em recentes pesquisas desenvolvidas na área. Por fim, uma análise crítica dos resultados de estudos experimentais apresentados pela literatura foi realizada. Verifica-se que as alterações da microestrutura estão relacionadas com as características constitutivas do concreto e com processo do aquecimento. Constata-se a potencialidade das técnicas microestruturais para as etapas de na inspeção e recuperação de estruturas incendiadas, entretanto verifica-se a necessidade de combinação de técnicas e o estabelecimento de métodos padronizados.Palavras-chave: microestrutura, concreto, estruturas de concreto, altas temperaturas.

Increasing population growth and rise global energy petition have made congregation the requests of energy generation and safety a major challenge global. In another hand nanogeoscience is commencing to develop a viable remediation approach of attention in coal mine drainage (CMD) around spontaneous coal combustion (SCC). On the ecological context, nanophases (minerals and/or amorphous phases) are more reactive than bulk compounds, a property that powerfully influences the fate of pollutants in topsoils and drainages. In this work petrographic and geochemical investigations of selected CMD sediments were conducted. The substitution of As, Cd, Hg, and Se in pyrite was most apparent in the CMD sediments. The sampled CMD contained carbon nanotube (CNTs) structures and many others C-nanophases. The CNTs contained several elements, including Hg, Pb, F, Cl, and halogens. While CNTs are known to be produced from coal fires of varying ranks, this seems to be the first report of naturally occurring CNTs. This work also denotes the occurrence of historical NPs-locations in near vicinity to all other, as for illustration deposits of C-NPs and non-crystalline compounds appear only nanometers separately from each other on the contradictory sides of nonaltered amorphous phases. In addition, non-conventional sources, including CMD hosting elevated concentrations of Rare Earth Elements and Yttrium (REY), have been explored as attractive secondary sources for elements recovery. Consequently, in this study we investigate CMD from abandoned coal mines in the South America as a potential REY resource. It is suggested that more work is required on CMD and a few research areas are proposed for future research.

Among the processes that involve the degradation of concrete structures subject to the high temperatures of a fire there is the spalling phenomenon. Its mechanisms are related to the thermal stress of the materials dilatations and pore pressure the process of vaporization of water during heating. The factors that influences in its occurrence are related to concrete properties, structural member characteristics or the exposure conditions, and their parameters are not clearly known yet. This paper aimed to study the influence of three concrete mixtures, four coating thicknesses and two bars diameters of longitudinal reinforcement in the spalling phenomena exposed to ISO 834 fire curve. The characterization of concrete were performed either of the axial compression strength tests, water absorption by capillary and mercury intrusion porosimetry, besides the fire resistance tests in real-scale specimens. It was concluded that the diameter of the bar does not have influence, while the mixture and the concrete cover thickness does. More spalling was recorded for the columns with thicker concrete cover and concrete compressive strength at 61,9 MPa, and although higher strength concrete have less permeability, this characteristic can be balanced with the higher tensile strength of this type of concrete.

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