Análise microestrutural de concretos com substituição parcial de cinza de casca de arroz em altas temperaturas

Abstract

The usage of supplementary cementitious materials, such as rice husk ash (RHA), as a partial replacement of Portland cement is commonly studied and at room temperature may provide mechanical strength and durability to concrete. However, it is necessary to further study the behavior of concretes with RHA when elevated at high temperatures. In this research, the X-ray diffraction (XRD) technique was used to mineralogically characterize concretes contaning RHA before and after being exposed to high temperatures and to correlate the identified mineral phases with the mechanical behavior of the samples. It is importante to note that these concretes were prepared in a previous research with a constant w/b ratio of 0.45 and 25% cement replacement with RHA. After curing for 91 days in a climatic chamber, the samples were heated according to the RILEM TC 129-MHT instructions (RILEM Technical Committee, 2004) and remained in the oven for 60 minutes when they reached the temperature levels of 200 °C, 400 °C, 600 °C and 900°C, at a 1 °C/min rate. The compressive strength test was conducted after the samples were slowly cooled. Small fragments of the specimens subjected to comperssion were collected, samples were prepared and X-ray diffraction analysis were performed. For the concrete samples containing RHA, it was verified on the XRD patterns that the intensity of the characteristic peaks of portlandite decreased, suggesting pozzolanic reactions. However, the increase in compressive strength was 6,58%. The first significant reduction in compressive strength occurred in samples subjected to 600 ° C, 58% for the reference concrete and 56% for the concrete containing RHA. On this temperature level the characteristic peaks of portlandite were no longer identified. For samples exposed to 900 ° C, the characteristic peaks of CaCO3 were no longer identified. At that same temperature level, there was the greatest reduction in compressive strength of concretes, 4,85% for the reference concrete and 7,60% for the concrete containing RHA.