Extração da água livre em cimentos álcali ativados e seu efeito na resistência à compressão

Abstract

Alkali Activated Binders (AAB) are sustainable alternative binders to ordinary Portland cement (OPC). They are binders with less environmental impact, clinker free, produced from the alkaline activation of an amorphous aluminosilicate source. Under adjusted synthesis conditions, these materials present similar, or even superior, performance than OPC materials. However, there are barriers to the effective application of these materials on large scale. The drying thermal treatment, with the objective of removing free water from AAB, presents itself as a promising alternative on performance improvement, compared to special applications that require thermal stability and refractoriness. Several works in recent years have evaluated the application of a drying pretreatment, aiming to reduce the growth of cracks and shrinkage. After that, the geopolymers are exposed to high temperatures (>500°C). The presence of free water in the microstructure of the AABs generates negative effects on its performance under high temperatures, as consequence of cracks caused by stress derivated from water evaporation. However, there is needed analysis to identify the best condition to the water extraction from the matrices. Thus, this work aimed to investigate the effect of free water removal, for AABs with different precursors: fly ash, slag and metakaolin. The matrices were molded and submitted to three different thermal treatments, for a period of 48 hours: 25, 60, and 150 °C. The effect of the applied treatment on the compressive strength was evaluated, focusing on stability and reliability of the results by Weibull distribution, as well as the validation of the quantified water removal, based on thermogravimetric analysis (TG). The results demonstrated that on the 60 °C treatment, for all three materials, the removal of free water was not effective to free water removal. At this temperature, the stability on Weibull analysis results was low, with water still existing in the material and interfering on the results variability. High values for Weibull modulus proved that after 150 °C treatment the results were less dispersed. For that last test, the TG analyzes demonstrated that the removal of water was more effective, making the material more mechanically stable.