Cimentos álcali-ativados preparados com resíduos de cimento-amianto inertizados

Abstract

The recycling of secondary raw materials, such as asbestos-cement waste (ACW) has become a priority due to its carcinogenic potential. Thus, the search for technologies that make the reuse of ACWs feasible is seen as a way to minimize environmental impacts and disease risks. In this sense, methods were developed to allow the ACW inertization, with emphasis on heat treatments, which are the most used. However, there is a lack of studies aimed at optimizing this procedure, since it is an expensive task. Thus, this work proposes to define the optimal parameters of ACW heat treatment to simultaneously inertize chrysotile and embryonicly, to evaluate the application of ACW treated as a precursor to the production of alkali-activated cement (CAA) of a part. For the thermal treatment of the ACW, the optimization was performed by applying a 2k factorial design with a central point having as factors the temperature (600 °C and 800 °C), the calcination time (1 and 3 h) and the mass of ACW (1 and 5 kg), considering minimum and maximum levels. Eleven experiments were used to elaborate models that relate the answers: calcite content (% calcite), chrysotile content (% chrysotile), belite content (% C2S) obtained through thermogravimetric analysis (TG) and X-ray diffraction. (XRD). Simultaneous optimization was used to identify the optimal levels of the factors for complete chrysotile dehydroxylation, to minimize energy expenditure and CO2 emissions, and to maximize the content of belita (C2S) in the treated ACW. Then, the treated ACW (ACWT) was used as a precursor to the production of CAA of a part. The mixtures were prepared using ACWT, solid sodium metasilicate (MSS) (SiO2 / Na2O equal to 0.98) and water. Eleven formulations established using the I-Optimal design, served as basic formulations for the statistical design of the mixture. The formulations established in the statistical planning were characterized in terms of physical-mechanical properties, thermal and mineralogical analysis. The results of heat treatment show the complete dehydroxylation of chrysotile, with an increase in the content of belita (C2S) and CO2 emission, for temperatures equal to or higher than 700 °C. It was observed that the optimal conditions for the treatment were 5 kg of ACW, 800 °C, for 1 h. Using these parameters, the chrysotile was completely eliminated, and the treated waste showed 40.42% of belite, which gives the material the agglomerating capacity, with an estimated CO2 emission of 175.6 gCO2/kg of treated waste. In the dosage of alkaliactivated pastes, formulations with an MSS concentration of around 5% and molarities close to 1M, showed the best mechanical properties. The addition of MSS had a strong influence on the accumulated heat rate, which may be associated with the speed of dissolution of the Si - O and Al - O bonds. The activated formulations showed superior mechanical behavior in relation to the mixtures without activator. Furthermore, pastes without activator have less shrinkage due to drying and chemical shrinkage. It was found that the ACWT can be applied to the production of CAA and the statistical design of the mixture is an efficient method in the prediction of the evaluated properties