Estudo da hidratação e microestrutura de pastas de cimento Portland com adição de cinza de casca de arroz de baixo teor de carbono grafítico e sílica ativa
Growing concern for sustainability has resulted in a search for alternative materials for the cement industry to reduce its environmental impacts and achieve greater durability of structures at lower costs. By partially replacing the cement with some mineral addition, changes in the microstructure and in the reactions of hydration of the materials occur, being essential, then, to evaluate its performance. Based on this premise, the present work proposes to analyze the effects of the incorporation of low graphite carbon rice husk ash (RHA) from Alegrete/RS Electric Power Generation Industry on the properties of cement pastes, comparing its performance with silica fume (SF) of national supplier through the calorimetry test, X-ray diffractogram (XRD), mercury intrusion porosimetry (MIP), chemically combined water, electrical conductivity and hydrogen potential (pH). For this, pastes with different RHA contents (5, 10, 20 and 30%) and SF (5, and 10%) were molded (in cylindrical samples of Ø4x8cm), as well as a reference sample containing 100% Portland Cement of Initial High Strength (CP V-ARI). Three ratios of w/b (0.35, 0.50 and 0.65) and different cure periods were adopted. From the results obtained, it was observed that the increase in the cement substitution content by RHA and SF resulted in lower initial and final setting times and decrease in the electrical conductivity and pH values of the aqueous solution of the pores. The chemically combined water content increased with higher w/b ratios and higher pozzolan contents. By the results of MIP, when compared to the reference mixture, pastes containing RHA and SF showed increase of the total pore area, but decrease of the critical diameters and the amount of macropores, evidencing the pore refinement promoted by the pozzolanic reaction of additions with CH. Heat release curves obtained by conduction calorimetry test showed that RHA and SF modified the rate of hydration of the cement, increasing the induction and acceleration period, and releasing greater total heat than the reference paste.