ABSTRACT The variety of sulfides (inorganic compounds formed by combining sulfur with a metal element) present in earth crust is not small, which makes it difficult to avoid the use of sulfide-bearing aggregates in the concrete, especially in places where another source of material is non-existent or economically impracticable, as is the case of dams in Brazil. Within this scenario, this work presents an evaluation of the long-term behavior of mortar produced with aggregates containing pyrite and pyrrhotite. For the study, rock samples from the Irapé Hydroelectric Power Plant area in Minas Gerais (Brazil) were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost 1300 days, subjected to three different exposed conditions: 1-temperature of 23° ± 2°C and relative humidity of 95 to 100%; 2-submerged in calcium hydroxide solution diluted in water almost 850 days and then kept in water up to 1300 days, both in room temperature; 3-submerged in calcium hydroxide solution diluted in water at 50° C during almost 850 days and then kept in water (50° C) up to 1300 days. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content. The results showed that the calcium hydroxide solution speeds up the expansion just like the temperature. It was also observed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not only by the total amount of sulfur.

ABSTRACT The variety of sulfides (inorganic compounds formed by combining sulfur with a metal element) presented in earth crust is not small, which makes it difficult to avoid the use of sulfide-bearing aggregates in the concrete, especially in places where another source of material is non-existent or economically impracticable, as is the case of dams. Within this scenario, this work presents an evaluation of the long-term behavior of mortar produced with aggregates containing pyrite and pyrrhotite. For the study, rock samples from the Irapé Hydroelectric Power Plant area in Minas Gerais were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost 1300 days, subjected to three different exposed conditions: 1-temperature of 23° ± 2°C and relative humidity of 95 to 100%; 2- submerged in calcium hydroxide solution diluted in water during almost 850 days and then kept in water up to 1300 days, both at room temperature; 3- submerged in calcium hydroxide solution diluted in water at 50° C during almost 850 days and then kept in water (50° C) up to 1300 days. The expansion caused by the iron sulfide oxidation as well as the content of each sulfide were evaluated over time. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content over time. The results showed that the calcium hydroxide solution speeds up the expansion just like the temperature. It was also observed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not only by the total amount of sulfur.

ABSTRACT Nanotechnology can significantly improve the mechanical strength and durability of concrete. At the nanoscale, good dispersion of constituent materials is an essential factor for the improvement of concrete properties. In this scenario, the objective of this research was to evaluate the mechanical properties of high performance concrete containing stabilized nano-silica. The incorporation of nano-silica to the polycarboxylate chemical admixture allowed its application in concrete, since the nano-silica was evenly dispersed in the concrete mass. For this study, three different concretes were employed: a reference concrete, one containing stabilized nano-silica and another containing a mixture of silica fume and stabilized nano-silica. The evaluated concrete mechanical properties were the axial compressive strength and secant modulus of elasticity measured in three different ages. The results indicated a substantial increase in concrete compressive strength and modulus of elasticity with the addition of stabilized nano-silica combined with a reduction in cement content. The results also showed that use of stabilized nano-silica proved to be very advantageous, especially when mixed with silica fume. When they were combined, a synergy effect occurred in the concrete that was of chemical nature in addition to the physical effect of better concrete particle packaging. Analysis by scanning electron microscopy showed the improvement in microstructure of concretes with stabilized nano-silica, which contributed to gains in mechanical properties.