Long-term behavior of mortars produced with sulfide-bearing aggregates

Resumen

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.
Keywords: sulfide-bearing aggregates, mortar, long-term behavior, dams.