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

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