Degradation processes of iron-sulfides and calcite containing aggregates from asphaltic mixtures

Abstract Iron-sulfide and calcite containing aggregates used for preparation of asphalt mixtures underwent degradation processes under atmospheric conditions. Four locations with specific secondary mineral phases situated on highways and roads from Transylvania (Romania) were investigated. The mineralogical investigations performed on aggregates as well as on the resulted secondary phases revealed the presence of two types of degradation processes: sulfate precipitation, and carbonation. Iron sulfides such as pyrite and marcasite of hydrothermal origin identified in aggregate grains are the source for iron and acidity. During the wet and warm season, in the presence of atmospheric water and oxygen, in a relatively short time (up to two months) iron-sulfides are transformed and sulfate minerals such as melanterite, rozenite, szomolmokite, copiapite, halotrichite and gypsum precipitate. They are associated with secondary calcite, clay minerals and sulfur. Secondary calcite is the result of carbonation process which is mainly controlled by the acid environment generated by atmospheric CO 2 dissolved in the rain water. Another source of acidity is the presence of sulfuric acid resulted during the iron-sulfides degradation process. All secondary products are accompanied by Fe-oxihydroxides of rusty color. Thereafter, the resulted sulfate minerals, which later are easily leached by rainwater, progressively consume the iron-sulfides from the aggregate. This process leads to mechanical disintegration of the grains, progressive stripping and potholes. Secondary calcite is found filling the pores of the asphalt mixture or coating the aggregate grains. Later, it could be dissolved by rainwater.