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

‘Aggregate’ is a collective term for the mineral materials such as sand, gravel and crushed stone that are used with a binding medium (such as water, bitumen, Portland cement, lime, etc.) to form synthetic materials (such as asphalt concrete and Portland cement concrete). In HMA, aggregates account for 92%-96% by volume of the total mixture. It has been well documented that the stability of aggregates after their incorporation into a synthetic material is dependent on both mineralogical composition and environmental conditions. Exposure of aggregates to atmospheric conditions may lead to transformations of their minerals. According to the genetic type of rock, these minerals can be of primary and secondary origin. Iron in combination with other elements (e.g. Sulphur) forms different mineral species (e.g. iron – sulphides), present in some rocks in use today for construction.

The most common iron – sulphides (pyrite and marcasite) are of hydrothermal origin into volcanic rocks where is frequently associated with calcite. The properties of rocks containing such minerals, upon exposure to oxygen, water and other environmental variables on the earth surface, are prone to alterations. To a large extent, it is rather unfortunate that the degradation processes of such rocks on asphalt are yet to be thoroughly studied.

Rock aggregates in asphalt mixtures are subject to degradation processes. To address this, researchers from the Department of Geology at Babeş Bolyai University in Romania: Associate professor Nicolae Har and Alexandru Lazarean, in collaboration with Dr. Mihai Iliescu, Dr. Nicolae Ciont and Ioan Florin Abrudan at the Technical University of Cluj-Napoca looked in depth on the degradation processes of iron-sulfides on asphalt. The studied processes are responsible for aggregate stripping, potholes and traffic safety issues. To be precise, their goal was to back trace various weathering degradation processes which have affected the original natural aggregate grains in asphalt mixtures, under atmospheric conditions, based on the resulting secondary minerals. Their work is currently published in the research journal, Construction and Building Materials.

In brief, iron-sulfide and calcite containing aggregates used for preparation of asphalt mixtures were subjected to 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 resulting secondary phases revealed the presence of two types of degradation processes: sulfate precipitation, and carbonation. The authors further reported that iron sulfides such as pyrite and marcasite of hydrothermal origin identified in aggregate grains were the source for iron and acidity. In addition, 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 were transformed and sulfate minerals such as melanterite, rozenite, szomolmokite, copiapite, halotrichite and gypsum precipitated.

In summary, the study by Associate professor Nicolae Har and his colleagues reported two chemical weathering processes responsible for aggregate degradation: precipitation of sulfate minerals, and precipitation of carbonate minerals (carbonation). Generally, the team established that acid environments generated by the degradation of iron sulfides accelerated the process of carbonation, and consequently the precipitation of calcite. In an interview with Advances in Engineering, Associate professor Nicolae Har emphasized that the quality of the raw materials used in aggregate production is essential for high quality asphalt mixtures. Thus, metal-sulfides and calcite free aggregates are recommended for durable asphalt concrete production.