Damaging Mineral Transformation of Greywacke Aggregate in SMA 11 and AC 11 Wearing Courses Used in A1 Motorway Pavement in Silesia Poland

Abstract

The paper covers the issues of mineral transformations in the natural aggregate made of greywacke rock used in A1 Motorway asphalt wearing courses (SMA 11 and AC 11) under hypergenic conditions (connected with external environment factors), typical for pavements. The results of these transformations have an appearance of rusty efflorescences on surfaces, which denote destruction process of some grains of the aggregate. This process can next initiate local damage to the wearing course. The research was focused on the experimental tests and mineralogical studies aimed at greywacke grains isolated from A1 Motorway pavement. Experimental tests in laboratory conditions allowed to simulate the process leading to arising of the efflorescences, which finally adopted an evaporite form. Precipitates recreated in laboratory conditions, acquired for mineralogical research, were similarly coloured to those observed on the pavements. Mineralogical studies – performed using optical microscopy, X-ray methods and a scanning electron microscope (SEM), for grains which were the sources of rusty efflorescences and the received precipitates – allowed to identify minerals involved in the transformations (primary minerals) and the minerals subsequently produced (secondary minerals). The most important primary mineral involved in the process leading to arising of efflorescences was pyrite, while the most important secondary minerals were gypsum, halotrichite and copiapite. What was noticed was that mechanism of mineral transformations leading to creation of hydrated sulphates on the surface and in the pores of the grains can be accompanied by the exudation process. The work is closed by short description of the proposal procedure according to PN-EN 1744-1 method modified for pavements applications, which allows to predict the tendency of natural aggregates to the mineral transformations. The modification involves activation of aggregate susceptible to mineral transformation using different than standard lime water factor which can imitate accurately hypergenic conditions typical for pavements.