[en] Andesites and basaltic rocks are commonly used as construction materials for roads and other civil engineering structures in Slovakia. Rock quality and durability vary in different resources. Yearly periodical testing of rocks by conventional techniques of rock mechanics is unable to indicate local, small scale degrading which is caused by neo-formation of clay minerals from less stable rock constituents. For recognition of tendency to rapid weathering and rock decay on so-called 'fresh rocks' the technique of grinding in water according to Kühnel and Van der Gaat (1987) was used. Several andesites and basaltic rocks of Slovakia were tested on clay mineral content in dry state and after grinding in water. A significant increase of intensity of main mines of clay minerals has been observed on less durable rocks. The quantified line intensities of smectite (so-called 'smectite index') show high correlation with other mechanical properties.
