Ancient clay bricks: manufacture and properties

Abstract

The physical, mechanical and chemical properties of ancient fired clay bricks are addressed. A literature survey is carried out and samples of old Portuguese bricks dated from the 12th to 18th centuries are considered. A high porosity (15-40 %) and water absorption (10-20 %) is found. The suction rate can be rather high (up to 0.35 g/cm2/min), while the specific mass is low (1,500 to 1,800 kg/m3). The compressive strength shows a huge scattering with frequent values ranging from 1.5 to 30 N/mm2. In general, raw clays used in old clay bricks seem to have some consistency with respect to proportion of the main chemical constituents, while bricks of the same origin generally exhibit a strong chemical similarity. Finally, it is shown that a minor-destructive test (microdrilling) allows to adequately assess the compressive strength of old clay bricks, using appropriate correlations.