Mineralogical and physical characterization of the bricks used in the construction of the “Triangul Bastion”, Riga (Latvia)

Bricks proceeding from one of the bastions of the Riga medieval defence wall were studied. Two main type of bricks were identified: one rich in quartz and red coloured and the other with a calcium silicate phase and yellow coloured. The survey of the bastion reveals a partial deterioration of the bricks. It was observed that differences in mineralogy and texture of calcareous and non-calcareous bricks were correlated with their behaviour in hydric and weathering tests. Data from powder X-ray diffraction (presence of high-temperature mineral phase in yellow bricks), optical microscopy (vitrification in yellow bricks), Field Emission Scanning Electron Microscope (development of secondary bubbles due to extensive melting in yellow bricks) and ultrasounds measurements (lower velocity in red bricks) allow to deduce that yellow bricks were fired at a higher temperature than the red ones. The study of the pore system revealed a bad interconnection among pores in yellow bricks (hydric data) since melt isolated the pores and filled the smallest, whereas large pores increased in size (porosimetric data). As a consequence, considering the climatic conditions of the region, yellow bricks are the materials most subjected to environmental alteration. It is in agreement with the freeze-thaw test results. Finally, low contamination of the bricks with water-soluble salts in situ leads to exclude salt-caused decay process. The relevance of these results in the conservation of Cultural Heritage is discussed.