Salt damage to brick kiln walls in Japan: spatial and seasonal variation of efflorescence and moisture content

To clarify the mechanism of salt fretting of a brick building, monthly observations were made over one year at a two-storey brick kiln in central Japan. Products of weathering that had fallen were collected and weighed; salts were identified using XRD and the moisture content of the brick was measured at its surface using a portable infrared optical moisture meter. Gypsum (CaSO₄·2H₂O) was found at the bottom of the first floor wall in the warm and humid season. Thenardite (Na₂SO₄) is dominant on the first floor wall; it is abundant in the cold and dry season but sparse in the warm and humid season. Magnesium sulphate is dominant on the second floor wall; epsomite (MgSO₄·7H₂O) was observed in the dry-to-wet season and hexahydrite (MgSO₄·6H₂O) in the wet-to-dry season. Seasonal changes in salt abundance result from the temperature dependence of the solubilities and equilibrium relative humidities (ERH) of each salt. The amount of brick decay material due to salt weathering is much greater from the second floor wall than from the first floor wall and is greater from spring to the end of summer. These seasonal and spatial variations of the rate of brick decay can be explained by a combination of factors: (1) the types of salt, (2) efflorescence or sub-florescence and (3) salt deliquescence.