Characterization of the superficial weathering of bricks on the City Wall of Xi'an, China

This study characterized the influence of salts on bricks in correlation with the efflorescence, exfoliation and pelletizing occurring in the ancient brick wall at Xi'an, China. After 640 years of atmospheric exposure, many bricks have suffered from efflorescence, exfoliation and pelletizing. Different scientific instruments, such as optical microscopy (OM), X-ray fluorescence analysis (XRF), X-ray diffraction (XRD), thermal analysis (TGA/DTG) and X-ray photoelectron spectroscopy (XPS), have been applied to investigate the chemical and mineralogical components of the salty crusts and evaluate their species and accumulative quantities. The results show that the detected salts are chiefly aggressive deterioration problems that are causing serious damage to the brickwork. Calcium-containing salts with a dominant amount of calcite (CaCO3) and ∼2.3 at.% of gypsum (CaSO4·2H2O) were detected as the main components in the efflorescence sample, demonstrating the significantly large amount of efflorescence that occurs on shining days after rainy days. This results from the almost insoluble CaCO3 that occurs via a neutralization reaction of calcium hydroxide [Ca(OH)2] that migrates from the puddled clay core and carbon acids on the surface of the bricks. The flaking and powdering of bricks demonstrate different weathered morphologies that have similar material compositions that are classed as exfoliation and pelletizing and are responsible for the presence of some salty ions, such as C, S and N, due to halite (NaCl), thenardite (Na2SO4) and metal nitrates [M(NO3)n] at a total of 10 at.%. With a comprehensive cognitive objective, this research contributes to a new understanding of weathered phenomena that occur on the surface of bricks in brick-clay structures without neglecting the effective removal of salty ions that regularly accumulate on the surface of porous fictile structures.