Influence mechanism of salt erosion on the earthen heritage site wall in Pianguan Bastion

The earthen heritage site wall of Pianguan presented salt crystallization and pulverization in China. In this study, rammed earth samples were collected from the heritage wall, and the salt solution erosion test of samples were carried out. The influence mechanism of salt erosion on the earthen heritage site wall was conducted on the rammed earth samples using portable digital microscope, scanning electron microscope, portable chromatic aberration instrument, strip-shaped adhesive tapes, hardness tester, electronic universal testing machine, X-ray diffractometer (XRD) analysis. The macro and micro observations showed the salt erosion damage of rammed earth goes through three stages roughly. The warping and hollowing were appeared on the surface and salt precipitation in the inner sample at the early stage; The soluble salt precipitation thin salt shell appeared on the surface and the pulverization in the inner sample at the middle stage; Salt particles covered the whole surface, the inner sample was wet and the pulverization was reduced at the later stage. The chromatic aberration analysis showed the chromatic aberration of the surface sample increased with the increase of salt supply time, but the variation decreased when the salt shell covered the surface of the sample. The adhesion analysis showed the number of particles fall off the sample surface increases first and then decreases, and the cohesion of the sample surface is overall decreased. The hardness and compressive strength analysis showed they decreased linearly with the increase of salt supply time. The component analysis of precipitated soluble salts showed sodium chloride in soluble salt mainly migrated and precipitated on the surface during the sample drying process, and the main factor leading to the pulverization of the sample.