Properties, characterization, and decay of sticky rice–lime mortars from the Wugang Ming dynasty city wall (China)

• Mortars used in the Wugang city wall are a calcium carbonate-sticky rice hybrid bonding material.
• Carbonation processing is extremely slow due to dense and compact microstructure of mortars.
• Decying of mortars results from the appearance of soluble salt from ambient environment.

Urgent restoration of the Wugang Ming dynasty city wall brings about the need for a study of the formulation and properties of mortars. In the present paper, mortar samples from the Wugang Ming dynasty city wall were characterized in a combination of sheet polarized light optical microscopy, scanning electron microscopy with X-ray energy dispersive spectrometer, thermogravimetric/differential scanning calorimetry, X-ray powder diffraction, Fourier transform infrared spectroscopy, and inductively coupled plasma emission spectroscopy. Results show that mortars are mainly built up from inorganic calcium carbonate based organic–inorganic hybrid material with a small amount of sticky rice, which plays a crucial role in forming dense and compact microstructure of mortars and effectively hindering penetration of water and air into mortars. Analysis of decayed products shows that the detrimental soluble salts originates from ambient environment.