Abstract: Inorganic materials with strong compatibility and durability are widely used in the conservation of earthen sites, and those with especially loose structure has been the research focus of earthen sites reinforcement. Micro-nano Ca(OH)₂ has a range of characteristics, including small molecular structure, significant reinforcement effect and good durability. Take the UNESCO world heritage site at Suoyang City as an example, we prepared the loose soil samples with the density of 1.5 g/cm3, then, the samples are enforced by micro-nano Ca(OH)₂ suspension with concentration of 5%, 7.5% and 10%, respectively. Through the tests of air permeability, color difference, unconfined compressive strength and shear strength, it is found that the decrease value of air permeability is within 2%, and the color difference (ΔEab*) treated by 5% and 7.5% micro-nano Ca(OH)₂ suspension is less than 4, to certain degree, which is acceptable. The compressive strength and shear strength are improved after three rounds of reinforcement using the suspension with concentration of 7.5%. Meanwhile, the unconfined compressive strength increases by 9.8%, and the cohesion of soil increases by 34%. The internal friction angle grows up by 9°. The soil-water characteristic curve shows that micro-nano Ca(OH)₂ has a good inhibitory effect on the volumetric shrinkage of soil samples. The reinforcement mechanism is revealed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis. It is found a series of physicochemical reactions occurred in the alkaline environment after the treatment of micro-nano Ca(OH)₂. From the physical perspective, loose samples are enhanced by filling, wrapping, and mosaic. In terms of chemical perspective, the carbonation of calcium hydroxide forms new calcareous cements, which is the primary reason of reinforcement. This study reveals that micro-nano Ca(OH)₂ has potential for the anti-weathering and reinforcement of earthen sites.

Key words: earthen sites, weatherproof materials, preservation of cultural relics, laboratory test