Visualization of capillary water migration in grotto sandstones

Moisture intrusion is a critical factor threatening the structural integrity and long-term preservation of sandstone grottoes. However, limited studies have addressed the dynamics and transformation characteristics of capillary water migration within such heritage materials. To elucidate the characteristics and mechanisms of capillary water migration, one-dimensional vertical capillary rise tests were carried out in the laboratory on fresh sandstones from the Yungang Grottoes and the Leshan Giant Buddha site. The development of water absorption rate and height of capillary rise over time was measured. In addition, nuclear magnetic resonance and infrared thermography techniques were used to obtain the evolution of spatiotemporal distribution of moisture within the sandstones. The results indicate that both Yungang sandstone and Leshan sandstone show a moderate capability of capillary water absorption. Due to a larger number of large pores and the non-uniform pore size distribution in Leshan sandstone, its water absorption rate and height of capillary rise change more rapidly, resulting in a highly irregular shape of wetting front. Throughout the testing process, the saturation degree of micropores remains higher than that of mesopores, and the differences in the increasing rate of saturation degree among different types of pores are more pronounced in Leshan sandstone. Additionally, some larger pores are unable to become fully saturated via capillary action. Capillary water migration consists of two processes, i.e., water rising and water diffusion. Within the sandstone absorbing water, moisture content is the highest at the bottom, which gradually decreases with increasing height. It leads to the formation of wet, transitional, and dry zones, with its corresponding surface temperature changing gradually from low to high. Changes of temperature can reflect fine variation in moisture content, and therefore, the infrared thermography technique exhibits good applicability in rapid investigation of moisture distribution. This study did some exploration on the deterioration mechanisms of sandstone grottoes and the applicability of non-destructive testing techniques, aiming to provide a theoretical basis and practical guidance for the preventive conservation of cultural heritage.