Capillary water absorption characteristics of sandstone in Nankan Grotto: impacts from salt types and concentrations

The capillary water absorption issues of the sandstone have significantly influenced their salt weathering conditions. This work aimed to explore the influences of different salt types and concentrations for the capillary water absorption of sandstone in Nankan Grotto. Three sets of capillary water absorption tests were designed. In addition, the mineralogical, major element, micro-structure, and physical properties of sandstone were also analysed. It is found that the mineralogical compositions of the sandstone were quartz, feldspar, illite-smectite mixed layer, illite, and chlorite. The water absorption weight curves showed three stages: rapid water absorption stage (0-4 h), slow water absorption stage (4-36 h), and inactive water absorption stage (after 36 h). The capillary rise height curves showed two stages: sharp increase in 0-20 min and very slow increase after 20 min. The capillary water absorption coefficient (Acap) results revealed that all of the salt solutions (Na2SO4, NaNO3, and Na2SO4 + NaNO3) promoted the capillary water absorption in sandstone. The promoting effect of Na2SO4 solution was the most remarkable, followed by Na2SO4 + NaNO3 solution. The promoting effect of NaNO3 solution was insignificant. The maximum water absorption velocities under the NaNO3 solution conditions were always greater than those under the Na2SO4 solution conditions, and smaller than that under deionized water. This was due to the highest viscosity of Na2SO4 solution. The Hall model fitted the capillary water absorption curves better than Feng and Janssen model. In addition, the modified model for capillary rise height prediction demonstrated good agreements with those obtained from experiments.