Study on the Characteristics of Damaged Sandstone in the Longshan Grottoes Using Water Chemistry and Freeze–Thaw Cycling.

Sandstone from the Longshan Grottoes in Taiyuan, China, was the research object of this paper. The sandstone samples were soaked in distilled water, Na₂SO₄ solution, and NaCl solution and subjected to freeze–thaw testing. Sandstone specimens were treated with 0, 5, 10, 15, 20, 25, and 30 freeze–thaw cycles. The mass ratio, P-wave velocity, surface hardness, uniaxial compressive strength, and other physical-mechanical features of rock samples were measured after different numbers of cycles. The results of mercury injection, scanning electron microscopy, and X-ray diffraction were combined to explore the damage mechanisms and characteristics of the rock samples under the combined action of chemicals and freeze–thaw cycles. It was found that the damage degree of sandstone increased logarithmically with the number of cycles, with Na₂SO₄ solution causing the most damage. Damage variables D of the samples soaked in distilled water, Na₂SO₄ solution, and NaCl solution after 30 freeze–thaw cycles were 3.89%, 6.51%, and 4.74%, respectively. The difference in damage between the solutions is caused by the combination of frost heave, dissolution, and salt crystallization, and the damage process generally occurs from the inside and the outside and is manifested as an increase in the number of macropores and the appearance of new pores. Freezing–thawing and salt action are important causes of the powdering and detachment of sandstone in the Longshan Grottoes. [ABSTRACT FROM AUTHOR]