Research on the Weathering Resistance of Phosphate-based Restoration Materials for Helankou Rock Art in Ningxia, China

Abstract

The main diseases of the Helankou rock art are related to delamination, cracking, and spalling caused by the expansion of structural fissures, posing higher demands on the compatibility and weathering stability of the restoration materials. This study investigated a metakaolin-monocalcium phosphate monohydrate (MK-MCPM) phosphate gelling system, formulating both neat paste and aggregate-mixed repair mortars. The materials were subjected to freeze-thaw cycles (20 cycles), wet-dry cycles (20 cycles), and 70°C dry heat aging (300 hours). The mechanical properties, surface hardness, mass loss, and volume stability of the materials were systematically evaluated. The results show that the system exhibits excellent weathering resistance: after freeze-thaw cycles, the material’s mass loss rate is consistently below 0.5%, and both strength and surface hardness are stable, with an increasing trend in surface hardness, indicating superior freeze-thaw stability. Wet-dry cycles have minimal impact on compressive strength, but the flexural strength of the aggregate-mixed mortar remains stable and even improves, indicating the effectiveness of the aggregate framework in constraining cracking caused by drying shrinkage. Under more severe dry heat conditions, the mortar system can limit strength loss to less than 10%, with volume changes below 0.6%, showing good high-temperature dimensional and mechanical stability. The study suggests that the MK-MCPM repair mortar has potential for application in the Helankou environment, and the durability evaluation and engineering formulation should focus on tensile performance and interface stability under dry heat conditions, with further improvement in long-term service reliability through aggregate optimization.