Evaluation of the environmental medium erosion resistance of natural hydraulic lime and metakaolin-air lime mortars

Abstract

Hydraulic lime has been successfully applied to repair masonry/geotechnical ancient architecture. However, chemical pollutants caused by modern industrial development have enriched the building base for a long time, and the mechanism by which corrosive media destroy the hydraulic lime repair material is unclear. In this study, the properties and structural failure of natural hydraulic lime (NHL) and metakaolin-air lime (MK-AL) due to NaOH, NaCl and Na2SO4 erosion were studied in terms of macroscopic variation, mechanical property deterioration and internal three-dimensional (3D) structure evolution. The results obtained by XRD, TG-DTA and MIP tests show that the hardening products and pore structure composition of NHL and MK20AL pastes were significantly different, which led to different erosion resistances. Based on the apparent morphology and X-CT test, no cracks were formed in the macroscopic structure of NHL and MK20AL mortar by NaOH and NaCl erosion after different periods. The mechanical strength of NHL mortar showed little change after erosion by the above two media. For the MK20AL mortar, the compressive strength increased by NaOH erosion and the flexural strength decreased significantly by NaCl erosion. No clear expansion cracks formed in the surface and internal structure of NHL mortar by Na2SO4 erosion, and the compressive strength increased by ∼20%. Gypsum erosion and secondary erosion (gypsum and C-A-H reaction to generate ettringite) formed in MK20AL mortar by a Na2SO4 medium. Additionally, volume expansion of solid phase (corrosion product) caused crack to form in the internal mortar structure, the cracks gradually became wider, and a crack network formed with the extension of erosion; thus, the internal structure and mechanical properties were seriously damaged. Overall, compared to the MK20AL mortar, the NHL mortar shows better resistance to erosion by environmental media, and NHL is the better choice for restoring ancient buildings and plastering mortar projects. The resistance of MK20AL to erosion by mortar environmental media needs to be optimized and modified.