Bricks are widely used in ancient constructions throughout the world. However, weathering patterns such as cracking, flaking, and powdering are common among them and conservation interventions are thus necessary. In this paper, the alcoholic solution of calcium hydroxy glycolate was investigated as a novel protectant. It features good penetration and controllability. The consolidation of the bricks can be achieved by successively introducing the alcoholic solution of calcium hydroxy glycolate and water. The consolidation mechanism was studied by scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. The protection effect was evaluated by determination of penetration depth, water absorption, porosity, water vapor permeability, mechanical strength, and color change. The results show that the consolidation is due to the reaction of calcium hydroxy glycolate with carbon dioxide in the air and active silicon in the brick (25 °C, RH = 60%, 1 atm). The cohesive calcite and calcium silicate phases are thus generated and weathering fragments are reunited together. After treatment, the mechanical strength and weathering resistance of the samples are improved greatly. The porosity, water absorption, and color, however, are affected insignificantly, indicating good compatibility. These satisfying results suggest a promising application prospect of the proposed method in the conservation of heritage brick constructions.

Gypsification is a common problem in weathered calcareous relics. In previous studies, the solutions of barium hydroxide in water and methanol were used as protectants for gypsification calcareous relics and showed significant differences in permeability. In this study, the underlying reasons for permeability differences between these two solutions were investigated using optical microscopy, ultraviolet–visible spectrophotometry, X-ray diffractometry, Fourier transform infrared spectroscopy, the phenolphthalein test and physical property characterizations. The results indicated that the permeability differences were primarily caused by the solutions’ reactivity. Specifically, owing to the high reactivity of barium hydroxide in water, it reacted rapidly with atmospheric CO₂ and gypsum (the weathering product) to generate barium carbonate, barium sulfate and calcium hydroxide precipitates. These precipitates hindered the penetration of solution into weathered relics. In contrast, barium hydroxide in methanol did not react with atmospheric CO₂ or weathered relics, which also kept the solution in a liquid state during the infiltration process. Therefore, the solution of barium hydroxide in methanol exhibited high permeability. Based on the above findings, this study is meaningful for applying barium protectants in the conservation of gypsification calcareous relics.

Gypsum mortar is widely used in structures of architectural heritage. However, due to the high solubility of gypsum in water, it is easily destroyed by water erosion in outdoor environments, leading to the instability or even failure of the ancient buildings constructed with it. To improve the water erosion resistance of gypsum mortar, the alcoholic solution of barium hydroxide was explored as the protective agent in this study. The method involves treating the gypsum mortar with the alcoholic solution of barium hydroxide and water in sequence. The mechanism of its action and protective properties were studied by infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, conductivity meter, colorimeter, etc., and conclusions were made that the alcoholic solution of barium hydroxide has high permeability and its subsequent conversion to insoluble barium sulfate and calcium carbonate helps to increase the water erosion resistance of the solution. Additionally, the positive results such as the increase in mechanical strength from 20.80 HD to 60.94 HD, the reduction in water absorption from 18.37% to 15.75%, and a total color difference (ΔE*) of less than 3.0 indicated the application prospects of the proposed method in the conservation of the historical buildings made from gypsum mortar.