Two birds with one stone: The method for both surface protection and integral reinforcement of gypsum mortar in historic constructions

• A novel conservation method for weathered gypsum mortar. • Preparation of continuous mineral phase HAP using calcium hydroxy glycolate and DAP. • High permeability and concentration of HAP by this novel method. • The treatment has the dual effect of reinforcement and surface protection. • The dissolution of gypsum mortar in water can be significantly reduced. Gypsum mortar was widely used in many historic constructions. However, it is easy to be corroded by water from rain or damp. For the purpose of both surface protection and integral consolidation of the weathering gypsum mortar, a novel conservation method was explored in this study. The strategy is introducing the solutions of calcium hydroxy glycolate and diammonium hydrogen phosphate into the gypsum mortar one by one. X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy coupled with energy-dispersive spectroscopy were employed to study the action mechanism of the method. Mechanical strength, compatibility and weather resistance tests were used to investigate the protective performance. The highly positive results show that the proposed method has a broad prospect in the conservation of the gypsum mortar in the historical buildings.