Development and Characterization of Colored Lime–Gypsum Mortars for Heritage Building Restoration in Humid Environments

Surface deterioration and paint peeling occur in historic buildings worldwide due to excessive moisture. Conventional coatings often fail to preserve these structures. In Mosul, Iraq, conventional paints often do not preserve historic structures. The article aims to use colored lime–gypsum mortar, which has significant potential to be used as a sustainable and appropriate candidate material for the restoration of historic structures. This is particularly relevant for the restoration of exterior elements or interior walls in humid environments. The flowability, strength (compressive, flexural, and tensile), and shrinkage cracking of several mortar mixtures with different lime–gypsum ratios and color additives were all part of the extensive testing. Every procedure closely followed the applicable international standards The mortar mixture identified as optimal (Mix A10), comprising a 1:1 lime-to-gypsum ratio with carefully calibrated pigment additives (0.5 g chromium oxide, 0.2 mL liquid oxide, and 0.5 g powder oxide), demonstrated superior mechanical properties and minimal shrinkage cracking. This composition was ideal due to its superior mechanical strength and reduced shrinkage cracking compared to pure gypsum mixtures. The colored lime–gypsum mortar is a sustainable material well-suited to the restoration of historic structures, and applicable to both interior and exterior elements in humid environments. Its low shrinkage cracking enhances durability and effectively prevents moisture ingress in moisture-sensitive cultural settings.