Experimental Optimization of Air Lime–Gypsum Mortars by Fresh and Hardened Tests

Air lime is a binder used in mortars for more than two thousand years and is still used for some purposes, mainly conservation and rehabilitation of built heritage. However, whilst air lime is a low embodied energy binder and sustainable alternative to cement, its slow setting and high shrinkage have limited widespread use, challenges that can be mitigated by incorporating fast-setting binders such as hemihydrate gypsum. The mechanical performance of air lime mortars depends on drying, which can take hours to complete, and carbonation, taking several days to achieve a substantial degree. To establish a framework for the rational design of air lime-based mortars, this work investigates the effects of varying binder:aggregate volumetric ratios (1:1, 1:2, and 1:3) and partial gypsum substitutions (5%, 10%, and 20% by volume of air lime) on the fresh and hardened properties of 12 mortar formulations. Key findings reveal that a 5% gypsum substitution reduces initial setting times by over 50%, while a 20% substitution eliminates measurable linear shrinkage and increases flexural strength by up to 148%. Moreover, this study shows that replacing part of the air lime with hemihydrate gypsum can reduce the embodied carbon footprint of the plastering mortars.