Lime-Based Injection Grouts for Historic Masonry Conservation: A Critical Review of Design, Compatibility, Performance and Durability

Lime-based injection grouts are widely used for the conservation and consolidation of historic masonry because they can fill cracks, voids, and detached interfaces while maintaining compatibility with historic substrates. However, the literature remains fragmented across architectural surface reattachment, structural masonry consolidation, grout rheology, injectability, mix design, durability, field application, and non-destructive assessment. This paper critically reviews lime-based injection grouts with emphasis on design principles, compatibility, constituent selection, fresh- and hardened-state performance, durability, and multi-scale validation. The evidence shows that grout design should be performance-based and substrate-sensitive, considering the intervention objective, substrate morphology, crack and void characteristics, moisture condition, environmental exposure, and expected mechanical contribution. Fresh-state properties govern injection success, whereas hardened-state performance depends on strength, stiffness compatibility, adhesion, shrinkage, pore structure, and moisture transport. Durability remains one of the least standardized aspects of the field. The review therefore highlights the need for integrated design and validation strategies that link material properties, substrate interaction, and post-injection performance.