Abstract
The conservation of Roman concrete structures demands repair materials that are both compatible with the original fabric and durable under environmental exposure. This study evaluates the compatibility and durability of five lime-based mortar formulations—aerial lime, natural hydraulic lime (NHL), lime-pozzolan, and two modified mixes with water-repellent and biopolymer additives—as candidates for repairing Roman concrete. Compatibility was assessed through physical (porosity, water absorption, capillary coefficient), mechanical (compressive and flexural strength, modulus of elasticity), and chemical (mineralogical analysis via XRD, thermal analysis) tests, comparing results with published characteristics of Roman mortars. Durability was evaluated through accelerated ageing cycles including freeze-thaw, salt crystallization, and wet-dry testing. Results show that NHL and lime-pozzolan mortars achieve mechanical strengths within the range of Roman concrete (5–15 MPa) while maintaining lower modulus of elasticity (3–6 GPa) that prevents undue stress on the substrate. Water-repellent admixtures reduced capillary absorption by up to 60%, enhancing freeze-thaw resistance. However, biopolymer additives decreased compressive strength by 20–30%, compromising long-term durability despite improved workability. Carbonation rates in lime-based mortars were consistent with historic materials, while cement-based reference mortars exhibited incompatible high strength and low permeability. Multivariate analysis identified the lime-pozzolan formulation with a mild water repellent as the optimal compromise, offering high compatibility (porosity 18–22%, vapor permeability equivalent to historic mortars) and superior durability (mass loss <5% after 50 salt crystallization cycles). These findings support the use of tailored lime-based mortars for the sustainable repair of Roman concrete heritage.
Keywords
lime-based mortars, Roman concrete, compatibility, durability, repair mortars, carbonation, pozzolanic reaction, heritage conservation