Mechanical and Microstructural Performance of Hydraulic Lime-Based Mortars Incorporating Mechanically Milled Mineral Additives and Graphene Nanoplatelets

The development of advanced hydraulic lime-based mortars incorporating mineral additives and graphene nanoplatelets offers potential opportunities for improving material performance in the conservation of historic masonry structures. This study investigates the mechanical and microstructural performance of hydraulic lime-based mortars incorporating mechanically milled mineral additives and commercially available graphene nanoplatelets (GNPs). Hydraulic lime, brick powder, repair mortar, injection mortar, slaked construction lime, and gypsum plaster were processed using planetary ball milling under identical conditions, whereas graphene nanoplatelets were incorporated without additional mechanical treatment. The additives were introduced at 1%, 2%, and 3% by mass of the original hydraulic lime binder. Compressive and flexural strengths were evaluated after 28 days of curing, complemented by scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The results demonstrated that mechanical performance was strongly influenced by additive type and dosage. The mortar containing 3% injection mortar additive (IM-3%) achieved the highest compressive strength of 9.75 MPa, representing a 4.4% increase over the reference mixture, together with a 25.8% improvement in flexural strength. The incorporation of 1% slaked construction lime (SCL-1%) produced the highest mean flexural strength of 3.02 MPa, corresponding to a 28.0% increase, whereas 1% graphene nanoplatelets (GN-1%) enhanced flexural strength by 19.1% despite reducing compressive strength. Most other modified mixtures exhibited lower mechanical strengths than the reference mortar. Microstructural characterization revealed variations in matrix morphology and local elemental distribution, while the principal carbonate- and silicate-related characteristics were largely retained. These findings demonstrate the composition-dependent mechanical response of hydraulic lime-based mortars incorporating mechanically milled mineral additives and graphene nanoplatelets, providing a basis for further investigations into their long-term performance and compatibility with historic masonry materials.