Comparative assessment of hemp shiv and basalt fibre reinforcements in lime-based mortars for sustainable building and heritage-compatible applications

The need to enhance both the mechanical resistance and the thermal performance of lime mortars has intensified interest in fibre-based reinforcement strategies, particularly for heritage-oriented and energy-efficient construction. This study evaluates the influence of natural and mineral reinforcements, hemp shiv and basalt fibre, on the fresh state, physical, microstructural, mechanical, and thermal behaviour of three lime-based mortar systems: natural hydraulic lime (NHL), air lime, and a mixed air lime–cement binder. Reinforcements were incorporated at two dosages (1 and 4 wt% of binder) to examine their interaction with distinct binder chemistries and pore structures. Hemp shiv decreased workability and increased open porosity across all mortars, while basalt fibre produced only limited changes in fresh properties. Microstructural variations arose mainly from increased air-void content due to weak binder–fibre adhesion and interfacial gaps. Compressive strength trends reflected these microstructural modifications; natural hydraulic lime mortars achieved the highest values up to 12 MPa, and high-dosage basalt fibre improved long-term strength through crack bridging, whereas hemp shiv reduced strength, particularly at higher dosages, by 21% in NHL mortars and about 60% in air lime-based mortars. Thermal behaviour, however, showed clear benefits across all reinforced mortars: thermal conductivity consistently decreased, with the most significant reductions observed in hemp-shiv mixtures, up to 30%, and in basalt-fibre mortars at low dosage by 45% in NHL mortars and 24% in air lime-based mortars. These improvements highlight the capacity of fibre reinforcements to promote more insulating pore networks while maintaining acceptable mechanical performance. Overall, the findings clarify how organic and mineral fibres modify lime-based matrices and demonstrate their potential for producing thermally efficient, sustainable, and compatible mortars for building and heritage-compatible applications.