Thermal and Hygrothermal Performance of Arundo donax Shive–Lime Composites for Sustainable Building Insulation

The construction sector is under increasing pressure to reduce energy consumption and carbon emissions, driving the development of bio-based, low-impact insulation materials. Lime composites incorporating plant aggregates are promising alternatives to conventional insulation; however, the potential of Arundo donax as a bio-aggregate remains underexplored. This study evaluates the physical, thermal, and hygrothermal performance of Arundo donax shive–lime composites produced with binder-to-shive mass ratios of 1:1, 1.5:1, and 2:1. Composite panels were prepared using CL90 slaked lime and cured under controlled conditions. Apparent density, water absorption, and thermal conductivity were measured after 28 days, complemented by temperature-dependent thermal conductivity tests over the range −10 to 50 °C. Microstructural and chemical characterisation was conducted using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR). Dry apparent density increased from 302 to 445 kg·m−3 with increasing binder content due to matrix densification and carbonation. Thermal conductivity rose linearly with density (0.072–0.092 W·m−1·K−1, R2 ≈ 0.99), confirming density as the dominant parameter governing heat transfer. Thermal conductivity also increased with temperature for all formulations, while temperature sensitivity decreased with increasing binder content, indicating enhanced thermal stability. Water absorption decreased markedly as binder content increased. SEM, EDX, and FTIR analyses confirmed effective lime coating of the shives, partial pore filling, and the formation of calcium carbonate, supporting the observed physical and thermal trends. Overall, Arundo donax–lime composites exhibit performance comparable to hemp–lime and other reed-based materials. The 1.5:1 binder-to-shive ratio provides the most balanced combination of low density, insulation performance, moisture resistance, and thermal stability, highlighting Arundo donax as a viable bio-aggregate for sustainable building insulation.