Triticale straw fiber reinforced concrete effect of fiber length on mechanical and thermal properties and microstructure

The incorporation of agricultural waste into cementitious composites represents a promising strategy for improving sustainability while enhancing selected engineering properties. This study investigates the influence of triticale straw fiber length (2.5, 5.0, and 7.5 cm) on the fresh, mechanical, durability, microstructural, and thermal properties of cement-based composites. Fresh concrete properties, flexural and compressive strength, water absorption, freeze–thaw resistance, microstructure, and thermal conductivity were evaluated after different curing periods. The results showed that increasing fiber length reduced workability but had no significant effect on fresh density. The highest flexural strength was achieved for composites reinforced with 7.5 cm fibers, reaching approximately 8.5 MPa after 21 days, representing an increase of about 18% compared with the reference mixture. Although compressive strength was reduced at early curing ages, prolonged curing improved the mechanical performance of all fiber-reinforced composites. Straw incorporation increased water absorption, with the greatest effect observed for mixtures containing 5.0 cm fibers, whereas specimens reinforced with 7.5 cm fibers exhibited comparatively better freeze–thaw stability. Microstructural observations confirmed that fiber geometry and fiber–matrix interaction governed pore development and crack-bridging mechanisms. Sensitivity-based thermal analysis indicated that straw fibers reduced the normalized thermal conductivity by approximately 5–7% for mixtures containing 2.5 and 5.0 cm fibers, while the reduction for 7.5 cm fibers was limited to approximately 1–2%. The results demonstrate that fiber length is a key parameter controlling the balance between mechanical performance, durability, and thermal efficiency of straw-modified cementitious composites.