Lime-free mortar renders reinforced with steel wool and luffa fibers: an experimental analysis

Lime-free mortar renders were reinforced with consumer-grade steel wool and untreated luffa fibers obtained from commercially available products. Nineteen mortar mixes were prepared with a sand-to-cement ratio of 3:1 by varying fiber type, content, and length. Steel wool provided the most favorable balance between mechanical performance and water absorption. At intermediate fiber dosage and length, notable improvements in both flexural and compressive strength were observed compared with the control mix, while water absorption was reduced below the reference level. In contrast, luffa fibers showed limited mechanical benefit, with only slight improvements in flexural strength at low dosage, and generally increased water absorption as fiber content increased. Statistical analysis confirmed that fiber content significantly affected both flexural and compressive strength, and quadratic response models were used to identify the optimum conditions within the tested range. Microstructure observations supported the macroscopic trends, revealing a denser interfacial transition zone for steel wool and micro-gaps around luffa fibers, explaining their higher moisture uptake. The study establishes a strength–absorption performance map for selecting suitable fiber reinforcement in lime-free CS III render mortars.