Influence of Thermal Treatment and Rapid Air Cooling on Waste Rock Wool Fiber-Reinforced Cement Mortar for Enhanced Sustainability

The disposal of waste rock wool insulation materials has become an increasing environmental concern, while their reuse in cementitious composites remains insufficiently explored, particularly under elevated-temperature conditions. Moreover, limited information is available regarding the influence of fiber pretreatment on the residual mechanical performance of cementitious mortars after fire exposure. Therefore, this study investigates the effect of untreated rock wool fibers (URWFs) and hydrothermally treated rock wool fibers (TRWFs) on the mechanical and thermal performance of cementitious mortar. The waste fibers were hydrothermally treated by immersing 40 g of fibers in 1 L of water, stirring for 10 min, followed by filtration and oven drying at 105 °C for 24 h. Mortar specimens incorporating eight fiber dosages (2.5%, 5.0%, 7.5%, 10.0%, 12.5%, 15.0%, 17.5%, and 20.0% by weight of cement) were tested for compressive strength after 28 days of curing and after exposure to elevated temperatures of 400, 500, and 600 °C for 2 h, followed by natural air cooling. The results demonstrated that hydrothermal treatment significantly enhanced the residual compressive strength of fiber-reinforced mortars compared with untreated fibers, with the greatest improvement observed after exposure to high temperatures. The optimum fiber content (2.5–5.0%) provided the highest retained strength ratio, improving residual compressive strength by 25.1% compared with the corresponding URWF, while exhibiting a retained strength ratio of 54.8%, slightly exceeding the control mixture 54.5%. TRWF mortars also exhibited lower water absorption of up to 18% reduction, and lower densities of up to 60% reduction compared to URWF mortars, indicating improved matrix densification, fiber–matrix bonding, and thermal stability. These findings demonstrate that hydrothermal treatment is an effective and sustainable approach for upgrading waste rock wool fibers into value-added reinforcement for lightweight, low-permeability cementitious mortars with improved fire resistance. The study is limited to compressive strength evaluation, and future work should investigate tensile and flexural behavior, ductility, energy absorption, crack propagation, and long-term durability.