Waste Stone Wool as a Co-Precursor in Natural Pozzolan-Based Alkali-Activated Materials: Balancing Performance, Resource Efficiency, and Carbon Intensity

Within the literature reviewed, no systematic study was identified that evaluates waste stone wool (SW) as a partial replacement for natural pozzolan (NP) while varying both SW content and NaOH concentration in the same alkali-activated material (AAM) mixture. To address this gap, this study examined SW replacement levels of 0–40 wt.% and NaOH concentrations of 5, 7.5, and 10 M. Fresh-state behavior, physical and mechanical properties, drying shrinkage, pore structure, and material-based environmental indicators were evaluated. Increasing SW replacement level reduced flowability, while mixtures containing up to 30% SW showed improved hardened-state performance. SW30-7.5M exhibited the best overall performance, with compressive and flexural strengths of 16.13 and 3.41 MPa, respectively, total porosity of 15.07%, and ultrasound pulse velocity of 3.19 km/s. It also showed the lowest 63-day drying shrinkage magnitude, with a reported value of −1014.9 × 10−6 mm/mm. Mercury intrusion porosimetry (MIP) analysis of this mixture gave an MIP porosity of 14.03% and an average pore diameter of 0.24 µm. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) indicated changes in the bonding environment after activation, but did not identify a specific reaction product or a new dominant crystalline phase. Within the defined material-input boundary, SW replacement reduced NP consumption and the material-based carbon indicator, with the lowest carbon intensity per unit compressive strength (0.0113 kg CO2-eq./MPa) obtained for SW30-7.5M. The overall performance of the investigated AAMs depended on the combined effects of SW replacement level and NaOH concentration.