Enhancing the mechanical and durability properties of fly ash/GGBS-based geopolymer mortars using dunite and polypropylene fibers

Polypropylene (PP) fiber-reinforced geopolymer mortar reduces the high brittleness of conventional geopolymer made with natural aggregates. However, the effects of different precursor materials on the performance of PP fiber-reinforced self-compacting geopolymer mortar (SCGM) remain unclear. This study investigates the feasibility of using dunite (D), a magnesium-rich ultramafic rock with consistent mineralogy and low processing energy demand that enhances thermal stability and durability, as an alternative precursor for fly ash (FA)/ground granulated blast furnace slag (GGBS)-based geopolymers. The study examined the workability, flexural and compressive strengths, and sorptivity of mortars cured at 20 ± 2°C and 60 ± 5% relative humidity, as well as their durability under freeze–thaw cycles and high-temperature exposure. Interfacial interactions between aggregates and the geopolymer matrix, along with microstructural and morphological changes caused by fire, were analyzed using scanning electron microscopy (SEM). X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were also performed for detailed material characterization. The mix containing 30% dunite exhibited the highest flexural strength, reaching 5.70 MPa at 28 days, the highest among all mixes. Under fire exposure, the mix containing 10% dunite significantly reduced mechanical strength loss and retained about 33.3% higher residual flexural strength than the FA–slag control mix at 600°C.