Mechanical behavior of K-geopolymers reinforced with silane-coated basalt fibers

A potassium-based geopolymer (KGP) was produced through the combination of metakaolin and a K-based alkali metasilicate solution (K₂O•Al₂O₃•4SiO₂•11H₂O). Two types of silane-coated chopped basalt fibers, manufactured for cement or epoxy-based applications, were used in order to compare their effects. The fibers had a 12.7 mm (½ inch) length and were incorporated initially in 10 wt % contents, due to the limited fluidity of the matrix. The effect of the addition of Sapetin® superplasticizer in varying weight percentages was examined through consistency tests. 0.5% by weight of the matrix was established to be an adequate amount to improve the geopolymer workability, allowing a greater incorporation of both types of fibers into the matrix (20 wt%). The mechanical properties were analyzed through compression and 4-point flexural tests. Pull-out and direct tensile tests were also performed. Additionally, X-ray diffraction (XRD) was conducted with the KGP material and scanning electron microscopy (SEM) was used to measure the fiber cross sections. Both composites manufactured with 10 wt % of fibers reached similar high flexural strengths (~30 MPa), suggesting a suitable crack propagation at higher stresses due to strong fiber-matrix adhesions. The fibers manufactured for epoxy applications presented a greater compatibility in 20 wt % contents, reaching 37.8 MPa in flexural tests. This was attributed to a better dispersion of such fibers in a fresh mix with reduced friction, such as KGP with the addition of superplasticizer, suggesting an improved use of this reinforcement in such contents.