Evaluation of Fiber–Matrix Bond in the Mechanical Behavior of Geopolymer Composites Reinforced with Natural Fibers

The development of new sustainable materials is fundamental for the evolution of the construction industry, allowing the use of green-friendly sources. Geopolymers have shown an excellent mechanical behavior when compared with traditional materials. They are obtained through the combination of aluminosilicate sources and an alkali solution, forming a stable inorganic polymer. This study presents an experimental investigation on different natural fiber reinforcements (jute, sisal, and curaua) in a 100 % metakaolin-based geopolymer mixture. The strength development of the plain material was studied through compressive strength tests. X-ray diffraction and scanning electron microscopy were used to characterize its microstructure. The composites’ cracking formations and mechanisms were studied through tensile and flexural strength tests. All composites presented a strain-hardening behavior with multiple crack formation, reaching a flexural strength up to 33 MPa (curaua geopolymer). The use of a styrene–butadiene rubber as a polymer coating was investigated. Additionally, direct tensile and pull-out tests were performed to evaluate the fibers’ stress transfer mechanisms. The use of the polymer coating resulted in an increase in the fiber–matrix bond, reaching 17.48 MPa in tensile stress, which is opposed to the previous result that achieved only 11.53 MPa. The geopolymer composites presented superior mechanical behavior when compared with portland cement-based materials, suggesting an intrinsic improved bond between their components.