Durabilidade de fibras vegetais em matrizes álcali-ativadas: efeitos da dosagem da matriz e da exposição ao envelhecimento natural

Abstract

Alkali-activated binders (AAB) have mechanical properties equivalent to Portland cement (PC). However, as with OPC, the brittle behavior of AAB demands the use of reinforcements capable of making them suitable for applications involving dynamic loads or tensile stresses. Vegetable fibers (VF) can potentially improve the mechanical properties of fragile construction materials, especially improving toughness and post-cracking resistance. Nevertheless, there is a shortage of knowledge regarding the durability of VF when employed as toughening elements in AAB. In this sense, this work aims to evaluate the effect of fiber treatments (alkaline and hornification), alkaline activators (Na and K), aggressiveness of the matrix, and aging of the composite (natural and laboratory) on the durability of VF. Statistical Mixture Design was used to characterize and control the aggressiveness of AAB produced with metakaolin, treated asbestos cement waste, and alkaline solutions. The matrix/fiber interaction was evaluated by pullout tests, direct exposure of the fibers, and aging of the composites for 120 days. The dosing method made it possible to control the aggressiveness of the matrix through the electrical conductivity of a leached solution, which varied between 7.26 and 41.10 mS/cm for sodium-based activated mixtures and between 6.42 and 33.50 mS/cm for potassium-based activated mixtures. Direct tensile tests and pullout tests showed that matrices dosed with maximization of alkalinity can severely degrade fiber components, and degradation mechanisms can be potentiated by applied treatments. Untreated fibers, exposed in potassium-based matrices with minimized alkaline ions, exhibited better mechanical performance. Mechanical evaluation of composites after aging demonstrated that matrix optimization resulted in maintaining tensile strength and composite toughness, with greater effectiveness for potassium-based matrices. These results are consistent with the higher thermal stability and better microstructural integrity of fibers extracted from composites. In this sense, it appears that matrix optimization is the primary tool that enables the production of composites based on AAB and VF. A model of VF behavior in AAB was proposed.