Optimization of Sisal Content in Geopolymer Mortars with Recycled Brick and Concrete: Design and Processing Implications

Geopolymer mortars were produced from recycled concrete powder (RCP) and recycled brick powder (RBP) at a 30/70 wt% ratio, activated with a hybrid alkaline solution (NaOH/Na₂SiO₃/KOH) and reinforced with sisal (Agave) fibers at 0–2 wt%. Mechanical performance (compression and flexural) and microstructure–phase evolution were as-sessed by XRD, FTIR, and SEM-EDS after low-temperature curing. Sisal addition de-livered a strength–toughness balance, with an intermediate dosage (~1–1.5 wt%) providing the best overall performance; higher dosages induced packing loss and fiber clustering. Microstructural evidence indicates the coexistence and co-crosslinking of N-A-S-H and C-(A)-S-H gels promoted by the RCP, which densifies the matrix and enhances fiber–matrix anchorage. Fractographic features support a crack-bridging/pull-out mechanism responsible for the improvement without penaliz-ing early-age strength. The study identifies a practical advantage of sisal reinforcement in RCP/RBP geopolymer mortars and links it to gel chemistry and interfacial phenom-ena, providing a reproducible pathway toward fiber-reinforced, low-embodied-carbon geopolymers derived from construction and demolition waste and suitable for durable construction applications.