Strength of Fiber Reinforced Modified Metakaolin‐Based Geopolymer Incorporating Crumbed Rubber and Mixed Plastic Waste Aggregate Under Sulfate Solution

Rising demand for sustainable construction materials, combined with the growing knowledge about geopolymers as a green substitute for ordinary Portland cement (OPC), which has an internationally well‐known high CO 2 footprint during manufacture, has increased research into this area. Specifically, this study is intended to investigate the behavior of metakaolin‐based geopolymer concrete (GPC) modified with 5% by weight of calcium oxide and silica fume after being immersed in a solution of 5% sodium sulfate (Na 2 SO 4 ). The parameters of the mix design included metakaolin, fine and coarse aggregates, superplasticizer, and water activated with sodium hydroxide and sodium silicate solutions in proportional quantities (372, 910, 603, 8, 56, 83, and 192 kg/m 3 , respectively). To make this further impactful towards sustainability, 10% of the voluminous natural coarse aggregate was replaced with recycled mixed plastic‐waste or crumb rubber from used tires. Increasing the volume fraction of carbon fiber (0%, 0.15%, and 0.2%) to enhance its mechanical properties. This study examined the compressive strength and mass change, as well as visual assessment after exposure to sulfate solution for periods of 28, 90, 180, 366, and 456 days. After 90 days of sulfate exposure, compressive strength improvements were seen, ranging from 3% in the reference mix to 9% with plastic‐waste aggregate and 0.2% carbon fibers in the GPC‐based matrix. Although strength gradually reduced over time, after 456 days, the maximum loss was only about 22% in mixes with crumb rubber aggregate and carbon fibers, with remaining strength still acceptable. No significant surface degradation or cracking occurred, and the addition of carbon fibers enhanced both the microstructure density and compressive strength. A fiber‐reinforced metakaolin‐based GPC, which incorporates recycled aggregates, has been shown to maintain structural performance while also improving resistance to sulfate attack, suggesting it is a viable environmentally friendly alternative.