Rice straw and single-use plastic waste co-pyrolyzed plastic-char composite for the partial replacement of cement in concrete: a life cycle perspective

This study examines the valorization of single-use plastic and rice straw waste via co-pyrolysis to produce a plastic-char composite (PRSB) and rice straw biochar (RSB) for sustainable building applications. The generated biochars were thoroughly analyzed for their physicochemical characteristics using SEM, FTIR, XRD, and Raman spectroscopy. RSB and PRSB were utilized as partial cement substitutes (1–7% w/w) in M30-grade concrete to produce high-strength and durable composites. Optimization of mechanical performance was executed via a Box–Behnken design, focusing on compressive, split tensile, and flexural strength, with curing time as a critical variable. Experimental results indicated that a substitution of up to 3% markedly improved mechanical characteristics relative to ordinary concrete. The algorithm forecasted an ideal combination with 3.8% substitution and 28 days of curing, optimizing strength across all metrics. Sustainability was evaluated by a cradle-to-gate Life Cycle Assessment (LCA). With a 1% substitution of PRSB, the Global Warming capacity (GWP 500) from fossil emissions decreased from 80% to 72%, while biogenic emissions declined from 78% to 60%. However, substitution levels over 5% elevated total GWP due to increased processing, resource, and durability-related stresses. The findings indicate that minimal PRSB integration (< 5%) can enhance mechanical performance while providing substantial environmental benefits, thereby promoting the circular economy by upcycling agricultural and plastic waste into sustainable, low-carbon concrete.