Evaluation and Optimization of the Replacement of Fine Aggregate by Waste Tire Rubber in Geopolymer Mortar with Metakaolin

The rapid growth in population, consumption, and economy have led to an increase in the extraction of natural resources, directly influencing the environment by generating waste and CO₂ emissions, particularly in the civil construction industry. The study aimed to evaluate the use of coarse and fragmented tire rubber waste, without chemical treatment, as a replacement for fine aggregate in geopolymeric mortar specimens. The central composite design, coupled with the response surface methodology, was used to determine the optimized values for compressive strength, water absorption, void index, and specific gravity. X-ray diffraction and scanning electron microscopy were used to characterize the geopolymeric mortar specimens. The optimized parameters for the best results were 1% replacement of fine aggregate with tire rubber waste, 0% coarse rubber, and approximately a 6-day curing time. This combination resulted in optimal values of 17.75 MPa for compressive strength, 10.48% for water absorption, 18.58% for void index, and 1.77 g/cm³ for specific gravity. The experimental validation of the models had an error of less than 10%.