Performance Evaluation of Soil, Fly Ash, and Recycled Concrete Aggregate Geopolymer Mixtures for Pavement Base and Subbase Applications

In regions where suitable natural aggregates are scarce or expensive, soils are often treated with cement to produce pavement base and subbase materials. This study evaluated soil–RCA geopolymer mixtures as a sustainable alternative for these applications. Response surface methodology (RSM) was used to prepare mixtures containing soil, recycled concrete aggregate (RCA), Class F fly ash (FA), sodium hydroxide, and sodium silicate. The effects of FA, RCA, and liquid sodium silicate (Na2SiO3) in the alkaline activator solution on compressive strength (fc), elastic modulus (E), and density were examined using regression analysis, Analysis Of Variance, and sensitivity analysis. Selected mixtures were further evaluated using wetting and drying, freeze and thaw, and scanning electron microscopy (SEM). Geopolymer stabilization increased strength and stiffness compared with untreated soil. FA improved both fc and E, while RCA produced a nonlinear response, with 15% RCA providing favorable mechanical performance. Increasing Na2SiO3 reduced both responses under the tested conditions. The mixtures remained within the mass and volume change limits after 12 cycles. SEM observations showed geopolymer reaction products, particle coating, and matrix densification. The findings indicate that soil–RCA geopolymer mixtures can provide suitable performance for pavement base and subbase layers while reducing reliance on cement and virgin aggregates.