Strength and Durability Assessment of Alkali-Activated Fly Ash–GGBFS Binders for Kaolin Clay Stabilization

The necessity for sustainable alternatives to cement-based soil stabilization has driven interest in alkali-activated geopolymer binders derived from industrial byproducts. This study evaluates the mechanical, durability, and microstructural performance of fly ash (FA) and ground granulated blast furnace slag (GGBFS) geopolymer-stabilized kaolin activated with sodium hydroxide (NH) for pavement base applications. Specimens were prepared with 10%–30% precursor contents and activated using 6–14 M NH solutions with curing periods ranging from 7 to 60 days. Compaction behavior, unconfined compressive strength (UCS), and resistance to wetting–drying (W–D) cycles were evaluated. Results show that increasing precursor content and alkali molarity reduced the optimum moisture content and increased the maximum dry density, attributed to improved particle packing and gel formation. UCS increased with curing time and molarity, with the optimum mix (30% precursor, FA : GGBFS = 40 ∶ 60 ) achieving peak strengths of 16.7 and 20.3 MPa after 60 days under 10 and 14 M activation, respectively. Based upon overall viability, 10 M NH was considered as the optimum activator concentration. Durability tests indicated mass losses below 5.7% after 12 W–D cycles. Microstructural analyses confirmed that there was a formation of dense N─A─S─H and C─A─S─H gel networks. Response surface methodology demonstrated a strong predictive capability ( R 2 = 0.99 ), which identified alkali molarity as the dominant factor. The study confirms that the FA–GGBFS based geopolymer stabilization of kaolin using Proctor compaction is an effective approach for pavement base applications.