Macroscopic and Microscopic Evaluation of the Effect of Mellowing Time on the Mechanical Properties of Expansive Clay Stabilized with Class F Fly Ash–Based Geopolymer

For construction quality control, the compaction delay referred to as mellowing time (MT) is crucial for achieving the desired outcomes of the chemical soil stabilization process in the field. In the current study, fly ash-based geopolymer (GFA) is used as a chemical stabilizer for expansive clay because of its significance in resource utilization and waste repurposing for soil stabilization through an enhanced process. The MT-influenced macroscopic physicomechanical properties and microstructural and mineralogical properties of expansive clay treated with varying GFA and curing period (CP) were investigated. The significant amelioration of strength and compression properties is observed through the unconfined compression test, California bearing ratio test, and one-dimensional (1D) consolidation test with an increase in GFA content and CP. This improvement is caused by the formation of cementitious [(N, C)-A-S-H] compounds as confirmed by SEM, EDAX, and XRD analyses. Meanwhile, as the MT increases, a decline in both the strength and compression characteristics of the GFA-treated specimens is observed. However, these specimens exhibit a reversal in deformability and brittleness with an increase in MT, which can be attributed to the development of a porous aggregated soil structure resulting from initial hydration before densification. In addition, a generalized mathematical modeling framework was established based on three-dimensional (3D) response surface modeling to quantify the MT-influenced strength and brittleness-related characteristics using MT, GFA, and CP as predictors. The established mathematical framework showed generality and reasonable accuracy in the prediction based on the experimental data. This article outlines the implications for practitioners and researchers of using GFA for the stabilization of expansive clay considering MT-influenced mechanical characteristics in the field.