Firdevs Uysal
Problematic fine-grained soils exhibit low strength and inadequate durability, highlighting the need for sustainable stabilization using eco-friendly binders. This study examined the strength development and durability of a natural CH soil (NSs) stabilized with fly ash (FA) based geopolymer exposed to sulfate attack and freeze-thaw (F-T) cycles. The effects of FA content (0-40%) and NaOH molarity (0-10 M) on unconfined compressive strength (UCS) were evaluated after 1, 7, 28 and 56 days of curing. Durability was assessed separately under accelerated laboratory conditions after 1, 3, 5, 7 and 11 F-T cycles and 7, 28 and 56 days of sulfate exposure. In non-activated specimens, FA contents of up to 30% enhanced the UCS primarily through the microfiller effect and possible time-dependent pozzolanic reactions. Alkali activation promoted the development of a compact binding matrix through the dissolution and polycondensation of aluminosilicate precursors, with the microstructural and chemical observations being consistent with the possible formation of C-(A)-S-H and/or N-A-S-H-type reaction products. F30M8 exhibited the highest strength, reaching a 56-day UCS of 1488.58 kPa compared with 282.46 kPa for untreated NSs. F30M8 retained approximately 94% of its UCS after 11 F-T cycles and 92% after 56 days of sulfate exposure. XRD, FTIR, and SEM-EDX analyses provided evidence of aluminosilicate restructuring and the development of a dense microstructure under alkaline activation. This refined matrix may have contributed to limiting sulfate- and ice-crystal-induced deterioration, thereby helping to preserve the structural integrity of the FA-based geopolymer-stabilized NS specimens, whereas untreated and non-activated FA-stabilized specimens disintegrated under sulfate exposure. These findings indicate that FA-based geopolymer stabilization has considerable potential for natural CH soil under the laboratory exposure conditions investigated in this study.