Investigation of geotechnical and microstructure characteristics of gypsum soil using ground granulated blast-furnace slag (GGBS), fly ash, and lime

This study investigates the synergistic effects of Ground Granulated Blast-furnace Slag (GGBS), lime, and fly ash on improving the geotechnical properties of gypsum soil. Gypsum soil samples, constituting 84% of the mixture, were blended with lime and GGBS in proportions ranging from 0% to 16%. These mixtures underwent curing for 1, 7, 14, 28, 56, and 90 days. Tests and analytical techniques elucidated mechanisms responsible for strength improvements. Findings revealed that when fly ash was used alone, there was no significant increase in Uniaxial Compressive Strength (UCS). However, fly ash expedited essential pozzolanic reactions for silicate gel formation. The most substantial increase in UCS occurred in the 28-day samples, contrasting with lower gains in the 7, 14, 56, and 90-day periods. Enhanced strength in lime-GGBS-gypsum soil mixtures before 28 days was attributed to cation exchange and initial flocculation. In the 28-day specimens, increased strength was linked to ettringite crystal and silica gel growth. Microstructural analysis confirmed Ettringite (E) crystal growth within intercrystalline and pore spaces. Cementitious compounds like CH, CSHH, CHS, CAH, and CASH displayed growth. The rearrangement of the clay mineral matrix, coupled with added sulfate, contributed to enhanced gypsum soil strength. Interestingly, decreased UCS in 56-day samples indicated increased ettringite crystal growth, resulting in reduced strength. Elemental analysis through Energy-dispersive spectroscopy (EDS) revealed shifts in Al: Si, Ca: Si, and Al: Ca ratios. The Al: Si ratio reduction confirmed the formation of more cementitious compounds in 90-day samples. Additionally, the Al: Si ratio in 28-day cured samples exhibited a significant decrease, signifying accelerated initial strength in lime and GGBS mixtures. The presence of sulfur (S) confirmed ettringite crystal formation, underscoring the intricate interplay between additives and curing durations, shedding light on mechanisms enhancing gypsum soil's geotechnical properties.