Solidification of Loess Using a Composite Geopolymer Based on Slag Powder and Fly Ash: Influencing Factors and Mechanism Analysis

To solve the problem of carbon emissions and pollution of traditional cement, composite geopolymer based on alkali-stimulated slag powder and fly ash was used to solidify loess. The effects of the alkali activator content, the ratio of NaOH to alkali activator, and the ratio of slag powder to silicon-aluminum (Si-Al) raw material on the unconfined compressive strength (UCS) of solidified loess were investigated through different tests. Then, the results from microscopic tests such as scanning electron microscopy (SEM), x-ray diffraction (XRD), and mercury intrusion porosimetry (MIP), as well as the variation results of the UCS, were used to analyze the influencing mechanism of the three factors on the solidified loess. The test results showed that the UCS of the slag powder– and fly ash–based composite geopolymer solidified loess first increased and then decreased with the increasing alkali activator content, and the UCS of the solidified loess also first increased and then decreased with the ratio of NaOH to alkali activator. The UCS increased as the ratio of slag powder to Si-Al raw material increased. The reaction of composite geopolymer solidified loess produces amorphous hydrated calcium silicate hydrates (C─ S─ H), calcium aluminum silicate hydrates (C─ A─ S─ H), and sodium-based aluminosilicate (N─ A─ S─ H) gels. These gels effectively bond and fill the soil particles, leading to a decrease in the number of macropores and total pores in the soil. As a result, the soil becomes more compact and increases the UCS of solidified loess. The results of this study provide a certain basis for engineering applications of the composite geopolymer to solidify loess based on slag powder and fly ash.