Solidification of sodium sulfate saline loess by biomineralization of reactive magnesium oxide binder

This study aims to solidify sodium sulfate saline loess by biomineralization of reactive magnesium oxide (MgO) binder. The impact of Na2SO4 concentration on the viability and urease activity of S. pasteurii, the mechanism and products of biomineralization of MgO, and the effectiveness of the biomineralization-MgO binder in solidifying saline loess with varying salt content (1%, 3%, and 5%) were investigated. Results showed that low and moderate concentrations of Na2SO4 favored bacterial proliferation. However, the presence of Na2SO4 inhibited bacterial urease activity, and the inhibition was more significant at higher Na2SO4 concentrations. In addition, low and moderate concentrations of Na2SO4 decreased the specific urease activity, whereas high concentrations of Na2SO4 significantly increased specific urease activity. S. pasteurii was able to use carbonate ions formed by urea hydrolysis for the mineralization of MgO and to form magnesium carbonate minerals dominated by rosette-like dypingite and hydromagnesite crystals. The primary mechanism involves microbial cells and extracellular polymeric substances leading to partial dehydration of Mg2+ ions from the Mg2+-H2O complex and allowing for further association with carbonate anions to from Mg-bearing carbonates. Unconfined compressive strength tests conducted on the saline loess samples after 7 days of curing revealed a significant influence of urea concentration on the strength of the solidified soil. The optimal urea concentration to obtain a better 7-day UCS ranged from 4 mol/L to 5 mol/L. Furthermore, solidified soil with 5% salinity yielded the highest 7-day UCS and soil with 3% salinity exhibited the lowest 7-day UCS at the same urea concentration. XRD and SEM analysis of the solidified soil samples indicated that the formation of magnesium carbonate minerals in the soil matrix by the biomineralization-MgO binder was responsible for the UCS enhancement. The remarkable 7-day UCS of saline loess solidified with biomineralization-MgO binder demonstrates the effectiveness of this material in curing saline loess.