Effect of Ammonium Ions on the Engineering Properties of Clayey Soil Improved with Calcite Powder Derived from the MICP Process
Keywords:
Ammonium, Soil stabilization, Microbially induced calcite precipitation (MICP), Clayey soilAbstract
Currently, improving the quality of clay soils to enhance their load-bearing capacity and reduce environmental impact is a widely discussed issue. Microbially Induced Calcite Precipitation (MICP), which facilitates the formation of calcium carbonate (CaCO3), is considered one of the most promising and environmentally friendly techniques for enhancing the engineering properties of soil. However, this process often produces ammonium ions (NH4+) as a byproduct, which may adversely affect soil behavior. This study aims to investigate the influence of NH4+ on the unconfined compressive strength (qu) and hydraulic conductivity (k) of clay soils by artificially introducing NH4Cl to simulate the potential impact of ammonium generated in the MICP process. The results show that clay specimens treated solely with CaCO3 exhibit a significant increase in qu, as the CaCO3 particles initially fill the voids between soil particles and further develop strength over the curing time due to long-term pozzolanic reactions. In contrast, specimens containing NH4+ showed a reduction in compressive strength, attributed to the moisture-retaining effect of NH4+ in the soil matrix, which inhibits strength development during curing. At the same void ratio (e), the hydraulic conductivity of NH4+ contaminated specimens was markedly lower than that of untreated clay. This is likely due to the swelling behavior induced by the NH4+ ions, which restricts the flow of water. Conversely, CaCO3-treated soils exhibited higher k values than untreated clay, as the relatively large calcite particles enhance water flow pathways. These findings underscore the importance of managing or eliminating ammonium ions in MICP-related applications to enhance the performance and sustainability of clay soil treatment, particularly for engineering projects that require both mechanical strength and adequate permeability.
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