Mechanical Performance and Environmental Effect of Coal Fly Ash on MICP-Induced Soil Improvement

Abstract

Coal fly ash (FA) is one of the main byproducts of coal burning. Nearly half of FA cannot be reused or recycled. The potential environmental leaching of trace elements in FA may limit its application. Microbially induced carbonate precipitation (MICP) is a promising technology to improve soil properties. This study was to investigate the effect of fly ash in MICP-stabilized soil on its mechanical and environmental impacts. Two kinds of fly ash were considered: FA1 was Class-F fly ash, FA2 was off-specific fly ash. The 0% (sand only), 3%, 6% and 9% content of FA were introduced to FA-sand mixtures to perform MICP process. Triaxial compression test was performed to evaluate the effect of FA content on the development of strength. The triaxial test results indicated that with 3% addition of FA, the peak deviator stress increased significantly. When MICP-treated sand mixed with 3% FA1, the deviator stress increased to 1,959 kPa compared to that of MICP-treated sand only samples of 800 kPa. The peak deviator stress increased by 154% and 115% when the additions of FA1 were 6% and 9%, respectively. The stress increase was caused by the bonding of precipitated CaCO3 in MICP However, higher content of FA1 (9% or higher) could restrict the activity of bacteria by reducing the void spaces. MICP-treated samples with the addition of FA2 presented a better enhancement in peak stress for its higher CaO content which could lead to additional cementation besides MICP. Leaching tests by toxicity characteristic leaching procedure (TCLP) and sequential extraction tests indicated that there was no potential risk to introduce fly ash into the MICP process during the soil improvement. MICP process resulted to the fraction change of trace metals which could make trace metal more stable. Microscale images at scale of 10 µm, 100 µm and 200 µm have clearly presented the precipitated CaCO3. It showed that large amount of precipitated CaCO3 coated the particle surfaces and filled the void spaces. Small particles were buried and formed aggregates. There was a highly cemented phases produced between soil particle matrix. XRD analysis also confirmed the presence of CaCO3 crystal after the MICP process.