Mechanical properties of Na-montmorillonite-modified EICP-treated silty sand

Abstract

The effects of Na-montmorillonite (Na-Mt) content and curing age on enzyme-induced carbonate precipitation (EICP)-treated soil were studied. First, the effects of Na-Mt addition on the urease activity, Ca2+ precipitation rate, and pH of the solution were analyzed through tube tests. Then, Na-Mt-modified EICP was used to reinforce silty sand in the Yellow River flooding area in China. The solidification effect and action mechanism of Na-Mt were investigated via the unconfined compressive strength (UCS) test, calcium carbonate content (CCC) measurement, X-ray diffraction, and scanning electron microscope analyses, wherein soil treated by conventional EICP and soil treated with Na-Mt alone were considered the control group. Na-Mt improved the urease activity and Ca2+ precipitation rate, lowered the pH, increased the CaCO3 production through chelation, then regulated the morphology of the CaCO3 crystals and facilitated the formation of densely aggregated calcite. The CCC and mechanical parameters increased rapidly during the first 7 days of curing, and then slowed down. The incorporation of 8% Na-Mt enhanced the UCS and Ca2+ utilization ratio at curing age of 7 days by 1.4 and 2.72 times, respectively, compared with that of traditional EICP; and the optimal Na-Mt content was identified to be 8%. At Na-Mt contents lower than 8%, the mathematically expressed improvement effect of the Na-Mt-modified EICP on the soil strength was greater than the arithmetic sum of that when these two approaches applied individually; this result confirms that the Na-Mt-modified EICP technique proposed herein is an efficient approach for solidifying fine-grained soil.