Effect of Phosphate Dispersant Amendment on Workability of Ca-Bentonite Slurry for Slurry Trench Cutoff-Wall Construction

Abstract

Commonly available Ca-bentonite can be considered as an alternate material for cutoff walls when high quality Na-bentonite is difficult to obtain. This study aims at investigating feasibility of using phosphate dispersant additive to enhance swelling capacity of the Ca-bentonite, and to improve performance of the Ca-bentonite slurry for soil-bentonite cutoff walls. A series of free swell index tests are conducted to investigate effects of the phosphate dispersant on basic properties of the Ca-bentonite. Subsequently, viscosity, density, filtrate loss and pH experiments are performed using slurries prepared with various phosphate dosages and bentonite contents. The results show that phosphate dispersant plays a positive role on improving swelling capacity and dispersity of the Ca-bentonite. The slurry exhibits significant decrease in filtrate loss, filter cake thickness, and pH value, but increase in viscosity with increase in phosphate dosage. All of the slurries in absence of phosphate dispersant failed to meet viscosity and filtrate loss requirements for slurry for cutoff walls construction, while the slurry that contains 20% bentonite amended by 2% phosphate dispersant is found to yield optimum slurry for use. In addition, filtrate loss and filter cake thickness display obvious negative correlation with viscosity, whereas filter cake thickness exhibits prominent positive relationship with filtrate loss, especially when viscosity <40 s or filtrate loss <25 mL. This study demonstrates that phosphate dispersant additive is promising in enhancing workability of the Ca-bentonite slurry for use during the construction of soil-bentonite cutoff walls.