Effect of polyanionic cellulose modification on properties and microstructure of calcium bentonite

Abstract

Soil-bentonite slurry trench cutoff walls have been extensively used as engineered barriers for contaminant containment. In some regions without appropriate sodium bentonite (NaB), local calcium bentonite (CaB) that is cheap and abundantly available may be considered as an alternative material. However, CaB has a poor swelling capacity and it is important to improve the properties to meet the requirements of the cutoff walls. As a highly effective filtrate reducer and viscosifier, polyanionic cellulose (PAC) has been used to modify NaB and Na activated CaB to enhance their chemical compatibilities, but its application to natural CaB has been seldom studied. Therefore, this study aimed to identify the optimal reaction condition for PAC-CaB preparation using a more systematic slurrying-drying-grinding method and preliminarily evaluate the application potential of PAC-CaB in slurry trench cutoff walls from the perspective of the workability of bentonite slurry. The effect of PAC modification on the physicochemical properties of CaB and workability of CaB slurry were explored by a series of laboratory experiments. The modification mechanisms of PAC were comprehensively revealed by means of scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The results show that as the PAC dosage (by mass) increased from 2 to 8%, the swell index of CaB increased almost linearly and exceeded that of NaB. Both 4% and 8%PAC-CaBs had a lower hydraulic conductivity and a better chemical compatibility (using 10 mM CaCl2) than NaB. Moreover, PAC modification could significantly enhance the workability of CaB slurry, approaching or even exceeding that of NaB slurry, and reduce the bentonite content. A planar or spatial network microstructure was observed in PAC-CaB due to the interactions between PAC and CaB involving bridging and bonding. Overall, PAC is a promising modifier for CaB and PAC-CaB has the potential to replace the NaB in stabilizing the trench wall during construction.