Resistance to Desiccation Cracking in Clay-Polymer Mixtures

Abstract

This thesis investigates how polymer treatment can mitigate desiccation cracking in Na-bentonite. It aims to address key unanswered questions, regarding conditions under which polymers are effective, underlying mechanisms, and durability of improvements through a three-part experimental investigation. Sodium carboxymethyl cellulose (Na-CMC) with varying degrees of substitution (DS) is added to Na-bentonite following by two different mixing methods, namely wet mixing (WM) and dry mixing (DM). Part 1 investigates petri dish-dehydration experiments to compare cracking intensity and evaporation characteristics of amended and unamended mixtures under different conditions. Part 2 examines the mechanisms contributing to the reduction in desiccation cracking and investigates water retention ability, swelling, shrinkage, tensile strength, and microstructural changes using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Part 3 studies the probability of polymer elution from DM andWMsamples, during consolidation and permeation, and its effect on hydraulic conductivity (k). The research findings indicate that adding 2% Na-CMC to bentonite had minimal impact on water retention, but reduced swelling/shrinkage potential, and improved tensile strength. The two latter parameters likely contributed to cracking resistance of amended mixtures in dehydration experiments. DM reduced bentonite k by 60%, but its polymer leaching was more pronounced than WM, leading to increased k over time. Despite polymer loss, both WM and DM samples retained lower k than untreated bentonite. Adding a slit-film geotextile did not significantly enhance polymer retention in amended samples, but reduced the polymer migration into the effluent of k test, suggesting polymer retention by geotextile, not bentonite. These results show Na-CMC as a promising agent for reducing desiccation cracking, but its practical viability and long-term sustainability require further investigations.