Bentonite pellets will not swell to their maximum size under which condition?

• A. In distilled water
• B. In air
• C. In hot water
• D. In highly mineralized water

Answer: (D)

Swelling of bentonite pellets depends on hydration and water chemistry. When bentonite is placed in water with few dissolved ions, the clay’s layers hydrate and push apart, allowing water to enter between layers and the pellet expands to its maximum size. In air there is no water to hydrate, so swelling won’t occur. In hot water the process happens faster and can still reach near maximum swelling; heat mainly speeds up the swelling rather than preventing it. In highly mineralized water, the dissolved ions increase the solution’s ionic strength, compress the clay’s diffuse double layer, and promote cation exchange that reduces the repulsion between layers. This causes the pellets to swell less and not reach their full swollen size.