Sodium bentonite generates a powerful suction force which can hydrate itself even if no water is added to it by drawing moisture from the soil in the surrounding area. Bentonite is capable of producing a suction force of 650psi — this is enough suction to draw water from even very dry soils.
As the bentonite hydrates, the suction force is reduced to the point where an equilibrium is reached between the suction of the bentonite and the suction of the adjacent soil. The point where Voltex reaches equilibrium is around 100% moisture content. While this may sound high to the layperson, it is worth noting that Voltex is considered fully hydrated at 150-250% depending on the confining pressure. Even if the Voltex loses some of its moisture over time, there is still a suction force present that would allow additional water to be absorbed if it were to become available.
For further information, read over either CETCO's Technical Reference on Wet / Dry Cycling or the Volclay BRANZ Appraisal on the Volclay Voltex product page.