Myth Busting with Bentonite
With over 1,800,000m² of membrane installed in New Zealand in the last 17 years, Volclay can rightly be seen as one of New Zealand’s most successful tanking products. Volclay has been used across a wide range of projects ranging from small residential homes right through to some of the largest and most complex construction projects. Even with such a long track record of successful projects there are some enduring myths about Bentonite that we would like to clear up once and for all. Over the next four issues we will break down some of the biggest misconceptions about Volclay Bentonite.
First up, lets clarify one point. Not all bentonite products are equal: The properties of sodium bentonite can be appreciably enhanced through a specialised treatment process patented by Volclay’s parent company (AMCOL International since 1964). The treatment process involves the application of chemical additives to specially selected and processed sodium bentonite. These additives work both individually and synergistically to render the bentonite more resistant to attack by chemical & salt water contaminants. Various formulations of the additives in this treatment process have been incorporated into Volclay’s sodium bentonite waterproofing membranes (including Voltex DS/SWR)
Myth 1: Bentonite Dries Out?
This one is perhaps the most misunderstood of all. Firstly “dry” bentonite has a 25% moisture content - it's actually very hard to dry bentonite out! So hard in fact that it took BRANZ over 3 whole days (using an industrial oven set at over 200°C) to remove the moisture from a hydrated sample of Volclay Bentonite and return it to a granular state.
Interestingly, bentonite can be re-hydrated and dried an infinite number of times without losing its original swelling capacity. It can similarly be frozen and thawed repeatedly without losing its ability to swell.
NO! The sodium bentonite that makes up the Volclay system is over 100 million years old. Sodium bentonite is most widely known for its ability to swell. It can absorb up to 5 times its weight in water and at full saturation may occupy a volume of 12 to 15 times its dry bulk. When bentonite absorbs water it takes on a gel like consistency which is resistant to fracturing or cracking. When installed correctly, the sodium bentonite will form a watertight barrier for the life time of the structure.
A common misconception is that bentonite is prone to drying out, there are two important aspects to consider in the wet/dry cycle of bentonite. Firstly bentonite creates a strong suction force, that will in most situations, prevent it from losing water once it hydrates. Secondly, any water that could theoretically be lost through evaporation will be quickly regained through absorption from any residual moisture in the surrounding substrate and/or rainfall.
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.