Swelling index measures how much bentonite expands in water. In the common test, 2 grams of dry powder are added slowly to a graduated cylinder of water, and the settled, swollen volume is read in millilitres after a set time.
The result is reported in mL per 2 g. The value you should ask for depends on how you use the clay. Liners and drilling grades need high swelling, while foundry and some other uses need less.
How the test works
The most widely cited method is ASTM D5890, the standard test for the swell index of clay mineral components of geosynthetic clay liners. Its main steps are simple:
- The sample is dried and ground so most of it passes a fine sieve.
- The powder is added in small portions to deionised water in a 100 mL cylinder.
- The cylinder is left to stand for a fixed period, usually at least 16 hours.
- The volume of swollen clay at the bottom is read and reported in mL per 2 g.
Why it matters
Swelling shows how much active sodium montmorillonite is present and how well the clay will seal or thicken. High swelling usually means a better seal in liners and slurry walls, and higher viscosity in drilling mud. Natural calcium bentonite swells much less than sodium bentonite.
Activation with soda ash raises the swelling of calcium bentonite, but the effect can fade if it was poorly done. Ask whether the value was measured on the activated product as shipped. Our article on activated bentonite covers this.
What value to ask for
There is no single right number. For geosynthetic clay liners, a widely used industry specification sets a minimum of 24 mL per 2 g. Civil and liner buyers often work to that level or above. Drilling buyers usually rely on viscosity and filtrate tests under API Specification 13A instead. Foundry, pelletising and litter buyers generally set their own swelling limits alongside other tests.
Start from your own process or tender. If you are unsure, test the material you already use and set your minimum just below its typical value. Then ask suppliers to show results on recent production lots, not on a hand-picked sample.
Common pitfalls
- Results vary with water quality, so the method must specify deionised or distilled water.
- Results vary with time, so the standing period should be stated on the report.
- A single high result on a sample does not prove the whole lot will match.
- Swelling can drop if activated bentonite is stored too long or gets wet.
Next step
Put the method, the minimum value and the sampling plan in your contract, then have lots tested independently. See what tests belong on a bentonite COA. To set a specification that fits your use, book a free sourcing review.