hydrophillic material

[johnnymat]

Adding particulate mineral to absorb water will not fgive you the the degree of swell you are looking for. You will need to add a water absorbant polymer such as crosslinked polyvinyl alcohol,this will swell considerably and swell your compound as a whole

Adjust the amount to suit.

There are other water swellable polymers, but this is the route to take

Johhnymat