||09-16-2010 08:36 AM
A colloid is a suspension. You can look up the exact definition of a colloid on Wikipedia. Clay is a family of minerals, consisting of several hundred different types. A colloidal clay is a clay which remains in suspension when agitated in water. This means that the clay is in the form of very fine particles which do not settle out of the water column.
A typical colloidal clay is bentonite, so named because it was originally mined in Benton Wyoming. It is essentially impervious, hence is used in waterproofing panels. It also holds water very well, but does not release it easily. Bentonite is used in kitty litter, well drilling fluid, waterproofing membranes, landfill covers, and many other applications, and can be purchased by the 50 lb bag at specialty stores that cater to drillers, or you can buy clay based kitty litter and take your chances on what you get.
Now as to why anyone would want to add such a product around their house, I have no idea. If added in sufficient quantity, it will make the soil effectively impervious, so water will run off rather than percolating downward. So far as I know, it will not improve the soil characteristics for growing plants. I have never seen it used for the purpose you suggest, but if the customer is convinced that they want it, well at least now you know what it is and where to get it.