I'm not sure what you mean by pre-mix, because there's nothing to mix in grout except to add water. I do think you can sometimes buy the already moistened grout, so maybe that's what you mean?? But mine was just white powdery dry grout that I added water to. It's the same kind I used in the studio classroom with my students, but can be bought at any hardware store. I think it doesn't really matter, whatever works is fine.
I did find this, though, from a website. It explains a lot especially about the porosity, I think the kind with sand is more porous:
**Grout is a type of mortar used to fill joints, cracks, and cavities in tiles, masonry, and brickwork. Grout can also be used in craft projects and is particularly useful in creating mosaics. It typically consists of water, cement, and sand; or cement and water. Used in semi-liquid form, it may be pumped, spread, or poured into cavities and allowed to harden, creating a tight, water-resistant seal.
The three main types of grout are epoxy, Portland cement-based, and furan resin. The epoxy type is strong and water resistant. It is available in 100 percent epoxy resin and modified epoxy emulsion form. Epoxy grout is generally more expensive than other types and can be difficult to find. However, it considered highly effective when a high level of water and stain resistance is desired.
Portland cement-based grout is available in sanded, unsanded, pre-mixed, or powdered form. It can usually be found in do-it-yourself stores. This type of grout is commonly used in creating mosaics, as well as other types of craft projects. Portland cement-based grout is weaker than the epoxy type, yet better suited to crafts.**
Here's the link where I found it:
http://www.wisegeek.com/what-a...
(Edited to correct the link.)
