Dear all, I have bought a box of AMACO CLAYCRETE INSTANT PAPIER MACHE intending to be used for my dioramas groundwork. However, when I mix it with water it just became a complete mess , and it doesn't dry out after a couple of days. Can I ask have I bought the wrong product or done something wrong? Thank you!

You may want to just make it old fashioned way - the way southern people in the US make gravy for their biscuits: flour and water.

Sounds like you added too much water. And if it's like Celluclay, it should her applied in thin layers with plenty of time to dry in between. If the base is wood, it must be thoroughly sealed with urethane varnish, or it will warp.
Some people get great results with instant papier mache products, but I consider them more trouble than they are worth. For ground cover, I like wall patching plaster products, preferably those with a plastic constituent, like Durham's Water Putty.
The other alternative, though a little more expensive, is epoxy putty. It was created for the plumbing industry, but it is now sold through art supply companies as well (in finer grades). Epoxy hardens chemically in a couple of hours, and requires no water at all (though dipping your tools in water will prevent the product from sticking to them)

This is the product I bought https://www.frontlinehobbies.com.au/images/r_545x/Product/85669/AME-CC1.jpg?ts=1329343251

I agree with Gerald Owens - it sounds a lot like Celluclay. Is it a very light powdery substance? Probably recycled and pulverized newspapers. Mix just enough water to make it a consistency of porridge, maybe rice porridge? Thick and gloopy! Try mixing in some white glue (school glue) - this will help shorten the drying time and make it stronger. And apply in thin layers; no more than 5mm, or 1/4 inch. Otherwise it will take forever to dry, and could even grow mould. Spreading it over a rough-cut styrofoam base will save on your papier-mache product, and shorten the production time of your base. It is also a good idea to mix in some cheapo craft acrylic paints to color the mix an earth tone, just in case it chips or cracks afterwards, to prevent telltale white (or light gray) spots.

I routinely use Celuclay for my ground work. As Biggles mentioned, a good dollop of white glue, aka Elmer's, helps it adhere. AS he also said, keep the mixture as dry as possible allowing for it to be spread over the base. In addition, I precolor it with brown craft paint and add fine model railroad ballast to give it sandy texture. If you drill multiple holes in your base,it will have more to grip for a more permanent application. Also, building up contours with styrofoam reduces weight and the need for deep applications that will take longer to cure. I typically get mine to cure in a few hours, overnight at most, under a desk lamp.

I might also add that Celluclay, and similar products, have a tendency to curl up at the edges and lift off the base, during drying. When that happens just mix a small amount, or save any left overs in a small food container, and use that to fill any gaps.

It seems that you have the right product. Drying time will depend on how much water was mixed into the Claycrete and how thick the Claycrete was applied to the base. The wet mixture should be like bread dough not like soup.

AMACO website for Claycrete has PDF ...

www.amaco.com/pdfs/345-1.pdf

Basically, it says 30 to 60ml of water for each 30gr of dry Claycrete. Mix by adding Claycrete to water. Wait 1 or 2 days for the mix to dry or dry more quickly in direct sunlight or 65C oven.

Anybody know how Claycrete compares with Celluclay, pricewise?

One online art supplier shows Celluclay being one dollar less than Claycrete for a one pound bags.