Hi Happy, I think what your are referring to are
Thermalite blocks, or their equivalent. If you are, then I would say that the material is not strong enough to make a shallow wide container without support every couple of inches.
Recently I have been experimenting with coating some chunks in qiuck dry mortar. I have not had much success yet, I'm still working out how to get a good final look. The material on its own will not look very attractive imo uncoated.
If you do attempt something yourself, I'd like to see your results.
Regards,
Chris.