I used to spend a lot of time on groundbaits in my former life as a match angler and did a lot of experimenting with break down times. With regard Leam there are a number of types most of which were used simply as a medium to help carry bait to the bottom quickly and then to break up quickly. Its most common use was with bloodworm and was simply a carrier for the worm to get it to the desired depth and sometimes an environment for them to stay in on the bottom. Grey leam , which I believe was powdered clay(certainly looked like it) was very sticky and could probably be used, if you can find it these days, to bind groundbait in the way you need, though I never used it for this as their were better(to me) alternatives.
If I wanted to make a groundbait to slow release over hours I would do so using a product called PV1 or Colant. Most of the manufacturers did a version though I haven't noticed it much recently amongst the 101 fishmeal recipes now taking up the shelves. I believe that it was a sugar based product that could be mixed into any other groundbait to stiffen it up. It could probably be used neat though I used in as part of a mix.
Almost as important as the groundbait is the way in which it is mixed the key principles for me would be:
Lots of water added over a period of time maybe an hour and always the night before for a specialist mix like this
Using hot/warm water makes most mixes stickier and slower to breakdown.
Be thorough with the mixing, any lumps or un-wetted bits could cause a ball to break up prematurely
Hope this is of help
Matt