Concrete will not adhere to your plastic bucket. Using grease, etc. to act as a release can also hurt the look and face of your concrete.

If you use a bucket, you are going to have to let the concrete set for several hours (4 to 6 depending); in other words, you will probably only be able to do one, maybe two blocks per day. If you pop your blocks out too soon, you are asking for trouble. And damn, they are going to be heavy and cost a small fortune, not to mention you can only mix up one small batch of concrete at a time! If you must use the bucket, line with a plastic bag (any imperfections can be smoothed/dealt with before bing fully cured), and this will also aid in removing the block from the bucket.

If the blocks aren't to be used structurally, try using 'hyperturfa which is a mix of Portland cement, and sifted peat, and/or some sand and/or some vermiculite and/or perlite. There are several sites with hyperturfa formulas. This will a) make your blocks much more lightweight and still very durable, b) end up costing you way less, and c) give a much more interesting finish. (Standard colorants or acid stains can be used, but I like to use natural things like rusty water, coffee, tea, berry juice, etc.). d) reduce curing time by about 30%. Note: curing will really depend on your conditions — the colder it is the longer curing is needed — moisture content in air, where you cure, etc.)

Instead of using a bucket, make forms out of cardboard reinforced with wood or something to keep cardboard from buckling if needed (I have never needed). Your blocks can be left in the cardboard (which makes them easier to move to your curing spot) and the cardboard peeled right off in two-to-three hours. Also, the moisture held in the cardboard will help with the curing. No, your cardboard will not be rusable.

Cure for several days in a dark, cool environment, covered with plastic, and misted occasionally.

As an afterthought, depending upon what you are using these blocks for, you can put balled up newspaper or styrofoam in the center (use less product and even more lightweight) or, like concrete building blocks, inset some pipes in the middle to create holes.

Once you start playing with hyperturfa, your wife will be thinking up all kinds of cool projects for you. Some of the things I have made are: faux rocks; faux stone walls, huge planters, bird baths, Hobbit houses, garden edgers, and have even done some sculpting.

Vaseline will work as a mould release in this instance.

Do yourself a favour, I would recommend that you look into lightweight concrete, depending on how structural it has to be.

Vermiculite, paper, perlite and/or wood shavings, added to the concrete in various proportions can make a variety of light weight concretes.

I made some test block 10 x 10 x 8 out of the light weight stuff and I can almost juggle it. If it was plain old concrete I would have found it harder to manage.

Concrete doesnt stick to plastic but if you want to insure a quick release just put a little veggie oil on the plastic. Thats a big block and it'll take at least 6 to 8 hrs to set up, overnight is safer unless you buy some accelerator powder and add it to the mix. It can shorten the dry time a lot.

That is going to be a heavy block.I would use 3/4 plywood for the form and line it with plastic.Screw it together so you can remove the form later.

Geez, I know there are products out there, but without sounding like a smart-ass….

Why not just buy the blocks?

Go to a Home Center or concrete supply and buy the correct Release product. The size of your blocks will make it hard to remove the block in less than a few hours and likely, overnight.