Lightweight concrete has the strength of regular concrete, but it's mixed with a lighter material to create a lightweight, more moldable concrete with practically endless decorative uses. Use it as a garden path tile grout, pour it into decorative molds to create garden sculptures, or make bricks for retaining walls and garden paths. If you don't feel like mixing up a batch of lightweight concrete and making your own objects, you can purchase pre-made lightweight statuary and pots from most garden and hardware stores.
To make lightweight concrete, you'll need a bag of ready-mix concrete -- the kind you just add water to. You'll also need something lighter to mix in with it. Perlite concentrate is a common additive and you'll find it at home improvement stores. Lightweight concrete isn't loadbearing, which means it can't be used to hold up the weight of other objects, so you can't make walls out of it. However, it's fine for creating pots, pathway bricks or statuary. You also can add vermiculite instead of perlite for a different texture. Also, add in peat moss, expanded shale, mason sand or smoothed glass chips to add visual interest. Lightweight concrete pots made with peat moss are called hypertufa pots.
Concrete has a powdery dust that shouldn't be inhaled, so always wear a dust mask, rubber gloves and eye protection when working with it. You're going to need a large tub to mix everything together. You'll find them in several sizes at the home improvement store. Get one that's larger than you think you'll need, so you don't end up making extra batches if you run out. You'll also need a mold, either store-bought or homemade, to form the concrete into shapes. A variety of brick molds are available at hardware stores. Use cardboard boxes, plastic pails or silicone molds to make pots and decorative elements. Pack concrete into a large pail and insert a smaller pail inside to form a basic pot. Use cardboard boxes for square pots. Line molds with a plastic bag or spray with cooking spray if you don't want to break or tear the mold when you release the dried concrete.
Ratios & Mixing
Use a basic 1:1 or 1:1:1 ratio for your mix. The extra "1" is for adding peat moss, shale or other decorative materials. Scoop equal parts concrete mix and perlite, with or without a third ingredient, into the tub. Mix the materials together gently with your gloved hands. Try not to create more dust in the air, and keep kids and pets away from the mix area. Peat and "chunky" materials should be separated with your fingers so they mix in evenly. Add one part water slowly, the same amount you used for the other materials. The mix should resemble cottage cheese. It should be wet, but not too wet. Let it stand for 5 to 10 minutes before packing the mold.
Lightweight concrete must set up for 24 to 36 hours before you remove it from the mold. Put the mold in a plastic bag or cover with plastic to keep the concrete from drying out too fast and cracking. If you're making bricks, cover the brick mold with plastic. After 36 hours remove the plastic and peel off or carefully remove the mold. Put the concrete form in a dry area and let the piece finish curing for another two to three weeks. After that, the concrete will be completely cured and ready to place in your garden.
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