Lightweight Concrete Stackable Garden Beds

Lightweight Perlite Concrete Recipe

Here’s the recipe using volume for measurement:
2 parts Perlite
1 part Masonry Sand,
1 part Portland Cement,
pinch of Glass Fiber.

A graphic of a perlite lightweight concrete recipe ratios
The perlite recipe by volume

Here’s the recipe by weight:
6.4 lbs – Perlite
28 lbs – Masonry Sand
22 lbs – Portland Cement
12 lbs – Water
2 fl.oz – Red Tint
0.5 oz – glass fiber

A graphic of a perlite lightweight concrete recipe
The perlite recipe by weight

And it will fill two 36″ molds or three 24″ molds. And you can scale the recipe up or down to suit your needs. I have included the weight of the water as it’s easy to measure this way.

I have a postal scale that if perfect for this type of measuring.

The amount of water can vary greatly depending on the dryness of the ingredients you start with. The sand I buy is often wet so that can change things a bit. But, this is a good starting point.

My Measuring Can

I’m using an old metal paint can for measuring my ingredients here. It’s 7 inches tall and 7 inches in diameter.

Measuring Lightweight Concrete Ingredients

I’ll start with four heaping cans of perlite. Followed by two cans of sand. The grade of this perlite is coarse.

I then mix those two ingredients in my wheelbarrow. Then wet it down with some water. This also helps to reduce the dust from the perlite.

Now I add one can of the portland cement. Give it a stir then add the glass fibre. Then the second can of portland cement.

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Now I’ll blend this well. I’ll add the rest of my coloured water. And add some fresh water to rinse the rest of the colour from the white pail.

Then I’ll slowly add more and more water until my mix is wet and has a shine to it. And it starts to resemble a mortar mix. I try to not have it too wet or sloppy.

This may take some practice to get it just right. Sometimes I’ve added too much water and then had to quickly add more dry ingredients to get it back to a thicker consistency.

Fill the Molds with Wet Perlite Concrete

It the shop now I’ll start filling the molds. It may seem too dry a mix at first but, a little shake makes it flow and fill into the corners.

With a small trowel I pack a bit more of the mix around the wires and pipes.

To settle the mix further I use a reciprocating saw without a blade, and push it against the table. Or a rubber mallet works well too.

With the mold half full I can rotate down the wires and set the grid in place.

I’ll add more mix to the mold and spread it with the trowel. Then vibrate it again.

Then I’ll repeat all that for the second 36″ mold.

And finally trowel the surface somewhat smooth.

Make a Second Perlite Batch

I mixed another batch of the perlite mix and filled some 24″ molds.

When done, I cover all the fresh lightweight concrete with plastic. And set the thermostat of my shop to 70 degrees.

Plastic sheets covering concrete molds in a workshop
Once filled and troweled smooth, the wet concrete is covered with plastic

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