Make Lightweight Concrete Garden Boxes – Vermiculite Aircrete PART 3

Lava Rock – 2nd test

Lava Rock Concrete - 2nd Test

I figured that I needed to add more sand to get a smoother mix. So I started the same. Mixed it all up in the wheelbarrow and it came out a little bit wetter than I liked so I added a bit more sand to absorb some of that water.

I could tell right away this consistency was closer to regular concrete. So same thing, fill the mold halfway, lay in the reinforcing wire grid, and then vibrate and smooth it out.

Troweling Lava Rock concrete in form
I used a little bit of the extra mortar type mix in the bottom of the wheelbarrow to improve the surface finish on the first form. And this seemed to work pretty well.

I covered the forms with plastic and left them to set up. The next day I removed the form sides, and ends, and these castings came out fairly easily.

Lava Rock Lightweight Concrete Garden Box Panel
Lava Rock Lightweight Concrete Garden Box Panel

I was pretty happy with how the panels looked but, I could tell right away that they were a little bit heavier than I expected.

I went back to our landscape supplier and got some lava rock that closely approximated gravel.

Red lava rock

This red lava rock has a lot of smaller pieces, instead of the same three quarter inch size. I compared the weight of gravel to lava rock for an equal volume. The lava rock weighed approximately half that of the gravel.

Weighing equal parts of Gravel and Lava Rock
Weighing equal parts of Gravel and Lava Rock
Lava Rock weighs approximately half that of Gravel
Lava Rock weighs approximately half that of Gravel

For my third attempt with the more variable size lava rock the mix I used is: one part Portland, three parts lava rock, two parts sand and again a pinch of glass fiber.

Lava Rock – 3rd test

Lava Rock Concrete - 3rd Test

I measured all the ingredients and mixed up a slightly larger batch. Again starting with lava rock, the sand with fiber, and then adding the Portland cement and then the water. And into the forms as before. It settled nicely with vibration and all looked good.

Settling Lava Rock Concrete mix with reciprocating saw minus the blade
Settling Lava Rock Concrete mix with reciprocating saw minus the blade

The next day, I remove the plastic and disassembled the forms. These panels came out of the forms quite nicely. And this was my final version of the lava rock mixture.

Lava Rock mix was, on average, 10% lighter than regular concrete
Lava Rock mix was, on average, 10% lighter than regular concrete

I substituted the gravel for lava rock in this first lightweight concrete attempt. The lavacrete, if you will, worked very well once I used a variety of rock sizes that approximated gravel in my mix. The finish is very good and the panel feels solid and durable.

This panel was approximately 10% lighter than regular concrete. Not a lot lighter but I’d say its almost equivalent in durability and strength. I had hoped that this panel would be much lighter given that the lava rock was about half the weight of the gravel.

Porous Lava Rock
Porous Lava Rock

I suspect that the voids in the lava rock were filled with sand, cement, and water during the mixing so I lost most of those air spaces that would have made it lighter.

Now let’s look at the vermiculite mix.

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