This technique may sound a bit strange to some, using cement blocks and earth, but we tried it and it worked.
But first of all, why mix natural materials with something like cement blocks?
We were making a bathroom outside of our house and wanted to use earth as much as possible, so for the upper half we made adobe bricks, but the lower half needed something more water resistant as it would be in contact with the ground underneath. We do not have many stones on our farm so unfortunately could not make the stem wall that way, but we do have a lot of clay. Something I had always wanted to try was using clay, or cob as a mortar with modern bricks, so we gave it a go on this small structure. I wanted to wait till the building had seen a whole year of temperature fluctuations before posting anything about it, and we are proud to say that after more than a year the building has fared well, with no cracks anywhere, but particularly between the bottom layer of rammed cement blocks, and top layer of adobe bricks.
This method was incredibly quick to build with and allowed us to use humid clay earth to fill the bricks, and wet cob as a mortar, meaning that the majority of the structure actually came from our land, but we also had the convenience, speed and straight lines that come with building with cement blocks.
The first job was to clear and level the ground. We used an a-frame level as you can see in the photo below.
Next we dug the trench down to hard ground, which was not too far at all in this spot. We wanted to add gravel for drainage but the bed rock was so close to the surface that it was not possible. This part of our land is the driest as it is on a small ridge so we weren´t too concerned about the site becoming too wet. So we filled the trench instead with a layer of limecrete. At this point we also dug and set in the drainage pipe for the shower and sink.
The first rise of cement blocks went straight on to the limecrete when it was wet.
For the first rise, as the block filler we used humid earth compacted lightly with a mallet.
It worked well but when we came to do the first layer of mortar to stick the bricks together we realised that the wet mortar wouldn´t have an extremely strong hold when meeting the dryish block filler, so for the second rise we made a wet cob mixed and poured it into the blocks. This actually was much quicker and easier and meant that the various rises really were stuck together as one.
Whilst we are here we may as well talk about the shower base. For this we started by compacting some sandy earth with a tamp
Then on went the ´vapor barrier´ which in this case was a few old compost bags. I would not recommend this in very humid areas but as we are building in a place that becomes completely dry for 6 months of the year, and on a piece of land that drains away on 3 of 4 sides I was not too concerned about rising damp.
Then another layer of sandy earth, compacted with a tamp to separate the cob from the plastic.
And finally the 15cm cob slab which the rest of the cob base would be built on top of
For the final layer of cement blocks we used slightly narrower ones to slowly taper the wall to meet the even more narrow adobe bricks we had made. As you can see the ledge can be joined with more cob to make for a smooth transition.
Once we had enough adobe bricks to get going we put one last thick (3cm) layer of cob around the whole wall and set the bricks straight onto the cob.
As a final note, we did then go on to plaster the whole structure in cob to smooth it all off, then lime plaster to waterproof it. The cob did stick well to the cement blocks but it did need a bit of help. We painted on a good layer of clay slip, and made sure it was not too wet, not too dry when applying the cob to ensure that it stuck.
On the rounded areas especially we really needed to add a lot of cob so to make this stick we had to build it up in multiple layers. Once we had made the building the shape we desired using the cob we were able to then apply the final layer of lime plaster, which stuck to the cob with no problems.
And there you have it, a building made with cement blocks filled with rammed earth and cob, complete with mint and banana growing outside in the shower grey water.