For a long time I have wanted to try mixing cob with a cement mixer but never had the opportunity. But today I was helping a friend render his wall with Adobe in a stone house and as he had a cement mixer there and I was tired I offered to try to mix it with the machine. It worked really well with no problems so in this post I will explain how I mixed it.
The issues I would predict and have heard people complain about are that the mixture sticks to the inside of the drum. This is logical as cob needs to be sticky and somewhat solid to make a wall, and still somewhat mouldable to render too. So to stop it sticking to the drum I had to prepare everything in a specific way.
It is worth mentioning that I was making a render mix which needs to be more liquid than a brick or cob wall mix. So it was easier using this method. If you needed a drier mix this may not work quite as well but I’m sure it’s possible somehow.
If you are able to grab a lump of adobe with your hands and tell whether it’s good to use or not then this helps a lot as you can add sand or earth as needed whilst using the mixer. If not then maybe mixing cob on a plastic tarp is a better start as you can prepare all your ingredients dry in the right quantities and then add water till its ready. Alternatively if you need to use the cement mixer then mix all your materials in the machine totally dry, remove them, then add them using the process shown below
First of all sift your sand and earth to your desired diameter. This is generally best at half the final wall or render thickness. So we were doing a thick first render around 1cm thick so we filtered the materials with a 0.5cm mesh. Get your water and a measuring bucket ready. You should already know your rough ratio of sand to earth unless you are doing it by eye like a boss.
This was a second mix so the mixer had some cob inside already but it is best clean. So first I added dry earth at one bucket. In our case we were using 2 earth: 1sand but this may be different for you.
After adding another bucket of earth I poured in a bucket of sand and turned on the mixer
You should also add your chosen fibre to the dry mixture
At this point I like to check the fibre quantity is good. When it is well mixed pull a handful out and see if there is enough fibres in your handful
So this is the important bit. You have your mix all at the right quantities but you need to mix in the water well and have everything tumbling around nicely before it is ready. If you want at this point, remove a bucket of dry mix before adding water.
Next add water, but add more than you think you will need. If not the mix will not tumble and mix well, and it may start to stick to the drum.
So now your Adobe should be tumbling and flopping inside the drum. Don’t add so much water that it’s washing around though! Then the final key is adding that bucket of dry mix you separated to dry out the batch to your desired consistency.
As mentioned above the mix was for a render which meant it could be fairly soft, for a clay brick or wall mix you may struggle with this technique as you need to get everything quite wet and liquid first.
As always, any questions, mistakes, confusion then get in touch