Hey there permies!
No sure if anyone can shed some light here…. We’re looking to build a small cob structure. We’re located in the wet tropics close to the equator. Our land is almost 100% clay under the few inches of top soil, the clay is very pure with almost no impurities. We have a lot of the common red (terra-cota) clay in the higher parts of the land, but we also have a clay that’s grey and one that’s blueish in the low swampy areas.
From my understanding reading several books, internet research and attending a workshop on cob building… the cob mix always contains sand. Right??
Over the several months we have experimented with different mixes, ratios, etc. Including:
CLAY + SAND + STRAW
CLAY + SAND + STRAW + CEMENT
CLAY + SAND + CEMENT
CLAY + STRAW
Out of this different mixes we experimented with different ratios and sample sizes from a few inches to full brick sizes.
After testing several combos and to our surprise the samples that were the strongest were the Grey and Blue clays in pure form. When mixed with sand and or straw the samples would crumble and break much easier. The ones with the pure grey and blue clay harden into a rock! Incredibly strong, and showed no cracks at all in small samples and some very minor cracks on the brick size samples.
Not sure what’s going on here… is possible this clay is non-expansive? Can we just use this pure clay to build with?
Any advise is much appreciated!
Best to everyone!