Compressed Earth Block Workshop

This natural building technique is relatively new to me, so I wanted to make some test blocks to see if they would work out for the finished surface of my porch. My first attempt was a failure after using the very clayey soil that I've used for the adobe floors because I couldn't get the cement mixed in with the soil adequately.  I found a source of a looser soil that still has a relatively high clay content.

Regardless, the first step is to pass the soil in the 1/4 screen to loosen it up. 

Interestingly, the water content in the soil was pretty high in the morning, but

by afternoon, it had dried out much more than I would have expected.

This is the mixing bucket.  We tried three recipes with varying ratios of cement (1:7, 1:11 and 1:15), and you can see the scale's readout that allowed us to measure precisely.  

This is one of the 1:11 mixes.   After mixing this thoroughly, we also added water to get the mixture to the optimum moisture content (more water was needed as the day went on).  It took some practice, but we found that if you squeeze a handful of mixed soil, it should form a coherent ball, and that ball should shatter when dropped onto a hard surface from about 6 feet high.

This is the simple mold: 2x4 sides screwed onto a 2x8 base.  The first blocks were pretty thin, so we ended up filling it, compacting it slightly by hand, and then re-filling.  Next time, I'll try 2x6 forms to save that step.  (FYI, to make a large number of blocks, you can buy a human-powered press, or gas-powered pneumatic presses.  Since I'm only going to need a few hundred, that's not worth it for now.)

Here, we're compressing the blocks with a hand-tamper hammering on a 3" thick board that fits inside the molds. It only takes about a minute of non-strenuous pounding, and you can feel when you are getting diminishing returns.  You can also hear the difference when it's time to stop, but that is pretty subtle.

I was surprised how hard it was to get the blocks out of the mold.  

We needed to remove two sides, or it was just too sticky to come out whole.

Knocking it with a hammer helped free the block.

 An array of block after a few days of drying.  Since the recipe includes cement, they should be kept shaded and moist, and they won't achieve their full strength for a month.  I've left a couple random pieces out in the rain, and they've held up with no visible degradation.  Once they've all cured, I'll try some compressive tests on dry samples and soaked samples, comparing the results of different cement contents.  

Many thanks to Kate and Bob for the photos and assistance!