posted 13 years ago
A number of people have tried and failed with portland cement. The properties do not seem to work well with the natural clay. Lime, much as used for the exterior renders, would be the best bet for a true strengthener.
Many unusual ingredients have been encountered inside the walls of ancient cob. One that I have actually used for a test wall out on the beach was fine chips of shale. Added to cob it is properly called shillet, and it seemed to help the cob dry much faster, strength change unknown.
Also, my first 10 dump truck loads for my home I built with pit gravel from my lower pit. I was able to build 5' plus per day with no ooging (bulging) at all, compared to 3' average for most mixes. The cob would set up after three days no matter how wet I kept the tarped mixes (using a forklift loader to mix 15 yard batches). Possibly the fines in the gravel or the fines in the clay I was using, but it set up no matter what. Right now I am using sand from my upper pit, it was sorted 50 years ago into a pile when the pit was being used commercially by a road-builder. I got 30 ten yard loads out of the first pile, 20 for my house and building projects, and 10 for the beach at our swimming pond.
On a recent post on the forum I asked about glass or poly fiber reinforcement as relates to cob, maybe it has been dealt with in an earlier thread but I couldn't find it.
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