[Cob] Differential adhesion qualities

[Mitch Ventura]

There has been some discussion and comments for sometime about using cob in conjunction with other materials (cob and cement, with wood, adobe, etc.), which causes me to ask for one of the Forum's experts to comment upon, as those of us with layman's or limited hands-on experience can go back and forth discussing what we "think" is best.
 
My "research" has lead me to read books by engineers and architects who insist that if other materials are used with cob, then there will be a separation, sooner or later, due to differential expanding and contraction qualities, which explains in part why a cement-based plaster is probably not good for cob structures (besides inhibiting the "breathing" that cob needs).
 
However, there are also experts who advise using cement and other materials (mainly referring to adobe).
 
In my own case, while not being able to complete a project that I needed, due to being out of the country, I ordered an adobe structure built around a water tank to hide its ugliness. (The builder refused to use cob even though I offered to pay for his training!!!). Although he is excellent with adobe, he did show me structures with much cracking in the "adobe" mortar which is why he uses a cement-based mortar. I explained to him that even though my experience is limited, it was easy to see that the adobe was not from the immediate area while the mortar was, accounting for two different mixtures (there was at least a 16 km separation between both locales), requesting that the adobe brick manufacturer supply him with almost the same clay-sand mixture (slightly elevating the clay percentage) in order to prepare the mortar.
 
Needless to say, upon my return I found the adobe structure constructed but with a cement mortar (that he cleverly disguised to make it look like adobe). Although I was not pleased with him using cement, I have to admit that other cabins and tanks that he has made in the area using that type of mortar shows no separation (even though some are 5 years old) as compared to the adobe mortar that I mentioned. This does not make sense., as I would expect the opposite to be true.
 
When it comes to mixing adobe bricks and cob (perhaps to save construction time), I would also expect a separation... am I also wrong on that?
 
I understand that there is always some "mixing" as many cob structures are placed on a rock, cement, or other foundation, but that is a different scenario as the weight is (or should be) laying evenly on a different surface... I would ot expect the cob to rise off the cement or rocks.
 
Thanks in advance.
 
Mitch

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