I am hoping to build a small (10x20 foot) greenhouse with cob. At this size I'm not bound by the construction code.
Last summer I built a concrete pad with 1 foot high walls with the intention of adding cob walls to it, but the end of the hot season came faster than I expected and I only got one shoddy mix of cob done before I gave up.
I made a mistake when ordering sand and got masonry sand instead of concrete sand, so I wonder if adding gravel to the mixture will help get the ratio to where it should be.
If anyone near me has cob experience and is willing to help, I'll provide the beer and food.
Mason sand would make good cob, very fine textured. Adding gravel would give more bulk with less clay and possibly greater mass structural capacity, not sure of that. Mason sand is the kind recommended for cob or lime plaster which you will also want for finished surface protection.
I have some concern about using cob in a greenhouse, which will naturally be quite humid. As long as you don't depend on the cob for structural support of the roof I think you can try it without undue risk. You need to make sure there is plenty of roof overhang on the outside so rain seldom hits the cob. You are obviously not going to have much wall on the south side; how much are you planning for the north and east/west? Cob is a good thermal mass, but not an insulator for long cold periods. I would hope you will use some form of insulation on the outside of major walls.
Masonry sand: http://samedayaggregates.com/what-exactly-is-masonry-sand/ (Masonry sand can be thought of as a finer version of concrete sand. While the rock composition of masonry sand is often similar to concrete sand, its smaller grains make it quite a bit more attractive when used in construction. Masonry sand is usually washed as well.)
From The Hand Sculpted House (pg 123 - attached).
And a source from Youtube:
From these sources what I understood is that Masonry sand is not a good choice for Cob and I should have ordered a sharp/coarse sand. Is this correct?
If I understand what you're saying is that the masonry sand is good which somewhat contradicts the sources I consulted.
I'm not saying you are wrong or they are, I'm honestly trying to get the correct material for the job.
I'm a bit worried about the humidity as well so I plan to install a fan in the wall near the ceiling to be used during the warmer months to ventilate. For the cooler months and winter it's downright dry so I'm worried as much about that.
Noted. Overhang has been planned from the beginning.
North, east and west are 18 inches thick, south is 12 inches.
Cold months and insulation
I was thinking of using a wood stove for those periods and just hoping that's enough to keep it warm. I think that having the cement foundation already done I'm a bit too far into look into adding straw bales on the north wall or something like that. Considering this is a proof of concept I'm hoping to get my "feet wet" in building with cob and move up to building a small home in the 5 year plan.