Many heating systems which use heated water under the floor prescribe the use of gypsum covering the pipes since it is good at radiating heat and it has a high heat capacity (.26 as compared to .2 for cob). This material is commonly purchased in bags just like cement and lime are packaged. It costs seven dollars for a 40 pound bag.
Now the free part. Actually it's much better than free! Used a drywall is made of gypsum and depending on where you live can cost anywhere from $100-$200 per ton for disposal. Where I live in Victoria BC I charge $200 per ton for the removal of this material. Using a small children's pool I soaked some of this material and then ran it through a bearcat garden chipper. The resulting paste was somewhat chunky but very easily troweled and there is no reason why this could not replace purchased gypsum. I will use more than 10 tons of this material as soundproofing and as thermal mass adjacent to my RMH. Houses built before 1980 often have asbestos drywall tape so don't get your material from houses this old.
I ran several strength tests on gypsum which I pulverized in the garden chipper. I made several hockey puck sized pancakes of this material and allowed them to dry. With some I stripped the paper off and pulverized only the gypsum and then mixed half of this batch with 10% Portland cement and the others I just allowed to dry without additives. The one with Portland required may be 20 pounds pressure to break with a twist of the wrist. The one without Portland required less than half as much strength and it crumbled. Next I ran some gypsum through the chipper paper and all. Again one batch of pancakes was mixed with 10% Portland cement and the other was just allowed to dry. The one with Portland cement and paper was so strong I couldn't break it with my hands. The one with just paper broke with a hard twist of the wrist and was probably twice as strong as they gypsum with Portland sample. So for most purposes drywall can simply be run through a hog with paper attached and a relatively strong and malleable paste results. This product is not sticky enough to use on its own as a plaster but it would certainly work for filling cavities anywhere in a house where you're looking for thermal mass and soundproofing. Since all paper is covered in gypsum these cakes were fireproof. Running gypsum through any sort of chopping device while dry is asking for a dust storm and would also wear out your equipment faster. The dampened material made no dust and it ran quieter then when chipping branches. Restricting the outflow allows for a very fine grind but for most purposes you're simply looking for lots of easily spreadable material so the stuff could be run through rather quickly.
Important note: although the resulting material looks a lot like concrete it is highly absorbent and completely unsuited for any exterior application. Even the cakes with 10% Portland cement become soft and breakable when wet.
Abe Connally wrote:Dale, do you know if gypsum from drywall can be used in agricultural applications?
Generally, this is not a good idea. Used gypsum will contain paint and sometimes fibreglass or asbestos. It does not do the same job as lime but may have many uses so long as ph and other factors are right.
New, unpainted drywall scraps should be safe to use in soil unless they are made from recycled drywall which is sometimes the case.
yeah, gypsum seems to be a remedy for clay soils to help loosen them up. I would be nervous putting scrap drywall in my soil, but I was just curious about your thoughts on it.
It is strange to me that gypsum is used in this manner, because it is also used to produce alker, which is basically clay soil + 10% gypsum, which creates a fast setting, extremely strong building material.
hey dale, have you found a good way to get the gypsum separated from the paper? In an attempt to recycle, I don't have a shredder like you have, so using a hammer, I beat the drywall board apart, stripping paper from the powder when the gypsum finally released. That paper is really heavy quality paper, although it has paint on it! The cleaned nearly white calcium carbonate powder I finally accumulated about 5 gallons of it after 2 days of beating the drywall apart.
You mentioned soaking your used drywall in water first then shredding it, wet. So I guess I'm asking you, is the shredded drywall, is the paper lumps easily removed/sifted while wet mush?
I would think if any part of the drywall is moldy, or could become moldy, it is the paint & paper, by removing the paper, the gypsum can be stored wet or dry without 'growing stuff' in it. I got the nails & staples out of my powder with a strong magnet.
It just seems such a waste of material/gypsum, like after hurricane Sandy, all that water damaged gypsum is typically, wrecked, removed, buried, & totally wasted. One thing I was able to do with my gypsum powder was to soak it with water, and it seems to have a little clay in it already, so I form it into a chunk of chalk, leave it on the stove till completely dry & harden, it is then pretty easy to carve or saw.
I'm hoping to accumulate enough to make my own lump free 'wall plaster in a bucket', to set aside with all the other useful-for-something, stuff I have. I guess it would of been easier to just buy a bag for $7, but where is the fun in that?