Earth Plaster - Can it be done without sand?

I have built an earthbag house in the mountains in Sichuan Province, China. The problem I have with making plaster and cob is that the soil is almost completely clay. And there is no sand to harvest nearby. There is something that looks like sand, it is white and has small stones and pebbles in it as well as a powdering sand looking substance. But the sand looking substance can be smashed with your hands into a fine powder. I have tried using this but I still had cracking in the plaster. The only way I was able to prevent cracking is to add about 3 parts sand for one part soil/clay. I also add a lot of straw.

I purchased sand but I have to carry it on my back up the mountain! This is hard work and after trying it this weekend I decided that the amount of sand I need is significant. I need to find another way.

Can I use something else to replace sand? My ideas include:

1. Sawdust
2. Shredded styrofoam
3. Perlite (but this is expensive so I need to find something similar if this is an option)
4. Some kind of waste shredded plastic
5. Others?

I am looking forward to responses from anyone who has suggestions.

Dustin

I'm sorry, I don't have any expert answers for you. You'll probably just have to try out several different proportions of materials that are available to you and see which of them stay on well after drying. Probably best to start with the most out of the way, least visible surface of your building, and as you get better at the mix, work around towards the more visible or important parts.

In my particular experience, we made some pretty big public buildings out of packed earth 20 years ago and they've held up great, but we found out years later that what we'd used as clay is technically very fine silt. And yet it worked as clay. We had sandy soil right on site, and deposits of this fine stuff that we thought was clay right up the road, so we made several test bricks using different proportions, let them dry, and then tested them: looking for cracks, standing on them, dripping from chest hight, kicking corners, and dribbling water. We found a mix that seemed good and went ahead with the building projects. This is the time-honored way of earth building, described by Laurie Baker. We also use that silt for casual clay modeling just for fun (not firing) in our school. We'd never have known it wasn't really clay except that years later the brains behind the whole thing, a local Ladakhi guy, went and did a master's at CRATerre in France and took samples for testing. Probably just as well that we proceeded and went ahead with building, and didn't rely on testing and expert advice, or we might never have built it!

My guess is that shredded plastic and styrofoam might not have good stickiness or bonding, but on the other hand, they might be flexible when the clay shrinks...? We found that sawdust when we were trying to make insulative adobes made them weak, but you should still try it.

Go for it!

If the bags you used for your earthbags are plastic and not specifically sold as UV-resistant, you risk them deteriorating if you don't get them plastered over asap. In our bright high desert climate on the Tibetan plateau, woven plastic sacks don't last a year outdoors before crumbling.

Rebecca Norman wrote:You'll probably just have to try out several different proportions of materials that are available to you and see which of them stay on well after drying. Probably best to start with the most out of the way, least visible surface of your building, and as you get better at the mix, work around towards the more visible or important parts.

Go for it!

Rebecca, the location you are in sounds very interesting. Thanks for the advice. I should have thought of this before, but what I can try is first do a rough base coat to fill in all the uneven surfaces and gaps. For that I can just throw in a bunch of the local pebble white stuff and straw. Even if this cracks, I still can fix the cracks with a final coat that uses. Sand. I don't mind carrying some sand up the mountain, it is only a 5 minute walk. If I reserve the sand for the final coats that may be a good solution.

Dustin

Does the white stuff have a glue type property when wet like clay?

What outside temp and humidity level are you plastering in?

Terry Ruth wrote:Does the white stuff have a glue type property when wet like clay?

What outside temp and humidity level are you plastering in?

Terry, the white stuff does have a glue type property. When wet and added to sand bags it gets very hard when dried. But I am not sure which elements have the glue properties. It may not be be white powder but something else also mixed in (such as clay). I could take pictures if that helps.


The temperature is subtropical humid. It doesn't get too cold, around 50 or 60 degrees in winter. Hot humid summers.

Dustin

Dustin, just a guess but perhaps the white stuff is a bentonite clay calcium or magnesium family that are light in color. Some bentonites are very expansive, shrink/swell is high, get hard as rock when dry and don't have much plasticity, need modifying or stabilizing.. Kaoline clay is the lightest and closest to white but it is not much of a binder but gives other clays more sheen and workability. It is used in paint for those reasons. All good stuff.... Did you try adding straw to it?

What happens is certain clay types like yours it sounds like are real expansive and shrink real fast that is when the cracks occur. If it is real dry and hot that will accelerate drying and cause cracks. You need a means to decelerate the drying slow down the desorption rate. or reduce the amount of expansion from liquid water uptake.

With that said the more water you add to these clay types the worse it gets. Did you try cutting back on water? Your mix looks very wet. You should only use enough water to activate the binder(usually 10--30% of mix) so it barely holds onto the aggregates being used or just to a point it barely sticks to the bags. If need be mist it down if it is drying too fast and work the cracks out with a trowel as it dries.

I don't know if this is more labor intensive than carrying sand bags up a hill but, if you could chop the straw up into smaller fibers add them to the finish coat that should help reduce the shrinkage. Use a square end shovel on a hard surface to chop the straw, or cutting sheers or something. The straw should act as an adsorber to hold water and retard drying, and hold the clay together. Perlite or other rock hygroscopic aggregates should have the same affect.

Foams and plastics will have the opposite effect, they will act as a barrier and insulation. In your mild climate there are better choices.

So here are some things I would try. If you can make some test form blocks out of wood or something, say .3 meters square.

1. 3 parts straw, 1 part clay brown coat plus 3 parts straw (50% chopped), 1 part clay(white or brown) finish coat. Wet as little as possible, slow drying by misting over several days - weeks.

2. Same as above with white stuff as binder.

3. Same as above but instead of chopped straw try sawdust using large as possible wood chips, little dust or screen the dust out.

Keep testing you should be able to do this without the use of sand. Please keep us informed on your progress so we can help the next guy/gal.

Terry, I have been told the place I am located has one of the largest deposits of Diatomaceous earth. The white stuff might be that. I need to research the uses for it in earth building.

Terry Ruth wrote:

Keep testing you should be able to do this without the use of sand. Please keep us informed on your progress so we can help the next guy/gal.

Terry, I am using a rototiller to mix the plaster. Instead of chopped plant fibers can I use chopped/shredded plastic? That is more convenient to buy. I have straw but my supply is limited and to try to harvest plant fibers other than straw will take too much time. If I need to add more fibers to prevent cracking then I definitely will need to buy something like shredded plastic fibers because straw is hard to come by here. I have been experimenting but so far I still get big cracks. I need to add more water in order for my rototiller to work.

I first mix the ingredients dry. But when I mix the straw sand and clay in a dry state and then add water the problem I have is that if the water is too little the clay clumps and clogs up the blades. If I add more water, the rototiller digs deeper and gets stuck in the mud. I added paddles on the ends which help prevent it from sinking, but now the width of the blades is so wide it makes it hard to turn the machine. The rototiller is heavy. I still haven't figured out the best way to do this. This is a separate problem, but the point I want to make is that I need to add enough water. I still haven't used as much fiber as your recommended. So I need to try adding a lot more and see if that helps.

Dustin

Instead of chopped plant fibers can I use chopped/shredded plastic?

Do you know what plastic family it is? Do you have any strong tensile strength grasses there and know the type? Do you have access to lime at a reasonable cost, if so which one, such as a NHL 3.5 or 5?

Did you try and add some diatomaceous earth? As far as I can tell it looks safe.

Doesn't China have alot of Magnesium Oxide free or low cost? Alot of MAG board comes from there. Can you get some fly ash or gypsum free from a local utility company?

Your clay content is too high we need to find another binder or filler to take it down so it does not stick to your paddles and is so heavy when wet.

you might be able to get some manure ? kinda gross if you think about it too much, but its basically just chopped straw, from an herbivore, horse, or cow manure. it would also help with the cracking.

it's not so good to think...even if it cracks i can fill it in with more stuff. theoretically that sounds like it would work, but often if it cracks badly to begin with, the new stuff you put on top will also crack, it will follow the same cracks as the bottom coat. only very tiny cracks can be patched up good.

Terry Ruth wrote:

Instead of chopped plant fibers can I use chopped/shredded plastic?

Do you know what plastic family it is? Do you have any strong tensile strength grasses there and know the type? Do you have access to lime at a reasonable cost, if so which one, such as a NHL 3.5 or 5?

Did you try and add some diatomaceous earth? As far as I can tell it looks safe.

Doesn't China have alot of Magnesium Oxide free or low cost? Alot of MAG board comes from there. Can you get some fly ash or gypsum free from a local utility company?

Your clay content is too high we need to find another binder or filler to take it down so it does not stick to your paddles and is so heavy when wet.

Terrry

1. The fiber is made with PP powder. See picture below.
2. I am going to try adding diatomaceous earth. How much should I add? It already has some mixed in, but maybe if I increase the ratio it will help.
3. What ratio of lime or gypsum would I need? If it is 3 or 4 parts per 1 part clay, I would be better off just hauling up the sand .

leila hamaya wrote:you might be able to get some manure ? kinda gross if you think about it too much, but its basically just chopped straw, from an herbivore, horse, or cow manure. it would also help with the cracking.

it's not so good to think...even if it cracks i can fill it in with more stuff. theoretically that sounds like it would work, but often if it cracks badly to begin with, the new stuff you put on top will also crack, it will follow the same cracks as the bottom coat. only very tiny cracks can be patched up good.

Leila, I can get cow manure, but I don't know if the volume I have available is enough, and I like to use manure for compost. What ratio of manure to clay would I need?

Dustin

Dustin, I’m not a big fan of plastics and PP has toxicity issues. It be better if you had a skid loader or mortar mixer drum. You’re limited by the horse power of your rototiller, If you can find a power drill with a mortar bit in a bucket that might help.

I think in this situation it is best to try and work with materials at hand. Your density and weight is too high for your tiller, we need to reduce it. Hold off the manure for now.

Try this: Get a bag or two of the PP add enough diatomaceous earth to a paste form and your tiller can mix it. Let it dry and see if the diatomaceous earth binds to the PP as an isolator. If it does not let me know, if it does continue to mix this premix until it is 70% of the mix when 30% clay is added. Test this mix with your tiller. If it is clogging or balling up drop the clay by 5% and add more diatomaceous, keep swapping diatomaceous for clay until the it tills easy. Add as little water as possible as you go. Try and test the mix on the bags wetting often to slow down drying for cracks when dry.

The main reason you don’t want any cracks in each coat has to do with undesirable chemical and physical reactions with vapor and/or liquid water after cure that cannot be solved with more fill. The PP is going to reduce some hydrothermal properties but, with your high clay content and diatomaceous acting as a microbial isolator it may not matter.

Wear a mask when working around the diatomaceous.

Dustin Mattison wrote:

leila hamaya wrote:you might be able to get some manure ? kinda gross if you think about it too much, but its basically just chopped straw, from an herbivore, horse, or cow manure. it would also help with the cracking.

it's not so good to think...even if it cracks i can fill it in with more stuff. theoretically that sounds like it would work, but often if it cracks badly to begin with, the new stuff you put on top will also crack, it will follow the same cracks as the bottom coat. only very tiny cracks can be patched up good.

Leila, I can get cow manure, but I don't know if the volume I have available is enough, and I like to use manure for compost. What ratio of manure to clay would I need?

Dustin

Manure is a major component of traditional cob. it's not so much used anymore, but some people still do. it's actually one of the best things to add to a cob mix, IMO, because the quality of the straw/grasses/plants after they have passed through the digestive system of the animal are ideal for cob. you could replace a portion of the straw with manure, but its actually (again hey, just my opinion, take it or leave it =)) way better than the straw. cause of that extra goopiness (yes not a very technical term, probably not even a word!)...

but i can understand prioritizing it for gardens, for sure. maybe you could find a free -for the shoveling- source. i do believe it would help with your cracking issue.

theres a lot of other additives that are sometimes used, maybe some of those would help. plants, and catcus and wheat gluten...too all sorts of animal furs, hairs, and various additives. i have heard of people using wood flour and sawdust for some purposes.

i cant see how you could get around not having sand at all, but i think you could greatly minimize it ?

it's hard to give exact recipes for cob, because everywhere you go the qualities of earth are so different, and the materials available are so different.
so you have to get the exact materials you are going to use, make records of different ratios and different mixes, and then test them all out by making many test bricks. thats the best way to find the right mix with the specific materials you have.

it's good to have some dry materials around for adding...if you are ending up with a situation where its too wet. its a bit like making bread dough, where you want some extra dry flour around to get it to the right feels....

google says --->
http://ilovecob.com/archive/cows-make

plaster recipes +

earth plaster how to

Terry Ruth wrote:... Your density and weight is too high for your tiller, we need to reduce it.

Terry, Thanks! I have a lot of testing to do based on your suggestions. One thing...why don't I just mix everything dry with my tiller, then use a shovel to move smaller batches to a tarp, add water and mix with my feet? I think that wouldn't be too much extra work because all the ingredients would be mixed already.

Dustin

Dustin, that is ultimately what we want to do but first I suggest you see if the d-earth bonds to the PP and your tiller mixes it wet, add small amounts of your high density/weight clay and water until you can still till. Once we know the d-earth and PP bonds along with some small ratio of clay and to your bags without cracks it does not matter how you mix the tested proven ratio. If you are getting alot of vertical cracks drop off more clay, too much weight still failing in shear. This wall you are building if your interested in learning the tech stuff is a "shear" wall and it will see alot of that in the future.

Bear in mind this test of d-earth/PP is to make sure that in the future the toxins in the PP do not leach or outgas out to your lungs and indoor air quality. If you do not find that to be a concern, or the occupants are not that chemically sensitive, use a ratio of PP. d-earth. clay that works. PP may in fact never leach, we do not know, its a guess, this is just a precaution, safeguard.

As Leila accurately suggest there may be other fibers if you can source and transport them easily that may work but, I suggest to just use the fibers and not any high density binders other than mostly d-earth and small clay.

I don't know how to get to the fibers(straw) in poop. I personally would never want around me thats just me

Terry Ruth wrote:Dustin, that is ultimately what we want to do but first I suggest you see if the d-earth bonds to the PP and your tiller mixes it wet, ....

Terry, I added lots of DE and chopped straw. I think this will work! I did a test patch of plaster and it hasn't cracked yet. It dried fast and is pretty hard. I still haven't tested the PP. I will try that next week.

These are some pics of my house and the de-earth source, about 20 meters from my house.

Awesome, very good news! Good job! My guess is DE bonds to PP rendering it inert and chemically stable.

Dustin, what ratio's of DE, straw, and clay did you use? I think it important to capture conclusions here for future use although that can vary it could be used as a start especially in China. You might consider testing a DE wash for adhesion on all your building materials like wood to improve the smoke index in case of fire. I would think your fire index is really high(5+) on the DE walls meaning very safe. The DD wash may also provide some additional moisture holding capacity to protect the wood from rot much like we do to chem treat it but with more natural materials. By wash I mean get a cheap agriculture pump spray applicator and dilute the DE with water to a point it sprays out a large nozzle and adheres.

Has anyone tried adding compost tea or other fermented liquids into plaster batches? I wonder if that will help, similar to using cow manure.

I might try a combination of wheat paste and fermented compost tea.

Dustin

Terry Ruth wrote:Dustin, what ratio's of DE, straw, and clay did you use?

Terry, I am using about 80-90% D-Earth to clay. So far it is working. I don't like using the PP, it has a strong chemical odor. So I will use straw. I have another question, is there a disadvantate to sawdust and wood chips? I did a test batch this weekend using 50% clay 50% sawdust/tiny wood chips/and some straw. If this works I won't have to dig up so much D-earth.

Dustin

We also have sawdust and wood shavings as a free resource, but straw is fed to local cows so it is expensive. We have tried making insulative adobe bricks with various materials mixed in, and have found that straw or cowdung make it stronger, while sawdust and wood shavings make it weaker, and dry autumn leaves makes it positively crumbly. We still use the wood shavings where strength is not an issue, but since you're talking about plaster I suspect it wouldn't be helpful.

What you've done so far works, so why not go with it? Or is it too expensive?

Rebecca Norman wrote:

What you've done so far works, so why not go with it? Or is it too expensive?

Rebecca, the D-earth is free. I just have to dig it up, put it in my bamboo backpack and carry it about 100 meters. I can do that, but I was just curious if sawdust would work. Does plaster have to be strong? It is just for decoration, but perhaps more strength is a good idea. I might keep using D-earth but also add a little sawdust. But is there an advantage to sawdust?

Dustin

The PP odor is probably formaldehyde. At 50-500 part per million (PPM) range people can smell it. In nature average is 3-12 PPM. There is no minimum PPM scientific prove it will not cause cancer. http://www.arb.ca.gov/research/indoor/formaldGL08-04.pdf

I was wondering if the DE would stabilize it and stop it from emitting, render it odorless. I leave that risk up to you.

Your DE success has nothing to do with similar clay mixes proven otherwise. You are using an entirely different binder if it is in fact DE. You are now the pioneer of DE plaster Add straw, sawdust, wood chips, sand, see how it performs. Conventionally, we go for more binder(DE/clay) at the finish coat to resist water and improve abrasion, add more fibers at the scratch and brown to increase insulation. If there is no concern about heavy wind driven rain, freeze/thaw, abrasion, or you need more insulation add more fibers to the final layer. Your plaster is not needed for strength in your design.

leila hamaya wrote:you might be able to get some manure ? kinda gross if you think about it too much, but its basically just chopped straw, from an herbivore, horse, or cow manure. it would also help with the cracking.

it's not so good to think...even if it cracks i can fill it in with more stuff. theoretically that sounds like it would work, but often if it cracks badly to begin with, the new stuff you put on top will also crack, it will follow the same cracks as the bottom coat. only very tiny cracks can be patched up good.

Leila, can cow manure be used in combination with DE-earth? I want to add white to the mix and I don't want to buy kaolin clay. Can I add linseed oil over the cow manure plaster to make it more water resistant and shiny?

Dustin