This article was originally posted in Instructables and is reposted with permission from Michael Warren.
Papercrete is just what it sounds like: concrete made with paper. I tell people to think of it as industrial papier-mache. It’s inexpensive to make, amazingly sturdy, lightweight and insulating.
Step 1: So, What Is Papercrete, Anyway?
Making papercrete consists of adding a certain ratio of paper and/or cardboard to water and then adding portland cement. The mixture is then stirred with a blade to re-pulp the paper and mix everything together. When properly mixed, it has the consistency of lumpy oatmeal, which can be poured into forms and cast into shapes, such as blocks or beams or dome sections. You use the same stuff as a mortar to glue the blocks together. It can also be used as a plaster to make a smooth finish coat on the inside and outside of a structure. Papercrete has an R value of 2 per inch so a 12" wall has an R value of 24. Not bad!
• It’s sturdy but lightweight. A block only weighs a few pounds but can hold up a car!
• It’s weatherproof. I’ve had blocks out in the elements for four years, and they haven’t changed at all. It does absorb water like a sponge so if they are going to be used for walls they need to be protected from moisture. Similar to wood they will decompose if buried underground so they need to be up on a raised foundation.
• It’s easy to make. As long as you stick to the rough proportions of water, paper and cement, you’ll end up with a usable product.
• It's easy to work with. You can use regular woodworking tools to cut or drill holes in papercrete blocks.
• It’s green. I use all of our paper trash for the year and then quite a bit more. It’s fun to go to the recycling center and see their faces when I ask for paper rather than dropping it off. Now we even grind up all of our plastic trash in a paper shredder and mix it right in!
• It’s cheap. It costs about a quarter to make an 8″ x 12″ x 5″ high block. I spent about $250 total for the whole project.
Step 2: Papercrete Mixer
The McCain mixer is sheer genius in its backyard engineering brilliance and simplicity. It consists of a trailer made from a truck rear axle with a stock tank mounted on it. The axle is rotated up 90 degrees so that the end where the drive-shaft would normally attach is sticking up through the bottom of the tank. A lawn mower blade is mounted on the differential stub so that as the trailer is towed it turns the lawnmower blade creating a giant blender.
First I gathered up my materials, which included:
A four foot diameter metal stock tank
A full sheet of 3/4″ plywood
The rear end from a Land Rover (I think I have the classiest trailer in town)
A trailer hitch
The rubber inner tube from a large truck tire
A couple hinges
A lawnmower blade
A small can of bondo
A tube of silicone and liquid nails
Assorted nuts and bolts and some wood screws
The first step was to assemble the trailer. I needed a contraption that could securely carry several hundred gallons of water. I used galvanized I-beams that were way heavier duty than I needed to build the trailer with, but hey, they were pretty cheap at the scrap yard and about the length I needed already. I welded them together along with the rear end from a Land Rover to create the framework.
Next, I cut the plywood sheet in half and glued and screwed the two halves together to make an inch and a half thick platform to hold the stock tank. I cut a hole in it where the end of the differential would stick through. I cut the plywood to fit snugly around the differential so that it would be relatively easy to seal later. After that, I cut a hole in the stock tank as well so that it sat on top of the platform and fit over the differential, too. Once everything was aligned I drilled through the tank, plywood and trailer rails and bolted everything together.
In order to get the papercrete out of the mixer I needed a drain. I took the tank off and cut a hole that was the circumference of the truck inner tube. I cut a third of the inner tube off and slid it through the hole in the plywood and secured it with a couple screws. It looks like an elephant’s trunk sticking out of the bottom! I cut a matching hole in the stock tank but made the hole an inch smaller so that I could cut tabs and bend them down to secure the tank over the drain. I bolted everything together and sealed the joint between the differential and the tank with bondo. I also made a flap under the trailer to hold the drain shut. All that remained was to attach the lawnmower blade to the differential, and I had a mixer.
Step 3: Making Papercrete Blocks
Making blocks is super easy. After mixing up a batch you just cast it into forms.
Block molds — mine are made from 2-by-6s and scrap siding
Bathroom scale — for measuring out the paper
Paper (used of course)
Shredded plastic (if you want)
95 lb. bag of cement (cement, not concrete — no rocks or sand in the mix)
1. Set out your molds. You will need enough flat space to drive over them and pull your truck and the mixer in all the way in front of the molds.
2. Fill the mixer 3/4 full with water. I just eyeball it.
3. Put in the paper/plastic. I use about 75 lbs.
4. Add the bag of cement. You don’t need to open it, just toss it in.
5. Cover the mixer securely. This is very important. Just think of what happens when you have a blender top malfunction and multiply it by 100… I use a canvas tarp with a cargo strap.
6. Drive slowly- 5-10 mph for one mile. The mixer will chop up the paper into a pulp and mix it with the water and cement.
7. Empty the mix into the molds. If you got the mix right you should be able to open the drain, fill some blocks, close it, pull forwards a bit, repeat. If the papercrete is too thick you've got some shoveling to do. The type of material you are using can make a difference too. Cardboard makes for a thicker, chunkier mix where newspaper is finer and smoother. Sometimes I use a plunger to force it through too. I get about 45 blocks per batch.
8. Remove the molds. I do this right away. I want as much air flow around the blocks as possible to help them dry.
9. Wait a few days for the blocks to dry.
10. Stack the blocks under cover to dry further. I like to wait a couple weeks before using them.
11. Do it again and again until you have several pallets stacked with bricks and your year’s supply of paper trash is gone. That’s 10-15 batches for me.
Step 4: Building Time!
You built the mixer, saved the paper and made the blocks. Now it's the fun time- putting them together to actually make something. The first step is digging a trench and casting a foundation for the building. I used a metal stake and a piece of string as a giant compass to scribe a building sized circle in the dirt. After shortening the string to match the inside of the building I drew a second circle. Now I knew exactly where I needed to dig. The foundation was one foot wide and six inches deep. It was filled with (real) concrete and had rebar reinforcing inside of it. I also cast a small front entry stoop at the same time. The foundation didn't need to be too heavy duty since the papercrete is so lightweight.
The door frame from the yurt was attached to the stoop and a ring of cinderblocks were put down as the first course on top of the foundation. The exterior of this initial ring was coated in roofing tar to waterproof it. It will act as a water barrier and keep moisture from wicking up into the papercrete blocks. Next it was just a matter of stacking and mortaring the blocks together. It was a lot of labor but went pretty fast. I was able to complete the wall ring by myself in just a couple days. I also used the papercrete to start to plaster the inside of the walls. After the walls were up I made a double layered ring of plywood that went all the way around the top. This ring is called a bond beam and it ties all the blocks together at the top so that the weight and pressure of the roof doesn't spread the walls apart. It's screwed into the walls with 6" long screws.
I wasn’t really sure how to build the roof. It needed to be self supporting and not have any columns holding it up because the original yurt structure needs to fit inside of it. It also needed to be very sturdy. We've gotten three feet of snow in a single storm and the roof needs to be able to handle that as well as our 50-60 mph spring winds. We started by making a central ring out of a couple layers of plywood that all the rafters would connect to. The ring was about 5 feet in diameter with a large hole in the center for a skylight. Next, we needed some way to hold this ring in the right place to attach the rafters to it. We erected some scaffolding and spent quite a bit of time getting the placement of the ring correct. It needed to be at just the right height and exactly in the center of the building. Oh, and level too.
You can see more photographs of this project on my original Instructables post.
Photos by Michael Warren