How to Make a Papercrete Mixer

There is a lot to love about papercrete as a building material. It’s made out of readily accessible and inexpensive materials: water, used paper and cement. To make papercrete you have to be able to grind up lots and lots of paper. In order to do that you need a mixer capable of shredding paper. A regular cement mixer won’t shred paper it will just stir it around. Luckily for us a guy by the name of Mike McCain invented a papercrete mixer that you tow behind a truck.

Papercrete tow mixer

The plywood top looked good but it didn't work that well

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.

Inside the papercrete mixer

I built my own McCain mixer a few years ago and it’s worked really well for me. I’m going to go over my build here and post more detailed instructions over at instructables as well (at some point) so that you can build one yourself if you get inspired. (I lost my !@%$&# photos from building my mixer so we’re going freehand from here people!) First I gathered up my materials which included: A four foot diameter metal stock tank A full sheet of 3/4″ plywood Metal beams 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. Once I welded on a trailer hitch from Pep Boys I had a trailer. Papercrete mixer trailerNext, 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. Papercrete trailer plywood platformIn 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.

Towing papercrete mixer

Now I use a canvas tarp to keep everything in

How long did it take? I took my time acquiring parts over a few months and then once I got started building it took me the better part of a couple of weekends to construct it. How much did it cost? I scrounged as much as I could.  I did buy the metal for the trailer but it came from the scrap yard so it only cost about $35. The trailer hitch was another $20 and the stock tank was about $175.  Maybe another $20 for bondo, bolts, glue and silicone.  So a total of about $250. What you do different next time? I would re-design the drain so it was easier to use and make it larger to let the mix out better. I also need to put a couple of chains on by the hitch in case something bad ever happens and the trailer comes off. What’s next?Next time we’ll go over actually making a batch of papercrete and casting it into blocks.

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8 Comments

  1. Posted September 23, 2011 at 7:01 am | Permalink

    Wow! Can’t wait to hear what comes next and how it is made. Because while I’m still pondering what a differential stub is, it’s all fascinating and I’m totally there on using up junk mail in a good way!

  2. Anonymous
    Posted September 26, 2011 at 8:24 pm | Permalink

    Wow!  That mixer is really cool.  I’ll have all my junk mail sent to you.

  3. Posted November 8, 2012 at 10:02 pm | Permalink

    Looks like making that mixer is quite a project in itself. I was wondering if you just ran the paper through a shredder first if you could use a regular cement mixer. What do you think?

    • Posted November 9, 2012 at 8:24 am | Permalink

      I’ve heard people doing it but have not tried it myself. Do a small batch and let us know!

    • Wes
      Posted September 15, 2013 at 10:21 pm | Permalink

      A shredder would work great, or you can just rip the paper up, it really doesn’t have to be ripped all that small because the blade will handle most of that for you. A regular cement mixer won’t work. The PC slurry just sloshes around and flings out. If you’re doing a smaller project and/or have more patience and elbow grease you can mix it in buckets using a paddle drill. And when making a tow mixer or other stock tank mixer I’ve found you don’t actually need a lawn mower blade, any relatively flat piece of bladesque metal will do because it doesn’t actually have to be sharp. I used one made with the suspension struts off an old truck and it worked just as good as one made with actual mower blades.

  4. wes
    Posted September 15, 2013 at 10:17 pm | Permalink

    I learned that when making a tow mixer you had to put a chamfer along the side to cause the pc to swirl and form a whirlpool, otherwise material would just sling out of the sides due to centrifugal force. Trust me, I’ve seen it happen. I noticed your two mixer had no such chamfer. Am I correct to assume that it works without one because the blade is very off-center?

    • Posted September 17, 2013 at 2:23 pm | Permalink

      I strap a tarp over the top of the mixer to keep the mix in. Otherwise I leave papercrete allover the road.(ask me how I learned that) There are no issues with the mix swirling w/ an off center blade. That was not a design choice so much as just the result of the differential placement on the axle.

  5. Ty A. Fry
    Posted January 15, 2016 at 2:15 pm | Permalink

    What about using an RV sewer trap valve for your drain?

2 Trackbacks

  1. By No More Trash « Stuff I Made This Year on September 30, 2011 at 7:49 am

    […] years ago I built a papercrete mixer which is essentially a giant blender that you tow behind a truck.  It gets filled with scrap […]

  2. By How to Make Papercrete Blocks - mikeandmollyshouse.com on September 30, 2011 at 8:05 am

    […] Papercrete mixer […]

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