Kinark Sustainable Living Centre

My Summer as a Mixmaster

On the first day at the construction site, if Chris Magwood had pointed to the dump truck load of clay that had just arrived and told me that I was to spend the summer shoveling it and three more mountains like it into a mortar mixer, I might have had second thoughts about why on earth I was paying money to do this.

Chris Magwood was our teacher for the Sustainable Building Design and Construction class at Fleming College, and our task this summer was to design and then build a sustainable classroom for Kinark Outdoor Centre, near Minden, Ontario, Canada.

As it turned out, we got our money’s worth of education and experience, as well as plenty of exercise and fresh air.

In designing the building for a lower environmental impact, we wanted to minimize the energy consumed and greenhouse gases produced.

One low-hanging fruit to focus on is cement. The manufacture of Portland cement is an energy intensive process and it adds 200 million tons of carbon dioxide annually to the atmosphere. For every ton of cement produced, one ton of carbon dioxide is emitted.

We chose to use locally available clay as a substitute. Clay is naturally formed by chemical weathering of rocks into very fine particles. When clay is mixed with water it coats larger particles such as sand. When the mixture dries, it forms a hard material similar to concrete.

Because of the large quantity of material that needed mixing, we rented a gasoline-driven mortar mixer for the summer. While we could have made our mixes in a wheelbarrow with a hoe using lots of muscle power, time constraints dictated that we use a machine. Still, test batches were always made by hand in a wheelbarrow or pail.

The first task was to make clay slip, a thin mixture of clay and plenty of water. This was where my initiation to mixing began.

We dipped two sides of straw bales in a bathtub full of clay slip and built up the walls using these bales. Having a mud-filled bathtub available at all times helped keep participants in line. Dunking “deserving” students in said mud-filled bathtub was a standard disciplinary measure.

One day I found my hands free and volunteered to take over mixing of the slip, which had to be about the consistency of a milkshake, thinly coating the straw. Too much water and the clay would not cover the straw; too little water and the bales would become unnecessarily heavy with too much clay. By sticking my hand into the mix and observing how it coated my fingers and dripped, I could tell when it was just right. With practice, I could tell when the mix was ready by watching how the slip flowed as the paddles stirred away in the mixer.

Operating and filling the mixer required constant vigilance. Because the clay was dug right out of the ground, it contained stones and tree roots that could cause the mixer to jam. In hindsight, we should have screened the clay for anything large enough to jam the mixer, though this would have required an extra person.

Much of my time was devoted to mixing two different types of plaster: the push coat and the finish coat. Sand was used as the main aggregate, in a ratio with water that would provide maximum workability with minimal cracking upon drying. The push coat was used to build up the plaster to about one inch (2.5 cm) of thickness over the bales, inside and outside the building.  The finish plaster coat was applied on top to make a smooth surface.

In addition to mixing clay and sand, we also made hempcrete by mixing Portland cement, lime, water, and chopped hemp fibres to create a spongy, lightweight insulative fill.

During the summer of building, I gained an appreciation for using simple, natural materials that have been used by humans for millennia. I learned that with all of our senses, including taste, we can quickly evaluate the quality of an earthen mix.

The actual process of mixing can be an almost Zen-like experience. Watching the paddles churn the mix can be hypnotic. The ear protection required when using a noisy mechanical mixer blocks out all other sounds on the job site, and can be almost peaceful.

There is ample opportunity for backyard research into the properties of natural building materials. For example, when fermented straw is added to the plaster mix (the more pungent the straw the better), the resulting plaster takes longer to dry and seems to become harder. Perhaps it is the sugars formed, but no one knows. A weekend experimenter willing to get a bit dirty with a shovel could discover valuable properties of natural building materials without any advanced degrees or teams of graduate students.

In the end, we need not fear simple earth as a building material. It’s possible to create lasting, comfortable modern structures that are easy to maintain and have minimal environmental impact.

David Elfstrom is a professional engineer living in Toronto who is fascinated by the science and practice of natural and sustainable building. You can reach him at david@elfstrom.com

 

Elfstrom’s mix recipes (for large quantities)

Clay slip for bale dipping
18 L of water
Add 20-22 spade shovels of clay, until it coats hand like a latex glove and doesn’t stick between fingers, and slowly drips off fingertip. Make slightly thicker for coating straw to stuff into holes.

Push coat of plaster
8 L of water
16 shovels of clay
11 shovels of sand
1 ‘wafer’ of a wheat straw bale sprinkled into final mix as a binder.
Mix water and half of the clay, add the sand, then the rest of the clay. Sprinkle the straw strands on top while the mixer is running.

Finish coat of plaster
10 L of water
3-4 whole cattails, split open to separate the seeds. This acts as a binder.
16 shovels of red brick fired clay (normally used for tennis courts)
20 shovels of sand
Mix water and cattails, add half of the clay, add the sand, then the rest of the sand.

Insulative hempcrete mortar
Nearly one 16L bucket of water
Nearly one 16L bucket of Portland cement & lime powder (50-50 mix is best, more Portland makes it dry faster and more lime makes it less crumbly)
3 heaping 16L buckets of hemp fibre
Make slurry of water and cement/lime mix, add hemp fibre, and top up mix with any extra water if needed. Clean out mixer to prevent damage from dried cement.

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