Introduction: Cast Concrete Pendant With Recycled Plastic

About: Huur... derrr.

Plastic is a big problem for recyclers. Falling oil prices means that making new plastic is sometimes more economical than collecting and recycling it. It's a very labor-intensive process to separate and clean all that material.

I started thinking about direct uses for post-consumer plastic waste. The problem with industrial plastic recycling is that it takes a lot of energy and resources to turn reprocess it. The solution is to find uses for plastic waste that requires little or no further processing. The idea of using plastic as an aggregate fill in concrete seemed like a logical step. The problem with using plastic in concrete is that it is considerably weaker than the surrounding material. This Would weaken the resulting concrete making it unusable for structural purposes. The idea of using shredded colorful plastic waste as an aggregate and then polishing the surface held promise as a decorative technique. I did a test casting here, but I wanted to find a practical application for the technique. Hmm... Jewelry! I figured a ring would be too dainty and earrings would be heavy. I thought about beads, but that would take a few things I don't have access to. A necklace seemed like an easy proof of concept.

Here's my cast concrete pendant with recycled plastic aggregate.

Step 1: Making the Molds

I used the side panel of an old computer tower as a base to work on as concrete doesn't stick to the powder coated surface. I used PVC rings with a split cut as molds. I taped the cut closed and when the concrete hardens it can be opened like a spring form cake pan. I made multiple molds as this an experiment and I'm not completely sure what I'm doing or what the tolerances of the material are. Combined with the completely random distribution of the plastic pieces, this could lead to breakage later in the process. Best to hedge my bets.

Step 2: Mixing the Concrete and Pouring

I mixed about a quart of Cheng Shapecrete to a smooth, pourable consistency, as per the directions. I added several handfuls of shredded, colored plastic, mostly spray paint lids. I mixed it smooth and filled a few of the molds. As I filled the mold, I held a straw in place to provide a hole to string the pendant later.

The first few molds didn't seem to be settling down so I added a bit more cement and even more water to make a mix with more cement with a thinner consistency. I poured the rest of the molds and tapped the table to release the air bubbles. This seemed to work much better. I set the molds aside to harden overnight.

Step 3: Removing the Mold and Polishing

I removed the tape from the hardened molds and popped off the pipe rings. I pulled out the straws and checked the results of the casting. As I suspected, the first few molds I poured were full of voids from air bubbles. The later pours were much better, I used my bench top belt sander to sand down the flat surfaces and round over the edges. I had a really nice shape going when the sander exposed a large piece of plastic under the surface and gouged out a big hole in the piece. I grabbed another mold and tried again.

The second piece went better and I ended up with a smooth 1 1/2"x1/3" disc with colorful flecks and a few minor air bubbles. I hand sanded it from here on with 100, 220 and 440 grit sandpapers. I finally wet sanded it in gentle circles building up a paste of cement dust that acted as a further abrasive to smooth the pendant. When I was satisfied with the surface texture- smooth enough to be comfortable but rough enough to be interesting- i washed it and let it dry completely in front of a fan.

Step 4: Sealing and Stringing the Pendant

I threaded the pendant back onto a full length straw and suspended it between two spray paint cans. I gave it several coats of clear spray glaze to give it a smooth, glossy finish. When it was dry I knotted it on a length of black cord. Ta da- a cast concrete pendant with recycled plastic inclusions.

I know that funky necklaces won't solve the recycling crisis but it's a step. What else could be done with this technique? Imagine a concrete bar top or counter with tons of colorful plastic bits and a 1/8" poured epoxy top coat. For that matter you could skip the concrete and use the shredded plastic in an epoxy suspension. to mold nearly anything. Furnishings, home decor, stepping stones, non-load bearing block and curbing- There's a huge market waiting to be tapped- go get a piece.

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