Currently, there is a huge demand for mechanically crushed rock, and a huge excess of mixed plastic waste.
Mechanically crushed rocks grip together when compressed, unlike smooth river stones and cobbles that slip apart. This makes them a crucial engineering material that adds strength and longevity to concrete mixes and the engineered soil surfaces which are prepared beneath foundations and roads. Unfortunately, it takes huge amounts of energy to break the stones apart.
Mixed plastics are our poorly sorted recycling waste. In order to produce high quality recycled plastic from mixed plastic, more sorting and lots of energy are required. We in the US have a lot of this waste because China has recently stopped taking it.
Can we take the mixed plastic, melt it down, combine it with a source of existing sand, form it into shapes that mimics or improves upon crushed stone and in so doing. produce a synthetic crushed stone? I think it is an idea worth exploring. You see, the quality of the plastic is likely to be compromised because it is unsorted, but the produced synthstone can be quality sorted by attempting to press it through a steel grate with gaps that are smaller than the desired stone size. The product that breaks and is pushed through the grate is rejected and either reprocessed or used for some non-structural fill.
Such production of synthetic stone could be incorporated into existing crushed stone production process, by using fines which are washed away from clear crushed stone as a waste product in place of the sand, and the synthetic stone could then be blended with standard crushed stone.
I’m not an expert on plastic manufacturing, but my first guess approach would be to heat sand up to the minimal mixed plastic melting temperature in a large rotating barrel, then introduce chunked mixed plastic, and rotate until a synthstone dough is formed. Then load the dough into a die extruder and produce the desired mechanical shapes by slicing a die formed extrusion. I’d call this the synthstone gnocci process, if it was effective.