[sdenton4]

In grad school I knew a guy who was reeeeally into the idea of building benches out of reused plastic and cob. The basic design was plastic bottles stuffed with plastic bags acting as bricks, which are then cemented together with cob.

Cleaning the bottles and bags was incredibly difficult, even at the scale needed for a single bench. Just a massive amount of effort... The difficulty was that any residual bacteria would multiply and off-gas, eventually causing structural problems.

Likewise, I expect the biggest problem for a home-LEGO-recycler would be dealing with the many random impurities in the input stream.

[TylerE]

LEGO is mostly ABS so its mechanical properties are a LOT better than the cheap stuff used for bottles.