Clay and other concrete based bricks must be cheap enough already, and work well.
If we intend to reuse plastic in bricks, maybe use those bricks for building things in and around landfills or things like storing nuclear waste - so that toxicity is concentrated. Otherwise, plastic bricks seem like one of those things that will be discovered to be toxic eventually.
The world is allegedly running out of sand appropriate for use in concrete products. 99% Invisible (podcast) has a episode on the topic I suggest checking out https://99percentinvisible.org/episode/built-on-sand/
If we intend to reuse plastic in bricks, maybe use those bricks for building things in and around landfills or things like storing nuclear waste - so that toxicity is concentrated. Otherwise, plastic bricks seem like one of those things that will be discovered to be toxic eventually.