[2021] Where to find concrete mix?

Originally posted by minamuckl: you can learn the schematic by leveling in intelligence and advanced engineer, concrete mix can be found around construction sites or you can by from trader sometimes
But you need the cement mixer to craft the blocks anyway.
You can also find working cement mixers at the trader sometimes

To add onto this, While the cement mixer is required to craft the blocks directly, you CAN upgrade other blocks into concrete as long as you have the concrete mix. So you can upgrade, for example, a cobblestone wall into a concrete wall. Likewise, there is the Rebar Frame block that upgrades directly to reinforced concrete with concrete mix.