The Red Concrete is a muted brick-red building-block. It can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that still exist in worlds created before R22 on September 16th 2015), but only after the crafting-recipe has been unlocked upon taking a (block of) Siltstone for the first time (like by mining it from the Stalactite layer with at least an Obsidian Mining Cell or harvesting from a Warmworm-pet). To craft 8 (blocks of) Red Concrete at a time, you'll need (as of R29 in February 2016):