Minecraft wiki entry for Concrete_Powder, associated text: If a concrete powder block comes into contact with water, it solidifies into a block of concrete. Specifically, the block has to be placed into, placed next to, or fall into flowing water, a water source block, or a waterlogged block. It does not solidify in midair falling past water. If it lands next to water, it solidifies only after a block update. Rain or splash water bottles also have no effect on concrete powder. Concrete powder in item form also does not become concrete. Concrete powder placed directly at water makes the placement sound of concrete.