An assembly which is adapted for mounting inside a mixing drum (18) and a mobile system for mixing and dispensing concrete includes a spiral fin assembly (36) for mixing and guiding a substance when the mixing drum (18) is rotated. The fin assembly (36) is constructed of a lightweight polymeric material which is resistant to abrasion, is many times lighter than steel, and tends to wear smooth rather than rough, which increases the cleanability of the fin structure over its entire design life. Novel structure for securing the fin assembly (36) to an outer wall (38) of the mixing drum (18), novel structure of fin assembly (36), and novel structure for connection of adjacent fin sections are also disclosed.