A mixer apparatus for mixing a high-viscosity fluid has a mixer shaft with a plurality of blades affixed thereto, a hydraulic motor drivingly connected to the mixer shaft so as to rotate the mixer shaft, a hydraulic pump connected by a fluid circuit to the hydraulic motor so as to deliver hydraulic fluid under pressure to the hydraulic motor, an electric motor drivingly connected to the hydraulic pump, and a hydraulic fluid reservoir connected to the fluid circuit so as to supply hydraulic fluid to the hydraulic pump. The plurality of blades are pivotally mounted to the mixer shaft.