Abstract: A method for reducing the carbon content of a combustion ash containing siliceous ash particles and carbon particles includes subjecting the combustion ash to comminution to reduce the particle size of the siliceous ash particles and the carbon particles followed by separating a portion of the carbon particles from the siliceous ash particles by subjecting the comminuted ash to a flow of gas which separates the carbon particles from the siliceous ash particles. The method preferably is performed in a unitary housing by employing autogenous grinding to effect the comminution of the particles and employing a classifying rotor to separate the comminuted portion of the carbon particles based on the density differential between the siliceous ash particles and the carbon particles. A method of making a concrete mixture employing the obtained combustion ash with reduced carbon content is also provided. An apparatus for reducing the carbon content of the combustion ash is also provided.