Method of producing filler

A method of producing filler, comprising precipitated calcium carbonate (PCC), said method comprising the steps of,

• providing fly ash generated in paper or paper board production, the fly ash comprising calcium oxide;
• fractionating said fly ash in at least one step, whereby a first fraction is separated from a second fraction, the first fraction comprising particles that are coarser compared with particles in the second fraction wherein the first fraction has an average particle size greater than 50 μm and less than 100 μm, and the second fraction has an average particle size of 50 μm or less;
• grinding said first fraction prior to the step of forming a suspension, wherein said first fraction is ground to exhibit an average particle size of between 10 to 20 μm;
• forming the suspension of only the grounded first fraction having the average particle size of between 10 to 20 μm, wherein the calcium oxide of the fly ash forms calcium hydroxide, wherein the suspension of the grounded first fraction further comprise a furnish that includes cellulose fibers;
• and,
• adding carbon dioxide to said suspension of grounded first fraction comprising the furnish to form precipitated calcium carbonate in the presence of the cellulose fibers.