Calcium contributes to scaling in various operations in a pulp and paper mill. CaCO₃ is water insoluble, and has inverse solubility with temperature. Consequently, it forms on heat transfer surfaces such as digester heaters and black liquor evaporators. CaCO₃ scales are also seasonal, and not easy to deal with. Calcium enters the mill through wood/ bark, chemical make-up and water. Depending on the type of wood, the method of pulping used and the efficiency of clarification for the white liquor, the calcium concentration can be high in the digester. Scaling is initiated when the concentration of free Ca and free carbonate exceed the solubility limit, to form calcite (CaCO₃). In green liquor processing, where large amounts of Na₂CO₃ are present, pirssonite (Na₂CO₃.CaCO₃.2H₂O) forms.

Calcium can form water soluble compounds with components of black liquor. Many of these compounds are not thermally stable and break-up at high temperatures, resulting in supersaturation of CaCO₃ in black liquor, which leads to scaling. The scaling tendency is high in the temperature range 110-140 C, where most evaporators operate. There is a patented method for calcium deactivation, in which black liquor is heated and stored at high temperatures to break up the calcium compounds and reduce scaling tendency

Calcium is purged with tall oil soap, the dregs, the grits and the pulp in the recovery cycle. CaCO₃ scaling can also occur in bleaching in stages employing high pH. In the bleach plant, the acid sewer is rich in calcium and other metals. If acidic effluents are recycled to the recovery cycle, a metals removal system is required.