AbstractCausticizing calcium carbonate (CCC), known as lime mud, is a by-product of the papermaking industry that comes from the green liquor causticizing process. In China, CCC has been used as a paper filler to replace precipitated calcium carbonate (PCC). This is beneficial for saving resources and preventing secondary pollution. Unfortunately, compared with PCC, CCC can reduce the sizing efficiency of alkyl ketene dimer (AKD). So the application scope and dosage of CCC has been limited in mill trials in China. In this study, CCC was prepared with green liquor and quicklime, which were obtained from an alkali recovery line of a pulping mill. The reason for the lower sizing efficiency of AKD when CCC was used as a filler was investigated. The results showed that when greater amounts of AKD were adsorbed by CCC, the AKD sizing efficiency was lower. The irregular CCC had higher (BET ) surface area and Barrett-Joyner-Halenda (BJH) method cumulative desorption pore volume, resulting in higher adsorption. The spindle-like or needle-like CCC had lower BET surface area and BJH method cumulative desorption pore volume that was beneficial for controlling its adsorption of AKD and improving the sizing efficiency.