AbstractLime sludge is a special type of precipitated calcium carbonate (PCC) recovered from black liquor, which can be used as paper filler in the paper-making process. However, one of the biggest problems when lime sludge is used as filler is that it is difficult to hydrophobically size the filled paper, especially in the case of paper with high filler content. Also, the efficient retention of the sizing agent AKD is a fundamental requirement for effective paper hydrophobic sizing. Therefore, in this work, a ternary retention aids system, cationic starch/CPAM/bentonite, was applied, and the hydrophobic sizing degree of lime sludge filled paper sheets and filler retention were evaluated. The results showed that the retention of lime sludge was significantly influenced by CPAM and was overall increased as the dosage of CPAM increased; cationic starch showed a more significant influence on paper Cobb value than filler retention; relative lower Cobb values and higher filler retention were achieved at lower bentonite dosage. At 0.75% cationic starch, 0.043% CPAM, and 0.3% bentonite, a substantial high hydrophobic sizing efficiency of Cobb value (45.72 g/m2) and filler retention (80.37%) were achieved simultaneously due to the synergetic performance of a cationic starch/CPAM/bentonite retention aids system.