The objective of this work is to compare the removal of Ca2+, Mg2+, Na+ and K+ by kaolinite and carbon-materials prepared from pyrolysis of sewage sludge mixed with kaolinite. Three carbon-based materials were prepared by pyrolysis of the sewage sludge, the sludge with a 5% of kaolinite and the sludge with a 10% of kaolinite at 650 degrees C, leading to SL-650, SL-650-5 and SL-650-10, respectively. The addition of kaolinite to the sewage sludge increased the cation exchange capacity of the final carbon-materials prepared by pyrolysis process. The increment raised to 37.6% and 45.3%, respectively, in the SL-650-5 and SL-650-10 with respect to the SL-650. The kaolinite and the carbon-based materials were mixed with solutions with different concentrations of Ca2+, Mg2+, Na+ or K+ (100 and 200 mg L-1). Results shown that the highest was pH, highest was the percentage of Ca2+ and Mg2+ removal by kaolinite and carbon-based materials. Nevertheless, the removal of Na+ and K+ was negligible. Results show that kaolinite and carbon-materials prepared by pyrolysis of mixtures of sewage sludge and kaolinite can be effective to the to reduce the hardness of water.