Biochar is getting increasing attention due to good performance in carbon retention and strong stability, and has been proposed as a promising material for long-term carbon sequestration. In this study, sewage sludge was used as feedstock, and Ca(OH)(2) was added to improve carbon stability in biochar through pyrolysis at temperature range of 300-700 degrees C. A systematical analysis was conducted for the sludge-derived biochar with and without Ca(OH)(2) addition, with respect to physiochemical properties, molecular structure, and chemical stability. Results indicated an obvious increase of DOC content and carbon retention in biochar when the Ca(OH)(2) was added in the sludge feed. The CaCO3 formation and an increase in carbon-containing functional groups were evidenced from X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra, respectively. Furthermore, the amount of carbon loss decreased over 3 times when H2O2 was added to the biochar as chemical oxidant, showing an obvious increase in carbon stability with mineral additive. Moreover, the Ca(OH)(2) addition also improved the surface area and alkalinity of the biochar, which may enhance the potential application of biochar products. Therefore, from effective increase in carbon retention and biochar stability, a novel idea can be provided for further production of sewage sludge-derived biochar with high carbon sequestration capacity and stability, and will help to further promote the utilization of pyrolysis as a promising strategy for the management of bio-waste. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.