Nowadays, environmental mitigation remains a key issue challenging sludge combustion toward its clean utilization. Herein, two opposite additives, CaO and kaolin, representing two different strategies, were employed for environmental mitigation of sludge combustion. An advanced TG-MS (thermogravimetric-mass spectrometer) system was used. The online MS analysis indicated that kaolin remarkably decreased the release of NOx whereas the fixation effect of CaO on SO2 was more pronounced. Furthermore, the experiments using a fixed tube furnace proved that overall kaolin had a better role controlling volatility of trace elements (Cr, Ni, Cu, Zn, Pb, As and Ba) than CaO and we found that the fixation mechanism of CaO changed at 750 degrees C, whereas that of kaolin changed at 500 degrees C. Then the kinetics of the various stages during sludge combustion were investigated. We found that low temperature stages were along with the formation of new phases, whereas, at high temperatures, the diffusion could be rate-controlling step. The present study thus provides not only a deep understanding of sludge combustion mechanisms but also two effective modifying strategies toward the environmental mitigation of sludge combustion.