Here we systemically elucidated the compositions and distributions of polluting materials during sludge combustion using both experimental and theoretical methods where the roles of ash were clarified in a category within aluminosilicate system. With regard to the polluting gases, an increasing temperature increased the SO2 release in replace of SO3 and sulfates while continuously enhanced the formation of NO and NO2. Comparatively, an increasing input oxygen amount had limited effect on S-bearing gases whilst continuously increased the NOx release. Additionally, the ash had a sulfur fixation effect through inducing the transformation from oxides to sulfates. The trace elements were overall categorized into four types based on their volatilities and the distributions and mineral phases of them were then clarified in detail. It was proved that with increasing temperature, they were increasingly distributed in gaseous state with ash providing a fixation effect. Moreover, 5 kinds of decomposition reactions related to polluting materials, namely sulfates, oxides, arsenates, silicates and hydroxides, were deeply analyzed from a respect of Gibbs energy where their stabilities and roles of ash were identified. Furthermore, the sludge combustion was experimentally clarified using a TG-MS system and here we defined a new parameter, namely peak index, to characterize the releases of polluting gases. This not only deepened the understanding of sludge combustion experimentally but also partially validated the theoretical analysis in quantity. Most importantly, the present study contributed to establishing a foundation for controlling and mitigating the environmental impacts of polluting materials toward efficient and clean sludge combustions.