Sewage sludge was solidified with soda residue (SR), ground granulated blast furnace slag (GGBS), and quicklime, which was used as a temporary covering material for landfills. The effect of leachate concentration, soaking time, and SR content on the long-term properties of solidified sludge was carried out. The micro mechanism of long-term performance was investigated. The results indicate that hydration products such as ettringite and calcium silicate hydrate are generated when the solidified sludge is soaked in the leachate. The strength of the sample increases with the increase in soaking time, indicating that the solidified sludge has good durability to the leachate. Nevertheless, a large amount of calcium carbonate and ettringite of different sizes are generated in the sample with 60% SR content after being soaked for 210 d. Then the structure becomes loose, and its long-term strength reduces to lower than the strength requirement for temporary cover. Too much SR content is not good for the long-term strength of solidified sludge. The strength of solidified sludge decreases as the leachate concentration increases due to the corrosive effects of leachate. The solidified sludge is structural, and the changes in the yield stress under leachate soaking are basically similar to the strength. During the long-term leachate soaking, SR-GGBS-quicklime can stabilize some pollutions such as chromium, copper, lead, and arsenic in the sludge and leachate.