Sludge is rich in organic matter

which would generate humic acid after decomposition. Humic acid will affect the reaction process and strength of cement-treated soil. The unconfined compressive strength (UCS) of sludge treated with 12% cement may decrease after 60 days curing. If adding both of lime (3%) and cement (12%) into sludge

the pH value of treated soils is higher than 10.5 after 180 days curing

and the UCS increases from 750 kPa (cured 28 d) to 1 500 kPa (cured 120 d)

which indicates that lime can improve the UCS of treated sludge significantly through establishing suitable strong alkaline environment. However

the UCS of treated sludge reduces to 1 250 kPa after 180 days curing

which may ascribe to humic acid dissolving hydration products of lime and/or cement. The calcium ion concentration increases with the elapsing of curing period

which could explain the behavior of humic acid attacking to hydration products. Metakaolin contains lots of amorphous silicon and aluminum oxides

with the addition of 3% metakaolin into sludge based on mixing with lime (3%) and cement (12%)

the UCS increases in the whole curing period (180 d). The results show that metakaolin can capture the calcium ion in short time and form stable cementitious materials in solidified soils. And these cementitious materials have strong anti-erosion capacity to humic acid effect. These above findings prove that metakaolin can improve the long-term performance of cement treated sludge.