Abstract: The soil-cement is one of materials that used for ground treatment in soft soil engineering. In the high degree of mineralization zone , the ettringite(3CaO•Al2O3•3CaSO4•31H2O ), which is the reaction product of hydration of cement(AlO3-,Ca2+) and SO42-, can cause expansion and crack to soil-cement and makes the strength of soil-cement decrease. The activated fly ash is mixed into the soil-cement, which can set free the activated SiO2 to increase the concentration of SiO32- and promoted the reacting speed of hydrated calcium silicate(CSH). Meanwhile, the probability of formation of ettringite is decreased. The expansion inhibiting mechanism of soil-cement is that the activated fly ash absorbs Ca2+ and changes the reaction process; and the reaction product (CSH) fills the hole of soil-cement with its micro-granule. The mechanism of activating fly-ash is the powdery sodium silicate destroys the high network polymerization surface of fly-ash, reaches the inner part of the fly ash and makes the active substance to set free.

Key words: soil-cement, sulfate erosion, activated fly-ash, inhibiting expansion