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The formation and transformation characteristics of ettringite played an extremely important effect on the accelerated hardening and expabsion of concrete as well as its long term durability.The stability and its decomposition mechanism of ettringite in alkaline (NH4)2SO4 solution were investigated.Results show that the stability of ettringte is largely influenced by the concentration: ettringite decomposes when the concentration is more than 0.090mol/L and it partially decomposes when the concentration of (NH4)2SO4 is between 0.021-0.090mol/L;and it remains unchanged when the concentration of (NH4)2SO4 is less than 0.021mol/L.Which shows that ettringite is not stabile in (NH4)2SO4 solution, and decompose and produce gypsum and Al(OH)3 as long as the concentration of (NH4)2SO4 reaches to a certain extent.Further experiment on the idenfication of reaction products formed in the surface area of the hardened cement mortar specimens respectively immersed in (NH4)2SO4 solution and Na2SO4 solution again showed that the ettringite detected in the former specimen was less than that of the latter.