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Two different mechanisms have been proposed for the formation of ettringite, i.e.through-solution reaction and topochemical reaction. In this paper, the formation mechanism of secondary ettringite in concrete due to sulfate attack was reviewed and analyzed. It was deduced that the formation mechanism of secondary ettringite is mainly by topochemical mechanism. The sample made from AFm (4CaO.Al2O3.CaSO4. 12H2O) and Ca (OH)2 was immersed in 5% sodium sulfate at 20 ℃ for 20 days. X-ray diffraction (XRD) was used to analyze the sample. The results verified that the formation mechanism of secondary ettringite should be attributed to topochemical reaction.