The pore crystallization of ettringite (AFt) and the material expansion are investigated for cement paste subject to external sulfate attack with and without chlorides. Specimens of 2mm thickness were exposed to solutions of 10g/L Na 2 SO 4 , 10g/L Na 2 SO 4 + 10g/L NaCl and 10g/L Na 2 SO 4 + 19g/L NaCl. The evolutions of AFt formation, pore structure, transport property and specimen expansion were characterized through 27 Al NMR, MIP, DVS and micrometer measurements. The analysis of results shows that: (1) chlorides decrease the AFt formation rate and the expansion capacity through depressing the supersaturation of AFt crystals; (2) AFt crystallization alters the pore structure in the whole pore size range, especially through the blockage of "ink-bottle" pores in micropore range; (3) AFt crystals change the vapour diffusivity by pore-filling and microcracking, and the linear stage of expansion is well captured by poromechanics with pore crystallization pressure incorporated.