Experimental studies on cement paste exposed to external sulfate attack (ESA), delayed ettringite formation (DEF), and the coupling effect of both reactions are presented in this paper. The objective is to compare the degraded state of specimens, the length and mass variations, and the pore size distribution (PSD) obtained by mercury intrusion porosimetry (MIP) of cement paste submitted to these different exposure conditions. By comparing the PSD of specimens before and after the sulfate attacks, a global expansion mechanism is proposed : ettringite first precipitates in the biggest pores without inducing obvious expansion, and then penetrates into capillary and gel pores leading to an accelerated swelling. In addition, the coupling effect of ESA and DEF is found to be the most damaging expansion, which raises a high durability problem for cement-based materials.