This paper aims to clarify the influence of the clinker sulfate on external sulfate attack of cement paste. The investigation shows that the clinker sulfate does not have any direct influence on the resistance of cement to external sulfate attack. Indirectly this sulfate reduces the amount of tricalcium aluminate (C(3)A) and tricalcium silicate (C3S), generating an improvement in the level of cement resistance to external sulfate attack. The mortar expansions, after 1 year of storage in 5% sodium sulfate (Na2SO4) solution, verify the mineralogical results. Additionally the investigation indicates that the combination of the three techniques - X-ray diffraction analysis, scanning electron microscopy and graphical treatment of microanalysis by energy-dispersive X-ray spectroscopy - leads to a better understanding of the causes of paste deterioration by external sulfate attack.