Additions of slag are usually considered to give cementitious materials which perform well in sulfate bearing environments. This paper compares the deterioration of slag cement blends to those of plain Portland cement and demonstrates that this occurs more through loss of surface than macroscopic expansion. When slag blended mortar is immersed in sodium sulfate solution, the sulfate ions penetrating into samples are mostly fixed by aluminate phases in a relatively narrow region close to surface, due to the refinement of pore size and buffering effect by slag addition. After all available aluminate phases have been reacted, the concentration of sulfate ion in pore solution will increase. Once the solution concentration reaches a critical level, fine AFm crystals confined within the C-S-H can react to form ettringite, exerting expansion force. If the system contains sufficient confined AFm phases, this process can cause spalling of the surface layer. Then, sulfate ions can penetrate into the sound area, manifesting another expansive area. Furthermore, the penetration depth of slag sample does not depend strongly on the concentration of sulfate solution, but higher concentration increases the crystallization pressure of ettringite, thereby causing more damage. (C) 2015 Elsevier Ltd. All rights reserved.