External sulfate attack (ESA) and delayed ettringite formation (DEF) are well-known concrete degradation phenomena induced by ettringite crystallization. This paper proposes a poromechanical model based on the surface-controlled ettringite growth mechanism and the chemical composition of the solution, explaining both ESA and DEF in a similar model. The driving force for this expansion is the crystallization pressure from the supersaturation of the sulfate solution that leads to the formation of ettringite in pores. The growth of ettringite is separated into two processes: (1) the pore invasion process, where ettringite forms in the existing largest pores and then penetrates progressively into the ever-smaller capillary and gel pores, and (2) the deformation process during which damage and cracks occur. Two kinetic constants corresponding to these two processes are proposed in this paper, and the predicted expansions show good consistency with the experimental measurements found in the literature.