Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder(w/b) ratio made a significant sense to the damage that low w/b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results demonstrated that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C-(A)-S-H, and thus, the damage of pastes hardly appeared while at later ones Al had been largely transferred from C-(A)-S-H into AFt, leading to expansive damage. Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder (w / b) ratio made a significant sense to the damage that low w / b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results form that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C- (A) -SH, and thus, the damage of pastes years Now at at later ones Al had been substantially transferred from C- (A) -SH into AFt, leading to expansive damage.