Abstract: The behavior of silica fume cement paste against sulfate at-tack has been studied. The experimental method used is long immersed tests, in which the hardened cement paste specimens are immersed in the 5% NazS04 solution. The strength change of the specimens with the time has been measured and phases of the specimens have been analyzed by XRD. Results indicate that
the strength of the OPC paste specimens increases at an early stage and then decreases greatly following sulfate attack. The appearance and XRD analysis have indicated that the cause of their deterioration is the expansion of ettringite and the obvious reduclion of strength loss in silica fume cement paste specimens. The bending strength does not reduce, while the decrease coefficient of strength increases a little. The factor is that the Ca( OH ) 2 in
the cement paste is reduced and the formation of expansive ettringite against sulfate attack is decreased yet by the dilute effect and the pozzolanic reaction of silica fume. Therefore, the silica fume may be mixed into cement concrete to improve their anti-sulfate attack.

Key words: cement paste, sulfate attack, silica fume, ettnngite