Influences of exposure condition and sulfate salt type on deterioration of paste with and without fly ash

• The deterioration of paste exposed to external sulfate attack is investigated.
• The influences of exposure condition and sulfate salt type are taken into account.
• The damage is evaluated by compressive strength, mass loss and Vickers hardness.
• The damage mechanisms of chemical and physical sulfate attack are analyzed.
• The influence of fly ash on different kind of sulfate attack is investigated.

The aim of this study is to investigate the influences of exposure condition and sulfate salt type on the deterioration features and damage mechanisms of paste exposed to external sulfate attack. Cement pastes with w/b ratios of 0.35, 0.45 and 0.55, and fly ash pastes with the w/b ratio of 0.45 were exposed to sodium sulfate solution and magnesium sulfate solution with different exposure conditions. Four exposure conditions, referenced as CI (continuous immersion), DWCI (dry-wet cycle immersion), PIAWL (partial immersion above the water level) and PIUWL (partial immersion under the water level), were designed for the exposure tests. The compressive strength, mass variation and Vickers hardness (VH) profiles of specimens were measured to evaluate the damage of the paste specimens. Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD) were used to analyze the degradation mechanisms. As the results shown, pastes exposed to external sulfate attack with different exposure conditions suffered from different damage mechanisms, including chemical sulfate attack and physical sulfate attack. The highest damage was observed on the specimens with the PIAWL condition when they were exposed to sodium sulfate solution. While in the cases of magnesium sulfate attack, the highest damage was found on the specimens with the DWCI condition. For chemical sulfate attack, a suitable content of fly ash in pastes could increase the resistance to sulfate attack, while the addition of fly ash might accelerate the damage when the specimens suffered from physical sulfate attack.