Studies on sulfate attack: Mechanisms, test methods, and modeling
Abstract
The objective of this research study was to investigate various issues pertaining to the mechanism, testing methods, and modeling of sulfate attack in concrete. The study was divided into the following segments: (1) effect of gypsum formation on the expansion of mortars, (2) attack by the magnesium ion, (3) sulfate attack in the presence of chloride ions—differentiating seawater and groundwater attack, (4) use of admixtures to mitigate sulfate attack—entrained air, sodium citrate, silica fume, and metakaolin, (5) effects of temperature and concentration of the attack solution, (6) development of new test methods using concrete specimens, and (7) modeling of the sulfate attack phenomenon. Mortar specimens using portland cement (PC) and tricalcium silicate (C 3S), with or without mineral admixtures, were prepared and immersed in different sulfate solutions. In addition to this, portland cement concrete specimens were also prepared and subjected to complete and partial immersion in sulfate solutions. Physical measurements, chemical analyses and microstructural studies were performed periodically on the specimens. Gypsum formation was seen to cause expansion of the C3S mortar specimens. Statistical analyses of the data also indicated that the quantity of gypsum was the most significant factor controlling the expansion of mortar bars. The attack by magnesium ion was found to drive the reaction towards the formation of brucite. Decalcification of the C-S-H and its subsequent conversion to the non-cementitious M-S-H was identified as the mechanism of destruction in magnesium sulfate attack. Mineral admixtures were beneficial in combating sodium sulfate attack, while reducing the resistance to magnesium sulfate attack. Air entrainment did not change the measured physical properties, but reduced the visible distress of the mortars. Sodium citrate caused a substantial reduction in the rate of damage of the mortars due to its retarding effect. Temperature and concentration of the solution were seen to change the rate and mechanism of the attack in both sodium and magnesium sulfate solutions. The test results from these experiments were used to generate models for prediction of physical properties such as expansion and mass change, which could be used either for service life predictions, or for designing more reliable laboratory tests. Lastly, mechanisms for the attack by sodium and magnesium sulfate were proposed, based on the observations in the various studies. These mechanisms were able to simplify the understanding of the sulfate attack phenomenon.
Degree
Ph.D.
Advisors
Olek, Purdue University.
Subject Area
Civil engineering|Materials science
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