Microanalytical and microstructural studies on the mechanism of sulfate attack
Abstract
The objective of this project was to develop a better understanding of the sulfate attack mechanism and to find effective ways to increase the resistance of portland cement concrete to sulfate attack. The scope of the investigation was: (1) Evaluation of the effects of gypsum formation to determine whether gypsum formation is a possible source of expansion. (2) Evaluation of the roles of cations during sulfate attack. (3) Evaluation of the influence of sulfate solution concentration on the alteration of sulfate attack mechanism. (4) Evaluation of the effects of pozzolans during sulfate attack and the relevant mechanisms. Paste and mortar specimens made of portland cement, alite, and C 3S, with or without pozzolans, were exposed to different sulfate solutions. Their physical and chemical properties were tested periodically. Samples were collected and analyzed using XRD, DSC, EDXA, and SEM. Gypsum was found to form as veins, layers, and rims around aggregate grains. The growth of gypsum crystals may apply strong tensile forces on the surrounding hydrated matrix and lead to expansion and crack propagation. Gypsum formation seems to be one of the factors causing expansion. Mg2+ attack can significantly influence the physical and mechanical characteristics of the specimens by causing expansion, strength loss, dynamic modulus of elasticity loss, and decrease in specific gravity. On the contrary, Na+ ions do not appear to cause any damage during sulfate attack. SO42- concentration in the exposure solution is a factor not only increasing sulfate attack intensity but also altering the mechanism of sulfate attack. With increase in SO42- concentration, sulfate attack mechanism is changed from being controlled by ettringite formation to being controlled by ettringite and gypsum formation. The pozzolans meeting the following criteria can increase the resistance to Na2SO4 attack: high SiO2 content, low CaO and Al2O3 content, high pozzolanic activity, and suitable fineness. Pozzolans may not increase the performance of concrete under MgSO4 attack. Instead, they may reduce the resistance to Mg2+ attack.
Degree
Ph.D.
Advisors
Cohen, Purdue University.
Subject Area
Civil engineering|Materials science
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