Mitigation of DEF in concrete: morphology and composition studies

Abstract

Experiments were performed to study the effects of select chemicals on the mitigation of delayed ettringite formation in Portland cement concrete (PCC) specimens. Specimens were prepared, conditioned and chemically treated with crystal growth inhibitors or waterproofing agents. They were then cured and physical properties were evaluated as a function of curing time. Reaction products were evaluated using scanning electron microscopy and energy dispersive X-ray fluorescence analysis techniques. While the chemicals did not mitigate delayed ettringite formation, the systems did yield large amounts of secondary ettringite of various morphologies. The lack of mitigation of delayed ettringite formation may be attributable to an overabundance of ettringite resulting from intensive conditioning protocol and/or too low a dosage of crystal growth inhibitor or waterproofing agent. Characterisation of the systems revealed some trends in morphological development, for example the samples cured for long times had a preponderance of lamella and laths involved. These and other morphologies observed were consistent with the literature, especially those studies based on scanning electron microscope evaluations of fracture surfaces. Chemical compositions, determined by energy dispersive X-ray fluorescence analysis, regardless of treatment or curing time, were generally quite similar, showing reasonable ratios of calcium: sulfur: aluminium. While delayed ettringite formation mitigation was not achieved, the results demonstrate the usefulness of scanning electron microscopy/energy dispersive X-ray fluorescence analysis examination of fracture surfaces for the evaluation of morphological and chemical features in concrete durability studies.