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Title: The deterioration of portland cement paste exposed to sodium chloride environments
Author: Zaman, M. S.
ISNI:       0000 0001 3396 3610
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 1979
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The use of de-icing salts such as sodium chloride and calcium chloride on concrete surfaces has promoted a considerable amount of research interest. Despite the extensive amount of literature covering the wide ranges of investigations on the strength characteristics of cement paste and concrete subjected to chloride environments, very little has been published regarding such investigations with sodium chloride when present in different extreme environmental conditions, although sodium chloride is most widely used as a de-icing chemical. In this thesis the effects of alternate freeze/thaw and wet/dry cycles in presence of sodium chloride solutions of different concentrations on the strength and microstructure of hardened portland cement pastes of varying maturity have been investigated. Microstructural changes were detected by scanning electron microscopy, x-ray spectrometry, mercury intrusion porosimetry and differential thermal analysis. Attempts have been made to relate the observed microstructural features and their subsequent effects to the strength behaviour of the paste specimens. Ordinary portland cement paste specimens of 0.3 and 0.4 water/cement ratios were fixed for the investigations. The freeze/thaw cycles alternated between 0°C and -18°C. The effects of freeze/thaw and wet/dry cycles in different sodium chloride solutions on the strength of mortar specimens of different sand/cement ratios and water/cement ratios were also studied in comparison with the effects already observed in the paste specimens. It was noticed that cement paste of both 0.3 and 0.4 water/cement ratios cured normally for 3 and 7 days before being subjected to freeze/thaw cycles in different salt solutions suffered loss of compressive as well as flexural strengths that was not completely recoverable even after prolonged subsequent normal curing. The loss in flexural strength was more than that in compressive strength and freeze/thaw cycles in saturated solution caused maximum deterioration in strengths whereas 4% solution caused minimum. Wet/dry cycles in the solutions also caused loss of strengths in the specimens but their effects were less severe than that of freeze/thaw cycles. Comparison with similar tests on mortar specimens showed that mortar specimens were more resistant than paste specimens to freeze/thaw or wet/dry cycles in the solutions of sodium chloride. Examination of the fracture surfaces of cement paste by scanning electron microscopy and x-ray spectrometry, and the study of the hydration characteristics by differential thermal analysis revealed that freeze/thaw cycles in the presence of sodium chloride solutions changed the morphology of cement pastes. Calcium chloroaluminates or calcium chlorosulphoaluminates formed and their needle like crystalline structures grew more with the increase in the concentration of the solution and increase of freeze/thaw cycles. Mercury intrusion porosimetry proved to be a sensitive technique for studying the development of hydration and the changes were induced by freeze/thaw and wet/dry cycles in the solutions of sodium chloride. The freeze/thaw cycles in different solutions resulted in redistribution of pores. The possibility of improving the cement paste to withstand freeze/thaw cycles in presence of different solutions of sodium chloride using air entraining agent was investigated. Air entrained paste specimens showed clear evidence of changes in microstructure as revealed by the scanning electron microscopy and was also found to resist freeze/thaw cycles in the solutions more effectively than the specimen without an air entraining agent. The electron micrographs of air entrained specimens also provided evidence to support the suggestion that tiny air bubbles served as escape reservoirs for the movement of solution from the capillaries during freezing cycles. Finally several suggestions were put forward to enable this line of investigation to be extended to include other aspects of concrete deterioration.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral