Title: Resistance of Fly Ash Concrete to Freezing and Thawing

Author(s): K. W. Nasser and P. S. H. Lai

Publication: Symposium Paper

Volume: 132

Issue:

Appears on pages(s): 205-226

Keywords: compressive strength; durability; ettringite; fly ash; freeze-thaw durability; lignite; scanning electron microscope; microstructure; Materials Research

DOI: 10.14359/2057

Date: 5/1/1992

Abstract:
A study was made of the effects of Saskatchewan lignite fly ash on the resistance of concrete to freezing and thawing. Concrete was made with either ASTM Types I or V cement and different percentages of fly ash with an air content of 4 to 6 percent. Performance of the concrete was evaluated by measuring the changes in its dynamic modulus and its mass. A scanning electron microscope was also used to examine the changes in the microstructure of the cement paste due to exposure to freezing and thawing. Results show that the use of high percentages of fly ash in concrete (35 and 50 percent) reduced its resistance to freezing and thawing even though it contained about 6 percent air and was cured in water for 80 days. However, concrete containing 20 percent fly ash gave satisfactory performance, provided its air content and strength were comparable to control concrete that contained no fly ash. Results from the SEM examination show that the decrease in resistance of fly ash concrete to freezing and thawing may be due to the slow migration of portlandite and ettringite crystals from the dense C-S-H zones to the air voids. Concrete with fly ash was less susceptible to the migration of portlandite, but its air voids contained more fibrous hydrates, which may have led to an increase in the past porosity.