Durability Properties of High Strength Concrete Containing Silica Fume and Lignite Fly Ash

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Title: Durability Properties of High Strength Concrete Containing Silica Fume and Lignite Fly Ash

Author(s): K. W. Nasser and S. Ghosh

Publication: Special Publication

Volume: 145

Issue:

Appears on pages(s): 191-214

Keywords: compressive strength; concretes; durability; ettringite; fly ash; freeze-thaw durability; high-strength concretes; matrix; microstructure; morphology; scanning electron microscope; silica fume; sulfate resistance; Materials Research

Date: 5/1/1994

Abstract:
A comprehensive study was undertaken to determine the effect of sulfates and freezing and thawing on the durability of high-strength concrete with silica fume and lignite fly ash. The concrete mixes contained normal CSA Type 10 (ASTM Type I) portland cement, 10 percent silica fume, and different amounts of fly ash from 0 to 80 percent of the binder weight in the mix. The aggregate-binder ratio by weight was maintained at 5 and the superplasticizer weight was varied between 1.5 and 2.2 percent of the binder, while the water-binder ratio was maintained at 0.27. Results of the freeze-thaw tests, as well as those of the sulfate tests (using Na 2So 4), were encouraging. Concrete with either 20 or 35 percent fly ash replacement along with 10 percent silica fume gave satisfactory resistance to frost action, while concrete with up to 50 percent fly ash and 10 percent silica fume considerably suppressed the sulfate attack. Concrete strength at 28 days with up to 60 percent fly ash and 10 percent silica fume was equal to or greater than that of control concrete of 100 percent Type 10 cement. A study of matrix morphology and microstructure bonding using a scanning electron microscope showed that fly ash + silica fume concrete had a denser paste microstructure (compared to the control), and this played an important part in enhancing compressive strengths and resistance to frost and sulfates. However, increasing fly ash content beyond 50 percent weakened the matrix bonding due to the presence of too many unreacted fly ash particles, which adversely affected the compressive strengths and resistance to frost and sulfates.