Optimizing the Amount of Class C Fly Ash in Concrete Mixtures

Abstract This paper describes the results of a two part research program on several laboratory air-entrained concrete mixtures designed to contain increasing amounts of moderately cementitious Class C fly ash. In the first part of the paper, a Type A admixture was used in half of the mixtures, treating both the portland cement Type I and fly ash (15% calcium oxide). In the second part of the paper, three other Class C fly ashes (9 to 12% calcium oxide) were extensively tested in concrete that contained a Type A admixture (but this time treating only the cement). Two very high lime fly ashes (25 to 30% calcium oxide) were also tested in concrete mixtures. All the fly ash concrete mixtures, provided they were properly air-entrained, showed excellent freeze-thaw durability (85 to 95% relative durability [RDF]) and good air-void parameters even at high fly ash contents. The setting times for the fly ash mixtures were longer by 4 to 6 h in the presence of a water reducer and very high fly ash contents (119 to 178 kg/m3 [200 to 300 lb/yd3]). Also in Part 1, the optimal fly ash replacement (criteria is 28-day compressive strengths and 1:1 replacement basis) was 25% without a water reducer and >33% with a water reducer at 356-kg/m3 (600-lb/yd3) cementitious material. For 297-kg/m3 (500-lb/yd3) cementitious material, these values changed to 20% and >33%, respectively. Again using 28-day strengths and pound for pound replacement as the criteria, the optimal fly ash replacement at 356-kg/m3 (600-lb/yd3) cementitious material was >50% for Crawford fly ash, 33% for Will Co. fly ash, and 25% for Joliet fly ash. At 297-kg/m3 (500-lb/yd3) cementitious material, these optimum replacements were 25%, >40%, and slightly less than 20%, respectively.