Assessing ash quality and performance

Document Type

Article

Publication Date

5-3-2017

Department

Department of Materials Science and Engineering

Abstract

This chapter reviews ash quality and performance issues, with a special emphasis on air entrainment and early-age performance. Recent developments in testing procedures are reviewed and recommendations are provided for standardized testing. One of the key contributors to variable air contents in concrete mixtures with fly ash is the presence of unburnt carbon. Here, a variety of techniques for assessing the absorption of air entraining admixtures by the fly ash are presented and critiqued. A second performance issue encountered with fly ash mixtures, particularly high-volume fly ash concretes, is early-age retardation that leads to delayed setting and reduced early-age strengths. Isothermal calorimetry is shown to be a valuable troubleshooting technique for evaluating the severity of this retardation. Various mitigation strategies are presented for restoring setting times and increasing early-age strengths to duplicate the performance of 100% Portland cement reference concretes. Finally, the contributions of fly ash to strength development via both pozzolanic and filler effects are assessed using both the standardized strength activity index test and the Keil Hydraulic Index test. The shortcomings of the former and the promising possibilities of the latter are discussed in detail.

Publication Title

Coal Combustion Products (CCPs): Characteristics, Utilization and Beneficiation

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