Ultra Fine Fly Ash for High Performance Concrete

Abstract

The incorporation of pozzolans such as fly ash imparts benefits to concrete such as reduced permeability, increased resistance to alkali silica reaction (ASR), sulfate attack, chloride ingress and corrosion. It has long been recognized that the activity of fly ash is much improved by increasing its fineness. Fine fly ash can be manufactured by grinding or by selective classification using air classifiers. In the past both these approaches have typically been cost prohibitive. However, recently it has become possible to commercially manufacture large quantities of an ultra fine fly ash (UFFA) with an average particle size of 3 micrometers (90% < 7 micrometers). This paper discusses the fresh and hardened properties of concrete made with UFFA at different dosages. Cement content was also varied over a broad range. Durability testing for ASR, sulfate attack, chloride diffusivity, and rapid chloride permeability (RCP) was also conducted. It was found that at a given workability and water content, concrete containing UFFA could be produced with only 50% of the high-range water reducer (HRWR) dosage required for comparable silica fume (SF) concrete. Similar durability measures as SF concrete were observed when a slightly higher dosage of UFFA was used with a small reduction (10%) in water content. Also, a recent application involving UFFA is documented.