INTERACTIONS OF CARBON-CONTAINING FLY-ASH WITH COMMERCIAL AIR-ENTRAINING ADMIXTURES FOR CONCRETE

The most important commercial outlet for coal ash is as a partial repl acement for Portland cement in the concrete industry. High levels of u nburnt carbon can render ash samples unsuitable for this high-value ma rket by interfering with the action of air-entraining admixtures, whic h are specialty surfactants used to stabilize air bubbles in concrete mixtures. An initial laboratory investigation was carried out to ident ify the fundamental interaction mechanisms between fly ash and air-ent raining admixtures. The results indicate that the interaction is time- dependent and occurs to a degree that correlates only crudely with the amount of carbon present. Measurements made on a variety of model add itives suggest that admixtures are preferentially adsorbed from the aq ueous phase on non-microporous carbonaceous surfaces. (C) 1997 Elsevie r Science Ltd.