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 replacement for Portland cement in the concrete industry. High levels of unburnt carbon can render ash samples unsuitable for this high-value market by interfering with the action of air-entraining admixtures, which are specialty surfactants used to stabilize air bubbles in concrete mixtures. An initial laboratory investigation was carried out to identify the fundamental interaction mechanisms between fly ash and air-entraining 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 additives suggest that admixtures are preferentially adsorbed from the aqueous phase on non-microporous carbonaceous surfaces.