Effects of carbon on air entrainment in fly ash concrete: The role of soot and carbon black

The most important commercial outlet for coal ash is in the concrete industry as a partial replacement for Portland cement. In many concrete mixtures careful control of air entrainment is required, but the presence of solid carbon disrupts the air entrainment process. An earlier study focused on the fundamental role of carbons, which were found to adsorb air-entraining admixtures, the specialty surfactants used in concrete mixtures, rendering them less active for their primary function as stabilizing agents for air bubbles. The present paper focuses on the specific sorptive behavior of carbon black, which is used as a pigment in some concrete formulations and also serves as a convenient model substance for soot, which may be present in fly ash from coal combustion. The paper also describes microscopic examination of several commercial fly ash samples for the possible presence of soot. The results show that eight commercial carbon blacks interact strongly with the air-entraining admixtures, the degree of interaction increasing with decreasing primary particle size or increasing nitrogen surface area. Overall, the adsorption of air-entraining admixtures is believed to be governed by the amount of accessible, hydrophobic, carbonaceous surface area. Examination of three fly ash samples from commercial pulverized coal fired combustion systems resulted in the conclusive identification of at least trace amounts of soot in each sample, with significant amounts found in one sample intentionally generated under very poor combustion conditions, Finally, a simple laboratory test was developed to screen ash samples for the presence of significant quantities of fine carbonaceous particles, which, on the basis of the results of this study, can have a negative impact on concrete performance in certain special cases.