Effects of Carbon on Air Entrainment in Fly Ash Concrete:  The Role of Soot and Carbon Black

Abstract

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.

High capacity mercury adsorption on freshly ozone-treated carbon surfaces

“…The surfactant adsorption capacity of the treated and untreated carbon black samples was determined by titration procedure referred to as the foam index test [18,21]. Foam index measurements involved placing the test carbon sample [usually about 90 mg] together with 8 grams of Portland cement and 25 ml of de-ionized water into a 70 ml cylindrical jar with a 40 mm I.D., 80 mm length.…”

Strategies and Technology for Managing High-Carbon Ash

“…"Ash #1": A commercial carbon-containing fly ash from bituminous coal combustion at the full scale. The fly ash is sample FA21 in the Brown University sample bank with 6.3% loss on ignition (approximately 6.3% elemental carbon), which has been used in previous studies [5,6]. 5.…”

“…In recent studies, surfactant adsorption on carbon has been identified as a key phenomenon determining the suitability of coal combustion fly ash as a concrete additive [5][6][7][8][9]. Here the carbon is a contaminant and its undesirable adsorption of surfactant from the aqueous concrete paste reduces the surfactant's ability to stabilize sub-millimeter air bubbles that help improve freeze-thaw resistance in set concrete [10].…”

Strategies and Technology for Managing High-Carbon Ash

“…Controlled ozone concentrations from 500 ppm -2 vol-% were generated in air and passed upward through fixed beds of ash (50 -200 gms), for fixed contact times (1 minute -20 hrs), while outlet ozone concentration was monitored in real time. The ozonated ash samples were removed and a standard surfactant adsorptivity determined by a simple titration procedure used previously [3]. Additional experiments were carried out on carbon black under the same ozonation conditions as an inorganic free model carbon for suitable for more detailed surface characterization.…”

“…The degree of interaction between an AEA and a specific fly ash or carbon solid sample can be estimated using the foam index test [3], a simple laboratory titration procedure. This procedure was used in this study to investigate the interactions between a commercial AEA (Darex II, trademark of W.R. Grace & Co., Cambridge, MA) and a variety of model carbonaceous materials and carboncontaining fly ash samples.…”