Effect of Carbon-Negative Carbon Black on Concrete Properties

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Carbon black is a fine, electrically conductive carbon-based material that can be produced from captured carbon dioxide, generating interest in potentially carbon-negative carbon black as an additive for lower-carbon concrete. This study characterized carbon black and evaluated the fresh, hardened, and durability properties of the Virginia Department of Transportation Class A4 General concrete produced with different cementitious binder systems with and without carbon black. Six concrete mixtures were prepared: three reference mixtures using Type IL cement alone or with reclaimed Class F coal ash or calcined clay as supplementary cementitious materials at 30% cement replacement by mass; and three carbon black-modified counterparts incorporating carbon black at 3% cement replacement by mass. Carbon black was characterized by particle-size analysis, scanning electron microscopy with energy-dispersive spectroscopy, and reactivity testing. The concretes were evaluated for slump, air content, unit weight, compressive strength, elastic modulus and Poisson’s ratio, surface resistivity, rapid chloride permeability, non-steady-state chloride migration, and drying shrinkage.

The carbon black was found to consist almost entirely of elemental carbon with negligible reactivity, functioning as an inert filler rather than as a supplementary cementitious material. At the dosage evaluated, concrete incorporating carbon black can meet the Virginia Department of Transportation Class A4 General concrete fresh and hardened property requirements. However, its inclusion sharply increased admixture demand—approximately 1.5 to 2 times more high-range water reducer and 20 to 45 times more air-entraining admixture—reduced compressive strength by 8 to 23% (least with calcined clay), and slightly increased chloride-transport indices and drying shrinkage, while having minimal effect on elastic modulus and Poisson’s ratio. Accordingly, carbon black provided no performance benefit beyond a possible reduction in embodied carbon, the magnitude of which depends strongly on the source of the carbon black. Carbon black may nevertheless offer transportation-relevant functionalities, such as self-heating for de-icing and self-sensing for structural monitoring; however, these effects generally require higher dosages than those considered in this study.

 

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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