Nanoengineering of concrete via topological constraint theory
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oduction Cement and concrete are by far the most manufactured materials in the world. With global production of 20 billion tons per year (approximately 1 m3/capita/year), concrete is produced at a rate ten times greater than all other construction materials combined.1 Unfortunately, this ubiquitous use comes with a heavy environmental impact; cement and concrete are responsible for 5–10% of the world’s total carbon dioxide emissions.2,3 This carbon impact is expected to increase by 13% by 2020, mainly driven by the high demand for infrastructure in developing countries.2 China used more cement from 2013 to 2015 than the United States used in the entire 20th century. Despite this environmental burden, the compositions and manufacturing process for producing cement have seen limited changes since the introduction of ordinary portland cement in the 19th century—cement still primarily relies on calcium silicate clinkers (the powder that forms a cement paste upon contact with water).4 This lack of innovation, which has resulted in a relative increase of the carbon impact of cement over the years as other industries are becoming more environmentally friendly, is mainly caused by (1) the low cost and ease of use of the material (concrete is about 100× cheaper than an equivalent weight quantity of steel), which impedes the emergence of alternative greener but more expensive materials; (2) the existence of decades of Edisonian “trial-and-error” optimization, thereby limiting the possibility of significant further improvements relying on empirical approaches; and
(3) construction safety regulations, which render the introduction of new building materials with unproven long-term performance to be challenging. Despite these drawbacks, no credible alternatives to concrete are on the horizon or likely to arise in the foreseeable future, so improving the properties and processes for existing concrete compositions is the only realistic option to reduce environmental impact.5 Due to the huge quantity of concrete produced, even small improvements would result in a major impact: for example, decreasing the carbon impact of concrete by 10% would be equivalent to removing all of the CO2 emissions associated with steel production.5 This situation calls for a systematic, rational optimization of concrete formulations, within the constraints of price, processability, and performance. Recently, concrete nanoengineering1,6 has emerged as an idea that may lead to the next step change in cement and concrete technology. Although poorly defined, this term comprises various experimental, numerical, and theoretical approaches to characterize how the nanoscale structure of concrete can be tuned to design concretes with tailored macroscopic properties, or with lower carbon impact. In this article, we review recent advances in the modeling and topological optimization of concrete. This approach was inspired by the topological optimization of glasses, which led to the discovery of Corning Gorilla Glass, a scratch- and damage-resistant glass used in more
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