The manufacture of cement is estimated to contribute about 5% of worldwide anthropogenic CO2 emissions, so there is a strong effort underway to find ways to reduce the amount of cement used in concrete. One of the most attractive approaches is to introduce waste products from other industries, such as the slag from steelmaking or the ash from burning of coal (known as “fly ash”). Fly ash is particularly useful, because it is rich in silica, which reacts with cement to produce a superior product. Unfortunately, some fly ash is contaminated with carbon from the burning process, and this can result in undesirable chemical interactions with additives that are used to control the flow and setting of concrete. The thesis research of Lori Tunstall is examining the chemical origins of the interaction of fly ash with the air-entraining agents used to provide frost protection to concrete. The goal is to understand the problem, so that we can develop a simple test that will allow producers to determine whether a given shipment of ash will cause problems, and will indicate how to compensate for it.
This is an ARRA/NIST project in which we are collaborating with Profs. Hamlin Jennings at MIT and Paulo Monteiro at UC Berkeley. The larger goal of the work is to understand the effect of fly ash on the structure of cement paste, from the atomic scale to the microstructural scale that controls strength and durability. For the use of concrete to be sustainable, it will be necessary to use high levels of substitution with mineral additives, such as fly ash, so the impact of those materials on the structure and properties of concrete must be understood at a fundamental level.
This is an image of carbon particles in fly ash. The bright white spheres are the glassy ash particles, many of which are embedded insidegraphitic particles of carbon