Impacts of Magnesium Chloride Deicer on the Durability of Nanosilica-Modified HVFA Concrete
Dr. Xianming Shi, Washington State University
In the U.S., approximately 20 million tons of sodium chloride used for every typical winter season, along with unconventional deicers for snow and ice control present new challenges for the durability of concrete infrastructure, beyond freeze/thaw (F/T) damage. For instance, deicer magnesium chloride (MgCl2) is commonly used when pavement temperature drops below 15F, and our recent study  has revealed that this chemical can compromise the strength of ordinary Portland cement (OPC) concrete without any visible surface distress, thus evading the traditional inspection methods. In this context, there is an urgent need to identify concrete mixes that are more resistant to MgCl2 by design. High volume fly ash (HVFA) concrete can be cast with denser microstructure and reduced pore sizes, but its resistance to the impact of MgCl2 remains poorly known. Compared with OPC concrete, HVFA concrete (with or without modification by nanosilica) features different microstructure as well as different chemistry of hydrates, and thus may exhibit different behaviors when subjected to physical loading (e.g., F/T cycles) and chemical loading (e.g., MgCl2). The overarching goal of this project is to investigate the impacts of MgCl2 on the durability of HVFA concrete in cold climates and the role of nanosilica in the HVFA concrete, in terms of both engineering properties and fundamentals at the micron and nanometer scales. To achieve the goal, this study aims to:
1) investigate the influences of concentrated and diluted MgCl2 solutions on the durability of HVFA concrete (with or without modification by nanosilica) under both constant ambient temperature and F/T cycling conditions,
2) characterize physical and chemical deteriorations of the microstructures and different phases of HVFA concrete and elucidate the role of nanosilica on the improved resistance against MgCl2 attack.
The engineering properties of HVFA concrete will be characterized through weight change, compressive and split tensile strengths, water sorptivity and surface resistivity tests. Afterwards, the physical and chemical deteriorations of the microstructures and different phases as well as the beneficial role of nanosilica for the durability will be examined and analyzed through microhardness test, SEM/EDX, XRD, DSC/TGA, EMPA/WDS, and 29Si/27Al MAS-NMR.
Dr. Xianming Shi