The unceasing quest for more durable, more environmentally friendly, and less expensive construction materials is compelling the concrete industry to adopt alternative formulations for concrete binders that depart radically from the traditional chemistry based on portland cement. The industry trend is to replace large portions of portland cement with industrial byproducts, natural pozzolans, and with chemically variable filler materials such as limestone and quartz. These modifications yield different—sometimes unacceptably different—performance characteristics throughout the life of concrete. Industry currently resorts to unguided trial-and-error approaches to designing these alternative formulations because it lacks the structure-processing-performance relationships needed to understand and anticipate the effects of mixture design on performance. This project will establish important subsets of these relationships and will thereby provide industry with structure-processing-property relationships and corresponding measurement science tools it needs to more efficiently and reliably design sustainable concrete binders.
Objective - Create new measurement technology and reference data that characterize the structure, composition, and reactivity of cementitious materials.
What is the new technical idea? The new technical idea is to create the measurement science, data and understanding required for the concrete construction industry to begin using structure-processing-property (SPP) relationships to develop materials having the desired properties, and to avoid unintended problems during construction and service. Nearly every advance in materials engineering, from metal alloys to biomaterials and pharmaceuticals, has been enabled by mapping out the key SPP relationships for the material that provide guidance on how to intentionally achieve desirable performance characteristics. However, SPP relationships are largely unknown for today's concrete binders, and the lack of such a knowledge framework increasingly hampers industry's ability to respond to frequent changes in raw material supply and to evolving mixture specifications. A suite of analytical methods for characterizing cementitious material structure, largely developed at NIST, has been firmly established over the last 20 years. However, methods for characterizing the chemical reactivity and interactions among cementitious materials, equally important for establishing the influence of structure on performance throughout the service life of concrete, are limited to calorimetry and, even then, only measure the net progress of all the combined processes that are occurring. The new idea here is to enable a new generation of concrete mixture design strategies, which are projected to lead to a 10 % reduction in the mass of cement used each year (Biernacki 2013), by developing the necessary measurement technology and reference data to map out the kinetics of binder hydration and thereby enhance our knowledge of SPP relationships in concrete.
What is the research plan? This project will (a) establish new measurement methods for detailed characterizion of material reactivity, (b) combine these new methods with established structural measurement procedures to determine the influence of structure and chemical composition on the rate and extent of hydration reactions in cementitious binders, (c) model the data from these combined methods to determine key SPP relationships and integrate them in a new data repository, and (d) begin developing best practices for using the repository for guided mixture design.
Three broad kinds of data must be acquired to determine SPP relationships for both portland cement minerals and fly ash: (1) structural (particle size distribution, density and morphology); and (2) compositional properties (distribution of oxide compositions of glassy phases and impurity levels in clinker phases); and (3) kinetic (rate laws for hydration reactions). Structural and compositional properties of selected cement and fly ash minerals will be obtained largely through proven characterization methods that include XRD, SEM, laser scattering for particle size distribution, nitrogen adsorption for surface area, and thermogravimetry for carbonation and prehydration characteristics. The kinetics of material reactions, however, are driven by solution chemistry and controlled by processes happening at the solid-liquid interfaces. Kinetic reaction data therefore must be obtained by combinations of solution chemical analysis and nanoscale measurement of mineral surface topography changes, so initial project activities will focus on establishing procedures for obtaining these data using inductively-coupled plasma optical emission spectroscopy (ICP-OES) or total reflectance X-ray fluorescence (TR-XRF) for liquid phase composition measurements, vertical scanning interferometry (VSI) for surface topography changes, and wet-cell petrographic microscopy for qualitative monitoring of hydration product growth and morphology. These latter techniques have not previously been applied systematically to cementitious minerals, so there is some risk that they will prove insufficient for the characterization requirements.
Data obtained from the combination of structural, compositional, and kinetic characterization methods will be used to determine SPP relationships that form the basis of a knowledge framework for guided mixture design. Such a framework will necessarily be sparse in the beginning, but the methods developed under this project will be leveraged in out years on other cementitious components and their data integrated to expand the knowledge base and make it increasingly powerful. Key industry and academic collaborators will be engaged through workshops to ensure that the fundamental data are ultimately distilled into useful design guides.
SPP relationships developed under this project will provide guidance for how raw materials and processing conditions, and mixture design parameters can be manipulated to achieve target performance reliably. Such guidance will be delivered to industry stakeholders in the form of special publications and active technology delivery tools (e.g., workshops, tutorials, webinars).
J.J. Birenacki, et al. Paving the way for a more sustainable concrete infrastructure: A vision for developing a comprehensive description of cement hydration kinetics, Special Publication 1138, National Institute of Standards and Technology, Gaithersburg, Maryland, March 2013.