Objective - Generate and publish reaction rate data and enhanced computer modeling tools to better understand and predict the rates of microstructure development and phase interactions in portland cement concrete binders.
What is the new technical idea? Previous STRS project research has developed and applied methods for analyzing rates of reactions in cementitious systems, including reflection digital holographic microscopy (DHM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). These methods, DHM in particular, have demonstrated that the rates of dissolution of cement component minerals are sensitively dependent on the type and density of structural defects at the reacting surfaces. This means that the shape and size distribution of powder particles in cement should have an even greater influence on cement reactivity than has been anticipated previously. This project, therefore, will not only continue the fundamental measurements on crystal surfaces by DHM, but will particularly emphasize reaction rates in single-component and multi-component suspensions. However, extracting meaningful reaction rate measurements from suspensions is complicated by (1) the more complex surface geometry, (2) the possibility of multiple simultaneous reactions, and (3) the inability to directly visualize the microstructure in situ. A rapid flow-through powder bed vacuum filtration system will be built to cope with these complications, whereby a liquid of known composition can be forced quickly through a thin layer of powder before any secondary components can precipitate. Analysis of the effluent solution by ICP-OES, and of the powder bed by quantitative X-ray diffraction, will indicate the extent of reactions as the liquid flows through the capillary porosity. Computational fluid dynamics (CFD) simulations of the experiments can help determine the flow patterns and residence time distribution of liquid as it flows through the powder bed, which will help make sense of the effluent composition changes. Using materials for which DHM dissolution data are already available, direct comparisons can be made on the influence of particle shape and size distribution in altering the specific reaction rates. In addition, important multi-component subsets of portland cement, such as tricalcium aluminate and gypsum, can be created and analyzed with the same apparatus.
What is the research plan? FY18. Powders of each of the four major clinker phases, as well important secondary phases such as gypsum and calcite, have already been obtained. The powders must be thoroughly characterized to be able to draw structure-property relationships from the experiments and simulations. This includes measurement of particle size distribution, specific surface area, and phase purity. In addition, the particle shape distribution will also be measured by X-ray microtomography in collaboration with Ed Garboczi at the NIST Boulder campus. Significant effort will be devoted in Year 1 to constructing and testing the vacuum filtration dissolution device and to establishing a protocol for solution sampling and measurement with that device. As this is happening, CFD simulations of flow past reacting surfaces of varying roughness will be undertaken to help understand and interpret DHM measurements made in FY17 of the dissolution of gypsum, calcite and C3A. Also during Year 1, kinetic Monte Carlo models of dissolution will be developed and tested on simple minerals such as calcite and gypsum, for which experimental data have already been obtained.
FY19. Mineral powders will be size classified, and each size class will be subjected in isolation and in selected combinations to vacuum filtration dissolution measurements in pure water and in electrolytes. Measurements will be made using at least three different temperatures to obtain apparent activation enthalpies for the reaction. The powders’ size distribution, specific surface area, and phase composition will be measured after the experiments to correlate solution compositions with particle erosion rates. Selected systems will be analyzed before and after by high-resolution X-ray microtomography to provide additional validation points for 3D microstructure development models such as HydratiCA and THAMES. This will also establish the foundations of a public dataset for cementitious mineral reactivity, which will also begin development in Year 2. Individual powder particles will also be observed in situ with polarized light microscopy to characterize changes in shape during reaction. The kMC models will be used to simulate retreat of nominally flat but structurally defective mineral surfaces, from which correlations will be made between reaction rate spectra and defect type and distribution.
FY20. Both kMC and cellular automata (HydratiCA) models will be tested against measurements of powder reactivity as a function of particle size made in Year 2. Results of this testing are expected to reveal general relationships between size and reactivity that can be embedded within coarser microstructure models for improved capabilities in predicting cement binder microstructure development. Further experimental data sets will be generated with mixtures that more closely approximate portland cement compositions, monitoring the time-dependent phase development by in situ XRD for at least 24 h, repeated at three different temperatures. These datasets will also be made publicly available and will form excellent, well-characterized systems for direct testing of hydration models.