Multi-scale kinetics-based model for predicting mechanical properties development of concrete containing supplementary cementitous materials.
Binary and ternary cements consisting of ground granulated blast furnace slag (GGBFS) have been found to develop improved long-term durability and strength. The base cementitious systems to be investigated are binary blends of slag-cement augmented with ternary compounds including fly ash (C or F type) or alkali-based activators. These blends, typically called slag-cements, are now widely used in practice. Few attempts have been made to optimize the use of fly ash or slag cement to produce concrete mixtures that meet specific performance objectives. With the growing availability of slag cement and fly ash the selection of materials for any given job has become more complicated. The objectives of this study are to link multi-scale hydration kinetics of binary blends of slag-cement and ternary blends with fly ash (C or F type) as the third phase to macro-mechanical properties and long-term performance of concrete. This will be accomplished through a joint study between Tennessee Technological University (TTU) and the University of Michigan (UM). The proposed research addresses topics across multiple scales: (1) phase resolved micro- and lumped parameter meso-scale chemical kinetics of hydration processes; (2) phase resolved nano- and micro-scale chemical composition and structure of hydrates; (3) macro-scale mechanical properties and thermal/shrinkage stresses and (4) multi-scale mathematical modeling to provide the link between nano- and meso-scale hydration kinetics and macro-scale mechanical properties. Nano- and micro- analytical methods, including scanning and transmission electron microscopy, electron microprobe analysis, wavelength dispersive and energy dispersive spectroscopy and quantitative synchrotron x-ray diffraction will be used to describe the chemistry and structure of phases and, where possible, their mechanical responses. These micro-scale responses will be linked to concrete properties through kinetics models (e.g. autocatalytic-like). Model parameters will be established through meso-scale evaluation using calorimetry (isothermal/semi-adiabatic) and thermal analysis (TGA/DTGA/DTA). Absolute rate constants will be obtained for the combined and distributed hydration phases. Heat of hydration is known to be a quantitative measure of the multiple hydration processes which develop both successively and simultaneously. Absolute rate constants will be obtained for the multiple hydration processes. For a given chemical composition and particle size the rate constant can be normalized for temperature effects for each individual process. The rate model function for each of the major hydration processes is then scaled for amount of SCM. Total heat development for variable temperature-time history is obtained through numerical integration, with the rate calculated by constantly updating the hydration level. The mechanics-based model will link heat of hydration to concrete strength for both OPC and blended cements. The kinetics-based hydration model will be used to develop a novel reactivity index for blended systems. This reactivity index can then be used to optimize cementitious blends for a given slag-cement-fly ash combination. Currently optimization of a given blend for strength development at different temperatures requires laborious testing. In this study, calorimetry will be used instead to assess reactivity for a wide range of blends, temperatures, and curing times. The effort will be a true integration of resources and expertise since the TTU PI (Biernacki, Chemical Engineering) is familiar with the nano-/micro-scale experimental methods as well as multi-scale modeling while the UM PI (Hansen, Civil Engineering) has expertise in areas of meso- and macroscopic hydration and mechanical properties. Student exchange and co-mentoring between the two universities will be a key element of the program so that students will have access to the appropriate facilities and a rich multi-disciplinary experience.
