Mikromechanische Analyse von Kompositmaterialien aus Mischzementen

Abstract

Zusammenfassung in tschechischer SpracheIn order to reduce CO2 emissions associated with the production of building materials, the cement and concrete industry has developed new binders by blending ordinary Portland cement with supplementary cementitious materials such as the industrial waste products blast furnace slag and y ash, and/or with finely ground inert materials such as limestone and quartz. This is setting the scene for the present thesis which is motivated by the fact that the new binders exhibit different hardening characteristics compared to their predecessors that were produced with ordinary Portland cement alone. In order to improve the predictability of properties of blended cementitious materials, a combined experimental-computational approach is used. In the experimental part, a comprehensive test database is elaborated by combining state-of-the-art microstructural characterization techniques and mechanical testing. Microstructural characterization combines methods including thermogravimetric analysis, X-ray diffraction with Rietveld analysis, and scanning electron microscopy, in order to determine the microstructural phase assemblages of the initial raw products as well as 1, 3, 7, 28, and 91 days after mixing with water. This allows for resolving phase volume evolutions point wisely. Mechanical testing, in turn, includes characterization of stiffness and strength. The early-age evolution of static unloading modulus is determined with a test protocol including cyclic loading-unloading tests which are hourly repeated from 24 hours after production up to material ages of 8 days. Dynamic stiffness is determined based on measurements of ultrasonic pulse velocities of longitudinal and shear waves, evaluated on the basis of elastic wave propagation in isotropic media. The uniaxial compressive strength evolution is determined both on pastes and mortars, crushed 1, 3, 7, 28, and 91 days after production. The tensile splitting strength, in turn, is determined 1, 3, 28, and 91 days after production. All the measured data is then stored in a newly established database \CemBase" along with additional data collected from the literature. At the end of this thesis \CemBase" contained information on 399 entries out of which approx. 20 % were measured during the experimental campaign and approx. 80 % were collected from available literature. The computational part focuses on multiscale strength homogenization in the frameworks of two complementary modeling methods: multiscale Finite Elementbased homogenization and continuum micromechanics. Finite Element-based homogenization uses nonlinear fracture mechanics with an isotropic damage model to establish a link between nanoscopic calcium-silicate-hydrates and the uniaxial compressive strength of cement pastes, emphasizing the nonuniform distribution of C-S-H around clinker grains. The model focuses on the special role of C-S-H as the main material phase contributing to the macroscopic mechanical properties as well as introduces the C-S-H/space ratio as the main microstructural descriptor. In addition, the model identifies key factors in uencing the compressive strength of cement pastes. Continuum micromechanics, in turn, is used for elasto-brittle modeling of both compressive and tensile strength. The method accounts for key features of cementitious microstructures in terms of their hierarchical organization, quasi-homogeneous material phases, their volume fractions, characteristic shapes, and mutual interaction. The compressive strength model considers that the macroscopic strength is reached once stress peaks in micronsized needle-shaped hydrates reach their strength. The latter is described based on a Mohr-Coulomb failure criterion including the angle of internal friction and the cohesion of low-density calcium-silicate-hydrates, as quantified by limit state analysis of nanoindentation tests. The model accounts for stress concentrations in the immediate vicinity of sand grains, and it uses strain energy-related stress averages for the scale transition from cement paste down to needle-shaped hydrates. It is found that microfillers effectively reinforce the hydrate foam, and that hydration of supplementary cementitious materials has a strengthening effect which is not only related to the increase of hydrate volume and the corresponding decrease of capillary porosity, but also to an increase of the cohesion of low-density calcium-silicate-hydrates. The tensile strength model, in turn, is based on upscaling elasticity and fracture energy from nanoscopic calcium-silicate-hydrates { as quantified by molecular dynamics simulations { all the way up to the uniaxial tensile strength of mortars and concretes. The model considers that cementitious materials suffer from pre-existing cracks which are only somewhat smaller than the maximum aggregate size, and that the tensile strength of the material is reached once these cracks start to propagate. Summarizing, the three developed models define a new state-of-the-art regarding multiscale homogenization of strength properties of cementitious materials.20