Institute of Solid State Physics

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Materials Properties from Microstructural based Experimental-Computational Synergy
Neven Ukrainczyk
TU Darmstadt
15:30 - 16:00 Wednesday 13 May 2020 

Video: https://www.dropbox.com/s/c9dq24zw6c1kt26/Ukrainczyk_uCTSeniorScientistPosition.mp4?dl=0

The porous microstructure of a cement-based material cover multiple length scales, ranging from nanometer-sized gel pores inside hydration products, via micrometer-sized capillary pores in-between the reactive cement grains, towards milimeter-sized air bubbles. A numerical scheme is used by means of either generating a porous network from advanced 3D numerical simulations (e.g. Hymostruc), or, by 3D sampling a porous network inside real cementitious samples, using experimental imaging via X-ray computed tomography. The algorithm starts with a discretization of the microstructure into a regular 3D mesh (stack of bitmaps), where each voxel in the mesh is assigned to a single phase. An assembled system of up to 1 billion equations are solved implicitly by a conjugate gradient algorithm employing e.g. an in-House developed OpenMP (shared memory) parallelized C-code. A novel method is proposed that mitigates resolution problems for numerical methods to calculate the effective properties of the multi-scale porous cementitious materials. The method refines initial mesh resolutions and includes the detailed sub-voxel into the transport calculation with a high accuracy and efficiency. The proposed method is validated against the effective diffusivity of a simple periodic arrangement of mono-sized particles, as well as against a number of complex 3D virtual cement paste microstructures. The method leads to very accurate results, significantly reduces computational efforts, and describes the porous network in a high level of detail. A new approach to calculate the effective diffusivity of cement pastes by considering a linear variation of the pore phase diffusivity with a grayscale level of the pore voxel that corresponds to the experimentally scanned CT image and that represents an intermediate level of diffusivity.

An efficient transient analysis algorithm for digital dissolution at pore-scale is demonstrated and was validated with an analytical solution for a Stefan moving boundary problem.

Other in-house developed pore/solid structure algorithms included characterization of a phase Size distribution, connectivity / isolated clusters, tortuosity, constrictivity and dead-end clusters. The effect of phase heterogeneities on the transport properties, pore structure and ageing mechanisms of cement-based materials are investigated. Examples of X-CT measurements are demonstrated on cement based materials with two extremely different porosities; from having no air bubbles to ultra-lightweight foam.

As a future research, X-CT experimental work proposes to study new porous cementitious materials with thermal and chemical energy storage functionality as an innovative added value to this general type of mostly used construction materials.

At 16:00, there will be an opportunity to ask questions at https://tugraz.webex.com/meet/p.hadley