Andreas Leemann

Analysis of cement-bonded materials by multi-cycle mercury intrusion and nitrogen sorption.

The pore systems of cement-based materials are studied by N(2) sorption and mercury intrusion porosimetry (MIP). Pore size distributions and internal surfaces are derived. Especially in materials with a broad pore size distribution, these (and other) methods generally do not lead to coincident results. It is shown here, how the interpretation of the experimental data of the two methods may be modified in order to obtain coincident pore size distributions from both methods. The studied pore systems are described as array of chambers which are connected by smaller throats. N(2) adsorption is used to calculate the size of the pores, whereby no distinction between throat or chamber type is possible with this method. Assuming mercury entrapping in ink-bottle type pores (pores that are connected to an external surface through smaller pores only) being the dominant process for mercury snap-off during extrusion and applying multi-cycle MIP, the calculation of the size of the entrances of these ink-bottles is possible. It is shown that similar results also may be derived from mercury extrusion data by applying a contact angle correction for the retracting mercury meniscus. A good agreement of the pore size distribution of the connected, non-ink-bottle type pores derived from either N(2) sorption or mercury intrusion is obtained. Samples of cement paste and mortar are analysed. A significant difference between cement paste and mortar regarding the neck entrances of ink-bottle type pores is found and attributed to the coarse pore space around the aggregates, the interfacial transition zone.

Focussed ion beam nanotomography reveals the 3D morphology of different solid phases in hardened cement pastes

Due to the development of integrated low‐keV back‐scattered electron detectors, it has become possible in focussed ion beam nanotomography to segment not only solid matter and porosity of hardened cement paste, but also to distinguish different phases within the solid matter. This paper illustrates a method that combines two different approaches for improving the contrast between different phases in the solid matrix of a cement paste. The first approach is based on the application of a specially developed 3D diffusion filter. The second approach is based on a modified data‐acquisition procedure during focussed ion beam nanotomography. A pair of electron images is acquired for each slice in the focussed ion beam nanotomography dataset. The first image is captured immediately after ion beam milling; the second image is taken after a prolonged exposure to electron beam scanning. The acquisition of complementary focussed ion beam nanotomography datasets and processing the images with a 3D anisotropic diffusion filter allows distinguishing different phases within the hydration products.

Segmentation of elemental EDS maps by means of multiple clustering combined with phase identification

An imaging concept is proposed for the phase identification and segmentation of elemental map images from energy dispersive spectroscopy. The procedure starts with presegmentation using common clustering algorithms, continues with automated identification of the chemical compositions, followed by their screening by professional expertise. The ultimate phases are finally clustered by applying a minimum Euclidean distance classifier. The potential, performance and limitations of the approach are presented on energy dispersive spectroscopy maps acquired by a scanning electron microscope and conducted on samples produced from cement clinker, natural rock and hydrated cement mortar. Nevertheless, the technique is suitable for arbitrary types of materials and general devices for energy dispersive spectroscopy acquisition. It is an approach for extending common energy dispersive spectroscopy analysis by means of visual examination and ratio plots towards quantitative rating.

Creep and Shrinkage of SCC

Strain in concrete can occur due to different reasons. When a stress is applied, concrete shows an immediate, reversible strain (elastic deformation), and a further deformation that increases with time (creep). After stress removal, part of this deformation is recovered immediately due to the elastic properties of the material, and a minor part is recovered in time (reversible creep). Another other part of the deformation, however, is not recovered (irreversible creep). In addition to creep, strain in concrete develops as well because of a volume decrease caused by shrinkage. Depending on the stress-strain situation at specific points in a structure, strain caused by shrinkage and creep may add up or creep may lead to relaxation as it can reduce stress caused by shrinkage strain. This interaction of stress and strain is of vital importance for concrete structures as it influences cracking, deflection and prestress loss. As a result, creep and shrinkage have been taken into account by numerous standards, and they are a focus point of research. With the appearance of self-compacting concrete (SCC) as a valuable alternative for conventionally vibrated concrete (VC), some of the established stress-strain-relations have to be questioned or at least reconfirmed. Looking at the scene in a broad sense, the most prominent changes in mix design from VC to SCC are the higher paste volume, the substantial use of mineral additions, and the high dosage of superplasticiser, often in combination with a viscosity-modifying agent (VMA). The changes in paste volume and binder composition influence the viscoelastic properties of the concrete.