Conference Agenda

The interfacial transition zone (ITZ), which is the weakest component of concrete, is inherently complex due to its spatially varying heterogeneity. It is therefore essential to gain a deeper understanding of the relationship between the microstructural characteristics and the macroscopic mechanical properties. In this context, the quality and resolution of both microscopic and micromechanical testing techniques have been improved, and nanoindentation has become an accepted measurement method for determining the micromechanical properties of materials. In the present study, nanoindentation is combined with scanning electron microscopy (SEM) to investigate the ITZ of recycled concrete made from recycled brick aggregate. The measurement procedure was adapted as follows: three suitable areas of ITZ and one additional area of bulk paste were selected for further analysis using optical light microscope. These areas were subsequently analyzed using SEM to identify and characterize the ITZ. Based on the image analysis, the distribution of porosity was determined. Nanoindentation grids with 5 μm spacing were then applied to the areas. In addition, deconvolution analysis of the probability density function obtained from more than 700 indentation results in each ITZ and bulk paste allows the quantitative characterization of the identified phases. The porosity analysis shows that the ITZ thickness is 65 μm, and the porosity of the ITZ is twice that of the bulk paste. The nanoindentation result shows that the porosity, low density calcium silicate hydrate, and calcium hydroxide are higher in the ITZ, while high density calcium silicate hydrate is higher in the bulk paste. The results provide a fundamental basis for the wider use of recycled brick aggregates in construction.

The rheological properties of cement slurry are primarily influenced by the spatial distribution of particles and their interaction forces. The Hamaker constant plays a crucial role in quantifying these interactions; however, its study has been limited due to technical constraints. Atomic force microscopy (AFM) can measure probe-to-particle surface interaction forces via the microcantilever deformation during the extension and retraction process, but it cannot directly measure particle-to-particle interaction forces. In view of this limitation, this paper introduces an AFM-based test method to quantify the interaction force between particles and a new approach for calculating the Hamaker constant. Using these methods, the particle-to-particle Hamaker constants of cement and fly ash in various environments were investigated. The measured Hamaker constants of different mineral materials ranged from 22.79 to 38.26 × 10−20 J in air and from 4.79 to 14.47× 10⁻²⁰ J in water. The primary factors influencing Hamaker constant were the material properties and the test environment.

The biomaterial “Biodentine” exhibits very interesting mechanical porperties, with a strength exceeding those of the biological tissue “dentine”. In the case of decay, replacement of the latter by the former is a clinically very interesting option. The question remains why Biodentine, being chemically very similar to ordinary Portland cement, outperforms the latter up to such a significant degree. The experiments (nanoindentation, ultrasonics, light microscopy, mechanical testing) and micromechanical models reviewed in this paper give indications towards answers to the aforementioned question. They concern the very high fineness of the cement power, with micrometer-to-submicrometer particle size, the formation of calcite-reinforced hydration products, and a pronouncedly uniform loading state throughout the microstructural hydrate networks, as fingerprint of an optimized load carrying mechanism.