TY - CONF A1 - Voigt, Marieke A1 - Hirsch, T. A1 - Meng, Birgit A1 - Stephan, D. T1 - Hydration and crystallization of Portland cement at 100 C N2 - For the construction of hot water storage tanks, ultra high performance concrete proofed to be a suitable building material to ensure water tightness. Common storage tanks build with concrete are designed to endure temperatures up to 90 °C. To increase the energy efficiency however, modern tanks need to withstand temperatures above 100 °C. With increasing temperature and correlating water vapor pressures, the cement must meet new requirements. Thermodynamic modelling is a promising tool for predicting the phase stabilities of minerals in the cement within these temperature and pressure ranges (T/P). Thus, mixture designs can be investigated and optimized with regard to the expected mineral phases in selected T/P-ranges. Additionally, predictions of the long-term material behavior can be derived from thermodynamic data and subsequently the experimental effort can be reduced notably. On the other hand, validation of predictions is needed. The base for the thermodynamic model is a thermodynamic equilibrium and thus, the experimental data for validation should approach the presumed conditions of the equilibrium. This study investigates variable experimental conditions for the hydration of Portland cement at 100 °C to reach a sufficiently progressed development of the phase composition. In case of Portland cement, the highest degree of hydration achievable within a reasonable time span is the most significant parameter. Therefore, Portland cement pastes with w/c ratios from 0.4 to 0.6 and dwell times up to three weeks were examined with powder X-ray diffraction to identify mineral phase changes. At 100 °C and high humidity, Portland cement paste shows the expected mineral phases like portlandite, katoite, and remaining clinker phases. The cement pastes with high water content and long dwell time seem to have the highest degree of hydration, even though the AFm phases stabilize with increasing w/c ratio. A dwell time of two weeks and a w/c ratio of 0.5 was found to provide the mineral composition with the highest degree of hydration including the major phase transitions. T2 - 4th International Conference on the Chemistry of Construction Materials (ICCCM) CY - Karlsruhe, Germany DA - 26.09.2022 KW - Hydration KW - High temperature KW - Portland cement PY - 2022 AN - OPUS4-56866 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Voigt, Marieke A1 - Gardei, Andre A1 - Meng, Birgit T1 - The adaption of the set-up for gas permeability measurements for ultra-high performance concrete N2 - In the framework of this project, a steam pressure vessel was to be developed from ultra-high perfor-mance concrete (UHPC) to withstand process temperatures of 200 °C and the respective steam pressure of 15,5 bar. To guarantee the long-term water vapour tightness of the system, the permeability of two UHPC mixtures were tested after long-term cyclic autoclaving. As UPHC shows a high density and therefore low permeability, measurements after the RILEM-Recommendation (TC 116-PCD) reached their detection limit. Therefore, the measurement set-up was adapted to measure the permeability of highly dense UHPC more reliably and quickly. This adaption includes measurements in a higher pres-sure range, the change of sample size thickness and the usage of Ar as a medium compared with the RILEM Recommendation. Additionally, the system was equipped with two pressure sensors and Ar-flowmeters to guarantee a continuous record of the experimental parameters. The new system was tested and evaluated by comparing measurements with the RILEM recommended system. Preliminary results show that the adapted system is capable to provide efficient and reliable results of concrete with low permeability. T2 - 14th fib PhD Symposium in Civil Engineering CY - Rome, Italy DA - 05.09.2022 KW - Hydration KW - High temperature KW - Portland cement PY - 2022 AN - OPUS4-56859 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Stephan-Scherb, Christiane T1 - Novel insights into high temperature corrosion phenomena by advanced X-ray methods N2 - A variety of materials of technological interest change their properties through contact with reactive media. Solid-gas reactions lead to a variety of reaction products on the surfaces and internal interfaces. The observation of nucleation and growth processes in the environment where they occur (in situ) from a chemical-structural perspective is especially challenging for aggressive atmospheres. The talk presents innovative approaches to study corrosion mechanisms using advanced X-ray methods. Using energy dispersive X-ray diffraction and X-ray absorption spectroscopy in different tailor made environmental reaction chambers, valuable insights into high temperature oxidation and sulfidation processes were gained. Fe-based alloys were exposed to hot and reactive atmospheres containing gases like SO2, H2O and O2 at 650°C. During the gas exposure the tailor made reaction chambers were connected to a high energy diffraction end station at the synchrotron. The crystallization and growth of oxide and sulfide reaction products at the alloy surfaces were monitored by collecting full diffraction pattern every minute. Careful examination of shape and intensity of phase-specific reflections enabled to a detailed view on growth kinetics. These studies showed, oxides are the first phases occurring immediately after experimental start. As soon as reactive gas media enter the chamber, the conditions change and different reaction products, such as sulfides start to grow. A comparison of different gas environments applied, illustrated the differences in the type of reaction products. The in situ observation of high temperature material degradation by corrosion made it possible to study the contribution of phases, which are not stable at room temperature. For instance, wuestite (Fe1-xO), was frequently observed at high temperatures in humid gases on Fe with 2 wt.% and 9 wt.% chromium, but not at room temperature. The strength of the occurrence of this phase additionally explains why, despite a higher Cr content, ferritic alloys with 9 wt.% Cr in a challenging atmosphere prevent the intrinsic formation of protective layers. The in situ observations were supplemented by careful considerations of thermodynamic boundary conditions and detailed post characterization by classical metallographic analysis. Additionally, the structure and chemistry of the dominant oxide layers were evaluated using X-ray absorption near edge structure spectroscopy. The talk will give an overview about chances and challenges for studying high temperature corrosion phenomena by advanced X-ray methods. T2 - MRS Spring Meeting CT08.02.01 CY - Online Meeting DA - 18.04.2021 KW - XRD KW - Spectroscopy KW - Corrosion KW - High temperature KW - In-situ PY - 2021 AN - OPUS4-52486 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Stephan-Scherb, Christiane T1 - Unravelling high temperature oxidation phenomena by in situ x-ray techniques N2 - Unravelling high temperature oxidation phenomena by in situ x-ray techniques. A multi techqnique approach to study high temperature gas corrosion is presented. T2 - Gordon Research Conference on High Temperature Corrosion CY - New London, NH, USA DA - 20.07.2019 KW - Corrosion KW - High temperature KW - Diffraction KW - Spectroscopy PY - 2019 AN - OPUS4-48772 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Pohl, Christoph A1 - Unger, Jörg F. T1 - Modeling thermomechanical damage mechanisms of concrete N2 - Compressive strength of concrete is highly temperature dependent. Using experimentally obtained relations as input for numerical simulations is problematic. A more accurate and reliable material model results from taking the coupling between thermal and mechanical behaviour into account. In this contribution, a coupled finite element solution on mesoscale geomtries is shown to exhibit a loss in compressive strength at higher temperatures. This is purely a result of the incompatible expansion of mortar matrix and aggregates, with no explicit temperature dependency of the employed constitutive models. T2 - VII International Conference on Coupled Problems in Science and Engineering CY - Rhodos, Greece DA - 12.06.2017 KW - Compressive strength of concrete KW - High temperature KW - Finite elements KW - Mesoscale KW - Thermomechanics PY - 2017 AN - OPUS4-40672 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -