TY - CONF A1 - Selleng, Christian A1 - Meng, Birgit A1 - Fontana, Patrick ED - Fehling, E. ED - Middendorf, B. ED - Thiemicke, J. T1 - Phase composition and strength of thermally treated UHPC N2 - Thermal treatment improves the excellent properties of UHPC. Recent studies have shown that an increase in compressive strength of more than 30 % is possible. However, the accurate conditions of thermal treatment for a maximal strength are considered as yet undetermined. A multitude of parameters can be varied: temperature, pressure, water saturation, and duration of the process steps. These parameters influence the phase development and in consequence the macroscopic properties of UHPC. The primary objective of the presented study was the optimisation of the conditions for thermal treatment, concerning compressive strength. It focuses on pre-storage time and duration of the treatment at defined temperatures and pressures (90 °C and 185 °C/1.1MPa). As expected, experimental results showed a fundamental change of phase composition in hydrothermally treated UHPC in comparison to standard cured UHPC: Ettringite decomposes at higher temperatures and is absent after thermal treatment; the amount of portlandite and clinker phases decreases. The change of phase composition is accompanied by increased compressive strength. Experimental studies of calcium-silicate-systems at hydrothermal conditions predict the formation of the crystalline C-S-H phase tobermorite. In fact, this is a typical phase occurring in other hydrothermally treated calcium-silicate-systems like Autoclaved Aerated Concrete (AAC). Commonly, high strength is attributed to the presence of tobermorite; however, in the presented study tobermorite was not detected in the hydrothermally treated UHPC. Therefore, tobermorite cannot be responsible for the increased strength of hydrothermally treated UHPC. In conclusion the development of phases and strength of UHPC at hydrothermal conditions differs fundamentally from AAC and the experimental studies with water saturation. Results of these systems cannot be transferred to UHPC. In thermally treated UHPC, the hydration of clinker is enhanced and the puzzolanic reaction is intensified. Hence, more C-S-H is formed that fills pores and cracks, leading to a denser structure and finally to higher strength. T2 - 4th International Symposium on Ultra-High Performance Concrete and High Performance Materials (HiPerMat) CY - Kassel, Germany DA - 09.03.2016 KW - Ultra-high performance concrete KW - Thermal treatment KW - Compressive strength KW - Tobermorite KW - C-S-H KW - Building Materials PY - 2016 SN - 978-3-7376-0094-1 VL - 27 SP - 7 EP - 8 PB - kassel university press GmbH CY - Kassel AN - OPUS4-36872 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - von Werder, Julia A1 - Simon, Sebastian A1 - Lehmann, C. A1 - Selleng, C. A1 - Fontana, P. A1 - Meng, Birgit T1 - Autoclaving of ultra-high performance concrete (UHPC) N2 - By the combination of an optimized granulometry, a reduced water cement ratio and the use of superplasticizers ultra-high performance concrete achieves a compressive strength of over 150 N/mm2 and a high resistance regarding acids and water-soluble salts. In different Research projects the effect of an autoclavation on the mechanical strength and the phase composition was analyzed. In systematic studies, the concrete mix and the process parameters were varied. The results show that autoclavation leads to an improved pozzolanic and hydraulic reaction and significantly improves the mechanical strength compared to a conventional thermal treatment. If a minimum time-span for hydration is ensured, the achievable strength level is not dependent on the prestorage time. However, the duration of the autoclaving is significant. After the Maximum strength is reached there is only a very slight decrease, even if unrealistically long autoclaving times are applied. T2 - ICAAC 6th International Conference on Autoclaved Aerated Concrete CY - Potsdam, Germany DA - 04.09.2018 KW - Heat treatment KW - Hydroxylellestadite KW - Tobermorite KW - Ultra-high performance concrete KW - Zonation PY - 2018 SN - 978-3-433-03276-3 U6 - https://doi.org/10.1002/cepa.866 SP - 131 EP - 136 PB - Wilhelm Ernst & Sohn - Verlag für Architektur und technische Wissenschaften GmbH & Co. KG CY - Berlin, Germanry AN - OPUS4-46357 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -