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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.
Die Festigkeit von Ultra-Hochleistungsbeton (UHPC) lässt sich
durch (hydro-)thermale Behandlung weiter steigern. In der Literatur (zum
Beispiel Yazici, 2013) wird diese Festigkeitssteigerung mit der Bildung von
Tobermorit, einer kristallinen C-S-H-Phase, in Verbindung gebracht, wie dies
auch für Porenbeton oder Kalksilikat-Baustoffe angenommen wird. Die
durchgeführten röntgenographische Untersuchungen geben jedoch keinen Hinweis
auf das Vorhandensein von Tobermorit, sodass diese Erklärung nicht tragfähig
ist. Die tatsächliche Ursachen für die real vorhandene Festigkeitssteigerung
bleibt vorerst unklar.
Ultra-high performance concrete (UHPC) is characterized by outstanding compressive strength of more than 150 MPa, double of normal concrete. Furthermore UHPC has a very dense structure which leads to an extremely low permeability for fluids resulting in with a very high durability.
Further structural improvement can be achieved with thermal treatment of UHPC. Recent studies have shown that an increase in compressive strength of more than 50% is possible. Nevertheless, the accurate conditions for an optimal thermal treatment are still not determined. A multitude of parameters can be varied: Temperature, pressure, water saturation, duration of the process steps.
Primary object of this study is the optimization of the conditions for thermal treatment focused to the treatment duration at a defined temperature and pressure (185°C / 1,1MPa). Therefore, the thermally treated UHPC samples are analysed by mean of mechanical properties and phase composition.
The development of phases and strength at higher temperature differs fundamentally from other cementitious systems (like normal concrete or autoclaved aerated concrete), although the chemical composition is similar. Because of the very dense structure and the low water/cement ratio the availability of water is low. This influences the hydration process. As a consequence tobermorite is absent in thermal treated UHPC in contrast to thermally treated normal concrete or predictions based on thermodynamic modelling.