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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.
Ultrahochleistungsbeton (UHPC) zeichnet sich durch eine enorme hohe Druckfestigkeit von über 150MPa aus, während er gleichzeitig eine hohe Dichtheit besitzt. Im Schwerpunktprogramm SPP 1182 der DFG sind umfangreiche Untersuchungen zu verschiedensten Aspekten des UHPC durchgeführt worden, sodass die Voraussetzungen für eine breite Anwendung gegeben sind.
A further improvement of the excellent properties of UHPC can be realized with thermal Treatment. Like for normal concrete, it accelerates the hardening and prevents shrinkage after the treatment. Additionally, an increase in strength can be achieved. Aim of this study was the optimisation of the thermal Treatment conditions of UHPC for very different Treatment methods; heat Treatment at 90 °C for unprotected and sealed samples, hot water bath at 90 °C and hydrothermal Treatment at 185 °C/1.1 MPa. The pre-storage time and the dwell time were systematically varied for each method to gain a higher strength. The compressive strength depends on the manner of Treatment at which higher water accessibility leads to higher strengths. The Phase composition changes considerably with different Treatment tempereatures. Finally, it can activate unhydrated binder components forming additional C-S-H, leading to higher strength.