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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.
Concrete technology results more and more in the development of materiale with enhanced properties, such as high strength, higher durability and increased ecological compatibility. One exceptional example is Ultra-High Performance Concrete (UHPC). Its advantages are very high compressive strength and an improved durability, caused by their dense micro structure. Curing under water vapor Saturation pressure (autoclaving) can improve these properties by affecting the micro structure and better the mechanical properties of UHPC. Autoclaving leads to an enhanced cement hydration and accelerated reaction rate of supplementary cementitious materials (SCM) and mineral additions, Used at the production of aerated autoclaved concrete (AAC) and sand-lime bricks, the process of autoclaving is well known, especially in highly porous Systems. One well known requirement to autoclave effectively is a porosity of at least 10%. There is no sufficient heat transfer and also not enough water available if the porosity is lower, Nevertheless, autoclaving also works in the dense structure of UHPC with porosities lower than 2%, as former investigations have shown. There is only a visible difference between the outer parts of the samples compared to their core. Investigations on the chemistry, the micro structure as well as the mechanical properties where performed to find an explanation to this effect and to explain the functionality of autoclaving UHPC in spite of the low porosity.
Composition and microstructure stability of cement compound under cyclic hydrothermal condition
(2020)
There have been many researches focused on the performance improvement of ultra-high performance concrete (UHPC) by autoclaving treatment. The goal of autoclaving is to increase the pozzolanic reaction, and to densify the cement stone and the transition zone which originates from the incorporation of supplementary cementitious materials (SCMs), such as silica fume, fly ash and blast furnace slag. Due to the superior properties, UHPC can also be utilized under high mechanical load and aggressive condition, for example, the fabrication of water tanks for thermal storage which is of great significance for saving energy and reducing CO2 emission. It is known that mineral stability of the hydration products of an inorganic binder is highly related to the temperature and pressure of the environment. A certain stable composition at room temperature, however, may undergo a phase transformation at high temperature and the performance decrease under this severe condition will generally be more severe. In this way, the rationale behind this deterioration under long-term hydrothermal condition involving many cycles and long duration has to be clarified, and then appropriate optimizing methods will be performed in order to obtain a kind of construction with high durability under aggressive environment. For this purpose, different types and amounts of SCMs are introduced into the standard mixture of UHPC and the phase compositions after autoclaving at 200 °C and 15.5 bar are determined by combined X-ray diffraction and scanning electron microscope. Mercury intrusion porosimeter is used to characterise the microstructure of the samples. In order to establish the relationship between microstructure and macroscopic properties, compressive and flexural strength are also investigated.
Gußeisen mit Kugelgraphit und ferritischer bzw. ferritisch-perlitischer Matrix gehören zu einer Werkstoffgruppe, die ausreichende Festigkeiten mit hohen Duktilitäten verbindet. Mit der vorliegenden Arbeit wurden die Kenntnisse über die Zusammenhänge zwischen der chemischen Zusammensetzung, der Gefügeausbildung und den mechanischen Eigenschaften erweitert. Dabei wurden sowohl unterschiedliche Gießverfahren (Sand- und Kokillenguß) in der Herstellung, als auch die Auswirkungen von Umwandlungsprozessen im Gefüge infolge von Wärmebehandlungen diskutiert. Die statistische Auswertung von Einzelergebnissen aus der qualitätssichernden Begutachtung von Lager- und Transportbehältern für radioaktive Stoffe wurden durch mechanische und bruchmechanische Untersuchungen an 24 Werkstoffsorten aus Gußeisen mit Kugelgraphit ergänzt.