A freeze-drying-based hydration stop technique used for the time-variant analysis of the early hydration products of cementititous suspensions

Abstract

The present dissertation is dedicated to the development of a lyophilisation-based hydration stop technique, which works as instantaneously, completely, and non-invasively as possible in order to enable a complete characterisation of the early hydration stage with a particular focus on changes to chemical composition and morphology of the sample and in particular of the early hydration product ettringite that is of technical importance due to its influence on rheological properties. Furthermore, based on the developed lyophilisation technique, the impact of ettringite on the rheological properties of a model system containing quartz sand is quantified. Lastly, changes to hardened cement paste induced by the addition of hard magnetic iron oxide nanoparticles are investigated.

It is essential to stop the hydration process instantaneously and entirely at any given time since the properties of hardened concrete can already be derived from the rheological properties of the cement paste suspension. However, it was not feasible with the state-of-the-art isopropanol-water exchange. Thus, a freeze-drying-based hydration stop technique had to be developed. In a first study, cementitious suspensions that were stopped at four different hydration times by lyophilisation, the state-of-the-art, as well as the combination of both techniques mentioned above, were compared. The ambiguous results of the samples’ comparison led, in the next step, to the examination of the influence of the levels and duration of treatment with low-pressure on pure synthetic ettringite. It was shown that a low-pressure of 400 Pa does not change the chemical composition nor the morphology of ettringite for at least 72 h. These results were published in a second article. Based on these, the freeze-drying technique was optimised to the extent that the ettringite content is highest after this hydration stop. The samples show the highest reproducibility compared to the state-of-the-art and the combined technique, leading to the third published article.

Further, the influence of ettringite on the rheological properties was quantified by characterising a model system based on quartz sand with different pre-set amounts of in situ formed ettringite. Derived data sets could allow for a differentiation between various influencing factors (exempli gratia simultaneous competitive hydration processes) being present in a cementitious suspension.

In order to generalise the applicability of the developed method, hard magnetic iron oxide nanoparticles were added to cementitious suspensions, in turn, to investigate their influence on physical properties in the hardened state. It was shown that adding magnetic nanoparticles significantly influences the mechanical and magnetic properties (as magnetisation) of hardened cement paste, with the extent of these effects depending on the amount added.

Five publications, which I was responsible for as the first author, form the basis of this dissertation. These have been published in internationally renowned journals and are presented in logically chronological order.