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- 2019 (2) (entfernen)
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- Englisch (2)
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- Alite (1)
- Hydrothermal treatment (1)
- Mechanical strength (1)
- Phase composition (1)
- Portland Cement (1)
- Steelmaking slag (1)
- Tricalcium-silicate (1)
- UHPC (1)
Organisationseinheit der BAM
Tricalcium-silicate (C3S) or Alite is the most important mineral in Portland cement. Since pure tricalcium-silicate is only stable above temperatures of 1250 °C, its decomposition has to be prevented technically by fast cooling after the sintering process. At room temperature, the decomposition velocity is very slow so that metastable tricalcium-silicate is obtained.
Although the mechanisms of clinker phase formation during burning process of Portland cement in a rotary kiln were solved and improved over the years, in view of possible economic and ecological benefits current projects aim to produce clinker phases from metallurgical slags. Recent studies discovered that the mineral phase which remained after a reducing treatment and separation of formed metallic iron from molten Linz-Donawitz (LD-) slags contained about 60 wt.% Alite despite it was cooled slowly. Because the results could be verified using slags from different origins and varying cooling velocities a chemical stabilisation of the Alite can be assumed. First tests in mortars indicate that workability, hardening and solid state properties are comparable with an ordinary Portland cement. An application of the observed phenomenon in cement production requires enhanced knowledge about formation and stabilisation conditions of Alite during crystallisation from melts in contrast to the sintering reactions in conventional Portland cement production. Therefore, this study focuses on the stabilisation mechanisms of Alite in consolidating melts. Samples from different melting experiments are analysed to determine stabilising factors.
The advantages of treating prefabricated components made of Ultra-High-Performance Concrete (UCPC) at 90 °C are largely recognized, while hydrothermal treatment at 185 °C and the corresponding saturation pressure of 1.1 MPa, in contrast, is not applied in building practice so far and was studied only by a few researchers.
In the research presented, the parameters pre-storage-time before treatment and dwell time in the autoclave were systematically varied. The results illustrate in which way compressive strength increases with the duration of treatment. Already samples which were only heated up and immediately cooled down show an increase in strength compared to the 28-day reference of about 10 %. After 20 h the maximum increase of about 25 % is reached. Interestingly the compressive strength of samples treated very long hardly decreases. It can be therefore concluded that for the composition investigated the achievable mechanical strength is not severely sensitive regarding the duration of the treatment.
Also, pre-storage times before autoclaving longer than 0.5 days showed no significant impact on strength development.
The phase composition measured by XRD exhibits significant changes compared to the reference. The amount of ettringite and portlandite have vanished, the cement clinker phases are substantially decreased and hydroxylellestadite and hydrogarnet are formed as new phases. The increase in strength can be assigned to an improved pozzolanic and hydraulic reaction. Contradictory to literature, there was found no tobermorite, a mineral which is known to be susceptible for disintegration at Long autoclaving duration. This explains why no significant strength loss appeared.