On the structure and the formation of the structure of C-S-H phases in future cementitious binders
Final Report Abstract
The aim of the DFG-funded project was to gain fundamental insights into the crystallisation of C-S-H. In addition, the increasing use of composite materials, such as calcined clays, was to be investigated with regard to C-S-H formation. In particular, the early time of C-S-H formation was the focus of the investigation, with special emphasis on the detection of precursor phases. In a first step, investigations were carried out on the influence of ground granulated blast-furnace slag (GGBFS) and especially calcined kaolin (MK) as representatives of cement composite materials on the hydration of Portland cements. The results of the chemical composition of the aqueous phase were subsequently used to adjust real experimental parameters for the targeted synthesis of C-S-H. The results showed that both GGBFS and MK were suitable for the hydration of Portland cements. It was found that both GGBFS and MK influence the aqueous phase and the hydration of Portland cement differently. Thus, the Na, Al and S concentration in the aqueous phase decreases when MK is used, which indicates an interaction (adsorption, formation of ettringite and monosulphate, respectively). At the same time, it was found that the sulphate supply in the aqueous phase was high and that the presence of MK or GGBFS did not lead to any impairment of the hydration kinetics. In the second step, the influence of MK on hydration was fundamentally investigated on the model system C3S (pure form of the main clinker mineral alite). It was shown that the reaction of MK is significantly dependent on the calcium ion concentration (and thus pH) of the aqueous phase as well as on the dosage of MK. It could be shown that C-A-S-H is formed within a few minutes. Furthermore, it could be investigated that C-S-H has an accelerating and C-A-S-H a retarding effect on the hydration kinetics. In this context, the importance of the correct sulphation of the system could be underlined. In the last step, methods were developed that enable a reliable synthesis of C-S-H. This step was essential for the synthesis of C-A-S-H. This step was crucial for achieving the main goal of the project, i.e. the detection of C-S-H precursor phases. It turned out that the synthesis of C-S-H from the hydration of C3S is a very suitable method, which, however, requires increased demands on the experimental work with increased effort. Since this solution was considered to be target-oriented, the main part of the project time was put into the method development. Particular attention had to be paid to ensuring that no secondary phases such as portlandite and calcite were formed, as the analytics used do not work in a phase-selective manner. In addition to the experimental set-up, the method development also involved optimising the measurement parameters of the analytical ultracentrifuge. This development work arose in the course of the investigations and could not have been foreseen in the application. It was finally possible to verify the existence of C-S-H precursor phases (clusters) with high reproducibility. Thus, for the first time, the crystallisation of C-S-H without foreign ions was described on the basis of the nonclassical nucleation theory. Future work should address the influence of foreign elements on the formation of C-S-H via precursor phases, which play an increasingly crucial role in practical Portland cements, such as aluminium, iron and sulphate. This is due to the fact that the use of Portland cement clinker must be reduced in order to reduce CO2 emissions, but at the same time its performance must increase.
