On the structure and the formation of the structure of C-S-H phases in future cementitious binders

• Applicants: Professor Dr. Helmut Cölfen (†); Professor Dr.-Ing. Horst-Michael Ludwig
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 401097244

Concrete is the main building material in our time. At a global level, cement is the most important binder within the concrete. But the manufacture of cement is associated with a strong emission of CO2. In order to reduce these emissions, alternatives are currently worked out worldwide. In a medium-term, a good alternative is to partly substitute Portland cement clinker by composite materials (SCMs). The reaction of Portland(composite) cements with water leads to the formation of nanoscaled Calcium-Silicate-Hydrate (C-S-H) phases -the main hydrate phases in hardened concrete. These phases are responsible for most of all macroscopic properties of concrete. Both structure and formation of structure of C-S-H are currently not sufficiently known, and are the main subjects of the proposal. It is essential to know both parameters for the successful application of the new cements and also to pave the way for these binders for new applications. New approaches regarding structure formation will be transferred from the fundamental science to engineering based on the collaboration of the two applicants. This is even more important, since it is found that crystal formation is not restricted only to monomer-monomer attachment. Also the aggregation of precursor phases is a potential route for crystallization (non-classical crystallization). This opens new perspectives in the design of the macroscopic properties of concrete.The proposal is systematically subdivided into five parts. The conditions of formation of C-S-H phases is investigated within cements with different SCMs during the first part (real system). In the second part, these phases are synthesized in a pure way (model system). Within the third part, the formation of the structure via potential precursor phases and the impact of foreign ions is mainly investigated by means of Analytical Ultracentrifugation and Dynamic Light Scattering. Here it is of central relevance if potential precursor phases do exist in cementitious systems, and thus are further indications for the non-classical crystallization theory. In the fourth part, the investigation of the structure (crystal lattice, solubility) of these C-S-H phases is mainly focused with special attention on the impact of the foreign ions introduced by SCMs (concentration and kind). Here high requirements have to meet regarding synthesis and methods. Thus, spectroscopy, electron microscopy as well as diffraction with synchrotron radiation will complementary be performed. The interfacial transition zone between aggregates and cement stone is investigated in the last part. Here we anticipate to build a bridge from fundamental science to concrete. The results of the project will be interdisciplinary discussed and published in appropriate journals. To get a step forward in understanding the structure and the formation of the structure of the most important reaction product of cement is the main aim of the proposal.

• DFG Programme: Research Grants