Investigations into the colloid-osmotic pressure of ASR products

• Applicant: Dr.-Ing. Colin Giebson
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448034144

The ASR in concrete produces products with different chemical composition and appearance. While the chemical basics of ASR has been largely revealed, the expansion mechanism has not been sufficiently understood. The results of the investigations conducted in the preceding project represent a decisive contribution to this understanding and can be summarized as follows: 1. Ca-free ASR products contain freely moving colloidal particles with a size between 50-600 nm. 2. The colloidal particles cannot pass through pores smaller than themselves. This results in semi-permeable conditions leading to the Donnan effect and osmosis. 3. With increasing Si concentration in the ASR products, the particle concentration and thus the osmotic potential increase. 4. By adding Ca, the particles are linked irreversibly, losing their osmotic activity and forming sponge-like structures with residual particles of colloidal size and crystalline phases. 5. Based on the colloidal nature of ASR products, the ASR expansion of concrete can be explained. The results of the project confirmed that ASR products can be understood as colloidal systems. This opens up a new approach in understanding ASR expansion in concrete. With the continuation of the project, the following two main objectives shall be pursued: 1. Investigations into the colloid osmotic pressure of ASR products shall clarify the magnitude of the osmotic pressures generated from ASR products as function of their Si and Ca concentration. The impact of the membrane (material, pore size) will also be taken into account. The main objective is to assess, whether the measured pressures can explain ASR damage of concrete and whether there is an exponential relationship between the Si concentration in the ASR product and the magnitude of the osmotic pressure. In order to do so, it is essential to determine the particle concentration in the ASR products more precisely. 2. Investigations into the temporal course of the ASR product formation shall clarify whether the formation of ASR products can be verified experimentally based on the Donnan effect. In addition, by using existing silica dissolution rates for a known alkali-reactive aggregate derived in a previous DFG project, it will be determined how long it takes in concrete until a critical Si or particle concentration respectively is reached. Based on the results, the model developed in project shall be quantified (measurement of osmotic pressures), verified (proof of the Donnan effect) and further studied in detail with comprehensive accompanying investigations. This can help later to break new ground in ASR prevention and to develop a new, correct ASR model.

• DFG Programme: Research Grants
• Major Instrumentation: Messgerät zur Nanopartikel-Tracking-Analyse
• Instrumentation Group: 1950 Partikelzählgeräte und -klassiergeräte (optisch, elektronisch, außer 35