Adsorption of superplasticizers in alkali-activated materials - Fluorescence microscopic in-situ investigations

• Applicant: Dr. Alexander Wetzel
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 460294323

Alkali-Activated Materials (AAM) have advantages compared to Ordinary Portland Cement (OPC) in terms of CO2 balance and in many cases in terms of durability. This increase in durability is due to the formation of strength-forming phases, which differ from the C-S-H phases in concrete made of OPC. The raw materials used are mineral materials rich in silicon and aluminium, which are ideally present as fine particles and in reactive form. Mainly used precursors are ground granulated blast furnace slag, fly ash or calcined clays. Since these materials belong to the group of latent hydraulic or pozzolanic materials, a stimulation of the chemical reaction is necessary. Alkali activators can be used for this purpose; alkali hydroxide or water glasses are mostly used, but, alkali sulfate or carbonate solutions can also be used. Despite the variety of possible combinations of the precursors and alkali activators, there are still no effective superplasticizers known for these systems, if one wants to compare the effectiveness with that of OPC. Most available superplasticizers lose their effectiveness, especially at higher concentrations of the alkali activators. A complex matrix of possible interactions results from the combination of precursors, types and concentrations of alkali activators as well as the types of superplasticizers, which require a high empirical effort in fresh and hardened concrete investigations. In the recently finalised DFG-funded project 389127734 the adsorption behaviour of superplasticizers on mineral particles was investigated in-situ by fluorescence microscopy. This method of analysis was established within the project and results on the adsorption behaviour of superplasticizers could already be directly correlated with rheological properties and the setting behaviour in cementitious systems. The influence of cation concentrations on the adsorption behaviour and thus on the effectiveness of superplasticizers could be proven. Thus, this method is suitable for time-resolved investigations of the adsorption behaviour of superplasticizers on various precursors and in different alkaline activator solutions. The aim of this project is the systematic investigation of AAM under variation of the precursors, the activator solutions and the used superplasticizers. Microscopic investigations will be correlated with investigations on fresh and hardened concrete in which the same parameters are varied. In addition to the further development of in-situ fluorescence microscopy for the investigation of the adsorption behaviour of superplasticizers, the correlation of the results of the various investigations will provide general insights into the mechanisms of action of superplasticizers in AAM. This will facilitate the use of alkali-activated binders and improve the performance of these systems. Especially with regard to the increasing importance of the use of OPC-free or -reduced systems, this objective is of interdisciplinary importance.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Bernhard Middendorf