Identification of mechanisms of action of conventional superplasticizers and novel bio-based flow agents in calcium-free geopolymer model systems

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In recent years, the focus in construction industry has transitioned towards innovative technologies and materials. It is well known that the production of cement has a significant impact on the environment; by manufacturing around 4 Gt of cement annually, the carbon emissions created during the production process are accounted to 1.5 Gt. Hence, the search for alternative binders, that are preferably independent from other industrial sectors, is essential. One such binder is calcined clays, which are available worldwide but vary widely in their chemical and mineralogical composition. Furthermore, no suitable admixtures currently exist for calcium-free geopolymer systems. The objective of this research project is to identify suitable superplasticizers for calcium-free geopolymers and to determine the mechanism of action using geopolymer model systems. As model systems, metakaolin- and metaton-like base materials are synthesized via a sol-gel process and mixed with water glass solutions. This allows the use of calcium-free systems. Commercially available polycondensate superplasticizers (melamine sulfonic acid and β-naphthalenesulfonic acid formaldehyde polycondensates) and PCE superplasticizers with variation of the backbone (MPEG, APEG, HPEG and IPEG) are utilized to disperse the geopolymer model systems. Initially, the flow effect and chemical stability in the highly alkaline environment is determined due to structural investigations. In addition, starch superplasticizers that exhibit high stability in the highly alkaline environment of calcium-free geopolymers are synthesized. Starch superplasticizers are synthesized by varying the molecular parameters of charge type, charge amount, and molecular mass (<100 kDa and >100 kDa), and key parameters, that induce high dispersion performance in the calcium-free model geopolymers, are identified. Rheological experiments are conducted to characterize the flow behavior of the model geopolymer systems with addition of the effective superplasticizers. The change in viscosity and yield point as well as the time dependence of the rheological properties are recorded and compared to a reference geopolymer glue. Investigations on the change of the zeta potential due to the addition of effective superplasticizers provide insight whether the flow effect is caused by surface interactions. Moreover, degrees of adsorption during the early geopolymer reaction are determined. The results can be associated with the structural parameters of the effective superplasticizers. Microstructural investigations can provide information if the effective superplasticizers change the contents of air, capillary, and gel pores in hardened geopolymer glues. Then, the findings on the effectiveness of superplasticizers in the geopolymer model systems can be evaluated and the results can be converted into a generally valid mechanism of action (empirical model).