Studies on the compatibility of different superplasticizers with alkaline activators for low calcium geopolymer binders.

The cement industry is coming into focus, as the annual production of around 4 Gt of cement is responsible for the emission of 1.5 Gt of CO₂ and thus for over 8 % of anthropogenic CO₂ emissions. This leads to the search for alternative binders. Such binders are calcined clays, which are available worldwide but vary greatly in their chemical and mineralogical composition. In many studies, particularly low‐calcium metakaolin is used as calcined clay, which reacts to form a low‐calcium aluminosilicate binder when mixed with a calcium‐free alkaline activator. The adjustment of the properties in the fresh state, especially regarding the consistency of these binders, is almost exclusively achieved by the addition of water, since commercially available superplasticizers are usually ineffective in low calcium geopolymer systems. The objective of this study was to investigate various PCE superplasticizers (MPEG‐, IPEG‐, HPEG‐PCE) with respect to their stability in different alkaline activators (NaOH, KOH, sodium, and potassium silicate solutions). The effectiveness of superplasticizers in low calcium geopolymer binders was verified by rheological tests. Size exclusion chromatography was used to investigate if structural degradation of the superplasticizers occurs. The investigated PCE superplasticizers showed no liquefying effect in the low calcium geopolymer system. This is due to a degradation process, i.e., the hydrolysis of the PEG side chains depending on the alkalinity of the activator.