Role of alkali cation in compressive strength of metakaolin based geopolymers

In this work, we carried out comprehensive studies on the role of alkali cations in the geopolymerization reaction and the subsequent network formation which affected the compressive strength of metakaolin (MK) based geopolymers. Rheological studies were used to provide real-time information for the geopolymerization from the initial stage. It was found that the difference in dissolution tendency between Na+ and K+ resulted in different morphologies, which affected the compressive strength. The compressive strength of the systems using Na+ was found to be higher than those using K+ under the same Si/Al ratio and alkali cation concentration. The highest compressive strength (~40.1 MPa) was obtained from a system using 12 M of Na+ with Si/Al ratio of 1.9:1 where a continuous monolithic microstructure was observed. Pycnometer was employed for the first time to gain an insight into the factors responsible for the compressive strength when similar morphologies were observed. The observation obtained from pycnometer compensates for the lack of visual evidence seen from similar morphologies and can serve as additional evidence for the different compressive strengths of MK based geopolymers.