Influence of sodium-based activators and water content on the fresh and hardened properties of metakaolin geopolymers

• Published in: SN applied sciences 2022-10, Vol.4 (10), p.1-16, Article 283
• Main Authors: Segura, Isabel Pol, Jensen, Peter Arendt, Damø, Anne Juul, Ranjbar, Navid, Jensen, Lars Skaarup, Canut, Mariana
• Format: Article
• Language: English
• Subjects: Applied and Technical Physics; Chemistry/Food Science; Compressive strength; Density; Earth Sciences; Engineering; Environment; Materials Science; Metakaolin geopolymer; Research Article; Rheology; Setting; Shrinkage
• ISSN: 2523-3963; 2523-3971
• DOI: 10.1007/s42452-022-05167-w

Several studies explore optimal molar oxide ratios for metakaolin geopolymer production. However, there is not a consensus on the optimal mix, and within similar range large differences in compressive strength are reported, and consequently in the overall performance. Hence, the present work selects a specific molar oxide ratio that leads to strengths above 30 MPa (SiO 2 /Al 2 O 3 ratio of 3.19, a Na 2 O/Al 2 O 3 of 1.00, and a water-to-solids ratio of 0.52), and investigates the individual effect of the sodium-based activators (NaOH and Na 2 SiO 3 ) and the water content on fresh and hardened properties of metakaolin geopolymers. The tested properties include the rheology, setting time, mass loss, shrinkage, density and compressive strength. The test results show that an increase of water content (water-to-solids > 0.52) and increase of NaOH (Na 2 O/Al 2 O 3  > 1.03) have the largest impact, showing a detrimental effect on both fresh and hardened properties. Moreover, the best results are obtained when using molar ratios of SiO 2 /Al 2 O 3 at 3.14, Na 2 O/Al 2 O 3 at 0.97 and a water-to-solids ratio of 0.51, which is within the range of optimum molar ratios from previous studies. The tests are further supplemented by Thermogravimetric analysis/Differential scanning calorimetry (TGA/DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Article highlights An elevated water content can improve the fluidity of the pastes, however, it increases pore formation and detriments the strength. A surplus of alkali content decreases the fluidity, accelerates the setting, and increases the shrinkage and surface cracking of metakaolin geopolymers. The best performance is achieved with a water-to-solids = 0.51, and molar oxide ratios of Na 2 O/Al 2 O 3  = 0.97 and SiO 2 /Al 2 O 3  = 3.14.