Effect of Sodium Hexafluorosilicate Addition on the Properties of Metakaolin Based Geopolymers Cured at Ambient Temperature

In this investigation, the effect of sodium hexafluorosilicate addition on the mechanical and microstructural properties of metakaolin-based geopolymers cured at ambient temperature was evaluated. The geopolymers were manufactured using metakaolin as a raw material with Sodium hexafluorosilicate Na(2)SiF(6)powder as an additive, and a mixture composed of NaOH, KOH, and sodium silicate together as an activating solution. The amount of Na(2)SiF(6)ranged from 1 to 5%, a (SiO2/ (Na2O + K2O)) modulus of 1.3 and a Si/Al ratio varied between 1.60 and 1.70. Geopolymer specimens cured at room temperature under air for 28 days were characterized, using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis (ATG-DTG), scanning electron microscopy (SEM), and compressive tests to assess the microstructure-compressive strength dependency. The findings indicate that the addition of 2% Na(2)SiF(6)improves the compressive strength by 28%, but beyond this value, the resistance progressively decreases, this observation was explained by the fact that this additive rises the Si/Al ratio, which led to an increase in the amount of strong Si-O-Si bonds. The reduced RC has been related to the excess amount of Na in the blend that promotes the formation of efflorescence, leading to crack formation, which negatively affects the mechanical properties.

Automated summary

The study evaluated the effect of sodium hexafluorosilicate addition on metakaolin-based geopolymers cured at ambient temperature. Results showed that adding 2% Na(2)SiF(6) significantly improved compressive strength, but beyond this percentage, the strength gradually decreased. This was attributed to the increase in Si/Al ratio from the additive, leading to more strong Si-O-Si bonds, as well as excess Na promoting efflorescence formation causing cracks and negatively impacting mechanical properties.