Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite

TLDR

Alkali activation of Al₂O₃/SiO₂‑rich materials can produce high‑strength cements. The study optimises compressive strength of sodium‑silicate‑activated metakaolinite polymers by varying Si, Al, and Na ratios. Polymers were prepared by activating metakaolinite with sodium silicate (made from silica fume and NaOH), varying Si:Al (1–3) and Na:Al (0.5–2) ratios, then curing at 75 °C for 24 h and testing compressive strength after 7 days. Compressive strength varied systematically with composition, peaking at 64 ± 3 MPa for a Si:Al:Na ratio of 2.5:1:1.3, and X‑ray data showed corresponding changes in the amorphous bonding network.

Abstract

High strength cements can be synthesised by alkali activation of materials rich in Al2O3 and SiO2. In this study, amorphous aluminosilicate polymers produced by sodium silicate activation of metakaolinite were studied, with particular reference to chemical optimisation of the compressive strength according to the relative concentrations of Si, Al and Na in the polymer. The sodium silicate was manufactured from silica fume and sodium hydroxide. The compressive strengths of polymers with Si∶Al molar ratios of 1.0–3.0 and Na∶Al molar ratios of 0.5–2.0 were considered. The polymers were cured at 75 °C for 24 h and their compressive strengths measured after aging for 7 days. The strength was found to depend systematically on the relative amounts of Si, Al and Na, with the maximum being 64 ± 3 MPa for an Si∶Al∶Na molar ratio of 2.5∶1∶1.3. X-Ray diffraction/scattering data indicate qualitatively that the bonding network in the amorphous aluminosilicate alters systematically with composition.