Influence of alkali activator on fly ash based geopolymer formation

The aim of this work is to study the influence of the type of activator on the formulation of geopolymers based on fly ash. Geopolymers were made from fly ash from thermal power plants and biomass. Basic activators (NaOH, quicklime, sodium silicate) were added as well as silicates in order to develop binding properties other than those resulting from the use of cement. The results showed that the basic activators based on 5 mol/L of NaOH and 22% of sodium silicate (N5-S22) gave a greater compressive strength. The compressive strengths of the regular fly ash mortar with activators N5-S22 and N5-C10 (5 mol/L NaOH and 10% quicklime) are respectively 14.3 MPa and 10.8 MPa while with biomass fly ash and activators N5-S22 and N5-C10, the compressive strength of geopolymers reached 6.8 MPa and 5.9 MPa respectively. The greater properties of geopolymers obtained with regular fly ash are due to the larger amount of amorphous silica and alumina phases. Scanning electron microscopy and calorimetry analysis provided a better understanding of the observed mechanisms.