Characterisation and properties of geopolymer composite part 1: Role of mullite reinforcement

Geopolymer-mullite composite was prepared using fly ash and mullite powders with sodium silicate and sodium hydroxide as alkaline activators. Mullite was used as a replacement to fly ash in the 20-60 wt% range. Sodium silicate to sodium hydroxide (12 M) ratio was 1:1 while the liquid to solid ratio was 0.6:1. X-ray diffraction (XRD) analysis revealed that the set of geopolymer specimens without mullite replacement (control) showed the co-existence of amorphous and crystalline phases of quartz, magnesioferrite (Fe2MgO4), Lazurite (Na8.56 (Al6Si6O24) (SO4)1.56 S0.44)) and calcium silicate hydrate. With an increasing amount of mullite replacement, calcium silicate hydrate and magnesioferrite diminished while the new phase of phillipsite (K, Na)2(Si,Al)8O16·4H2O) emerged. Microstructural analysis revealed Si-rich mullite needles possibly occurred by recrystallization of the original mullite. This suggestion was also confirmed by the change of the crystallite size as analysed using an X-ray diffraction technique. The ambient compressive strength was found to increase from 58 ± 21 MPa for the control geopolymer to 72-76 MPa, with a much smaller uncertainty, for the geopolymer-mullite composite. Modulus of rupture (MOR) was found to improve significantly from 0.7 ± 0.3 MPa to 3.7 ± 0.5 MPa in the 20% replacement and further to 7.8 ± 1.3 and 8.1 ± 1.1 MPa in the 40% and 60% replacement respectively. Improvement of fire resistance was observed in the 40-60% replacement thermal shock resistance property, however, was unchanged in these geopolymer-mullite composite.