Ultrasound enhanced geopolymerisation

Abstract

The feasibility of using ultrasound to enhance the geopolymerisation of metakaolinite/sand and fly ash/metakaolinite mixtures was investigated. The introduction of ultrasonication into the geopolymerisation systems increased the compressive strength of the formed geopolymers and the strength increased with increased ultrasonication up to a certain time. The dissolution of metakaolinite and fly ash in alkaline solutions was enhanced by ultrasonication, hence releasing more Al and Si into the gel phase for polycondensation. SEM analysis demonstrated that ultrasonication improved the distribution of the gel phase in the geopolymeric matrices and strengthened the binding between the particle surfaces and the gel phases. XRD patterns showed that ultrasonication enhanced the formation of semi-crystalline to crystalline phases in the formed geopolymers. The 27Al MAS-NMR spectra showed 27Al chemical shifts at around 55 ppm for the geopolymers synthesised with and without ultrasonication, indicating that Al was tetrahedrally coordinated in the form of Al(4Si). 29Si MAS-NMR studies showed that ultrasonication largely improved the interlinkage between Si and Al species, increased the concentrations of polysialate species and enhanced the ordering of the Si and Al tetrahedra in the gel phase in geopolymerisation. Both 27Al and 29Si MAS-NMR spectra indicated an increased extent of polymerisation between Al and Si species in the presence of ultrasonication. The thermal analysis indicated that ultrasonication improved the thermal stability of the formed geopolymers. The improved performance of the ultrasonically formed geopolymers in terms of compressive strength and thermal stability could be attributed to the accelerated dissolution of the Al—Si source materials, the strengthened bonds at the solid particle/gel phase interfaces, the enhanced polycondensation process and the increased semi-crystalline and crystalline phases.