Microstructure and physical properties of fly-ash-based geopolymer with the addition of expanded vermiculite

Fly-ash-based geopolymer pastes with the addition of expanded vermiculite (EV) powder were synthesised and their microstructure, compressive strength, setting time, moisture control, extent of efflorescence and thermal conductivity were studied. It was found that the addition of EV resulted in an increase in the standard-consistency water consumption and setting times. As a consequence, excessive addition of EV resulted in a larger amount of harmful pores, which was detrimental for the compressive strength of the paste. However, geopolymer pastes with an appropriate amount of EV (2–7 wt%) showed a slight increase in compressive strength because of the filler effect. Mg²⁺ and Fe³⁺ diffused from the EV interlayers through ion exchange between the EV and the geopolymer, and participated in geopolymerisation. This was reflected by the formation of N-(M)-A-(F)-S-H, as evidenced by scanning electron microscopy with energy dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy. In addition, Na⁺/Mg²⁺ or Na⁺/Fe³⁺ ion exchange reduced the mobility of Na⁺ the ions and therefore decreased the extent of efflorescence. Moreover, EV addition favoured an improvement in moisture content and the thermal conductivity properties of the geopolymer paste.