Effect of kaolin and argillite mixtures on the dielectric properties of geopolymers

Abstract The use of geopolymer in radiofrequency applications and more precisely in antenna is innovative. The objective of this study is to highlight the impact of mixtures of kaolin and argillite on the structure and dielectric properties of geopolymers for antenna application. For this, six geopolymer formulations based on mixtures of kaolin and argillite with different proportions (33, 50 and 67 %) and calcined at 600 and 750 °C were studied using mercury intrusion porosimetry, dielectric measurements and NMR spectroscopy. Low porosity values were obtained whatever the mixture. At 600°C, the porosity increases with the increase of argillite content from 18 to 23%. At 750°C, no impact of argillite content on the porosity (16%) was detected. However, the pore size increases with increasing the argillite content regardless of the calcination temperatures due to the unreacted secondary phases present in argillite. MAS-NMR experiments on geopolymer samples and calcined mixtures have evidenced a strong correlation between reactive aluminum resulting from dehydroxylation of kaolin and clay minerals of argillite and the geopolymerization rate. These structural variations influence the dielectric properties. Indeed, e and tan δ values increase with the increase of argillite content and calcination temperature due to the increase in the number of moles of free alkali earth cations (Ca2+ and Mg2+).