Geopolymer materials based on different aluminosilicate sources

Geopolymers synthetized from the reaction of metakaolin (or alumino-silicate) in an alkali medium are studied since decades [1]. However, in order to develop composites, alkali medium is undesirable due to the detrimental effect of alkali ions on fibers. As an alternative, the use of an acid medium seems promising as some authors demonstrated an increase of mechanical properties with acid-based geopolymers [2, 3]. The aim of this work is to investigate the various formulations and to understand the reactional mechanisms. The work is based on the activation of an alumino-silicate source with phosphoric acid to obtain geopolymer The various formulations consist to determine the Al/P ratio leading to consolidation at various temperatures (from 20 to 70°C). The samples were characterized by Fourier Transform Infrared and Nuclear Magnetic Resonance spectroscopies and X-Ray Diffraction measurements in order to study their structure. The thermal and mechanical properties were correlated with the microstructure [4]. Consolidated geopolymers were synthetized with different Al/P ratios. The consolidation time decreases with increasing Al/P ratio. For instance, the material realized with Si/Al=1,17 consolidates at 20°C in 15 and 8 days with Al/P=4 and 1, respectively. Tow behaviors can be distinguished samples presenting good fire resistance (Si/Al=1,17 and Al/P=1) or presenting poorly water resistant (Si/Al=1,17 and Al/P=4). Consequently, the impact of the reactivity of the alumino-silicate source on the geopolymerisation kinetics leads to different types of structures (secondary and metastable phases) in relation with the phosphor content. References: [1] J.Davidovits, J. Therm. Anal. 37 (1991) 1633-1656. [2] D. S. Perrera, J Mater Sci, vol. 3, pp. 6562-6566, 2008. [3] H. K. Tchakouté and C. Rüscher, Applied Clay Science, vol. 140, pp. 81-87,2017. [4] H.Celerier and al., under submission