Water molecular dynamics of metakaolin and phosphoric acid-based geopolymers investigated by impedance spectroscopy and DSC/TGA

• Published in: Journal of non-crystalline solids 2016-08, Vol.445-446, p.95-101
• Main Authors: Douiri, H., Kaddoussi, I., Baklouti, S., Arous, M., Fakhfakh, Z.
• Format: Article
• Language: English
• Subjects: Boards; Conduction heating; Dielectric properties; Dielectric relaxation; Dielectrics; Differential scanning calorimetry; Differential Scanning Calorimetry (DSC); Geopolymers; Interstitials; Molecular dynamics; Percolation; Thermal properties
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/j.jnoncrysol.2016.05.013

In this paper, thermal behavior and dielectric response of a series of metakaolin-phosphoric acid geopolymers with different amounts of H3PO4 were investigated. Two thermal phase transitions were detected in geopolymers by means of Differential Scanning Calorimetry (DSC) in a temperature range (−50°C; 175°C). These transitions were absent for metakaolin and were attributed to free and interstitial water evaporation. These results were confirmed with mass losses in geopolymers observed through Thermogravimetric analysis (TGA). In order to probe molecular mobility and dielectric relaxations, samples were characterized by dielectric spectroscopy in a board temperature range (−50°C–200°C) and in a frequency range of 0.1Hz to 1MHz. Results showed that in the case of metakaolin, only a percolation process was detected occurring as a result of the migration of protons and ions filling the porous space. For geopolymer materials, different dielectric processes were observed. In fact, an ice like water relaxation processes appeared at low temperatures followed with a percolation processes. At temperatures above 100°C, two new dielectric relaxations were detected and were attributed to the reorientations of free and interstitial water molecules, respectively. Besides, a conduction phenomenon noted for high temperatures occurring as a result of the carrier charges diffusion. All these processes showed a dependency on the amount of phosphoric acid added to the metakaolin. Indeed, activation energies and relaxation times calculated basing on Arrhenius law and Macedo and Litovitz laws as well as conduction process were to be found strongly dependent on the H3PO4 concentration. •Molecular dynamics in metakaolin-H3PO4 geopolymer materials were studied.•DSC-TGA results revealed two thermal phase transitions related to water evaporation in geopolymer materials.•Metakaolin showed a percolation process resulted from the migration of protons and ions filling the porous space.•Geopolymers presented new water dielectric relaxations in addition to a percolation process and a conduction phenomenon.•All these dielectric processes were strongly dependent on the H3PO4 concentration.