Study of Carbonated Clay-Based Phosphate Geopolymer: Effect of Calcite and Calcination Temperature

Abstract

This study aims to use natural carbonated Tunisian clay as an aluminosilicate precursor for the elaboration of phosphate-based geopolymers, which yields to the valorization of this common material in Tunisia. In addition, the presence of calcium carbonate in this clay allows the investigation of this associated mineral’s effect on the properties of geopolymeric materials. To achieve these purposes, several experimental techniques were used, namely fluorescence (FX), particle size analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA), dilatometric analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The mechanical strength and the open porosity of the obtained geopolymeric samples were tested by the compression test and the standard test method for water absorption, respectively. The findings of this work show that the used Tunisian clay can present an attractive aluminosilicate precursor for the synthesis of phosphate-based geopolymers. It also shows that the chosen calcination temperature of the raw clay considerably modifies the reactivity of minerals during geopolymerization and, consequently, strongly affects the properties and structure of the geopolymeric samples. These effects were attributed essentially to the formation of new calcium crystalline phases in the obtained geopolymeric samples. In fact, the anorthite (CaAl2Si2O8) phase appears in all the samples but in greater abundance in those obtained from the clay calcined at 550 °C, and the brushite phase (CaHPO4·2H2O) appears only in the samples obtained from the clay calcined at 950 °C. All these new crystalline phases are strongly dependent on the state of the calcite present in the calcined clay.