Synthesis of geopolymer with KOH by two kaolinitic clays from the Amazon: Influence of different synthesis parameters on the compressive strength

The term geopolymer refers to "inorganic polymers " formed by a chemical reaction between solid aluminosilicate compounds and a highly concentrated alkaline hydroxide solution. The properties of geopolymers mostly result from the chemical and mineralogical composition of the material used as a source of aluminosilicates. Considering the influence of the raw material for the synthesis of geopolymers, the present study evaluates the use of two different types of kaolin in the synthesis of geopolymers using KOH as an alkaline activator. For the present study, a sample of Belterra clay (KLN-1) from the bauxite deposits of Rondon do Par & PRIME;a, a sample of processed kaolin (KLN-2), and microsilica (MCR-1) (Ecopodium) and PA reagent from Sigma (KOH) were selected. The samples were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. For the synthesis of the geopolymers, the KLN-1 and KLN-2 samples were calcined at 750 C for 2 h. The syntheses of the geopolymers were performed at different KOH molar concentrations (12, 13, 14, 15 and 16), at different curing temperatures (60, 70, 80, 90 and 100 C) and for different curing times (24, 48, 72, 96, and 120 h). The geopolymers were characterized by mechanical strength testing and FTIR. The samples had different mineralogical compositions, which contributed to different compressive strength results. The samples showed greater resistance at concentrations of 12 and 16 M. The curing temperature of 80 C exhibited better mechanical compressive strength. At 120 h, the best compressive strength value was obtained, indicating that a longer time favored the geopolymerization reaction. The greater mineral diversity affects negatively the geopolymer reaction contributing to lower mechanical resistance.