Geopolymers from low-T activated kaolin: Implications for the use of alunite-bearing raw materials

The feasibility of using kaolins activated at T = 550 degrees C to synthesize geopolymers has been assessed, opening the way for utilizing low-grade kaolins containing sulfates. The effects of temperature (in the range 40-90 degrees C) and time (in the range 1-19 h) of curing, as well as of a secondary drying treatment on compressive strength and linear shrinkage of geopolymers have been studied by experimental design technique using a high-quality kaolin as starting material. Geopolymers characterized by high strength (>60 MPa) and low shrinkage (<1%) have been obtained when long curing times are applied at T<65 degrees C, while the oven-drying treatment is demonstrated not to improve geopolymer performances. Geopolymers synthesized starting from an alunite-bearing kaolin activated at 550 degrees C and cured at the conditions suggested by the statistical model showed good compressive strength and shrinkage. All geopolymers have been characterized by thermal analysis, X-ray powder diffraction and infrared spectroscopy.