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

•Kaolin activated at low-T is successfully used to produce geopolymers.•Alunite-bearing kaolins are suitable raw materials for geopolymer production.•Optimal curing conditions are conveniently identified by DoE technique.•T–t interaction and T-square terms influence product performances the most.

The feasibility of using kaolins activated at T = 550 °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 °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 °C, while the oven-drying treatment is demonstrated not to improve geopolymer performances. Geopolymers synthesized starting from an alunite-bearing kaolin activated at 550 °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.