Coal fly ash as raw material for the manufacture of geopolymer-based products

Abstract In this work coal fly ash has been employed for the synthesis of geopolymers. Two different systems with silica/alumina ratios stoichiometric for the formation of polysialatesiloxo (PSS, SiO 2 /Al 2 O 3  = 4) and polysialatedisiloxo (PSDS, SiO 2 /Al 2 O 3  = 6) have been prepared. The alkali metal hydroxide (NaOH or KOH) necessary to start polycondensation has been added in the right amount as concentrated aqueous solution to each of the two systems. The concentration of each alkali metal solution has been adjusted in order to have the right liquid volume to ensure constant workability. The systems have been cured at four different temperatures (25, 40, 60, and 85 °C) for several different times depending on the temperature (16–672 h at 25 °C; 72–336 h at 40 °C; 16–120 h at 60 °C and 1–6 h at 85 °C). The products obtained in the different experimental conditions have been submitted to the quantitative determination of the extent of polycondensation through mass increase and loss on ignition, as well as to qualitative characterization by means of FT-IR spectroscopy. Furthermore, physico-structural and mechanical characterization has been carried out through microscopic observations and the determination of unconfined compressive strength, elasticity modulus, apparent density, porosity and specific surface area. The results have indicated that the systems under investigation are suited for the manufacture of pre-formed building blocks at room temperature.