۱٫ Introduction

Red mud is an inorganic residue generated during the industrial production of alumina by the Bayer process. Red mud is classified as a toxic waste with elevated disposal costs due to the high basicity and leaching potential [1]. An annual world production of 21 million tons of aluminum corresponds to 82 million tons of red mud waste, so that new applications of this industrial residue are urgently needed. Construction materials such as cements, lightweight aggregates and geopolymers are considered interesting solutions for red mud safe disposal [2]. The term ‘geopolymer’ identifies an alumino-silicate material with distinctive environmental advantages compared to ordinary Portland cement. The synthesis involves the dissolution of alumino-silicate raw materials in an alkaline medium, followed by condensation and polymerization to achieve a highly stable three-dimensional network structure, where AlO4 and SiO4 are linked together sharing oxygen ions. The network has the capability to immobilize possible pollutants when wastes (usually industrial residues) are used as alumino-silicate source [3]. Red mud has been already proposed as raw material in geopolymer production, but always with a supplementary Al source, such as metakaolin [4] or fly ash [5], mainly to enhance the mechanical strength of the products. The present study aimed to use red mud as the only Al source and to eliminate the use of relatively expensive Na-silicate activating solution. This was supported by the use, as silica source, of soda lime glass (SLG) from urban waste collection. The specific waste glass fraction employed, where plastic and ceramic impurities are concentrated, is currently mostly landfilled [6].

۲٫ Materials and methodology

Red mud (RM) with chemical composition: SiO2=5.21%, Al2O3=15.21%, Fe2O3=52.94%, CaO=2.95%, Na2O=2.40%, K2O=0.63wt.% (Gardanne, France) and soda lime glass (SLG) (SASIL S.p.a, Biella, Italy) with hemical composition: SiO2=70.50%, Al2O3=3.20%, Fe2O3=0.42%, CaO=10.00%, Na2O=12.00%, K2O=1.00 wt.%. were dissolved in NaOH solution of desired concentration (4M and 6M) [7]. The SiO2/Al2O3 theoretical molar ratio of the final geopolymers was fixed at 5, 6 and 7, which represents initial mixtures of SLG/RM in wt.% of 40/60, 45/55, 50/50, respectively. The sample designation was established as xSyM, where “x” is associated with the SiO2/Al2O3 molar ratio, “y” refers to the molarity of the activating solution, varying from 4M to 6M. The liquid to solid ratio was fixed at 0.50 for all mixtures. Geopolymer samples were prepared by mechanical mixing waste glass and red mud in a sodium hydroxide solution for 4 hours before being casted in cylindrical polyethylene moulds, cured at 75°C for 10 days. The compressive strength was measured by using an Instron 1121 UTS (Danvers, MA) testing machine, operating with a cross-head speed of 0.5 mm/min. 10 cylindrical samples, with diameter of 14mm and height of 20mm, were tested for each batch. Microstructural evaluation was performed by means of scanning electron microscopy (SEM) (LEO 435 VP, Cambridge, UK and Ultra Plus, Zeiss, Jena, Germany), equipped with EDX. Solid-state Magic Angle Spinning (MAS) NMR spectra were recorded on an Agilent DD2 500WB spectrometer equipped with a commercial 3.2 mm triple resonance MAS probe at 27Al resonance frequency of 130.24 MHz. The heavy metal release was evaluated according to the European Standard for waste toxicity evaluation (EN 12457-2) and analysed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (SPECTRO Analytical instruments GmbH, Kleve, Germany).