High-performance alkali-activated composites containing an iron-ore mine tailing as aggregate

Abstract

High-performance cementitious composites have been developed to overcome the brittleness of mortars and concretes, thus improving the deformation and toughness of these materials under flexion and tension. Poli Vinyl Alcohol (PVA) fibres are employed in the production of such “Engineered Cementitious Composites” - ECC; the PVA fibres have a loadcarrying capacity after the first crack (matrix failure), which changes the mechanical behaviour of the composites from brittle to ductile and significantly increases the ultimate strength. This deflection or strain-hardening behaviour is accompanied by a multiple cracking of the composites, which results from the design of a proper formulation, with correct amount of PVA fibres (usually 2% vol. fraction) and employment of a very fine sand (passing 0.6 mm). Recent developments in the area of ECC comprise the replacement of Portland cement (PC) matrices with alkali-activated materials (AAM). The idea is to produce composites with similar performance but with improved chemical durability and lower environmental impact. A more sustainable solution would consider the replacement of the fine sand with mine tailings in the production of ECC-AAM. Some tailings from the iron-ore mining activities in Brazil are significantly finer than those aggregates used for PC mortars and concretes; therefore, they cannot be employed in traditional PC-based materials. Nevertheless, those fine materials could replace the fine natural aggregate used in the production of ECC. This paper investigates the replacement of a natural quartz sand with an iron-rich mine tailing in PVA-reinforced AAM. Four composites were studied from a combination of two different matrices and 2 different aggregates. The matrices were obtained from the alkaline activation of metakaolin (MK) with sodium silicate (Na2SiO3) and sodium hydroxide (NaOH); silica fume (SF) was used to adjust their composition: SiO2 / Al2O3 molar ratio equal to 3.0 or 3.8. The aggregates used were either natural quartz (passing 0.6 mm) or tailings produced during the mining activities of iron ore in the state of Minas Gerais, Brazil. The mine tailing studied is much finer than the natural sand (passing 0.3 mm) but it was used as received in the production of ECC-AAM. The aggregate to binder ratio was kept constant (equal to 1.0 in mass) irrespective of the type of aggregate. All mortars were reinforced with 2% vol. of PVA fibres; extra water was added to the mixes to maintain the same consistency for the composites. The mechanical properties investigated are compressive strength, flexural strength and toughness. The apparent dry density of the mortars was also assessed. The preliminary results presented in this paper indicate that iron-rich tailings may be effectively used in the production of ECC-AAM; however, durability tests are still necessary.