Valorization of Boron Mine Tailings in Alkali-Activated Mortars

Abstract

Mine tailings cause significant storage and environmental problems because a large amount of tailings containing potentially numerous hazardous contaminants is generated during mining operations. Therefore, the utilization of mine tailings in different sectors should be taken into consideration to minimize the storage efforts and environmental pollution problems. In this two-stage study, the alkali activation opportunity of waste clay containing boron (WCB) discarded from boron mine production was investigated. In the first stage of this study, alkali activation of WCB calcined at different temperatures between 500°C and 800°C was investigated under steam-curing conditions. It was found that a calcination temperature of at least 700°C is necessary to initiate the alkali activation of WCB, whereas the maximum compressive strength value was obtained at 800°C. Subsequently, the effects of alkaline activator solution type (sodium hydroxide and sodium silicate) and quantity on the development of engineering properties of alkali-activated WCB mortars, such as compressive and flexural strength, water sorptivity, and drying shrinkage, were investigated by using WCB calcined at 800°C. Furthermore, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA/DTA), and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) analyses were carried out on the hardened paste mixtures. The minimum activator concentration for alkali activation of calcined WCB was 6% Na2O by weight of WCB with a minimum silicate modulus (Ms=SiO2/Na2O) of 1.2. A compressive strength value up to 38 MPa along with acceptable water absorption characteristics and drying shrinkage values can be obtained from alkali activation of calcined WCB at 800°C. NaOH activator was not as effective as sodium silicate activator in activating WCB, even in high dosages.