Upcycling Mineral Wastes: their Characterisation and Activation for use in Sustainable Cementitious Binders

Abstract

The widespread use and production of Portland cement, amounting to 4.1 billion tonnes annually, poses challenges in acquiring quality raw materials. The emissions of carbon dioxide, particulate matter, nitrous oxides, and sulphur oxides associated with cement production also contribute to climate crisis and pose major health risks related to respiratory and pulmonary diseases. Despite exploring various options such as alternative cements, fuels, and technologies to reduce the impacts of cement production, the current solutions meet only 10 -15% of the projected demands. The declining production of well-established supplementary cementitious materials, such as fly ash and blast furnace slag, adds to the complexity of the situation as alternative sources of these materials are required. On the other hand, the demands associated with the global development had led to intensified mining and significant mineral waste production, the quantity of which is about 217 km3 at present with a predicted annual addition of 12.3 km3 to this volume. This poses major environmental, economic, social, and legal concerns. Construction and demolition waste (CDW) is another significant source of waste globally and the recycling plants mainly focus on the recycling of coarse aggregates and certain finer fractions. The finer fractions in the size range of silt particles and below are unused and are disposed of in landfill, leading to an increasing proportion of finer fractions generated from CDW recycling in landfill. The Construction and Building materials research under Horizon 2020 program of Europe 2020 strategy promotes the materials for energy efficiency including materials with low embodied energy and materials capable of reusing a high waste content. Mineral waste, with a composition similar to supplementary cementitious materials, emerges as a potential solution. However, documented challenges, such as crystallinity, variations physical, chemical, and mineralogical characteristics, acid mine drainage, and heavy metal leaching, hinder its immediate use as a cement precursor. This study focuses on developing a suitable treatment for mineral waste for its use as a binder precursor. A three-phase experimental methodology was designed to meet the objectives also in consideration with the scope and limitations. In the initial stage, the raw and activated mining tailings and CDW derived silt were characterized. Mineral wastes investigated in this study, namely mine tailings and CDW derived silt exhibited a high silica composition, containing approximately 54-58% SiO2. The initial screening by XRD analysis guided the selection of calcination and milling regimes. A calcination temperature of 650oC, 750oC and 850 ºC for mine tailing and 750oC and 850 oC for CDW derived silt were adopted. Milling balls and a milling container made of tungsten carbide were used for grinding using milling times of 30 minutes, 60 minutes, and 90 minutes. The median particle sizes (D50) of mine tailings decreased by 55.54% and CDW-derived silt, decreased by 72% after 90 minutes of milling. The results obtained from X-ray diffraction and Rietveld analysis revealed the oxidation of graphite and decomposition of clinochlore, and chlorite in mine tailings, as well as the decomposition of calcite and gypsum in CDW derived silt. Alterations in the quantities of all the mineral phases have been observed in both mine tailing and CDW derived silt with the activation methods adopted. It was found that the mine tailing samples calcined at 850 oC (MTCT850) and CDW silt samples calcined at 750 oC (STCT750) exhibited higher heat of hydration compared to other samples within a simulated cementitious matrix in adapted R3 tests. These samples also indicated Si and Al solubility higher than the raw mineral waste during dissolution in an 8M NaOH solution. Based on these observations MTCT850 and STCT750 were determined to be the samples subjected to optimum treatment. Following this, paste mixes were prepared by replacing 10%, 20%, and 40% of cement with activated mineral waste and with water to binder ratio 0.3, 0.4 and 0.5. The findings revealed that the setting time increased and the heat of hydration decreased with increase in percentage of cement replacement. The examination of mineralogical properties and microstructure of the cementitious pastes over a period of 7 to 90 days at different ages revealed an ongoing process of hydration. The XRD analysis of the pastes showed presence of biotite, quartz, feldspar, oligoclase from the mineral waste. This observation was also confirmed by SEM images which showed the presence of unhydrated MTCT850 and STCT750 particles in the cement paste samples. However, FTIR studies revealed ongoing variations in the chemical makeup and molecular bonding in the hydration products confirming the densification of the matrix and ongoing hydration as observed in the microstructure. The strength development and durability properties of mortar mixes with various percentages of cement replacement (10%, 20%, and 40%) and a water to binder ratio of 0.4 were evaluated in phase III. The strength tests revealed at the end of 90 days the OPC samples had a strength of 70.68 MPa, and the MT mixes exhibited strength in the range of 51.8 MPa and 63.8 MPa, and the same for ST mixes were 63 MPa to 69.8 MPa. Based on the observed strength development, it can be determined that the mixes with 20% MTCT850 (MT20) and mixes with 20% STCT750 (ST20) can be considered the optimum mixes. The MT20 and ST20 mixes also displayed better performance compared to OPC in terms of carbonation and demonstrated satisfactory performance in terms of sorptivity. The current study also estimated that the use of mineral wastes can reduce carbon dioxide emissions by 6% to 27% and energy by approximately 3% to 15%, equivalent to a range of 122 to 579 MJ/ tonne of binder. The aforementioned results collectively and individually suggest that the treatment processes employed, (i.e.) calcination and ball milling, has resulted in effective alterations to the particle size, morphology, and mineralogy of the material. The findings of this research emphasize the potential of mineral waste as a sustainable component in modern and high-performance cementitious binders, contributing to a greener and more environmentally responsible construction industry.