Making pavements with mine tailings
The novel idea of using mine tailings (MT) as a road-base material through geo-polymerisation was presented at the Cement and Concrete SA (CCSA) Young Concrete Researchers, Engineers & Technologists Symposium (YCRETS) 2023. The research was produced in collaboration between the Department of Civil Engineering, Durban University of Technology and the Department of Civil and Mining Engineering, University of Namibia, by Gbenga Aderinto, Jacob Ikotun and Valentine Katte.
Pavement construction usually involves using large quantities of natural aggregate materials extracted from quarries, and usually with the use of conventional stabilisers such as Portland cement, lime and bitumen. In many developing countries, the exploitation of quarries has been so massive that there exists a shortage of these aggregates.
Hence, the use of conventional stabilisers is a common practice, but this is a major source of environmental contamination. Alternative aggregate materials and stabilisation techniques in pavement construction are therefore highly sought after.
Recently, some researchers have proposed a new approach as an alternative to using conventional binders via alkali activation called geo-polymerisation to produce geopolymers. Some studies have shown that MT can be used as a road-base material through geo-polymerisation. Although there are different types of tailings, specific interest is on copper, iron and gold tailings, due to their dominance in the mining areas of South Africa.
Pavements are structures designed to withstand traffic loads and are composed of several layers made up of natural aggregate materials such as sand, cobblestones and laterites. Construction requires a large quantity of natural aggregate materials, with an average of 20 000 tons of aggregate needed for every kilometre of the roadway.
In areas facing a shortage of natural construction materials, two options exist: Finding new quarries or using conventional stabilisers like ordinary Portland cement (OPC) and lime to enhance the mechanical properties of low-grade materials such as soils and MT.
The mining industry produces vast amounts of MT annually during the extraction, chemical and physical treatment of mineral ores. In South Africa, over 315 million tons of tailings are generated every year. Due to the existence of hazardous materials in tailings, they cannot be utilised in civil construction projects.
Tailings are therefore required to be reprocessed into less hazardous waste before disposal or reuse for construction purposes, which is costly and can result in various environmental problems such as land use, air pollution, water bodies contamination and tailings dam failures. Geo-polymerisation of MT offers sustainable and eco-friendly solutions to environmental problems caused by MT disposal.
The lack of high-quality natural construction materials and the disposal problem associated with MT prompted the interest of researchers in the field of transportation engineering to find an alternative solution. These reviews showed that geo-polymerised MT as an alternative material for road construction has good mechanical properties compared to construction materials being stabilised with conventional stabilisers.
In terms of unconfined compressive strength (UCS), it was observed that geo-polymerised MT conforms to the specifications of various countries, indicating the feasibility of MT as a suitable aggregate material for road construction. As with any waste, the use of MT for pavement construction should incorporate environmental considerations such as leachate of toxic metals/elements and durability for long-term use. The standards related to the use of wastes as alternative aggregate materials in construction are limited.
As a result of systematic reviews on the use of geo-polymerised MT as a material for road construction, the following conclusions have been derived:
- The most significant benefit of geo-polymerisation is the use of a variety of industrial by-products as the basic raw materials in their composition.
- It has good compressive strength suitable for road construction.
- It represents an important step towards sustainability since conventional stabilisers are characterised by high energy consumption levels and leave a large carbon footprint (the geo-polymerised binder effuses 80% less CO₂ than ordinary Portland cement.
- The utilisation of industrial by-products in the geo-polymerisation allows waste to be recycled and lowers the surface area of landfill space required, supporting environmental conservation.
Despite the general acceptance of ordinary Portland cement, geo-polymerisation has the potential to revolutionise cement production due to its advantages. However, insufficient information is evident in some areas of geo-polymerisation. More studies are needed to understand the compressive strength of geo-polymerised tailings with multiple precursors and their behaviour under both laboratory and field conditions.
Read the full paper online.
Issue: With a shortage of natural aggregates, alternative aggregate solutions in pavement construction are highly sought after.
Solution: Geo-polymerised mine tailings offer a sustainable and eco-friendly solution with good compressive strength.
Full acknowledgement and thanks go to www.cemcon.sa.org.za for the information in this editorial.