Valorization of mine tailings as cement replacement in concrete

Cement and concrete are the most widely used construction materials globally and are in growing demand. However, cement's energy-intensive production contributes to approximately 8 % of global CO2 emissions. One solution to reduce emissions is to replace clinkers, the primary cement component, with alternative materials.

This thesis, therefore, investigated the potential of mine tailings' (MT) valorization as cement replacement material (CRM) in Portland cement (PC). MT, a fine-particulate byproduct from metal and mineral ore processing, are rich in silica, alumina, calcium, and iron and produced in billions of tons globally. The study screened 12 distinct and untreated MT for their 1) unique characteristics relevant to CRM, including their MT and metal properties, physical and chemical reactivity and alkaline activation potential 2) effect on cementitious properties, including their hydration behavior, hardened properties, hydration phase assemblage, and the influence of their metal and sulfur content, 3) environmental risk as CRM including metal leaching tests and utilization potential after metal removal.

The main findings were:

1) MT generally resembled the properties of sand, showing no pozzolanic potential and lower cumulative hydration heat than inert quartz (IQ) and limestone filler (LF), indicating no physical contribution. Their alkaline dissolution extent was low compared to commonly used precursors but showed a high dependence on the mineralogical composition rather than initial elemental content.

2) Despite the wide variation in characteristics, MT showed similar hydration properties. Due to their low reactivity, most MT showed lower heat of hydration than reference materials, resulting in low compressive strengths. However, finely particulate MT generated higher hydration heat resembling that of IQ. No MT resembled the properties of LF. Most MT increased the formation of carboaluminates and stabilized ettringite compared to PC. High metal contents in MT
retarded the induction period and later hydration product formation, while high sulfur contents delayed the aluminate reaction.

3) Mine tailing-blended mortar generally inhibited metal leaching, although a significantly high initial lead content was observed to be leachable, albeit remaining within EU leaching limits. Indications of increased metal leaching under concrete carbonation could be of concern.

The study found that MT in their untreated condition demonstrated low utilization potential as a pozzolanic CRM due to their crystalline origin from rock particles and corresponding low alkaline dissolution. However, their inert nature and minimal effect on hydration suggest a potential as fine particulate filler material, especially if further ground, since fine particulate material showed indications of a physical contribution. The aluminum content in MT and carbonate content in specific MT contributed chemically to hydration phase formation, indicating possible synergetic effects. Although metals were inhibited by the incorporation into mortar, MT with high metal, sulfur and magnesium contents must be addressed to ensure the environmental and durability safety of the cementitious material, as these could adversely affect early- and long-term material properties.

Given the vast amount of MT produced globally, they could serve as a promising source of CRM for sustainable construction practices, provided specific characteristics are carefully considered.