Oxidation of sulfides from secondary materials in cementitious binders as a function of environmental conditions

The process of mining produces large quantities of waste, which may be utilized as secondary materials in the building sector. Mine tailings have been investigated in the context of construction materials in different forms. One way of using these materials is to reduce the clinker quantity of cement used in concrete or mortars to reduce the “embedded” CO₂ footprint of infrastructure projects. Corresponding requirements have been implemented for example in the Danish construction regulations, with a planned reduction for allowable CO₂ quantities per m² and 50 years of operation.

Utilizing secondary materials can be seen in this context as a step towards the circularity and symbiosis of industrial sectors while also reducing embedded CO₂ in concrete. However, many mining ores contain sulfide minerals, which may have a negative influence on the performance and service life of cementitious materials. Hence, it is crucial to understand the oxidation of sulfides in cementitious binders.

This study looked at two storing conditions of cementitious materials, underwater and in the air at varying relative humidities, with the aim of investigating the oxidation of sulfides in cementitious materials in aqueous environments and during carbonation.

Storing the samples underwater, with partial solution exchange, lead to steady sulfate release during the period of testing. Changing the relative humidity and allowing the carbonation of cement hydrates, which occurred simultaneously with the oxidation of the sulfides led to higher concentration of sulfates in the leachate solution and small increase of ettringite in the cement pastes.