COLMAN Charlotte PhD

One of the key points within the framework of a sustainable construction sector is the recycling of its waste products, completing the life cycle of these materials.

 

Recycled concrete aggregates, obtained by the demolition or deconstruction of older concrete structures, can be used in a new concrete as a replacement for natural aggregates. 1.7 tonnes of these recycled aggregates are produced per person per year in Europe, waiting to be valorized.

An important reason fine recycled aggregates (FRA) of 0-4 mm are up to now not valorized, is their contamination with gypsum (CaSO4.2H2O) residues from the construction site. These gypsum particles are a source of water soluble sulfates, which are responsible for sulfate attack – a deteriorating reaction for cementitious materials. Sulfates react with water and C3A from cement to form ettringite, an expansive mineral causing microcracks and swelling of the material (Fig. 1).

As opposed to the better known external sulfate attack or Delayed Ettringite Formation (DEF), the sulfates in this context are provided by a contamination of the aggregates. For this reason, the maximum allowable water soluble sulfate limit in recycled aggregates is set at 0.2% by EN206. Characterization of industrial samples of FRA, collected from recycling centers troughout France and Belgium, show that almost all FRA samples surpass this limit (Fig. 2).

This research project aims to study the potential of sulfate attack deterioration in mortars and concretes, caused by the gypsum contamination of FRA. Industrial FRA samples are characterized for their physical and chemical properties. Next to those industrial samples of FRA, a 'pure' laboratory-made FRA was produced by fabricating and crushing concrete. The effects of gypsum contaminations as well as other fabrication parameters are studied. These results evaluate the limit posed in EN206, but also propose different mitigating or aggravating circumstances when handling contaminated FRA. Ultimately, providing a better understanding of the effects of sulfate attack in the context of demolition waste will promote the use of these recycled materials in the building sector.