Gypsym residues in recycled materials: effects on microstructural and mechanical properties of cementitious mixes

Researcher: Charlotte Colman

Institutional partners: École des Mines de Douai (France)

Funding: University of Liège – Help FRIA

Total project duration: 4 years

Start of project: September 2016

Keywords: gypsum, recycled concrete aggregates, sulfate attack, delayed ettringite formation

This PhD research is part of the INTERREG VALDEM project.

Globally, half a ton of construction and demolition wastes (C&DW) are generated per person per year. Reusing this waste into new concretes would prove a big advancement for the construction industry, but up to now, most European countries do not yet implement a real circular economy. Fine particles present in this construction waste may cause problems, making it harder to reuse them in the manufacture of concrete or other applications. These problematic fine particles are often contaminated with substances such as bitumen, wood, glass, gypsum or organic matter. Some of these contaminants are harmful to the durability of concrete. Gypsum, or CaSO4.2H2O, is a factor that can threaten the chemical and mechanical stability of cement-based materials via a process called sulfate attack.

The figure above show a SEM picture of ettringite formation following sulfate attack (Schmidt et al, 2007)

Sulfate attack, and the related Delayed Ettringite Formation (DEF), is a deteriorating process that causes the expansive formation of sulfate containing minerals, such as ettringite. The high volume of these minerals will cause an expansive stress within the concrete, with degradation and cracking as a result. The incorporation of fine recycled particles in concrete adds extra complexity to this degrading process and is up to now generally avoided.

In this project, we research how much residual gypsum contamination from demolition waste is acceptable for designing new concretes. Providing a better understanding of the effects of sulphate attack in the context of demolition waste will promote the valorisation of these recycled materials in the building sector.

The fraction 0/4 mm of C&DW from different recycling companies will be analyzed to determine the amount of sulfates present. Mortars will be manufactured where different percentages of natural sand are substituted by this waste, and the development of sulfate attack will be followed with several analytical techniques (SEM-XRD, oedometer tests, petrography, ...). Different parameters (cement type, curing humidity, ...) of the mortars will be varied to assess their influence on the swelling process. In a final step, a quantitative relation will be established between gypsum content and swelling dimensions.