Engineering

3D crack-tip microscopy shows a crack (purple) growing in a composite material containing silicon carbide fibres.

Company

University of Mancherster and Rolls-Royce

Challenge

Crack propagation in metallic materials is well understood. But aircraft manufacturers are increasingly turning to more complicated composite materials that are lighter, stronger and can operate at higher temperatures. Lower weight reduces fuel consumption, while higher engine operating temperatures allow aeroengines to be more efficient. The challenge is to understand how cracks propagate in such materials.

Sample

Titanium reinforced with silicon carbide fibres. This composite material can operate at higher temperatures than titanium alone, making it a promising candidate for jet engine parts.

Solution

Synchrotron X-rays penetrate tens of millimetres into a sample where the behaviour of cracks can be very different - this in contrast to eectron microscopy which only reveals the surface features of micro-cracks. On beamline ID15, scientists can use imaging, to see how cracks grow, and diffraction, which tells them about the local stresses that the cracks grow under.

Benefits

The ability to monitor cracks under load at high temperatures allows researchers to evaluate the potential of these materials under realistic conditions. It also helps to make realistic estimates

of the lifetime of existing components and to design safer, more crack-resistant materials for the future.

Better knowledge of crack propagation transfers directly to other industries in which failure is unacceptable, notably the nuclear industry.

Proc. R. Soc. A 468 2722.

Acta Materialia 60 958.

Company

ECOCEM, LMT (ENS Cachan, CNRS, Université Paris Saclay), LMDC (Université de Toulouse INSA/UPS Génie Civil).

ECOCEM Materials is the first independent producer of Ground Granulated Blast Furnace Slag (GGBS) in Europe. ECOCEM recycles a by-product from steel industry: granulated slag from a blast furnace which is in glassy form. The aim is to produce concrete with lower environmental impact: 1 ton of ground slag in the concrete industry avoids the emission of 750 kg of CO₂ due to the production of Portland cement.

Challenge

In the quest for environmentally friendly materials, ECOCEM is using slag rich concrete, mortar and other materials. These materials might display a blue/green colour when they get consolidated. When the concrete is exposed to air, the blue/green colour turns to grey (in the case of cement concrete) or white (slag concrete). However, if the concrete is let to set for a long time in a form work or cured under water, it keeps the colour for up to its entire service life. This specific colour is concerning in applications where visual aesthetics are important. The origin of this blue/green tinge was not resolved until now, even if it has been attributed to the presence of polysulphur species in the slag, but without any scientific evidence.

Sample

• Ground granulated blast furnace slag (GGBS). It is an alumino-silicate glass containing in particular less than 1% of total sulphur species.
• Non-ground granulated slag collected from a steel factory without grinding.
• The last sample of slag was ground under nitrogen atmosphere in the laboratory to avoid sulphur oxidation.

A cafe built with ECOCEM cement. Credits: ECOCEM.

Solution

In order to find out the origin of this coloration, scientists went to ID21 and used K-edge X-ray absorption near edge structure (XANES) spectroscopy to follow the evolution of the speciation of sulphur as the slag was hydrated.

The team used three methods to activate the slag: thermal, Portland cement and sodium silicate. First, they compared each method and then they compared the XANES results on blue and white samples and on zones that were not coloured.

They found that, independently of the activating method, the blue colour was related to the presence of a pre-peak in the corresponding XANES spectrum. This did not happen anywhere else in the samples. They linked this pre-peak to the presence of the polysulphur radical anion S₃^- (as shown in the figure). This blue chromophore is known to be in particular at the origin of the deep blue of the stone lapis lazuli or the ultramarine pigment (derived from the Lazurite mineral).

As for each specific sample, the team found that the oxidation state of sulphur in a granulated slag obtained just after quenching was higher than in a ground slag or when ground under nitrogen atmosphere. This suggests that the superficial sulphur is oxidized under quenching. 

Benefits

This research did not exclude the possibility of other elements influencing the coloration of slag-based concretes, in particular the presence of transition metals, but it showed clearly that the trisulphur radical anion is present and may play a major role.

In-depth understanding of the coloration mechanism was the key point if an effective decolorizing treatment was to be found. Through the study in ESRF by XANES, ECOCEM finally found an explanation of the blue/green colour: the presence of S₃^- in the concrete matrix. This study helped ECOCEM to explain clearly the reason of blue colour formation to their clients. And it also helped indirectly ECOCEM to find the solutions to eliminate the blue/green colour. 

Elemental distribution obtained by μ-XRF with an exciting beam of 4040 eV for K and Ca, and 2480 eV for S, Si, Al, O and Na obtained by μ-XRF with an exciting beam, map size 2.6 x 3.5 mm2 with a step of 100 µm.

Reference:

Chaouche M, Gao XX, Cry M, Cotte M and Frouin L, On the origin of the blue/green color of blast-furnace slag-based materials: Sulfur K-edge XANES investigation,  J Am Ceram Soc. 2017; 1-10. doi: 10.1111/jace.14670.