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Particle size outweighs chemical properties in aggregate strength

Geophysicists and fluids mechanics experts have provided new insight into how tiny ‘bridges’ help particles form aggregates, in a discovery that could have applications in cement production and construction.

Research from the University of Pennsylvania has revealed that when particles are wet and allowed to dry, their size can determine how strongly they stick to one another and whether those bonds will be maintained when water is reapplied.

According to the research team, the aggregate’s strength is caused by thin bridges formed when particles of the material are suspended in a liquid and then left to dry, leaving thin strands of particles that connect larger clumps. The strands, which the researchers call solid bridges, increase the aggregates’ stability 10- to 100-fold.

“We found that a particle’s size can outweigh the contribution of its chemical properties when it comes to determining how strongly it sticks to other particles,” Douglas Jerolmack, a geophysicist in the School of Arts and Sciences and the paper’s corresponding author, said.

The researchers believe their work could open up new possibilities to enhance stability of materials such as soil or cement.

“You could envision stabilising soils before a construction project by adding smaller particles that help bind the soil together,” Jerolmack added.

Further describing the process involved in forming aggregates, the team found the extreme suction pressures caused by evaporation pulled the small particles so tightly together that they fused in the capillary bridges. They leave behind solid bridges between the larger particles, to which they also bind, once the water is evaporated completely.

When the team rewet the particles, applying water in a controlled flow, they found aggregates composed solely of the 20-micron particles were much easier to disrupt and resuspend than those consisting of smaller particles or mixtures of small and larger particles.

“We found that if aggregates composed of only particles larger than five microns were re-wet, they collapsed,” Jerolmack says. “But under five microns, nothing happens, the aggregates were stable.”

The study was published in the journal Proceedings of the National Academy of Sciences.

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