Silicon fluorescence shines through microcracks in cement, revealing
early signs of damage

Date:
January 25, 2022
Source:
Rice University
Summary:
Scientists and engineers discover fluorescence from silicon
nanoparticles in cement and show how it can be used to reveal
early signs of damage in concrete structures.



FULL STORY ========================================================================== Concrete fractures that are invisible to the naked eye stand out in
images produced through a technique created at Rice University.


==========================================================================
A collaboration between research groups at Rice and the Kuwait Institute
for Scientific Research discovered by chance that common Portland
cement contains microscopic crystals of silicon that emit near-infrared fluorescence when illuminated with visible light. That led to two
realizations. The first was that the exact wavelength of the emission
can be used to identify the particular type of cement in a structure.

The second and perhaps more important is that the near-infrared emission
can reveal even very small cracks in cement or concrete. The trick is
to apply a thin coat of opaque paint to the concrete when it's new. In near-infrared scans, intact concrete appears black and glowing light
reveals the tiniest of cracks.

The open-access study by the labs of Rice chemist Bruce Weisman, Rice structural engineer Satish Nagarajaiah and Kuwait Institute of Scientific Research investigator Jafarali Parol appears in Scientific Reports.

Wei Meng, the paper's first author, found the phenomenon while pursuing
the Rice team's long-standing work on optical strain sensing with carbon nanotubes.

"This arose from a project in which we were trying to apply our
strain measurement technique to cement and concrete, but we ran into an unexpected problem when we illuminated a specimen coated with a nanotube
film," said Weisman, a pioneer in nanotube spectroscopy. "We found that
one of the peaks in our film spectrum was obscured by much stronger
emission coming from somewhere.

We never expected it would be from the cement itself." He said he was
not aware of any other lab reporting the phenomenon.

"Eventually, we were able to mask off the specimen so the emission didn't interfere with our strain measurement," he said. "But we kept in the
backs of our minds that maybe this could be interesting on its own."


==========================================================================
The emission's unusual spectral signature let the researchers deduce
that the source was pure silicon crystals. "Minerals called silicates
are major components of cement, and we hypothesized that during the
high temperature production process, very small amounts decompose
to form microscopic silicon crystals," Weisman said. "Their emission
wavelength tells us that they're larger than about 10 nanometers, but
they can't be much bigger or people would have noticed them long ago."
Meng experimented on small concrete blocks painted black and with holes
drilled in the middle. These served as focal points to form microcracks
that would propagate outward when the blocks were compressed, also
cracking the paint. He found the fluorescent signal came through the
tiny cracks and could easily be mapped with a raster-scanning laser.

"Concrete structures need monitoring, and this is one way of monitoring
them," said Nagarajaiah, who specializes in infrastructure/structural monitoring, system identification, damage detection, and adaptive
stiffness structure systems to withstand seismic events. "Getting a clear
idea of where cracks are can be quite important in structures, especially
in the critical places where we know they're going to be stressed."
He said the benefits of better crack detection could extend beyond bridges
and buildings to containment structures at nuclear power plants or on
ships or the insides of wells and pipelines that are difficult to access.

The researchers said a practical approach is to shine light on critical structures and photograph them using a near-infrared camera and
narrow-band spectral filter.

"Cement cracking can be an early symptom of failure, so people who are concerned with the structural integrity and safety of concrete structures
want to detect microcracks before they grow," Weisman said.

Rice research scientist Sergei Bachilo is co-author of the
study. Nagarajaiah is a professor of civil and environmental
engineering, of materials science and nanoengineering, and of mechanical engineering. Weisman is a professor of chemistry and of materials science
and nanoengineering.

The Kuwait Institute for Scientific Research and the National Science Foundation (CHE-1803066) supported the research.

========================================================================== Story Source: Materials provided by Rice_University. Original written
by Mike Williams. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Tiny_cracks_in_painted_cement_block_become_visible ========================================================================== Journal Reference:
1. Wei Meng, Sergei M. Bachilo, Jafarali Parol, Satish Nagarajaiah,
R. Bruce
Weisman. Near-infrared photoluminescence of Portland
cement. Scientific Reports, 2022; 12 (1) DOI:
10.1038/s41598-022-05113-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220125151010.htm

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