Hi everyone,
We are finally fully up and running with solid-state NMR capabilities here
at Virginia Tech (running a Neo400 with a 4 mm MAS probe), and I have a student
requesting to run 29Si MAS NMR on some 60-year-old concrete samples.
They mentioned that the samples contain about 5% Fe2O3, as determined by XRF.
I have very little experience with concrete samples in SSNMR and wanted to
tap into the community's expertise regarding a few points:
1. Spinning Safety: My understanding is that an iron oxide content of around
10% is the threshold where magnetic susceptibility begins to put the instrument
at risk of a rotor crash or stator damage, but that anything below that is
generally safe to spin. Can anyone shed some light on this or share their
safety cutoffs for paramagnetic/ferromagnetic samples?
2. Relaxation Times: I've also read that there may actually be a benefit to
this Fe2O3 content—namely that the paramagnetic iron will reduce the normally
agonizing 29Si T1 relaxation times, allowing for shorter recycle delays for
quantitative direct-excitation experiments. Can anyone confirm if this
provides a meaningful practical benefit for cementitious samples?
Finally, since these are crushed concrete samples (aggregate included) rather
than extracted cement pastes, I anticipate a massive Q4 quartz background.
If anyone has tips on the best way to handle or filter this (we are planning
to try 1H-29Si CP-MAS to selectively highlight the C-S-H gel), I would greatly
appreciate it.
Thank you in advance for your insights!
Best regards,
Ken Sharp-Knott
Manager of Analytical Services and the NMR Facility
Department of Chemistry
Virginia Tech
(540)267-6502 (Cell)
(540)231-0885 (Office)
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Received on Tue May 19 2026 - 11:25:43 MST