monitoring cement hydration with embedded nmr probes
TRANSCRIPT
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Monitoring Cement Hydration
with Embedded NMR Probes
B. J. Balcom
UNB MRI Centre, Department of Physics,
University of New Brunswick
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Outline• Magnetic Resonance
• Spin Spin relaxation, pore structure
• Portable Magnetic Resonance
• Embedded magnets, in situ measure
UNB MRI Centre www.unb.ca/fredericton/science/physics/mri
• Hydration measurements
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no field B0 field
a b
Magnetic Field
M0
Magnetic Moment of Nuclei in a Static
Magnetic Field
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7.0 Tesla UNB MRI Magnet
• 20 cm horizontal
bore.
• Optimized for
materials imaging.
• Laboratory
technique.
M0
N 2h 2I(I 1)B0
3kT
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1H Magnetic Resonance of
Cementitious Materials
Magic Angle Spinning
Spectroscopy. C3S hydrated
one year. Korb, Current
Opinion Colloid Interface
Science, 2009.
SPRITE MRI of water content
distribution. White Portland
cement based concrete,
isotropic FOV 6 cm,150 sec
acq. Balcom et al JMR 2004.
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Bo 2.4 Tesla, RF signal 100 MHz
90o
z
y
x
T2* time constant
= Bo
M0
Time Domain Magnetic Resonance, TD-NMR
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Sample and RF Probe
Voltage induced in RF coil
by precession of sample
magnetization - Faraday’s law.
Faster precession yields
higher voltage (signal) for
same M0.
voltage M0( )
Signal varies as 3 , 1H has the largest and the
highest concentration, 1H is the most sensitive measure.
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Bo 2.4 Tesla, RF signal 100 MHz
90o
z
y
x
T2* time constant
= Bo
M0
Time Domain Magnetic Resonance, TD-NMR
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MR Signal Lifetimes
• T1 spin lattice relaxation time, governs
recovery of Mz after RF perturbation.
• T2* effective spin spin relaxation time, decay of
observable transverse magnetization Mxy after
RF pulse.
• T2 spin spin relaxation time more fundamental
decay of observable transverse magnetization.
1/T2* = 1/T2 + B0
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Time Domain Magnetic Resonance, TD-NMR
‘NMR studies of hydrating cement: a spin-spin
relaxation study of the early hydration stage’ Lasic,
Pintar et al, Cement and Concrete Research 18, 951-
956 (1988).
‘Microstructure determination of cement pastes by
NMR and conventional techniques’ Bhattacharja,
Halperin et al, Advanced Cement Based Materials 1,
67-76, (1993).
‘NMR and nuclear spin relaxation of cement and
concrete materials’ Korb, Current Opinion in
Colloid and Interface Science 14, 192-202 (2009).
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TD-NMR T2 Measurement
Initial amplitude is Mo, quantity.
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Pore Size Distribution in Cement
Based Materials
Air
Void
10-2 m 10-3 m 10-5 m
Coarse FinePaste
x 1000
micro &
mesopores
10-1 m
Gel pore network evolves during
hydration, consuming liquid water.
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Surface Relaxation
Assumes rapid exchange of bound water surface layer and bulk
water in pore centre. S/V is pore size. ‘Solid’ 1H not observed.
MR relaxation times T2i are determined by the
size of the water-containing pores.
1
T2i
fbulk
T2ibulk
f surf
T2isurf
1
T2i
eS
V
1
T2isurf
e
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MR Fluid Dynamics in Porous Media
D=D/a2, C=SD/VL, = A/V
Common regime, rapid diffusional averaging within pores, no exchange.
McCall et al Phys Rev B (1991) 44, 7344-7355.
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Pore Size Distribution - T2 Distribution
S(t) i
i
etT2 i
T2
T2
T2
time
time
time
timeT2
Inverse Laplace transform
recovers T2 distribution.
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Unilateral Magnetic Resonance
Simple mobile instrumentation. Sample is removed
from the main field magnet.
Magritek
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Unilateral Magnetic Resonance
magnet
spot
Experimental measurement is taken from a sensitive spot
inside the sample of interest. The ‘spot’ is defined by the
intersection of the Bo and B1 fields, which are at right angles.
B1
B0
~ 1 cm3 spot displaced 0.8 cm
from surface, 0.9 kg magnet.
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The Big Idea - Embedded Magnets
• Avoid near surface measure with a sensor
that is positioned ’deep’ in a cement
paste/mortar/concrete sample at the time of
placing.
• A simple sensor which is cheap can be
‘sacrifical’.
• An array of such point sensors can provide
depth and lateral spatial resolution.
Cano-Barrita, Balcom et al Cem. Concr. Res. (2009) 39, 324-328.
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The Big Idea - Embedded Magnets
Cano-Barrita, Balcom et al Cem. Concr. Res. (2009) 39, 324-328.
• T2 measurements will yield evaporable
water content, and pore size distribution, as
a function of hydration or wetting/drying
over short (hours, days) or long (weeks,
months) time scales.
• Such sensors will bridge laboratory and
field measurement.
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Embedded Magnet Unilateral MR
• Materials < $50, a disposable single sided sensor.
• Lee Valley Tools, Nickel plated NdFeB, $8 each.
• Immersed or embedded in sample.
Cano-Barrita, Balcom et al Cem. Concr. Res. (2009) 39, 324-328.
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Embedded Magnet Unilateral MR
• Avoids the ‘depth of the sensitive spot’ problem by
insertion into the sample.
Stack of disc
magnets
3 mm above
surface, 10 MHz
Linear gradient from 2–7 mm above magnet, 2000 Gauss/cm.
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TD-NMR T2 Measurement
Cano-Barrita, Balcom et al Cem. Concr. Res. (2009) 39, 324-328.
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Calibration: Water in Sand
32 echoes, TE 200 µs, 512
avg, 9 min acq
Calibration: Cement Pastes
32 echoes, TE 168 µs, 2048
avg. 7.5 min acq.
Linearity 1H Density MR
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Curing mortar (w/c = 0.45). Avg.
first 2 echoes, 8192 avg.
Embedded Magnet 1H Evolution
Evaporable water content (non
chemically combined water)
decreases as the sample hydrates.
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Early Age Mortar Hydration
Type 1 cement mortar, w/c=0.6 Type 1 cement mortar, w/c=0.45
Sealed samples
Reaction progress is measured not as heat evolved, not as product created,
rather it is reagent consumed.
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Evolution of Pore Structure
Relaxation time evolution Type 1 mortar w/c=0.6, whole sample
desktop Bruker Minispec measurement. Bi-exponential fitting model.
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Evolution of Pore Structure
Embedded magnet T2
distribution, w/c = 0.6.
Table top magnet T2
distribution, w/c = 0.6.
Decreased T2 indicates
smaller pores.
Type 1 Portland cement mortar.
Cano-Barrita, Balcom et al Cem. Concr. Res. (2009) 39, 324-328.
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Conclusions
• Embedded magnets provide an in situ measure
of water content variation and pore size evolution
during hydration.
• Anticipate an order of magnitude improvement
in sensitivity.
• Technology is still developing, applications are
in their infancy. Bridge laboratory and field
applications.
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UNB MRI Centre
Andrew Marble, NSERC Doctoral Prize winner, 2008.
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Acknowledgements
Industrial and Academic collaborators
UNB MRI Centre
T.W. Bremner, Michael Thomas, Jesus
Cano, Joshua Young
NSERC of Canada
Steacie Fellowship (2000-2002)
Canada Chairs Program (2002-2016)
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MR Active Nuclei
Sensitivity -> 3 * I(I+1) * natural abundance * [conc.]