A Review of the Analysis of Cement Hydration
1
Kinetics via H Nuclear Magnetic Resonance
J.O. Ojo and B.J. Mohr1
Abstract. To date, the lack of experimental data concerning entrained water transport through a cementitious microstructure during self-desiccation has limited the
understanding of the mechanisms of internal curing. To improve the current
knowledge state regarding the moisture transport kinetics of internal curing, novel
1
in situ nanoscale characterization techniques, primarily H nuclear magnetic resonance (NMR), are being applied to elucidate changes in the early age hydration effects in the porous cementitious matrix due to internal curing. Relaxation time
analyses can indicate the relative intensities and percentages of free water, C-S-H
interlayer (physically bound) water, and C-S-H gel (chemically bound) water.
Many developments have taken place both with NMR equipment and testing technique. Consequently, this is making NMR a very useful tool in the studying permeability and moisture movement in the concrete matrix. This paper will review
the current state-of-the-art regarding the application of NMR to the analysis of
cementitious materials at early ages.
1 Introduction
In order to tackle the challenges of investigating the kinetics of hydration and internal curing at early ages in high-performance cementitious materials, the use of
nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging
(MRI) as novel analytical techniques has been proposed. These measurement
techniques, which have been flourishing in the fields of medicine, chemistry, and
biology, have more recently been utilized as an important tool for material scientists, particularly for particulate composite material studies. NMR and MRI are
versatile tools due to their sensitivity to molecular movements and chemical
composition as well as their ability to spatially mapping porous systems nondestructively. Though solid-state NMR is more commonly used to analyze cementi1
tious materials, in the case of early age hydration kinetics, proton ( H) analysis is
J.O. Ojo and B.J. Mohr
Department of Civil and Environmental Engineering, Tennessee
Technological University, USA
e-mail: [email protected]
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J.O. Ojo and B.J. Mohr
1
of particular interest. This review surveys the current applications of H NMR
analysis in cement-based materials research. Proposed applications to internal
curing transport kinetics based on the current literature will also be discussed
2 Nuclear Magnetic Resonance Technique
Nuclear magnetic resonance (NMR) refers to the orientation of nuclei particles
(e.g., protons) that occur in the nuclei of atoms when they are subjected to a static
magnetic field and also exposed to another fluctuating magnetic field. During this
processes, both the resonance frequencies and relaxation times for the protons are
monitored and recorded. In any compound, the resonant frequency of the protons
in the two atoms is not always the same. As a result, instead of a single NMR
peak, two peaks are produced and the resulting images can be spatially localized.
Application of gradients in orthogonal directions can lead to the reconstruction of
2-D and 3-D images using 2-D fourier transform techniques. If the applied field
gradient is linear, the frequency spectrum obtained after fourier transform becomes a 1-D profile of proton (i.e., water) concentration versus distance in the
chosen direction. This is known as 1-D mapping. In practice, the fourier transform
experiment pulse has proven so versatile that many variations of the technique,
suited to special purposes, have been devised and used effectively.
In addition, magnetic resonance imaging (MRI) is one of the more recent developments in NMR research and is a logical extension of the basic principles of
magnetic resonance. The MRI scanner provides a non-invasive and nondestructive method of imaging that is sensitive to subtle differences in water distribution, structural integrity, local flow, and other measurements.
The simplest form of NMR experiment is called continuous wave (CW). A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of
a powerful magnet, and is spun to average any magnetic field variations, as well as
tube imperfections. Radio frequency radiation of appropriate energy is broadcast
into the sample from an antenna coil. A receiver coil surrounds the sample tube,
and emission of absorbed radio frequency (RF) energy is monitored by dedicated
electronic devices and a computer. An NMR spectrum is acquired by varying or
sweeping the magnetic field over a small range while observing the RF signal
from the sample. An equally effective technique is to vary the frequency of the RF
radiation while holding the external field constant.
3 Application of Nuclear Magnetic Resonance to Cement-Based
Materials
As stated previously, NMR is commonly used in the fields of biology and chemistry [1, 10] but is increasingly utilized for the analysis of cementitious materials
due to the ability to monitor samples nondestructively and in-situ at early ages.
Modern NMR spectroscopy is frequently divided into several categories: (1) high
A Review of the Analysis of Cement Hydration Kinetics
109
resolution mode on homogenous solutions, (2) high power mode on highly relaxing nuclei which exhibit very broad lines, (3) the study of solids, for example,
magic angle spinning (MAS) techniques, (4) chromatographic separation of impurities coupled with proton detected NMR experiments, and (5) NMR 3-D imaging
(i.e., MRI).
Solid-state NMR of portland cement-based materials has been used to investigate changes in the atomic structure of C-S-H and other hydration products (e.g.,
27
29
33
1
Al, Si, and S NMR) [2, 4, 5, 11]. However, for this review, only H (proton)
NMR will be discussed, as related to the kinetics of cement hydration, and the potential application to internal curing of high-performance cementitious materials.
4 Application of 1H Nuclear Magnetic Resonance to Early Age
Cement Hydration
Using proton spin-spin NMR relaxation time analysis and thawing point suppression measurements, it has been shown that hydrating cement pastes have three distinct water components: (1) pore solution/capillary (unconfined/free) water, (2)
water adsorbed to hydration products (gel water), and (3) water located within hydrates (chemically combined water) [3, 8, 9, 12]. The water components have been
s, respe
respecassociated with T2 relaxation times of approximately 9, 80, and 350 µs,
tively. Furthermore, a clear evolution of the relative abundance (based on normalized amplitudes) of the three water components as a function of hydration time has
been observed [3]. Subsequently, it is anticipated that these results could be used
to estimate the degree of hydration based on the amount of chemically bound water. There is also additional solid-like water phases associated with the conversion
of ettringite to monosulfate and/or changes in C-S-H structure at an early age [6].
It has been stated [6] that these water phases are expected to play important
roles in the hydration process and their direct observation and characterization offers an additional avenue for understanding details of cement hydration at standard
conditions as well as at elevated temperature and pressures or with addition of
chemical admixtures.
Gussoni et al. [7] used a combination of NMR relaxation spectroscopy and
MRI techniques to characterize cement matrices prepared with and without organic solvents (i.e., chemical admixtures). It is also reported that residual dipolar
interaction and transversal relaxation decay can be used to provide relevant information about a cementitious paste. These findings emphasized how admixtures,
which are widely used in the concrete industry, can influence the cement hydration kinetics and the pore structure in concrete, particularly at very early ages.
5 Proposed Application of 1H Nuclear Magnetic Resonance to
Internal Curing
Among the numerous relaxation parameters that can be quantified by NMR, the
spin-spin relaxation parameter (T2) has been employed for the analysis of water in
cement pastes and mortars. Previous research has shown that T2 times for water in
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J.O. Ojo and B.J. Mohr
cement-based materials are a function of their respective locations within the
microstructure. That is, T2 relaxation times vary with the mobility of the water
protons. It has been shown [3] that chemically combined water, gel (C-S-H interlayer) water, and capillary water are characterized by T2 times of 9, 80, and 350
µs, respectively. Thus, the T2 relaxation times are an important parameter for analyzing water in the microstructure. Furthermore, based on these relaxation times,
the relative bulk percentages and evolution of the mobility types can be plotted as
seen in Figure 1.
Fig. 1 Example of T2 relaxation time
analysis for determining the relative
percentages of water mobility phases
as a function of hydration time (● – interlayer water; □ – water in C-S-H gel
pores, at early ages in capillary pores
and at later ages in nanopores; • - secondary hydration water; Δ – water in
large pores and intergrain space at
early ages, water in C-S-H gel capillary pores at later ages; + - protons in
solids environment) [6]
The purpose of these experiments is to primarily investigate changes in the
amounts of free and bound water in the samples beginning as early as reasonably
possible until approximately 7 days. It is anticipated that the addition of water
held in the internal curing materials will delay the transition from free to bound
water (compared to the w/c=0.36 control) and ultimately increase the amount of
bound water in the sample (compared to the w/c=0.30 control). For the first anticipated result, the time of delay will lead to an improved understanding of the
kinetics of moisture release and transport from the internal curing materials to the
hydrating cement paste. As for the latter result, this would be a direct indication
of an increase of the degree of hydration (amount of reaction).
Though no current literature has applied proton NMR to internal curing, it is
anticipated that the NMR technique shows great promise in resolving differences
in the state of water (i.e., how water is bound), as a function of hydration time for
internally cured samples.
One of the drawbacks associated with this technique is that heavy water (D2O)
is generally required for NMR analysis as the broad line signal generated is more
significant than that of regular water (H2O). The partial or complete replacement
of H2O with D2O has implications for the actual hydration kinetics, which can be
decreased by a factor of almost seven with the use of D2O. However, this disadvantage is not expected to minimize the applicability of NMR for the analysis of
internal curing. For the proposed internal curing research, the relative differences
A Review of the Analysis of Cement Hydration Kinetics
111
between samples would be of interest. Therefore, development of NMR in situ
experimental techniques to monitor the early stages of cement hydration in the
presence of additional water bound within the internal curing particles would
significantly contribute to the understanding of moisture transport away from the
internal curing materials. Improved understanding of the moisture transport kinetics through high performance cement pastes is critical in developing an appreciation of the mechanisms responsible for self-desiccation and autogenous shrinkage
mitigation.
6 Conclusion
1
H nuclear magnetic resonance (NMR) has been shown to be a relatively novel
technique for investigating the hydration kinetics of cement-based materials, particularly at early age. T2 relaxation time analyses can indicate the relative intensities and percentages of free water, C-S-H interlayer (physically bound) water, and
C-S-H gel (chemically bound) water. In addition, potential uses of proton NMR
applied to elucidate changes in the early age hydration effects in the porous cementitious matrix due to internal curing have been presented. It is anticipated that
NMR will be increasingly utilized as an in-situ and non-destructive analytical
technique in the studying the permeability and moisture movement in the cementitious materials.
Acknowledgments. The authors would like to acknowledge the National Science Foundation (CMS-0556015) and Tennessee Technological University for their financial support.
Any opinions, findings, and conclusions or recommendations expressed in this material are
those of the authors and do not necessarily reflect the views of the sponsors.
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