11th INTERNATIONAL BRICKJBLOCK MASONRY CONFERENCE TONGfl UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

MOISTURE AND SALT TRANSPORT IN BRICK: A NMR STUDY

L. PeJl, H. Brocken2 and K. Kopinga l

1. ABSTRACT

Moisture and salt transport in masonry can give rise to damages. While drying , salt crystallisation may occur at the surface, causing defacing, or just under the surface, where it may cause structural damages , e.g. , delamination, surface chipping or disintegration. Therefore a detailed knowledge of the moisture and salt transport is essential for understanding the durability of masonry. For studies of the moisture and salt transport in masonry it is important to measure the dynamic moisture and ion concentration profiles in a quantitative way. Nuclear magnetic resonance (NMR) offers a powerful technique to measure these profiles. Using a specially developed NMR scanner the moisture and ion transport was measured. In this paper some preliminary results will be discussed of the salt (NaCI) water absorption in fired-clay brick and sand-lime brick. .

2. INTRODUCTION

Most previous research has been focused on the moisture transport in brick and/or mortar, e.g. , [1 , 2, .3]. Up to now little attention has been given to the study of combined salt and moisture transport in masonry. A problem for such a study is the determination of the salt profiles. Often destructive techniques are used. However by measuring dynamic moisture and salt profiles during transport, more direct informa­tion is obtained. Nuclear magnetic resonance (NMR) off; rs this possibility.

Keywords: moisture, salt transport, non-destructive measurement, NMR

I Department of Physics , 2Department of Architecture, Building and Planning Technology, Eindhoven University of Technology, P.O. Box 513 , 5600 MB , Eindhoven, The Netherlands.

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This study focuses on the feasibility of NMR for measuring simultaneously the moisture and salt transport. First the theory of moisture and salt transport will be briefly discussed. In section 4 a description of the NMR technique will be given. Finally in section 5 some preliminary results will be discussed of absorption of a sodium chloride (NaCI) solution in fired-c1ay brick and sand-lime brick.

3. THEORY

The mathematical formulation of mass transfer at the macroscopic leveI in porous media is usually based on diffusion equations. A more fundamental basis for these equations was given by Withaker [4] and Bear [5]. If the gravity is neglected, the liquid moisture transport for the one-dimensional isothermal problem considered in this paper can be described by a nonlinear diffusion equation:

aa1 = ~(D aa1) at ax 6 ax

(1)

In this equation 81 [m3m-3] is tne volumetric liquid moisture content and D8 [m2s-1]

the moisture diffusivity which is a function of the actual moisture contento The salt concentration, will influence the moisture diffusivity as it changes the surface tension and the viscosity (see e.g. [2]). The salt transport can be described by:

a(a1c) =-~a(-D ac +CV)-f at ax I h ax I l~. (2)

Here c is the concentration of the ion of the salt, Dh the hydrodynamic dispersion coefficient which is a function of the actual moisture content VI the velocity of the liquid and fI-, a sink due to adsorption to the solid surfaces. (Dh" = De" + Dh ;

where D: is the molecular diffusion coefficient in a porous medium of a ion and Dh the dispersion coefficient of an ion within the liquid).

3. NUCLEAR MAGNETIC RESONANCE

3.1. General characteristics

In a nuclear magnetic resonance (NMR) experiment the magnetic moments of the nuclei are manipulated by suitably chosen radio frequency fields, resulting in a so­called spin-echo signal. The amplitude of this signal is proportional to the number of nuclei excited by the radio frequency field. NMR is a magnetic resonance technique, where the resonance condition for the nuclei is given by:

(3)

In this equation f is the frequency of the radio frequency field, 'Y is the gyromagnetic ratio (see table 1) and Bo is the externally applied static magnetic field . Because of this condition the method can be made sensitive to one type of nnclei and therefore

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to hydrogen (and thus to water), sodium or chloride. With NMR also a distinction can be made between free, physically bound, and chemically bound nuclei. When a known magnetic field gradient is applied, the resonance condition wil\ be dependent on the spatial position of the nuclei and the moisture/salt distribution can be measured without moving the sample. Assuming a single exponential relaxation, the magnitude of the NMR the spin-echo signal (for TI> >T2 ) is given by:

(4)

In this expression G is the rei ative sensitivity of the nuclei, p the nuclei density, TI the so-called spin-lattice relaxation time, TR the repetition time of the spin-echo experiments, T2 the so-called spin-spin or transverse relaxation time, and TE the so­called spin-echo time. In table 1 the relative sensitivity, the natural abundance and the gyromagnetic ratio are given for the isotopes considered in this paper. As can be seen, the sensitivity of sodium and especially chloride is small in comparison to hydrogen. Therefore, in this study we have only focused on the measured of mois­ture and sodium profiles.

Isotope natural abundance (%) G (-) 'Y (MHz/T)

IH 100 1 42.58

23Na 100 0.09 11.26

3sCl 75 0.005 4.17

Table 1: The natural abundance, relative sensitivity, G, and gyromagnetic ratio , "(, for various isotopes.

Obviously small T2 values lead to a decrease of the spin-echo signal, whereas , on the other hand, small TI values are preferred, as this parameter limits the scan repetition time: usually TE "" 4TI. In table 2 the relaxation times of hydrogen and sodium as determined in fired-clay brick and sand-lime brick are given.

fired-clay brick TI (s) T2 (p.s)

H 0. 15 760

Na 0.03 2400

sand-lime brick TI (s) T2 (p.s)

H 0.02 1600

Na 0.01 1950

Table 2: The relaxation times of H and Na in fired-clay brick and sand-lime brick determined from NMR, assuming a single exponential relaxation.

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As can be seen the TI values for sodium are relatively small in comparison to hydro­gen and therefore the low sensitivity can be compensated by averaging the signal over a larger number of spin-echo's. For the experiments described here, a home built NMR apparatus was used, which operates at a magnetic field of 0.7 T (33 MHz for IH and 8.9 MHz for 23Na). This apparatus was especially designed for quantitative measurements in porous materiais with short T2 relaxation times [2, 6] (unlike standard Magnetic Resonance Imaging (MRI) which is generally used in a qualitative way). A well defined magnetic field gradient is applied, offering a one­dimensional resolution of 1 mm for water and 4 mm for sodium. The experimental set-up for measuring the moisture profiles during absorption is given in Fig. 1. To obtain the moisture content with an inaccuracy of 2 %, the obtained signals from 12 subsequent spin-echo sequences were averaged, giving a measurement time of about 10 seco For the determination of the sodium content 164 spin-echo's, giving a measurement time of 30 sec., were averaged. After determina­tion of the moisture and sodium content at a position the sample is moved vertically over a few mm with the help of a step motor. This is repeated until a complete moisture and sodium profile has been measured. Subsequent moisture/sodium pro­files are measured by repeating the procedure mentioned above. During the measure­ments a time stamp is added to each point of the experimental profiles. An extensive description of the NMR apparatus can be found in [2, 6].

RF coil

,,/f

Faraday shield

stepmotor

teflon holder

, ' ,*-1 , ' , ' I~B , , ,

:"---!

""'--- w ate r

Figure 1: Experimental probe head for measuring moisture and sodium profiles dur­ing absorption. Using an-electrical sensor and a pomp the water levei is maintained constant.

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4. RESULTS

In the experiments cylindrical bars with a diameter of 20 mm and a length of 180 mm were used. These bars were drilled out of fired-c1ay brick and sand-lime bricks. The experiments were performed using a saturated NaCI (36%) solution.

4.1 Adsorption of sodium to surfaces

First the interaction of sodium ions with the solid surface was determined. Therefore brick samples were saturated with salt water. The sodium concentration was subsequently measured at one position for 48 hours. Since only free sodium ions can be measured the interaction with the pore surfaces can be determined. In Fig. 2 the results are plotted.

I: .2 iã E 20 CD () I: o ()

OI

Z 10

°0L-~~50~~~1LOO--~~15~0~~200 (Thouaand.)

time (sec)

Figure 2: The measured sodium concentration plotted as a functkm of time for frred-c1ay brick (O) and sand-lime brick (t.).

As can be seen the sodium concentration in fired-c1ay brick remains constant where­as for sand-lime brick, which has a larger internai surface, within 48 hours a small (- 5 %) decrease is found.

4.2 Absorption of sodium chloride solution

In absorption experimenis initial dry cylindrical samples were allowed to freely absorb a sodium chloride solution through one end. The experimental set-up is given in figure 1. The resulting moisture and sodium concentration profiles , each measured at a different time, are given in figure 3 for fired-c1ay brick and sand-lime brick. As can be seen from these figures a very steep wetting front for moisture is formed. The experimental error in the measured sodium profiles is in de order of 15 %. However, a decrease of this error by averaging over more spin-echo's would give rise to an unacceptable long measurement time. From these profiles it can be seen

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that sodium chloride is transported with the water. For both fired-c1ay brick and sand-lime brick the sodium concentration decreases towards the water absorption front. Since the adsorption to the pore surfaces can be neglected within the time­scale of this absorption experiment the decrease towards the front can be contributed to the hydrodynamic dispersion.

0.25 ,---0-------------------, 40

8° fired-clay brick

;)' \'8~~~~Q,... 'E 0.20 I

'" E --c: 0. 15 Q) -c: o 0 0 .10 Q) .... ~

Ui 'õ 0.05 E

• 20 40 60 80

position (mm)

30

20

10

0.30 ,----.----=.-... ----------------, 40

q-- 0.25 E ., E -0.20 -c:

CI) -c: 0.15 o o CI) :; 0.10 -VI

o E 0.05

10

sand-iime brick

30

20

10

20 30 40

position (mm)

........ ::R. o -c: o -.... ~ c: Q) o c: o o as Z

c: o -.... «l -c: Q) o c: o o as Z

Figure 3: Moisture (O) and sodium (., ~ , • , .) concentration profiles measured during the absorption of saturated (36%) sodium chloride (NaCI) solution for fired-c1ay brick and sand-lime brick. The curves are only given as a guide to the eye, whereas the times are only given -as an indication of the elapsed time.

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For water absorption ali measured profiles for one material can be related by the Boltzmann transformation (see e.g., [2, 7]:

(5)

In Fig. 4 the Bóltzmann transformations are plotted for the measured moisture profiles for fired-clay brick and sand-lime brick given in Fig 3. For both materiaIs the transformation yields a distinct curve on which the data from the various profiles collapse. This indicates that the moisture diffusivity does not depend on the position, and supports the modeling of the moisture transport during water absorption by a diffusion equation.

0.25 0.30 o flred-clay brlck •• nd-lIme brlck o II

0.26 0.20

;;- ;;-'E 'E

"E 1 0.20

=0,15 c c: ID ID C 'E 0.16 o o (.) (.)

! 0 .10 ~ :> ~ 0.10 o; o o E E

0 .05 0 .05

c5ID

1.50 0,08.00 0 .10 0 .20 0.30 0.40 0 .50

poslllon (mm) posltlon (mm)

Figure 4. Boltzmann transformation of the measured moisture profiles for the fired­clay brick (O) and sand-lime brick (to). (-) Boltzmann transformation of the simulated moisture profiles based on an exponential relation between moisture diffusivity and the moisture contento

The liquid moisture diffusivity for absorption is commonly approximated by an exponential relation [2, 7, 8]:

(6)

Based on this exponential relation, the parameters De,o and {3 for fired-clay brick and mortar were fitted by computer simulations of the absorption. In table 3 the coeffi­cients De.o and {3 corresponding to these simulations together with the capillary moisture content, Ocap, are given. The Boltzmann transformations of the simulated moisture profiles are added in Fig. 4. For both materiaIs these simulations give an adequate description of the observed moisture profiles .

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material De.o (m2s-l) (3 (-) Ocap (m3m-3)

fired-c1ay brick 3.0 x 10-9 31.0 0.22

sand-Iime brick 0.12 x 10-9 31.5 0.26

Table 3: The coefficients of the exponential function, De = De.oexp({30), describing the moisture diffusivity for absorption, and the capillary moisture content, OcaP'

5. CONCLUSION

Nuclear magnetic resonance is shown to be an accurate and reliable method for measuring the moisture profiles in porous media. The observed moisture transport can be modelled by a diffusion equation. The moisture diffusivity for absorption can be approximated by an exponential function. With NMR also the sodium profiles can be determined. From the observed profiles it is seen that during water absorption also salt is transported into the material. However in future research the signal-to­noise ratio has to be improved for the sodium measurements. Also experiments have to be performed over a larger period of time. The determination of the hydrody­namic dispersion coefficient from these profiles will be the next subject of this study.

6. ACKNOWLEDGEMENTS

Part of this project is supported by the Dutch Technology Foundation (STW) and TNO Building and Construction Research.

7. REFE~ENCES

1. Gummerson R.I. , Hall C. , and Hoff W.D., Water movement in porous build­ing materiaIs - hydraulic suction and sorptivity of brick and other masonry materiaIs , Bldg. Envlr. 15, 101-108 (1980).

2. Pel L., Moisture transport in porous building materiaIs , Ph.D. thesis, Eindhoven University of Technology, the Netherlands (1995).

3. Garrecht H., Porenstrukturmodelle für den Feuchtehaushalt von Baustoffen mit und ohne Salzbefachtung und rechnische Anwendung auf Mauerwerk, Ph.D. Thesis , Univeristãt Fridericiana, Karlsruhe, Germany (1992).

4. Whitaker S., Simultaneous heat, mass and momentum transfer in porous media: A theory of drying porous media, Adv. Heat transfer 13, 119-200 (1977) .

5. Bear I. and Bachmat Y., Introduction to modeling of transport phenomena in porous media, Vol. 4, Kluwer, Dordrecht, the Netherlands (1990) .

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6. Kopinga K. and Pel L. , One-dimensional scanning of moisture in porous materiais with NMR, Rev. Sei. lnstrum. 65, 3673-3681 (1994).

7. Gardner W.R. and Maylugh M.S ., Solutions and tests of the diffusion equation for the movement of water in soil , Soi/. Sei. Soco Am. Proc. 22, 197-201 (1958).

8. Pel L. , Kopinga K., Bertram G. and Lang G. , Water absorption in fired-c1ay brick observed by NMR scanning, J. Phys. D: Appl. Phys. 28, 675-680 (1995) .

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