11 th INTERNA TIONAL BRICKJBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

SALINIZATION EFFECTS ON THE SORPTION OF POROUS BUILDING MATERIALS - PRELIMINARY SORPTION TESTS-

H.J.P. Brocken'·2, O.C.G. Adan2, W. Rook3 and M.R.A.M. Maassen'

ABSTRACT

Deterioration of materiais such as salt cryptoflorescence and fungai growth is often strongly associated with transient humidity conditions and consequently with sorptiod' characteristics. Under equilibrium conditions, this sorption of porous materiais can be described by sorption isotherms. In the present study, adsorption branches of sorption isotherms were determined on the basis of a microcalorimetric method. This is done for one type of machine moulded fired-clay brick, with and without additionalload of salts. Experimental results indicate that in the latter case dissolvation and dilution of salts determine the sorption isotherm.

1. INTRODUCTION

With respect to salt crystallisation in porous building materiais, sustainability tests use to be performed by means of cyc1ic immersion and additional drying [1]. Since salinized bricks may absorb rather large amounts of water from humid air, variations of ambient humidity conditions may cause altemating crystallisation and dissolvation of salts. Obviously, the corresponding deterioration process is found in building practice [2]. The hygric response of porous materiais depends on the pore structure, including the degree and nature of salts in pores. The equilibrium moisture contents of such materiais is a function of relative vapour pressure and is, under isothermal conditions, given by

Keywords: Salt load, Sorption calorimetry, Sorption isotherm, Solution-vapour interface

'Department of Architecture, Building and Planning of the Eindhoven University of

Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands, 2TNO Building and

Construction Research, P.O. Box 49,2600 AA Delft, the Netherlands and 3TNO-PML,

P.O. Box 45, 2280 AA Rijswijk, the Netherlands.

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the sorption isotherm. In this paper sorption isotherms of salinized brick are determined on the basis of sorption calorimetry. Furthermore, preliminary results of exposure of salinized bricks to cyc1ic hurnidity conditions are added. Sodium chloride is used as a starting point.

2. CAPILLARY CONDENSATION IN SALINIZED POROUS MATERIALS

Generally, the sorption isotherm of a porous solid is c10sely bound up with the concepts of surface adsorption and capillary condensation. The Brunauer-Emmet-Teller theory [3] is widely used for description of vapour adsorption, whereas the principie" of capillary condensation is formulated in the Kelvin equation. This concept of capillary condensation implies that a vapour is able to condense in capillaries, when its relative vapour pressure is less than unity. Usually the correct application ofthis Kelvin equation refers to higher relative vapour pressures, whereas the BET theory is primarily used for lower ranges of relative vapour pressure. On the basis of the defmition of water activity [4], for a solution-vapour interface the Kelvin equation is written as:

RH pv(PB + Pc)

P"vs(PB )

(1)

In this equation RH is the relative humidity [-], Pv the vapour pressure above the solution-vapour interface in the capillary [pa] at temperature T [K], p'vs the vapour pressure offree pure water [pa] at temperature T [K], Pvs the vapour pressure ofthe free salt solution [pa] at temperature T [K], Y the surface tension of the solution-vapour interface [Jm'2], \)1 the molar volume of liquid water [m3mol'I], rm the mean pore radius [m], R the gas constant [JK'mol" ], Ps the barometric pressure [Pa] , Pc the capillary pressure [Pa] and a,. the water activity of the solution [-] . The right hand side of equation (I) consists of two terms. The first term accounts for the water activity, a,., of a free (rm -+ cx:» salt solution at hydrostatic pressures equal to the barometric pressure, Ps. For pure water this term equals unity and equation (1) reduces to the regular Kelvin equation as given by the second exponential termo As can be seen from equation (1), addition of salt to the capillary water, gives an extra reduction of the vapour pressure above the liquid-vapour interface by a factor a,.. Note that in this equation, both the water activity of the free solution, a,., and the surface tension of the solution-vapour interface, y, are a function of the concentration or molality of the solution.

3. MATERJALS, METHODS AND SAMPLE PREPARATION

Materiais In building practice a lot of different porous materiais are being used such as rock stone, concrete and brick. All these materiais have different pore size distributions and therefore show different sorption characteristics. The present paper focuses on fired-c1ay brick, i.e. , one type of machine moulded, soft mud, red fired-c1ay brick. Some general characteristics of thismaterial are summarized in Table I.

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Material Do .10.9 [m2s-I]

fired-clay brick RS 7.3

p [-]

29 0.26 0.42 3.3

Table I : The coefficients of the exponential relation De.o = Do exp(J39I)' describing the liquid moisture diffusivity for water absorption, the capillary moisture content, 8"p, and both the sorptivity, S, and the initial rate of absorption, IRA, that are determined using the (experimental) moisture diffusivity.

Methods Determination of sorption isotherms was performed on the basis of a microcalorimetric method [5,6]. The principie is schematically presented in Fig. 1. An evaporation vessel (E), containing the liquid adsorptive (I), and adsorption vessel (A) with the adsorbent (a) and a connecting tube including a valve with adjustable restriction (V,) form a closed thermodynamic system_ Heat exchange takes place only through two identical thermopiles, acting as heat flow meters. The cold junctions of both piles are connected to the constant temperature heat sink (Tc) ofthe calorimeter, whereas the other junctions are in contact with the vessels. Compensation for spurious heat flows due to uncontrolled temperature fluctuations is obtained by coupling two reference cells with similar thermopiles in electrically opposite sense.

o hol junction

o cold junclion

liquid adsorptiva adsorbant

cal! E cal! A

Figure 1. Schematic presentation of the sorption calorimeter. Legend: E =

evaporation vessel, containing the Iiquid adsorptive; A = adsorption vessel containing the adsorbent; V, = adjustable restriction; P = pressure gauge; Te = constant temperature heat sink.

After evacuation of air and thermal equilibrium of the system, the restriction V, is opened, admitting diffusion of adsorptive to the adsorbent. Consequently, two simultaneous heat flows are introduced caused by evaporation in cell E and adsorption in cell A respectively. Thesc heat flows evoke two opposite electromotive forces

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(e.m.f.'s) in the correspondent thermopiles. Integration of the evaporation-side e.m.f. (EMFE) over the time interval needed for re-equilibration represents a measure of the amount adsorptive that has been evaporated; taking the dead volume into account, the amount adsorbed can be easily deduced as well. Apressure gauge (P) connected to the adsorption vessel measures the actual adsorptive vapour pressure, that is required for deduction of the adsorption isotherm. Additional gravimetric measurements of sample weights after each measuring cyc1e provides a reference for overall control of sorption isotherms.

The experimental set-up for measuring the hygric response to transient humidity cyc1es is depicted in Fig. 2. Series of salinized brick samples were placed in a sample container and exposed to high and low relative hurnidities. Each sample container could be opened separately to determine the mass of the samples. In the present experiments the high and low relative humidity were adjusted to 93% and 53% respectively. The RH's were defined using saturated aqueous salt solutions in two containers. Each container was part of a separate main system with a permanent circulation of air, driven by a small fan . Both systems, i.e., the high and the low RH, were equipped with a by pass, connecting the sample container to the main systems. By means of pneumatic valves, controlled by time switches, the RH and the corresponding system was selected. At the inlet of the sample container the RH was monitored. The whole experimental set-up was placed in a c1imate room set at 20°C. The air flow was set to 1.5 I min·1 (i.e. ± 2 cm S' I) by means of adjustable flow restrictions in both the by-pass and the main system.

v v

RH HIGH

Figure 2. Schematic presentation of the experimental arrangement in the transient RH experiments. Legend: CS = containers with series of samples under test; C = containers with saturated aqueous salt solutions; S = RH­sensors; V = pneumatic valves.

The cyc1ic humidity experiments consisted of three separate tests. In the first two tests, the equilibrium water sorption at both the high and low RH was determined, whereas in the third test the response to changing humidities was determined. In this case the intermittent high and low RH periods each lasted for 24 h.

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-,

Sample preparation To measure the adsorption isotherm of salinized fired-clay brick, dry samples were immersed in solutions of sodium chloride (NaCl) for at least 10 days. Due to the limited size ofthe sample holder ofthe calorimetry equipment, on1y some small bars of2-3 mm diameter were used, altogether weighing 2 grams. As pulverizing would destroy the pore structure of the brick, this preparation was preferred. Two different concentrations of sodium chloride solution were used, i.e., 0.2M and 1.0M. Within 10 days of immersion, the concentration in the capillary water is assumed to equal the bulk concentration of 0.2M and 1.0M, respectively. Subsequently, samples were air dried at (moderate) room conditions of20°C and 50% RH for one day, followed by oven drying at 105°C. The dry sample mass was determined before and after salinization, giving an overall control of the salt deposited in the sample. After this preparation, samples were kept under vacuum. For the exposure to transient relative humidities, salinized samples of fired-clay brick were prepared similarly. In this case samples of 45 mm diameter and 10 mm thickness were made. The experiments started with completely dry bricks kept under vacuum before testing. Due to this initial condition, the establishment ofthe dynamic equilibrium took some time, i.e., 3 to 4 cycles. .

~ 1 c: CI> c: o '-' ~ :::> Ü; '0 E

250

r[CJ i ., caplllary 200

0 .30 ~ .. . molsture conte"t 0 .30 '", . ' Ê ;; ~ ... o 0.70 0 .10 0 .110 t.oo 1.0M! fired-clay brick

RH ., 150 E

0 .20 0 .20 ~

Õ > .,

100 .:! ãí :;

0 .10 dissolvation 0 .10 E ~

" 50

0.00 0 .00 o 0.00 0.20 0 .40 0.60 0.80 1.00 10·009 1.·008 10·007 10·008 10·005

rei ative humldity (-) pore radius (m)

Figure 3. [lefi] Sorption isotherms as measured for blank fired-clay brick (O) and fired-clay brick immersed in a O.2M (o) and 1.0M (6) solution of sodium chloride respectively. The indented line indicates the capillary moisture content of fired-clay brick. The inset shows the quotient of the water sorption for brick immersed in l.OM and O.2M solution respectively. The dashed \ines indicate the simulated water sorption according to experimental data for dilution of a free aqueous sodium chloride solution L7]. Figure 4. [right] The cumulative pore volume of fired-clay brick determined using Hg-porosimetrry. Data for three different brick samples are plotted.

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10·004

4. EXPERIMENTAL RESUL TS

Sorption calorimetry The adsorption isothenns of salinized fired-c1ay brick are plotted in Fig. 3 for untreated (i.e. without load of salts) samples and samples immersed in 0.2M and I .OM solutions of sodium chloride, respectively. The curve for the untreated sample hardly shows any sorption of water at high RH. Apparently most pores are bigger than 10-7 m so that at RH's < 0.98 capillary condensation hardly occurs. Results of mercury porosimetry experiments as plotted in Fig. 4, agree with this conc1usion. The curves of the salinized samples in Fig. 3 show that in the RH region up to approximately 0.75, obviously no significant effect of the salt load occurs. At this 0.75 RH the sorption of water steeply increases corresponding to the introduction of a free (rm ~ 00) saturated solution of sodium chloride. The sorption of water appears to be linearly related to the salts load, reflected in a 1 to 5 ratio (see inset in Fig. 3). Furthennore, with increasing RH up to approximately 0.85 this ratio still remains the same, suggesting further dilution ofthe solution. For higher RH values, deviations occur since for the l.OM salt load the capillary moisture content ofthe brick eventua1ly limits the water sorption.

~

c li> c o (J

~ ::J (i) 'õ E li> .~ iõ ~

1.00

0 .80 ",+ .. .

0 .60

0.40 ~ o . o o . ." +

.0 0 .20 ...

o & "*'

a

"'"

1.00

'E 0.80 li> c o (J 0.60 ~ ::J (i) 'õ 0 .40 E li> .~ C; 0 .20 .

~

b

o '- o

o

\.1 "

0 .00 - - - - -.111.\ - ....- - - - - - - - - ..... - - 0.00 - - - - - - - - - - - - - - - - - - - -

""'" 12 24 36 48 60 72 84 96 ~ ~ 24 36 48 60 72 84 96

time (h) time (h)

Figure 5. The sorbed relative moisture content (eq. 2) measured during the sorption cycles for fired-c1ay brick immersed in a O.2M (a) and l.OM (b) solution of sodium chloride respectively. The symbols indicate different samples. As the establishment of the dynamic equilibrium takes some time due to the initial sample conditions, the results after 3 to 4 cycles (i .e. 8 days) are considered only.

Transient relative humidities In Fig. 5, the respo'1Se. of salinized fired-c1ay brick is plotted as a function of time. In this figure, the equilibrium water sorption at the high and the low RH, i.e. at 93% and 53%, are taken as the upper and the lower reference respectively (dashed lines). With respect to this upper and lower reference, the water sorption is plotted as a dimensionless parameter ranging from O to 1, according to:

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RMV m(t) - mO•53

mO.96 - m0.53

(2)

In this equation RMV is the relative moisture content [-] plotted at the y-axis in Fig. 5, m(t) is the time dependent mass of water [gram] sorbed during the cyclic sorption test and mO.53 and mO.93 are the equilibrium masses of water [gram] sorbed at RH's of 0.53 and 0.93, respectively. Fig. 5a gives the water sorption for fired-clay brick immersed in a 0.2M solution of sodiuhl chloride whereas Fig. 5b gives the water sorption in case of immersion in a 1.0M solution. Obviously, within a 24 h period of drying, the 1.0M salt load shows a residual water sorption. In both figures the maximum water sorption is less than the water sorption at RH = 0.93 . In fact, in both figures this maximum water sorption differs for each single sample. (Note that the sample containers had to be opened to perform the mass registrations.)

5. DISCUSSION AND CONCLUSIONS

For the fired-clay brick considered in this paper, capillary condensation appears to play . a minor role in the sorption of water vapour. However, an artificial load of'sodium chloride introduces a steep increase ofwater sorption at approximately 0.75 RH. As the equilibrium RH or water activity of a saturated free sodium chIoride solution equals 0.75, equation (1) shows that the correct application ofthe Kelvin equation refers to a smaller range of RH's than in the case of pure water in capillaries. Additionally for this RH range, the assumption of a saturated capillary (i.e. not free rm < 10-7 m) sodium chloride solution involves ano upper RH !imit smaller than 0.75. The observed increase of the moisture content at a value of approximately 0.75 RH, corresponds to the filling of a volume of bigger pores outside the conunón capillary region mentioned above, Le. the filling of pores with diameters above 10-2- m. The steep increase of water sorption at the 0.75 RH value suggests the existence of a saturated solution in these non-capillary pores. The gradual increase of the moisture content with increasing RH's above this value might be related to dilution ofthe pore solution. In Fig. 3, starting from the saturation point at 0.75 RH, simulations based on experimental data for dilution of a free aqueous sodium chloride solution [7] are added. Obviously, in this region the simulated (represented by the dashed !ines) and measured water sorptíon fairly agree. With increasing RH's, the curves corresponding to the 1.0M salt load deviate towards a final value limited by the capillary moisture content of the brick. It is concluded that the upper part ofthe brick's sorption isotherm following the capillary region, may be described by dilution of a free saturated sodium chloride solution. In this case, the statements in previous work of Garrecht [8] are confirmed experimentally. Future experiments on true capillary porous material, such as sand-lime brick, should reveal effects of salt loads in the capillary region. Preliminary cyclic sorption tests with salinized brick samples indicated that, within 24 hours, fired-clay brick with a I .OM load of sodium chloride does not completely dry whereas fi red-clay brick with a O.2M salt load does. In addition to sodium chloride, future experiments should include hydrating salts like sodium sulphate and magnesium sulphate.

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ACKNOWLEDGEMENTS

The authors wish to acknowledge the experimental ,help of H. Smulders and A. W.B. Theuws of the departrnent of Architecture, Building 'and Planning of the Eindhoven University of Technology and J.P.G.M. v. Eijck of TNO-TPD Building Ceramics.

REFERENCES

1. Binda L. and Baronio G., Mechanisms ofmasonry decay due to salt crystallization, Durability of Building Materiais 4, 1987,227-240.

2. Winkler E.M., Stone in Architecture, Springer-Verlag, Berlin, 1994. 3. Brunauer S., Emmett P.H. and Teller E., Adsorption of gases in multimolecular

layers, J Am. Chem. Soco 60, 1938,309-319. 4. Thain J.F., Principais of Osmotic Phenomena, The Royal Institute of Chemistry,

Heffer, Cambridge, 1967. 5. Bokhoven J.J.G.M. van, A method to measure the net heat of adsorption and the

adsorption isotheml simultaneously, Thermochim. Acta 34, 1979, 109-126. 6. Adan O.c.G. , On the fungai defacement of interior finishes, Ph. D. thesis,

Eindhoven University of Technology, the Netherlands, 1994. 7. RobÍnson R.A. and Stokes R.H., Electrolyte solutions, the measurement and

interpretation of conductance, chemical potential and diffusion in solutions of simple electro!ytes, Butterworths Scientific Publications, London, 1959.

8. Garrecht H. , Porenstrukturmodelle für den Feuchtehaushalt von Baustoffen mit und ohne Salzbefachtung und rechnische Anwendung auf Mauerwerk, Ph. D. thesis, Universitãt Fridericiana, Karlsruhe, Germany, 1992.

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