experimental research on hygroscopic behaviour of porous specimens contaminated with salts

Construction and Building Materials 18(2004) 339–348

0950-0618/04/$ - see front matter� 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2004.02.007

Experimental research on hygroscopic behaviour of porous specimenscontaminated with salts

B. Lubelli , R.P.J. van Hees *, H.J.P. Brockena,c b,c, c

Delft University of Technology, Faculty of Civil Engineering, P.O. Box 5048, 2600 GA Delft, The Netherlandsa

Delft University of Technology, Faculty of Architecture, P.O. Box 5043, 2600 GA Delft, The Netherlandsb

Netherlands Organisation for Applied Scientific Research, TNO Building and Construction Research, P.O. Box 49, 2600 AA Delft,c

The Netherlands

Received 17 February 2004; received in revised form 20 February 2004; accepted 20 February 2004Available Online 21 April 2004

Abstract

Salt crystallisation is one of the main causes of masonry decay in monuments. The salt content in a wall is a very importantparameter that should be known in order to give a reliable diagnosis on the causes of damage; if a restoration work and a surfacetreatment are necessary, the salt content has to be known. In fact the presence of salts in the wall can not only reduce theefficiency of a treatment but also form a potential risk for the occurrence of a damaging salt crystallisation process. Differentmethods and instruments are available to detect salt content and type of salt present, but most of them are quite expensive andtime consuming. For this reason, other methods, cheaper and quicker, are preferably adopted. In building practice often thehygroscopic moisture content of powder samples is measured in order to have an indication of the salt content. This method ischeap and simple and, in case of material contaminated with a single salt, gives reliable results on the quantity of salts presentbecause of the linear relation between HMC and salt content. However, in case a mix of salts is involved, as usual in reality, therelation between the HMC and the salt content is not clear yet and the HMC measurement can give only an indication and not aquantitative value of the salt content. This paper presents the experimental results of a research carried out on the hygroscopicbehaviour of sodium salts introduced in a typical clay-brick either as single salts or as a mix. The obtained results point out aclear linear relation between salt content and hygroscopic moisture content. This proportionality is verified for(brick) specimenscontaminated with pure salts as well as with salt mixtures. Salt mixtures appeared to correspond with hygroscopic moisturecontents that are higher than the one calculated according to the amount of single salts present in the mixture. As expectedbeforehand no difference was found between the hygroscopic behaviour of solid brick specimen and brick powder, only a longertime to reach equilibrium in case of brick powder. This paper suggests in the case of the presence of single salts to use the HMCmeasurement instead of more complicate and expensive analyses(as for example in laboratory experiments). In building practice,the measurement of the HMC can help in selecting the most interesting spots on which to focus the research avoiding extensivecampaigns of sampling and analyses. Besides, HMC measurements performed at different RH, can give suggestions about thetype of salt present.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Salts; Hygroscopic behaviour; Hygroscopic moisture content; Porous materials; Experimental research

1. Introduction

Water soluble salts present in substrate material suchas brick, stone and mortar are a major cause of decay.The salts originate from ions that have leached out fromweathering rocks, from soil, from building stone, mortarand brick. They can also originate from deposit either

*Corresponding author. Fax:q31-15-2763017.E-mail address: [email protected](R.P.J. van Hees).

from the compounds of natural and polluted atmosphereor be generated by organic metabolisms. The ions arecarried in diluted aqueous solutions and may penetrateand be transported into the material. Where waterevaporates they concentrate so that their solution canbecome(super)saturated with respect to a certain saltphase within the system. This particular phase willprecipitate on or beneath the material surface and forman efflorescence or sub-florescence. With variation of

340 B. Lubelli et al. / Construction and Building Materials 18 (2004) 339–348

Table 1Osmotic coefficient of the salts used in the experimental research

m (molykg) f NaCl f Na SO2 4 f NaNO3

0.1 0.9324 0.921 0.7930.5 0.9209 0.873 0.6901.0 0.9355 0.851 0.6422.0 0.9833 0.826 0.6213.0 1.0453 0.810 0.6614.0 1.1158 0.797 0.7405.0 1.1916 0.7886.0 1.2706 0.788

relative humidity, salts may dissolve and re-crystallise.That implies that the microclimate contributes essentiallyto the weathering activity of soluble salts. As a conse-quence salt weathering cannot be understood withouttaking into account the interaction between microclimateand salt concentrations.The presence of soluble salts in a wall, their quantity

and distribution, is an important parameter to be knownbefore application of surface treatment. It may in factbe considered as a contraindication for the applicationof a surface treatment. Previous laboratory investigationson specimens with an artificial salt load and analysis ofsamples from real case studies have shown that a highsalt content in the wall cannot only reduce the effective-ness of a treatment but also cause an increase of thedamaging processw1x. To analyse the presence of salts,their type, quantity and distribution in masonry, deter-mination of the hygroscopic moisture content of smallpowder samples of the material, is an easy test that cangive an indication of the local salt content in thematerial. In fact water-soluble salts have a certainhygroscopic behaviour; that is at certain conditions oftemperature and RH they absorb moisture from the airand their dissolution takes place. The result is anincrease of weight that depends on the quantity and typeof salts. For a ‘single’ salt solution it is quite simple todetermine the quantity of salt present in the material, aslong as the relation between salt content and hygroscopicbehaviour is known. However, in reality different saltsare often present as a mix so that their analysis becomesvery complex. From literature it is known that theequilibrium RH of a saturated solution of salts in a mixis different from the one measured on the single saltsw2–4x. Consequently also their hygroscopic behaviourwill be different and not simply predictable.The aim of the research is to develop a methodology

that is suitable to analyse the salts that are present in asubstrate material. This means:

i. find out a relation between the quantity and the typeof soluble salts and the hygroscopic behaviour ofmaterials contaminated with them;

ii. point out the differences between the behaviour ofsingle salts and salts combined in a mix;

iii. evaluate the influence of each salt on the hygroscop-ic behaviour of the mix; and

iv. check if there is any difference in the hygroscopicbehaviour of brick specimen and brick powder spec-imens contaminated with the same salts.

Because of the complexity of the problem thatinvolves many variables, at first a research on singlesalts was carried out. The results obtained from this firstphase were used to develop a methodology for theanalysis of salt mixtures.

2. Theoretical outline

Most salts are able to dissolve not only in liquidwater but also at a RH below 100%, by absorbingmoisture from the ambient air. This happens when theRH of the ambient air is higher than the water activityof a saturated solution of that particular salt. The wateractivity is different and characteristic for each salt andmay also depend on temperature. If the RH drops belowthe water activity of the saturated solution, all waterevaporates and finally the salt will precipitate. If the RHrises above this value, at first salts will dissolve andsubsequently the solution will absorb more water andget diluted. A salt solution becomes more and morediluted with rising RH, and when RH is close to 100%,the concentration is close to zero, i.e. the solution isinfinitely diluted. The molality of a solution(moles ofsaltylitre of water) can be calculated by the equation ofRobinson–Stokesw5x

ln wsln a synfØM Øm (1)w w

where:w (%)srelative humidity;a (%)swater activ-w

ity of the solution;n (–)snumber of ions in the salt;F (–)sosmotic coefficient depending on molality andtemperature;M (kgymol)smole weight of water;mw

(molyKg)smolality of the solution.The osmotic coefficient expresses the deviation of the

electrolyte solution from ideality and must be foundempirically. Knowing the osmotic coefficient, Eq.(1)gives a functional relationship between the molality ofa solution and the RH of the ambient air. From thisrelationship one can easily deduce a linear curvebetween the salt content in a substrate and its equilibri-um hygroscopic moisture content in a certain environ-ment of constant RH. With respect to the salts used forthe research described in this paper Table 1 gives asurvey of the osmotic coefficient found in literaturew6x.For a solution with a mixture of three or more ions,

the analysis becomes much more complicated. In thiscase, the solubility of each salt depends on the concen-tration of the various ions. The RH over a solution of asalt mixture can still be calculated by the equation ofRobinson–Stokes, but the solubilities are not the same

341B. Lubelli et al. / Construction and Building Materials 18 (2004) 339–348

Fig. 1. The Na SOyH O system from 0 to 508C w3x; lines A, B and2 4 2

C represent the equilibrium state between the three phases.

as for the pure salts and the osmotic coefficient mustrelate to the specific solution. The situation can beanalysed by the ion interaction model of Pitzerw7x thatis based on experimental data of both single and mixedelectrolytes. This model was used by Steiger and Zeunertw2x to calculate the solubilities and the equilibrium RHof salt mixtures with three ions. In the present paper thetheoretical concentration of solutions as calculated forthe salt mixtures are based on the single salt values;instead of verifying the Pitzer model, the research wasmore focused on determining the difference of thehygroscopic behaviour between single salts and theircombination in a mixture.

3. Test on single salts

In building practice, the HMC is usually measured onpowder sampled from the investigated building, but canhappen that the same measurements are performed onsolid sample of stone, brick, etc.In order to check for possible differences in the

hygroscopic behaviour, the test on hygroscopic behav-iour was carried out on small brick specimens as wellas on powder samples obtained by grinding.

3.1. Description of the experiment

Since this study on hygroscopic behaviour was per-formed in the framework of a EU Projectw8x, theselection of substrates, salts and concentrations wasmade according to the parameters already chosen in thecorresponding research program.

3.1.1. SubstrateThe selected substrate is a Dutch red brick. Its

physical characteristics are the following:

● Porosity(V%): 36%.● Water absorption coeff.: 2.9(gycm h ).2 0.5

● Free water absorption after 24 h of complete immer-sion: 0.15(wyw).

3.1.2. SaltsThe following sodium salts were selected:

● NaCl (RH 75.5%).sat.sol

● Na SO (RH 82%) wNa SO 10H O: RH :2 4 sat.sol. 2 4 2 sat.sol..

93.6%x, see Fig. 1.● NaNO (RH .75.4%) w9x.3 sat. sol.

● Blank specimens were used as reference.

3.1.3. Concentration of solutionFor the present research, the selected salt concentra-

tions were based on the range of salt contents found inthe analysis of case studiesw8x. The following salts andconcentrations of solutions(weight percentage of saltreferring to the weight of water) were used:

● C1: 1% (0.14%: weight percentage of salt referringto the weight of the specimen).

● C2: 2.5%(0.35%: weight percentage of salt referringto the weight of the specimen).

● C3: 5% (0.71%: weight percentage of salt referringto the weight of the specimen).

3.1.4. Temperature and relative humidity of conditioningThe temperature used for the test was 208C. The

relative humidities of conditioning were chosen accord-ing to the ones available at TNO.

The climatic chamber conditions were:● 20 8C and 50% RH;● 20 8C and 65% RH;● 20 8C and 80% RH; and● 20 8C and 98% RH

The climatic boxes conditions were:● 20 8C and 85% RH;● 20 8C and 93% RH; and● 20 8C and 96% RH;

The RH and temperature values in the climatic cham-bers and boxes were monitored continuously so thatvariations and deviations from these values were known.The range of variation of temperature was"3 8C in theclimatic boxes and"1 8C in the climatic chambers; therange of variation of RH was"3% in both cases.

3.1.5. Number of specimensFor each salt concentration and RH one brick speci-

men was used. Bricks specimens that were not contam-inated with salts were used as reference. In order tohave a comparable situation all seven specimens to bestored at different RH were taken from the same brick.


Fig. 2. NaCl contaminated brick specimens: H.M.C. measured at 80%of RH.

Fig. 3. NaCl contaminated brick powder specimens: H.M.C. measuredat 80%.

3.1.6. Test procedureThe test procedure is different for solid brick and

brick powder specimens.

Brick specimens:● cilindric specimens of� 3 cm, height 2 cm, were

drilled;● the specimens were contaminated with the salt

solutions(the amount of solution used was 80%of the CMC of the material) and dried at 1058C;

● after thermal equilibration at 208Cy50% RH(forapprox. 2 h) the samples were put at differenthygric conditions in the climate rooms and boxes;and

● the specimens were weighed periodically until aconstant mass was reached.

Brick powder:● prismatic specimens(dimension: 2.5=2.5=2

cm ) were cut;3

● the specimens were contaminated with salt solu-tion and dried at 1058C;

● after drying each specimen was manually grinded(in order to avoid any loss of material) and putinto a glass container and then in the differentclimate chambers and boxes; and

● at every RH the moisture content of specimenwas determined gravimetrically.

3.2. Results of tests on single salts

The results obtained from the tests on single salts arepresented in this section.

3.2.1. Solid brick specimensDuring the first two weeks after putting the specimens

under different humidity climates, their weight was

determined every day. Figs. 2 and 3 present the observedincrease of weight respectively, for the brick specimensand the brick powder specimens.As can be seen, the increase of weight for the brick

specimens occurs within two days, while for the brickpowder the increase until constant weight takes approx-imately 12 days. Presumably, as a result of the dryingprocess, salts are concentrated on the surface of thebrick specimens accelerating their hygroscopic response.The obtained values of hygroscopic moisture content

(sequilibrium value moisture content at constant RHlevels) can be expressed as a function of salt content(as w% referring to the weight of the specimen). Atfirst this relation is presented in Fig. 4a–c for the brickspecimens, showing that:

● For all the salts at different relative humidities thereis a very clear linear relation between salt contentand H.M.C.; this confirms results obtained by Nun-berg and Charola for NaCl and Na SOw10x.2 4

● Even in specimens contaminated with Na SO and2 4

stored at 80% RH, an increase of weight is measuredwhile the RH of dissolution of Na SO is 82%. In2 4

this case, water adsorption can however, be explainedby hydration of thenardite to mirabilite(at 208C forRH)71.3%w11x). The latter is in contradiction withresults of previous experimental researchw12x whichsuggested that mirabilite can only be formed bycrystallisation after thenardite dissolution hasoccurred.

● Although NaCl and NaNO have almost the same3

equilibrium RH of saturated solution their influenceon H.M.C. is different.

To verify the reliability of the results, the obtainedvalues of H.M.C. are compared to the values calculated


Fig. 4. Test on brick: relation between salt content and H.M.C. at 80(a), 93 (b) and 96%(c) of RH.

Fig. 5. Brick specimens: relation between salt content(w%) and H.M.C.; comparison with calculated values of free salt solutions dissolved at80 (a), 93 (b) and 96%(c) of RH.

for free solutions of NaCl, NaNO and Na SO dissolved3 2 4

respectively, at 80%, 93% and 96% RHw6,13x (Fig.5a–c).As can be seen, only at 80% RH the theoretical curves

correspond with the experimental values; for NaCl andNaNO the theoretical curves represent dissolution of3

the salts, while for Na SO the theoretical curve repre-2 4

sents hydration of the salts(see the comment mentionedabove). For 93 and 96% RH the experimental values ofhygroscopic moisture content are lower than the theor-etical prediction. This can have two differentexplanations:

● The average value of RH measured in the period inwhich the test was performed, was 91.4% RH forthe 93% climatic box and 94.8 for the 96% climaticbox; the theoretical curves calculated for these values(dotted lines) are more similar to the experimentalvalues in the case of experiment performed at 96%(94.8%). Nevertheless for the case of 93%(91.4%)there is still a relevant difference;

● At 20 8C sodium sulfate, present in the form ofthenardite at the beginning of the test, hydrates tomirabilite and may stay in that condition at RH’sbetween 71.3 and 93.6%w2x (Fig. 1). At RH’s above93.6% (for instance at 96%) mirabilite dissolves toform a diluted Na SO solution. The theoretical2 4

curves in Fig. 5b,c are calculated, assuming that at82% Na SO completely dissolves. This however, is2 4

not true, because the hydrate mirabilite is formedbefore complete dissolution of Na SO takes place.2 4

The theoretical curves calculated at 93% RH forbrick substrates contaminated with Na SO may2 4

therefore show a great deviation with the experimen-tally determined values; the formation of mirabiliterequires less water than the complete dissolution ofNa SO does.2 4

In addition to the experiments performed on brickspecimens, also in the case of the experiment on brickpowder a linear relation was found between the quantityof salt (expressed as a percentage of the weight) and


Fig. 6. Test on powder brick: relation between salt content and H.M.C. at 80(a), 93 (b) and 96%(c) of RH.

Fig. 7. Brick powder specimens: relation between salt content(moleykilogram) and H.M.C.; comparison with theoretical curves at 80(a), 93 (b)and 96%(c) of RH.

the hygroscopic moisture content(Fig. 6a–c). In con-trast with the results for brick specimens, here noincrease of weight was measured for brick powderspecimens contaminated with sodium sulfate and storedat 80% RH. Therefore, hydration does not seem tooccur.The experimental data are compared with theoretical

curvesw6,11x in Fig. 7a–c. The same conclusions as theones described for the solid brick specimens can bedrawn.For this reason, the use of brick powder can be

suggested for further laboratory experimental works: infact this situation is more similar to the reality, wherethe H.M.C. is mainly measured on powder obtained bydrilling.

4. Test on salt mixture

In building practice salts are generally present as amix and not separately. The hygroscopic behaviour of asalt mixture cannot be determined from that of the singlesalts in the mixture.

In order to understand the relation between the com-position of the mix(type and quantity of salts) and itshygroscopic behaviour, experimental tests have beenperformed. In this paper, a laboratory research on aternary salt system is presented. The main aims of thiswork were the following:● verify the relation between salt content(of a mixture)and hygroscopic moisture content;

● verify and evaluate the different behaviour of a saltconsidered separately or combined in a mix; and

● evaluate the influence of each salt on the hygroscopicbehaviour of the mix.

4.1. Description of the experiment

4.1.1. SubstrateIn order to compare the results with the ones for the

single salts, the same substrate material was used, thatis red brick.

4.1.2. Salt mixture compositionThe same salts used in the test on single salts were

chosen. In this way the data on hygroscopicity for all


Fig. 8. Salt mix contaminated brick powder specimens: H.M.C. meas-ured at 80%.

Fig. 9. Salt mix contaminated brick powder specimens: relationbetween salt content and H.M.C.

the salts used are available and a comparison can bemade between the behaviour of salts considered sepa-rately and mixed in a cocktail.The salt mixture was composed out of:

50% Na SOq25% NaClq25% NaNO2 4 3

Note that in this case the negative ions SO , Cly y4

and NO can be combined only to Na and no salts,y3

different from Na SO Na SOØ10H O, NaCl and2 4, 2 4 2

NaNO can be formed.3

4.1.3. Concentration of solutionThe tests performed on single salts show that there is

a very clear proportionality between the salt content andthe H.M.C.The following concentrations of salt solution(weight

percentage of salt referring to the weight of water) wereused:

● C2: 2.5%(0.35%: weight percentage of salt referringto the weight of the specimen);

● C3: 5%(0.71%: ditto); and● C4: 7.5%(1.06%: ditto)

4.1.4. Size of specimen and test procedureSince there were no significant differences between

the results obtained with single salts on brick and onbrick powder the tests were performed on powdersamples only.The same size of prismatic specimens used for test

on brick powder(2.5=2.5=2 cm) and the same testprocedure were used.

4.1.5. Temperature and relative humidity of storageThe selected values of RH were:

80%: dissolution of NaCl (RH : 75.5%) andsat.sol.

NaNO (RH : 75.4%).3 sat.sol.

85%: dissolution of Na SO(RH 82%).2 4 sat.sol.

93% : this RH of storage was adopted to have somefurther support to the results and to compare them tothe ones obtained on single salt.96% : dissolution of Na SOØ10H O (RH : 93.6%).2 4 2 sat.sol.

4.1.6. Number of specimensFor each concentration and RH one specimen was

used. Reference(specimens not contaminated with salts)were not used because these tests were already per-formed in the case of single salts.

4.2. Results

The specimens contaminated with the salt mixturewere weighed until they reached a constant weight. Theincrease of weight at 80% RH is reported in Fig. 8.Also, in this case, as in powder brick specimens contam-inated with single salt, the increase of weight as afunction of the time proceeds gradually and reaches anequilibrium within 2–3 weeks.Again also in the case of the salt mix a very clear

linear relation between the salt content and the H.M.Cwas found(Fig. 9).An important aim of this research was to point out

the difference between the behaviour of single salts andsalts combined in a mix. This comparison is presentedin Fig. 12, both with data from the test on single saltsand with theoretical curves calculated from the dissolu-tion of single salts respectively, at 93% and 96%.

– (with experimental data of single salts). Since thequantity of each salt(expressed as a percentage ofweight with respect to the weight of the specimen)is not the same in the single salt test and in themixture, at first the relation between the salt content


Fig. 10. Linear equations fitting the experimental data of single salttest at 96% of RH; here, for example, the percentage of Na SO pres-2 4

ent in the mix and the relative H.M.C is calculated and reported initalic.

Fig. 11. Calculated value of H.M.C at 96% of RH. for the differentpercentage of salts in the mix.

Fig. 12. Comparison of H.M.C. of salt considered singly or combined in the mix.

and the H.M.C. was determined from the experi-mental data of test on single salts(see Fig. 9).Therefore the experimental values were fitted witha linear equation and, on the basis of this the H.M.C.values were calculated for the percentages of saltpresent in the mix. The example for 96% of RH isreported in Fig. 10. The sum of these values ofHMC of the simple salts was taken in order toestimate the total HMC(Fig. 11). In this way theH.M.C. of the mixture can be estimated.

– (with theoretical curves). The same procedure wasadopted to calculate the theoretical value of H.M.C.(determined from theoretical curves for single salts(see Fig. 12).

At 80% no dissolution of sodium sulfate occurs, andtherefore only the content of NaCl and NaNO were3

considered(Fig. 12a). At 93 and 96% RH all the saltsof the mix are assumed to dissolve, so the content ofall salts is taken into account(Fig. 12b–c).The first comparison between experimental data of

salt mixtures(q) and calculations based on experimen-tal data for single salts(h) is possible assuming thatthe temperature and RH conditions in the climatic boxeswere identical during the test on single salt and on saltmixtures. From the figures in Fig. 12 it can be seen thatthe hygroscopic moisture content of a salt combined ina mix is always higher than the hygroscopic moisturecontent calculated on the basis of the result of themeasurement on single salts. Differences are more evi-


dent at 93% RH This can be explained considering theeffect that the presence of NaCl and NaNO can have3

on the RH of mirabilite. As reported by Steiger andsat.sol

Zeunert w2x the RH of mirabilite, 93.6% at 208Csat.sol

w9x, is lowered by the presence of NaCl, so in this casethe mirabilite, formed from the hydration of thenardite,will dissolve at a RH lower than 93.6% increasing theHMC at 93%.In addition, Fig. 12 also compares the experimental

data determined for the salt mixture(q) with thetheoretical curves(—). As can be seen at 80% RH thehygroscopic moisture content of a salt combined in amix (q) is similar to the one obtained by calculationstarting from theoretical curves for single salts(—).Furthermore at 93% and 96% the differences betweenexperimental data(q) and theoretical curves(—)increase. This can be explained by:

● the humidity conditions in the boxes may have beenlower than 93% and 96%, respectively;

● the hydration of thenardite(Na SO) to mirabilite2 4

(Na SO 10H O) at RH between 71.3 and 93.6%2 4 2

instead of the presumed dissolution of Na SO(as2 4

explained in Section 4.1); and● the mutual influence of each salt on the hygroscopicbehaviour of the mixture has not been considered inthe theoretical calculation.

This experimental work takes into account only asimple ternary system of salts in which no salt differentfrom the ones introduced in the specimens can beformed. In reality the salt mixture present in the masonryare much more complex. Further studies are needed torelate the type and quantity of salts to their hygroscopicbehaviour.

5. Conclusions

Salt crystallisation is one of the main causes ofmasonry decay in many monuments. The presence ofsalts in a wall is a very important parameter to beknown to give a reliable diagnosis on the causes ofdamage and to plan an effective restoration. The saltcontent is an important parameter in case surface treat-ments have to be applied: in fact the salt can influencenot only the efficiency of the treatment but also formsa potential risk for the occurrence of damage due to saltcrystallisation.In order to know the salt content, the measure of the

hygroscopic moisture content can be performed on little(powder) samples of material. This is a simple andcheap method especially when a mix of salts is involvedas is usual. The work described in this paper is anexperimental laboratory research on the hygroscopicbehaviour of NaCl, Na SO and NaNO , considered as2 4 3

a single salt or combined in a mixture. From these teststhe following conclusions can be drawn:

● sodium chloride is the most hygroscopic salt amongthe ones studied;

● there is no important difference in the hygroscopicbehaviour measured on solid brick specimen and onbrick powder specimens;

● in both cases(pure salt and mixture) there is a clearlinear relation between salt content and H.M.C.;

● the H.M.C. surveyed on single salts is proved to belower than the one measured if the same salts arecombined in a mixture: the difference at 93% is morepronounced than at 80 and 96%. This can be due tothe effect of NaCl and NaNO on the RH of3 sat.sol

Na SO ; and2 4

● with respect to the single salts, a good correspon-dence between experimental and theoretical data hasbeen found at 80% RH. Differences between theor-etical curves and experimental data were observed at93 and 96%. These can be due to the variations ofthe RH in the climatic boxes(during the experimentthe RH appeared to be somewhat lower than 93 and96%) and to the hydration of thenardite to mirabiliteinstead of dissolution of Na SO .2 4

The results of the present study can be useful topractice research. It has been verified that, when thetype of saltyelements are known, it is possible to obtainan indication of the salt content. For single salts as wellas for mixtures of Na salts the relationship betweenHMC and the salt content proved to be linear. Inlaboratory research, when a single salt or a knownmixture of salt is used, the HMC measurement cansubstitute more complex chemical analyses for the eval-uation of the salt content.In building practice, where unknown salt mixtures are

present, the HMC measurement, giving an indication ofthe total salt amount, can be useful to select the rightsample on which to perform a more detailed study andto avoid extensive, and time consuming analyses.Measurement of the HMC at different RH gives an

indication on the RH of dissolution of salt(s) presentand, if combined with chemical analyses, this can insome case be enough to identify the salt type.From this experimental study emerges the necessity

to determine the HMC at relative humidities of 80 andyor 96% (96% is necessary when Na and SO are both4

present), anyway avoiding RH’s(for ex. 93%) coincid-ing with the RH of dissolution or of hydration of thesalts.This experimental work considers a quite simple case

since the salts composing the mixture all have a commonanion (sodium) and can not combine to each other toform new salts. In the reality the situation is much morecomplex and further experimental research is needed torelate the type and quantity of salts to their hygroscopicbehaviour.


Acknowledgments

The research was carried out at TNO Bouw in Delft(the Netherlands) in the framework of the SCOSTEuropean Contract(ENV4-CT98-0710).

References

w1x Binda L, Baronio E, Ferrieri ED, Koek JAG, van Hees RPJ,Franceschi P. Crystallization tests on treatd and untreatedwallettes, in Evaluation of the Performance of Surface Treate-ments for the conservation of historic brick masonry, Finalreport Contract EV5V-CT94-0515, paper 7.

w2x Steiger M, Zeunert A. Crystallisation properties of salt mixture:comparison of experimental results and model calculations, 8thInternational Congress on Deterioration and Conservation ofStone, Berlin, 30 September–4 October 1996, Berlin 1996, pp.535–544.

w3x Charola E, Weber J. The hydration–dehydration mechanism ofsodium sulfate, 7th International Congress on Deteriorationand Conservation of Stone, Lisbona, Portugal, 15–18 June1992, pp. 581–590.

w4x Sawdy A. An examination of the kinetics of deliquescence andrecrystallization of some soluble salts found in wall paintings,M.Sc. Thesis, Conservation of Wall Painting Department,Courtauld Institute of Art, London 1995.

w5x Horvath AL. Handbook of aqueous electrolyte solutions; phys-ical properties, estimation and correlation methods. Chichester:Horwood, 1985. p. 184.

w6x Robinson RA, Stokes RH. Electrolyte solution, the measure-ments and interpretation of conductance, chemical potentialand diffusion in solution of simple electrolytes, London 1959.

w7x Pitzer KS. Ion interaction approach: theory and data correlation.In: Pitzer KS, editor. Activity coefficients in electrolyte solu-tion. Boca Raton, Florida: CRC Press, 1991. p. 75–154.

w8x SCOST ‘Salt compatibility of Surface treatments’, EU ProjectENV4-CT98-0710.

w9x Arnold, A, Zendher K. Salt weathering on monuments, In: Attidel I Simposio internazionale, Bari, 1989, pp. 31–58.

w10x Numberg S, Charola AE. Salts in ceramic bodies II: deterio-ration due o minimal changes in relative humidity. Int JRestoration Build Monuments 2001;7(2):131–45.

w11x Goudie, A, Viles H. Salt weathering hazards, Chechester, UK,1997.

w12x Rodriguez Navarro C, Doehen E, Eduardo S. How does sodiumsulfate crystallize? Implications for the decay and testing ofbuilding materials. Cement Concrete Res 2000;30:1527–34.

w13x Schippers R. De Hygroscopische curve van zoutbelaste poreuzematerialen, Rapport nr. 96.07.M, Faculty Bouwkunde, Techn-ische Universiteit TUE, Eindhoven, The Netherlands, February1996.

experimental research on hygroscopic behaviour of porous specimens contaminated with salts

Documents