907

ELECTRICAL CHARGES ON MATERIAL SURFACES AND TREATMENTS Material studies at the Royal Palace, Stockholm

Josef Puhringer, Stockholm, Sweden Frantisek Makes, Royal Armoury, Hallwyl Museum, Stockholm, Skokloster Castle, Sweden

Summary The distribution of electrical charges on material surfaces determines the properties of the material and its mode of action during changes. Formulation of materials science problems (G-egradation-weathering, protection-conservation) is made easier if information regarding these charges and their magnitudes, their distribution and possible changes, is available. A standard method for the electrochemical analysis of solutions, polarography, has been modified for use in

detennining the distribution of charges on particles or small surfaces. The potential use of the method is demonstrated by results obtained in electrochemical analyses on different sandstones, mainly from the Royal Palace in Stockholm.

1. General

1.1 Material changes Material changes such as the degradation of materials are in most cases the result of a system of cyclically repeated dynamic physico-chemical processes, an end result which is in practice irreversible. The conditions governing this process, its course and its effects, can be described in physico-chemical material terms, i.e. the (physical) form of the material and the (chemical) composition of the material, in pairs of terms which indicate the possible changes in complex material systems, such as

(reactive) material surface : (transfer) volume reaction : diffusion

concentrations/possible reactions : structures/possible transfers (charged interfaces: electrokinetic phenomena)

1.2 Material changes and contact phenomena Material changes/degradation are primarily the result of contact phenomena (in porous material layers).

What essentially governs these contact phenomena in material surfaces is the mode of action of electrical charges on the contact surfaces, i.e. the way in which the material systems participating in these processes interact with one another electrochemically.

1.3 Electrical charges on material surfaces, charged interfaces Electrical charges on material surfaces may be created in various ways, such as ionisation, ionic adsorption and ionic dissolution.

In charged interfaces, ions can interact with one another both electrophysically (e.g. adsorption) and electrochemically (e.g. redox reactions).

1.4 Electrochemical redox reactions One special type of material change which is the consequence of electrochemical reactions in the contact zone between two materials which are electrochemically reacting with one another are redox reactions, i.e. pairs of (sometimes irreversible) processes in which electrons are exchanged in contact zones. The term reduction in this context denotes that part of the process which results in the uptake of electrons, and oxidation that part

which results in the removal of electrons.

1.5 Electrochemical mode of action of material surface Material changing/material forming processes can in many cases be described in terms of redox rea~tions in contact surfaces. Description of the possible redox reactions on material surfaces, apart from. a descn~lion of material properties, is also a description of the mode of action of materials in different electncally active

material combinations. This applies both to degrading, corroding processes and the effect of protective measures.

908

2 Measurement of electrochemical material properties

Measurement of the electrical properties of a material before, during and after a process can provide information regarding the details of the degradation process, and can indicate what protective action may be possible.

For the description of the electrochemical state of material surfaces, i.e. the charges on the material surfaces, there are some methods available which are in most cases based on the kinetic mode of action of an electrical double layer.

2.1 Calculation of zeta potential A comprehensive description of the theory of electrical double layers and the practical procedure for electrokinetic detennination of the zeta potential is given by (7) , cogether with very comprehensive results of systematic zeta potential determinations for minerals (as the basis for optimisations of flotation processes). The significance of zeta potencial calculations for mineral building materials is described by [6]. Similar electrokinetic criteria have been in [I OJ in assessing the practicability of using electro-osmotic measures in moisture control. On the basis of the zeta potential and the electrokinetic charge calculated from this, no direct conclusions can be drawn regarding surface charge density, surface potential etc for mineral building materials. (6, 7]

It may be assumed that the material required for zeta potential analyses, of l-2 my particle size (together with the associated preparation method (7) ) does not represent the mode of action of the material over relatively large surfaces (material changes over surfaces. e.g. weathering).

2.2 Polarography By modifying a standard electrochemical method of analysis, polarography, it is possible co study, more or less direccly, not only the charges on material surfaces but also the changes in these during a reaction process. Polarography is a method of measurement which utilises the effect of redox reactions on the surface of an electrode.

3 Conventional polarography

3.1 Apparatus The special principle on which the design of the analytical apparatus. the polarograph. is based is the dropping mercury electrode (Hejrovsky 1926) which constantly renewes itself and has the property of not becoming polarised during the analysis.

The original polarograph has undergone a lot of development. The fundamental principle of the non­polarisable and constantly renewed dropping electrode has however been retained, with dropping rates ranging from tenths of a second to several minutes. Potential is scanned in increments of mV and the electrode current can be measured with an accuracy of a few nA.

In the polarographic investigations described below, the DPP (differential pulse polarography) method has been used.

6

(fi g 2: 1 Polarographic principle Heyrovsky, 1926. Nobel prize 1959)

fi g 2:2 Modern polarograph

909

3.2 Method of analysis The electroactive ionised medium in solution which is to be analysed is subjected to a varying potential. The reactions on the electrode of the instrument, expressed in terms of applied potential and corresponding current and current direction can provide information regarding the type, composition and quantity of the dissolvedsubstance; each type of ion requires a specific (activation) potential for the uptake or release of electrons.

The type and extent of the redox process on the measuring electrode can be related to the redox state of the analytical material.

3.3 Analytical procedure The standard analytical procedure is based on two ways of varying the analysis potential, 0 .... +E and O .... -E.

The reaction current and the direction of the flow of electrons on the electrode surface indicate the concentration and the type of reaction: the magnitude of current is dependent on the concentration of the analysed substance (Faraday) in the solution, and the direction indicates whether oxidation or reduction is taking place on the electrode surfaces.

3.4 Polarogram The results of analysis are given in the form of a polarogram which shows the change in electron flow between the electrode and the specimen during the analysis.

The relationship between the varying measuring potential E and the analytical current i and its direction is given in conventional polarography according to certain conventions regarding electrode reaction and the corresponding redox reaction [9].

Solution polarography Electrode reactions +i

cathode reaction

-E reducing

anode reaction

-i

+E oxidising

4 Particle(suspension)polarography for determination of charges on material surfaces

4.1 Analytical principle The standard polarographic method for the analysis of solutions can, with minor modifications in electrode construction, also be used to provide - at least qualitative - information regarding charges on solid material surfaces.

A reactive material which is however insoluble in water is suspended in particle form in an eletrolyte. If there are electrochemical reactions (e.g. hydrolysis, neutralisation, hydration, condensation, adsorption of

surface active preparations etc) in or on the analytical material, this is manifested by changes in the electron

flow of the polarograph. In solutions, electrons are not taken up/released from/to ions until the electrode has reached a certain

activation potential. The same applies to ions on the surfaces of particles which must first be activated in order that they should be able to react with the electrode: in principle, the investigated particles with their net charges behave like an electrode. This behaviour is dependent on the rate of analysis.

The electrode reaction, i.e. the magnitude and direction of the analytical current, provides information regarding the type of redox state on the surface of the analysed material, i.e. the surfaces of the suspended

particles. In principle, the charges on material surfaces are measured by the effect of redox reactions on a measuring

electrode when the analytical material is in contact with the electrode. . Since the polarograph scans a potential range, the corresponding variations in the redox state on matenal

surfaces can in some way be defined.

910

4.2 Range of analysis In electrochemical analysis of large particle surfaces or material surfaces, the distribution of charges on the material surfaces influences the analytical process: in principle, the material surface with its charges behaves like an electrode. Apart from the conventional analytical range 0 ... +E/-E, there is also a special method of varying the electrode potential, in which the range is scanned with both positive and negative potentials and also with overvoltages, i.e. initial (and end) potentials which are sometimes greater than the charges on the particle surfaces, i.e. -E .... O .... +E and +E .... 0 .... -E.

Broadly speaking, these two diagrams must be mirror images of one another if the analytical reactions are ideal redox reactions; otherwise the differences provide a basis for an assessment of the reaction process (as, for instance, with overvoltages).

Polarograms obtained with two opposite scaning directions for the potential can be plotted on the same graph (one of them to opposite hand) in order to facilitate interpretation of the results.

4.3 Analytical variants Electrochemical reactions on particle surfaces can be of two kinds, with water soluble (hydrolysable) and non­water soluble reactants. The first kind of reaction is accessible by measurement of redox reactions on the electrode of the polarograph with the conventional standard procedure, solution polarography, and the other by suspension polarography. (The two types of reaction may occur simultaneously, e.g. when the electrolytes are aggressive; the analytical result in suspension polarography is then a combination of these two types of analysis)

The charges on particle surfaces are complex; they cannot be revealed by a single analysis. The more analyses - with different conditions - there are, the more concise and detailed will be the interpretation of the results.

The accuracy of analysis and the wealth of detail provided by the results will be greater if one and the same material is analysed by means of different analytical variants: the pH value of the electrolyte, the method of varying the electrode potential, or the type of electrolyte. The electrolyte may contain aggressive substances (for

instance those which occur in the environment). Use of redox reactions on electrodes, as an analytical principle, gives two possible methods of analysis:

Distribution of charges on one material surface can be determined Redox reactions in the contact zone between two materials can be simulated, by making the

measuring electrode represent conditions on one of the materials. In this way, the principles governing weathering and corrosion processes can be determined, and the effect of protective measures can also be checked.

4.4 Presentation of the resuJts of analysis (polarogram) The polarogram describes the redox reaction on the electrodes of the polarograph which, in principle, represent the redox conditions (distribution of charges) on the material surfaces. The following qualitative and quantitative information is given regarding the activated ion and the charges on the particle surfaces.

The simplified relationship between the charges on particle surfaces and graphical representation of the analytical material is as follows.

Suspension/particle polarography

Reactions and conditions on material/particle surfaces

+ charged, takes up electrons is reduced

electrode potential -E activation potential

- charged, releases electrons is oxidised to a lower reduction state

electrode current +i

electrode current -i

+ charged, takes up electrons is reduced to a lower oxidation state

electrode potential +E activation potential

- charged, releases electrons is oxidised

911

For certain extreme overvoltages for +/-E, these relationships must be slightly modified. The way this is done is not described here.

4.5 Reaction-time relation Different analyses can be compared more easily if the time dependence of the electrode reactions is plotted in separate diagrams. In this way it is possible to apply a special form of interpolation between the times at which the electrochemical data are measured. (e.g see subclause 6.2)

5 Comments on the method

5.1 Field of application Knowledge of charge distributions in the contact zone between different material systems can be very useful in different applications. The method of particle polarography can be used for

Analysis of the electrochemical behaviour of large material surfaces in the form of large particles or thin sheets. In this way, geometrically relevant simulation of degradation and weathering processes is made possible.

Assessment of the conditions for compatibility between different materials, for instance adhesion between mortar and blocks of mineral materials, the effect of aggressive substances (for instance wet or dry deposits).

Detennination of the degree of weathering. Assessment of the conditions governing the use and application of various (historic) surface treatments. Assessment of the protective effects of these surface treatments.

Determination of the mode of action of materials and material combinations in electrochemical treatment methods (electro-osmosis).

5.2 Advantages of the method One advantage of the analytical method is that it provides, directly and without conversion. specific and relevant information regarding charges and charge distributions.

Another advantage of the analytical method is that it can also handle large particles, e.g. those obtained in drilling in conventional material investigations. The required quantity of particulate sample is in the order of 50 micrograms, and the volume of the standard measuring cell is 50 cm3.

Because of the large possible grain size, it is possible to study conditions on fracture surfaces and in crevices. (In the samples required for zeta potential investigations, of 1-2 my in diameter, the effects due to these surfaces are eliminated).

By using special fixtures, the methods can also be used for the analysis of large material surfaces. The analytical methods can also be used to analyse the results of various ongoing material conversion

processes on one and the same analytical material, such as hydrolysis, hydration, dissolution and oxidation etc on material surfaces/mineral surfaces. It is not only the type of the reactions that can be determined, i.e. the reactivities of the materials in different contexts, but information can also be obtained regarding the rates at which these processes occur.

6 CHARGE DISTRIBUTIONS • EXAMPLES Polarograms , activating potential (E) vs reaction current (i)

6.0 General remarks It must be emphasised that full and complete interpretation of the complex polarographic analyses presumably demands a more rigorous and extended formulation of the theories, and it must be possible for the results of analyses to be evaluated with a computer program. Certain comments on the following analytical results should therefore be seen for the time being more as hypotheses.

In comparing different polarograms, it should be noted that because of the apparatus used they may at present have different scales for the reaction current (i .) In certain cases the analytical range was reduced in order to prevent abnormally high special peaks from dominating the graph and restricting detailed presentation of other processes (e.g. see fig 12). It is however possible that interesting analytical results would be found outside the selected extreme potential of ca +/-1 V. This will be examined in further studies. The effect of overvoltage on

912

"normal" i peaks in the vicinity of the boundary of the measurement range must be interpreted in each individual case. Standard polarographic analyses are normally performed in triple distilled water to which electrolyte is added. In particle polarography, by replacing the normal electrochemical process electrode-electrolyte-analytical material by direct contact between electrode and particle surface, it is possible to directly analyse charged surfaces on materials in only water.

6.1 (Water repellent treated) natural stone and facade material It appears from extensive literature

that there are no unambiguous relationships between the type of a treatment preparation and the type of stone material as regards the effect of treatment [5];

that there are no unambiguous relationships between type of material and type of climatic load (wet and dry deposits) as regards weathering of stone.[l]

The facades of the Royal Palace in Stockholm are being renovated, extensive rehabilitation work is in progress at Fredriksborg Castle to the north of Stockholm. Historic material is available for investigation.

The way particle polarography can be used in practice to create a basis for discussion regarding this problem complex is shown below. It must be pointed out that all analyses were performed in water without the addition of electrolytic substances.

6.1.1 Silane/siloxane (fig. 2) In order to find a partial explanation for the interaction between different types of sandstone and monomeric silane, a test was performed to determine polarographically the change in the electrical activity of silane in at least the hydrolysis process. The analysis could only be performed with emulsions of monomeric silanes in very low concentrations (1 drop/50 cm3 of water); there is a risk that silane will destroy the surfaces of the glass capillary which feeds mercury in the polarograph. For the same reason, it is at present difficult to study directly the continued (complete) condensation processes. An indirect test was made; see below (Subclause 6.1.5).

Current [nAJ

•t.OlEt-0-4

+ _)_ _.____

g ~ ~ ~ ; ~ ~ ~ & ~ g ~ ~ ~ ; ~ a ~ g ~ ? y y y y y y y y y 7 7 7 7 7 7 7 7 7

Polet"I iel

"' 0 v -1 v - v

Potent; al (VJ

Current [nAJ

0 v

0 - :;; ? y

~ ?

~ . ~ ? 'i' ?

Potent;al [VJ

0

~ ~ ~

~ ? -1

fig 2. I Hydrolyzing/hydrolyzed silan fig 2;2 Detail of fig 2: I (see also fig.4) (i)-unit 2.000 nA (i)unit 100 nA

- ~ PolenF •I

' 'f IYI

v

Analytical result: After hydrolysis and partial condensation over a large range of activation potential, silanes have a negative charge but with a high positive charge peak at a large negative activation potential.

The shape of the polarogram for silane at "normal" activation potentials is very similar to the shape of the polarogram for Gotland sandstone; this is not really surprising since they both have the same chemical building block, SiO- (see fig. 4).

Remark: Siloxane which has completed the reaction on sandstone evidently also has strong positive surface charges; see the example of Roslag sandstone, Subclause 6.1.5.

6.1.2 Gotland sandstone - untreated and after water repellent treatment (with concentrate) (fig. 3) A large proportion of the facade of the Royal Palace in Stockholm is covered in Gotland sandstone, a calcite bound sandstone (quartz) which in itself is not a uniform material.

Drillings from samples of Gotland sandstone were treated with both monomeric silane and monomeric silane with added catalyst, and were analysed electrochemically.

Current (nA)

.. :i.40E .. 03

-2. 60E .. OJ

-4.60E .. OJ

- 6. 60E+OJ

913

Current (nAJ

c c c c c c c c c c c c ~ N n Ill IJ) .... c 0 0 'i' 0 0

c c c c c c "! "! ~ c c 'j

c c ~ c

N

~

~ - a ~ ~ * ~ ~ ~ ~ ~ ~ ~ ,.. ~ i ~ i ~ g Pot.nt l•t

'i'.; .; .; .; .;.; .; .;.;.;.; .; .;.;.; 'i'.;.;.;.; '" 0 v .., v + I

0 v . I I I

Potential (VJ

I I I I

- 1 v Potential (VJ

fig 3: 1 Gotlandsandstone, 0 ... -1 V fig 3:2 Gotlandsandstone, O ... + 1 V (i)unit 2.000 nA (i)unit 200 nA

Analytical result: The surface charge on untreated Gotland sandstone is mainly negative, the extent of the positive portion of surface charge is considerably smaller than that of the negative charge.

Hydrophobation of Gotland sandstone with monomeric silane in concentrated form increases the negative activation potential (E); at the same time the proportion of negative charge (i) increases and the proportion of positive charge decreases. This effect is greater for silane without catalyst.

Hypothesis: Negatively charged hydrolysed silane both co-condenses with some of the negatively charged (SiO-) portions of the stone, and condenses on its own. The negative charge of the stone is blocked.

Negatively charged hydrolysed silane is to some extent adsorbed by the positively charged portions of the stone, without co-condensing with the stone: the positive surface charge of the stone is partly blocked and diminishes (unsuccessful water repellent treatment).

Fully condensed siloxane also has positive surface charges, see Subclause 6.1.7. Silane which has partly condensed on a small part of the substrate surface has both a positive and, predominantly, a negative surface charge. The net effect of water repellent treatment is an increase in negative charge and a decrease in positive charge.

The catalyst "activates" the negative charges of the hydrolysed silane, i.e. it promotes internal condensation of siloxane at the expense of its condensation with the substrate; the negative substrate charge increases (reduced reaction) and the positive charge decreases (increased adsorption).

6.1.3 Water repellent treatment (in low concentration) of Gotland stone (fig. 4) The importance of concentration in a hydrophobic preparation is evident from the polarogram of a Gotland sandstone which was given water repellent treatment with a low concentration of monomeric silane in ethanol.

Current (nA)

·•.00<•0> Got 1 and sandstone weathered

_, . 00£•02

~ " R § ~ . ~ !: 0 - i! ~ ~ /il . .; i .; .; 'i' .; 'i' .; .; 'i' ? ? ? ? ? .. 1 v 0 v

Potential [VJ

ethanol

§ ~ i Potent l ei

• IV I 'i' 'i' 'i'

Current [nAJ

-t, JOE +OJ

- 1.80E+O J

-2.JOE•03

' -1

:;; g ~

'f ? ? v

~ 0

~ ~ n ~ n i: 0

? 'i' 'f ? ? 'i' ? .; .; 0 v

Potential (VJ

fig 4. Gotland sandstone weatered, treated with diluted silanesolution, silane concentrate

(i)unit 100 nA

. /il ~ ~ ;;; ~ Pot~nt ••I

.; 'i' 'i' .; 'i' IYI

.. v

914

Analytical result: The treated Gotland sandstone is negatively charged in its entirety, the high peak for negative charge on the Gotland sandstone trated with silane is outside the range of analysis.

Hypothesis: A "surplus" of reacting and (incompletely) co-reacting hydrophobic preparation alters the

distribution of charge on a treated material.

6.1.4 Water repellent treatment of 18th century brick (fig. 5) That the way monomeric silanes react with Gotland sandstone is not generally valid is evident from corresponding comparative analyses of hard burnt brick from around 1700 from Fredriksborg Castle.

Drillings, both untreated and treated with silane with and without catalyst, were analysed.

Current [nA)

5000.00

0 . 00

- 5000.00

- 10000.00

-16000. 00

- 20000. 00

-_/

~ '\ v-silane+

v catalyst

si fne

untreatld

-1 v -0.80 - 0.60 -0.40 - 0.20 0 v 0.20

Potential [VJ

Fig 5. Brick from Fredriksborg

Current [nA) -1000.00~~~~~~~~~~~

- 11100. 00 -l...,..,..,.,.t,.,.,.,..,..,.,.t,.,.,.n'TTM,+,.,.,.,.,.,.,.,,+,n,.,.,.,.,,+.,., 0 v 0.20 0. 40 0 . 60 0 . 80 +1 v

Potential (VJ

5.1 - IV .. . 0 (inverse!!), (i) unit 5.000 nA 5:2 O ... +l V, (i)unit l.000 nA

Analytical result: The negative surface charge and the corresponding activation potential is considerably greater in the brick than in Gotland sandstone.

In contrast to Gotland sandstone, the negative activation potential and the size of the negative portion of the surface charge are smaller after water repellent treatment. The positive portion of surface charge increases to a corresponding extent. These effects - again in contrast to Gotland sandstone - are greater with a catalyst than without.

Hypothesis: The hydrolysing silane co-condenses with negatively charged stone surfaces and partly condenses on its own. The negative charge of the brick is blocked and diminishes. Silane condenses to a considerable extent (owing to the large negative charge of the brick) and acquires a positive surface charge; completely condensed silane also has positive surface charges (see Subclause 7.1.7, Roslag sandstone), and this occurs by way of augmentation of the positive surface charge of the brick.

With a catalyst, silane-brick co-condensation is promoted, with greater blocking of the negative surface charge and more positive charge after condensation.

6.1.5 Untreated and water repellent (concentrate) treated Roslag sandstone (fig. 6) Certain parts (mainly the base) of the Royal Palace in Stockholm are clad with Roslag sandstone, a dark red sandstone with siliceous binder.

Visible differences in weathering between parts of the facade are that Roslag sandstone is coated with a hard black incrustation while Gotland sandstone has no incrustation. The effect of water repellent treatment (contact angle) is seen almost immediately on Gotland sandstone, and hardly at all on Roslag sandstone (only with a catalyst).

The material analysed was drillings, both in its "natural" state and treated with concentrated monomeric silanes, one with catalyst added

Analytical result: The polarogram for this type of sandstone is different from that for Gotland sandstone. While Gotland sandstone has a negative net charge, which is normal for silicates, Roslag sandstone is positively charged!

Hypothesis: The probable explanation is that the dark red Roslag sandstone contains iron (iron oxide hydroxide) [8] . Trivalent iron (Fe3+), in the same way as trivalent aluminium (Al3+), can through reaction with negatively charged silicates give rise to a reversal of the surface charge. [7, 4]

Current (nA]

Current [nA]

915

Current [nA]

1609.00.,...--~----,r---.....--~ 1200.00~---.--~--~-....- 900.00~---.-----.--.---....-

-

e. 00-'tTT ...... ....m,..,.,........,...,........,.+,,..,.,..........+..- 200. 00~,..,.,....m+......,.......+.....,,..,.,....~,..,.,....m., 300.00~. _.....,...,.tm-. ....... -t.TTI,..,.,....m.. ....... ..+.n -1,0 v 0 +1,0 v -1,0 v 0 +1,0 v Potential Potential [VJ (VJ

fig 6: 1. Roslagsandstone fig 6:2 treated with silane (i)unit 400 nA (i)unit 200 nA

-1,0 v Potential (VJ

0 +1,0V

fig 6:3 treated with silane/catalyst (i) unit 100 nA

The positive surface charge has two intensities; a smaller one that can be reduced polarographically (-E) and a larger one whose degree of oxidation can be reduced ( +E). Water repellent treatment changes the charge, at an activation potential of about +0.5V its intensity is reduced. Hypothesis: At this activation potential the positive surface charge of the sandstone is not reduced; what happens is that it is replaced by positive charges through the completely condensed siloxane.

The positive surface charge of the sandstone has been blocked by the silane, its negative charge has been adsorbed on the highly positive (Fe3+) of the stone. That silane contributes to the positive charge is evident from the amount of charge (i) at an overvoltage. E of+ l V; for Roslag sandstone treated with silane there is a high activation potential with positive surface charge, in contrast to the untreated stone. Fe3+ in the sandstone delays/inhibits condensation through binding to the hydrolysing silane. Silane does not react with the sandstone but is bound to this by physical adsorption (with a poor water repellent effect in consequence).

(The fact that all three polarograms differ with regard to the current i for different scanning directions for the potentials E cannot be explained at present).

Summary In this series, the water repellent effect of monomeric silane diminishes from brick through Gotland sandstone to Roslag sandstone, parallel with a reduction in negative charge intensity on the material surface, fro~ mainly negative to positive; however, no relationship has been proved.

6.1.6 Gotland sandstone treated with linseed oil (fig. 7)

Current 20000.00~-~---.--.----....-( nA]

- 10000. 00 '4....~m+.~,,,,..+.....,,..,.,....~,..,.,....,.,..,

- 1,0 v 0 Potential [VJ

+1,0V

fig 7;1 Linseed treated Gotlandsandstone (i)unit 5.000 nA

Cur rent 12000. 00-n---r---r----.----..-

[ nA]

- 4000. 00 -'+T. ...... ....+r..,...,.,..,,.+,,..,.,......m,..,.,....,..,.,....+... -1. 0 v 0 Potent i a 1 [VJ

+1,0V

fig 8:2 Linseed (pigmented) treated Gotland sandstone. (i) unit 4.000 nA

916

Hypothesis: The reversal of surface charge by longtime wetting by acidic rains may be one of the causes for a very hard, thin and black crust on exposed areas. [8]

6.2 Lime, mortar and marble

6.2.1 Lime mortar from Fredriksborg Castle (fig. I 0) The dominant influence of Ca2+ ions in determining potential is also evident from the analysis of mortar

from the 18th century Fredriksborg Castle. (7, 2] Even this mortar which is obviously thoroughly carbonated has predominantly positive surface charges in

spite of a - relatively slight - proportion of siliceous sand in the mortar which is manifested by a small "silicate peak" in one scanning direction.

Current [nAJ

concrete

0

~ il ::: ~ ~ <i i i i i

+1 v

~ Ii i ?

~ 0

~ il ~ 0 il ~ . ii ::: g Pot9"t lel

i i i i i i i 'f 'f i 'f i IVI

0 v -1 v Potential [VJ

fig I 0. Lime mortar (Fredriksborg) , cement mortar

·Current [nA)

+S. 50E+Ol

• 4. '50E+Ol

•l. '50E•03

+1. :MJE+OJ

0 ~i~;R~~ ' i i i i i 'f i

-1 v

~ ; g ; ~ i i i i i

0 v Potential [VJ

10:1 +lV ... 0 ... -IV, (i)imit 2.000 nA 10:2 -lV ... O ... +lV, (i)unit 1.000 nA

6.2.2 Cement mortar/concrete (fig. 10) A control sample of cement mortar (drillings from concrete) also has extensive positive surface charge, in spite of the fact that isolated calcium silicates in general and also absolutely have a negative zeta potential. The probable reason for this also is that the lime phase of the hydrated/carbonated cement has a Ca2+ content which determines the potential. In this case also the presence of aggregate is manifested as a negative charge peak. (Measurement of streaming potential in concrete containing lime may produce values different from those in pure siliceous aerated concrete).

6.2.3 Ekeberg marble (dolomite) (fig. l l) Measurements of zeta potential on Swedish Ekeberg marble (dolomite}, in the same way as those on calcium carbonates, also produce a positive surface charge throughout. (Information in the literature indicates that certain carbonate or marble variants may also have a negative zeta potential). (6, 7)

Current (nA) 1200. 00

1000.00

v 800 . 00

I-"'"...__,

600.00

-400.00

200. 00

\

_/\J

+

-1, 0 v 0 Potential [VJ

~

fl \

~

1 v

fig 11.. Ekebergsmarmor (dolomit) , (i) unit 200 nA

917

Analytical result: The polarogram of a historic sample of Gotland sandstone which had been treated with linseed oil shows the effective influence of old linseed oil treatments in determining potential; the material surface has a strong positive charge (oxidised linseed oil?). The polarogram also shows the differences between unpigmented and (oxide) pigmented linseed oil treatments. The analytical result may be dependent on scanning direction, i.e. oxidation ... reduction or reduction ... oxidation, which is typical of organic substances; for instance, to a certain extent, of the unpigmented oxidised linseed oil treatment.

By way of comparison, see the treatment of Gotland sandstone with silane, Subclauses 6.1 .2 and 6.1.3.

6.1.7 Dry and wet deposits on sandstone 6.l.7.1 Gotland sandstone (treated, weathered, historic) (fig, 8)

Current [nAJ

•8.00 0•01

.,,00 ; •01

-2. 00 : +01

-1.00:+01

- 1.20! •02

-1 . 10: .01

-2. 20':•02

-2.70C:•02

Current [nA)

•1. 1[£•03

•1.5£•03

•1.3t!•03

•1.1[£•03

•9.0tE+Dl

+7.0l£+02

•S.0(£+02

•l.0(£•02

•1.0t£~2 surface

g ~ g ~ ~ ~ M ~ ~ ~ g ~ ; ~ ~ ~ ~ ~ i ~ ? ? ? ? ? ; ; i i y ? ; ; ; ; ; ; ; ; i

Pa enll•I

" g ! ~ ~ ~ ~ M ~ 5 ! ~ ft ; ~ ~ ~ i ~ ii ~ g •· -"•' ; ; ; ; ; ; ; ; ; ? ; ? ? ; ; ; ; ; ; ; i ,.

0 v + v 0 v +1 v Potential [VJ

fig 8:1 Gotland sandstone, dry (i)unit 50 nA

Potential (VJ

fig 8:2 Gotland sandstone, water wetted 48 h (i)unit 200 nA

Analytical result: Wetting of Gotland sandstone reverses the negative surface charge to positive values. It is known that water ions can determine the potential of mineral surfaces. Intensive contact between e.g. Gotland sandstone and water, corresponding to thorough wetting, reverses the negative surface charge of Gotland sandstone.

Hypothesis: This reversal of surface charges due to long term moisture load may be the explanation for the differences in the behaviour of sandstone in relation to wet or dry deposition of aggressive substances.

6.1.7.2 Roslag sandstone (weathered, historic) (fig, 9)

Current [nA)

•1, IOE+Ol

•8.00Et02

0 ~ ?

+ a ~ ? ?

; ~ 0

; ; ? il ll ~ ;

}; fJ Po enl l•I . " 'I

Current [nA)

inner layer

g ~ i ~ ~ ~ ~ R ~ ~ g ~ ~ ~ ~ ~ ~ ~ i i f ? ; ; ? ? ? ; ; i ? ? ? ? ? ? 1 ? ? ?

Pol~l l •I

"' +1 v

~ g - ~il~ i?1 ? ? ?

0 v Potential

+1 v 0 v - v

[VJ

fig 9: l Roslag sandstone, dry (i)unit 500 nA

Potential [VJ

fig 9:2 Roslag sandstone, wetted pH 4,5 (i)unit I 0.000 nA ! ! !

Analytical result: Wetting of Roslag sandsten with acidic (rain)water with pH 4,5 reverses -as expected

- the positive surface charge to negative values.

918

6.2.4 Comparisons with other lime mortars (fig. 12) Comparisons with other types of lime mortar such as pure lime or lime mortar with carbonate aggregate, with a few isolated exceptions, exhibit the same tendency for accentuated positive surface charge.

Current [nA) 6000.00-n-~~~~~~..--~....-

-20000. 00-'tn..m-,.yt,.,.,.,.,,M'TTf'TTTMM'TTTm..M'TTT'1'TT' -1,0 v 0 Potential (V]

fig 12. lime mortars

+1,0 v

Current [nA] 30000. 00~~~~~~~..--~-,--

-20000.00~M"TTTrn'IT.,....,.,.rm1'TTTT"""''hTT"""'"TtTT"

-1,0 v 0 Potential [V]

+1,0 v

12; 1 carbonatic aggregate), (i)unit 5.000 nA 12:2 pure lime mortar, (i)unit 10.000 nA

One type of mortar examined shows different reaction processes for different scanning directions of the analysis potential; the type of reaction in the mortar could not be identified although the curve in one scan is similar to that for a carbonate, and in the other (to some extent) similar to that for a hydroxide. See below.

One mortar exhibits a very pronounced negative peak. This peak may be explained by comparison with the polarogram for Ca(OH)2 particles.

6.2.5 General remarks on lime (fig. 13) Ca(OH)2 has a negative peak at a large negative activation potential.

Current [nA) 40000. 00....--~~~~~~..--~~

-20001!J .00...,,.,TTTrrn'tTTTTMM°TTf'"""''TTTT""".,....,.,.'1'TT' -1,0 v 0 Potential [V]

+1,0V

fig 13. lime Ca(OH)2, (i)unit 10.000 nA

Hypothesis: Concrete and mortars, in spite of extensive positive charge over the range of measurement -1 V ... + 1 V, might also have had a (further) negative peak outside the range of analysis (and "outside" the silicate peak).

6.3.1 Gypsum incrustations.and Gotland sandstone (fig. 14: I) In the same way as silicate, Gotland sandstone has negative net surface charges. Gypsum - in concentrated solutions of gypsum - has a negative zeta potential. [7] It is therefore rather difficult to explain why adhesions of gypsum incrustations to sandstone should act as points of initiation of damage.

919

Current [nA]

Current [nA)

1298118."8 100000.00.,.-~-.-~--r--.-~~~.--~~

T -~ I\ ~ -

60000.00

1eeeee.00

eeeee.00

-.00

l 20000. 00 ~

0.00

l }J -l --20000.00 fTTTTT" fTT' - 2000:J. 00 -tTn'TTTTn+n.,...,.,.,.,.m,.,.,.+,,...+~.,....,h-....~+.....

-1,0 v 0 Potential [VJ

+1,0V -1,o v Potential [VJ

' 0 +1,0 v

fig 14: 1 Gypsum , (i)unit 20.000 nA fig 14;2 Gypsum, Ca-acetate, (i)unit 20.000 nA

Analytical result: Polarographic investigation of crystalline gypsum shows almost exclusively positive surface charges and hardly any negative surface charges.

In this case also, Ca2+ ions largely determine potential. Hypothesis: The difference between zeta potential measurements and polarographic detenninations may be

due to the orientation of Ca2+ ions in the crystal surface. Shredding of analytical material for zeta potential investigations perhaps "smooths out" the distribution of charge. Streaming potential investigations in gypsum plaster may clarify the situation.

6.3.2 Charges on gypsum crystals (fig. 14:2) An indication that Ca2+ charges on indissoluble gypsum are associated with the distribution of charge on a crystal surface is given by a comparison between the polarogram for calcium sulphate and calcium acetate crystals.

Analytical result: In spite of the fact that the analytical quantities were largely the same, it was found that acetate crystals which dissolve easily have smaller surface charges than gypsum at exactly the same activation potential.

7 Development potential of polarography

7.1 Direct determination of surface charge (fig. 15) An analysis of a sample of Gotland sandstone in the shape of a surface fragment - a 5 mm thick slice - shows that it is possible to analyse charge distribution on larger material surfaces.

This avoids the risk that charges will be equalised when small particles are produced for purposes of analysis.

Current (nA)

linseed .oil +1.70£404

•1.J0[.04

• 2•00 ... , weathered

~~z=~==:::;:;:~ § M i ~ § § i R R ; § ; fi Ji i R i ~ i i § Pal9'1 l•I

f iii iii iii iii iii iii i . i'~ -1 v 0 v +1 v

Potential (VJ

fig. 15 Flake of linseed treated Gotlandsandstone, (historic) (i)unit 5.000 nA

920

Analytical result: Apart from its "normal" negative charge, the outside surface of the sandstone which has been treated in some way has a pronounced positive charge. (Hypothesis: linseed oil residue). The inside surface of the sandstone has a weak negative charge. (Hypothesis: treatment residue?).

7.2 Three dimensional (reaction-time) representation of polarographic analyses. (fig, 16)

0. 00 62. 50 125. 00187. 50250.00312.50375. 00437. 50500. 00

252.50

-1555.63

0 . 00 62.50 125. 00187. 50250.00312.50375. 00437 .50500.00

___ _..,..,_ time (sec)

...... ct: c:

..., c: QI L. L. :J ()

~r -2 ~ c: ~ ., .. & cl

' t -1v ~

fig 16. Current- potential- time- diagram 16: 1 Gotland sandstone 15:2 Roslag sandstone

Comparison of analyses on different analytical materials is made easier by a special three dimensional representation of the analytical curve. The E-i curve is augmented by plotting the time (rate) along the third axis.

8 Conclusion

The above examples demonstrate the following The results of measurements vary in a way which is somehow characteristic for the type of substance and

groups of substances, and are unambiguously reproducible. The results of charge measurements do not agree with measurements of potential according to current zeta

potential calculations. The differences can however be explained in some way, for instance on the basis of the following quotation regarding zeta potential calculations: "this means that the electrokinetic charge (sum of positive and negative charges) seldom exceeds 0.1 % of the total charges that can (and probably do) exist in most solid/aqueous interfaces". [3, Cook 1968]

The relationship between the net charge of a material and activation potentials, as well as the relationship between the pH value of the electrolyte (water) and the measuring potential used, should receive further study.

It should however be possible for the results of analytical measurements - in the same way as those of zeta potential calculations - to be applied at least as material criteria for ·certain building materials.

Even though many details remain to be solved before the results of polarographic analyses can be easily interpreted, it is very probable that, after the necessary additional institutional research and development work has been performed, the method will prove to be one of many other aids for perhaps not the solution, but at least the formulation, of materials science problems.

9 Acknowledgements Thorborg von Konow has made available samples of lime mortar) from an ongoing research project. Bertil Johnson has helped with graphical treatment of analytical diagrams and layout.

921

10 Bibliography

l.BR0GGERHOFF ST, MIRW ALD p W, Examination of complex weathering processes on different stone materials by field exposure studies. Proc. 7th Int. Congress on Deterioration and Conservation of Stone, Lisbon 1992 (ed. J Delgado Rodrigues). pp 715-724. 2.CLARK SW, Adsorption of calcium, magnesium and sodium ions by quartz. Trans. AIME 241 (1968), pp 334-341 3.COOK M A, Hydrophobicity control of surfaces by hydrolytic adsorption. Journ. Colloid Interface Sc 28 (1968), pp 547-556. 4.MACKENZIE J M W, Zeta-potential of Quartz in the Presence of Ferric Ions. Trans AIME 234 ( 1966), pp 82-88. 5.MARSCHNER H et al, Natursteinshydrophobierung mit silicium-organischen Verbindungen, Proc II Int. Colloquium on Material Sciences and Restoration, Esslingen 1986 (Ed. F H Willman), pp 509-527. 6.NAGELE E, SCHNEIDER U, Das Zeta-potential mineralischer Baustoffe- Theorie, 6. Eigenschaften und Anwendungen. TIZ International, Vol 112 No 7 ( 1988), pp 458-467. 7.NEY P, Zeta-potentiale und Flotierbarkeit von Mineralen, Springer-verlag Wien, New York, 1973. 8 NORD GA, TRONNER K, Characterisation of thin black layers, Proc. 7th Int. Congress on Deterioration and Conservation of Stone, Lisbon 1992 (Ed. JD Rodrigues), pp. 217-225. 9.POHRINGER J, MAKES F, JOHNSON B, Electrical Charges and the Degradation and Conservation of Mineral Materials, UNESCO/RILEM Int. Congress on the Conservation of Stone and other Materials, (Ed. M J Thiel), Paris 1993, pp 137-143. IO.WITTMAN F H, Zeta-Potential und Feuchtigkeitstransport durch porise Baustoffe, Proc. I. Int. Coll. Material Sciences and Restoration (Ed. F H Wittman), Esslingen 1983, pp 417-?