induced damage to materials in marine environments - part ... · *lab. mineralogia e petrologia and...

A two-case study on the environmentally-

induced damage to materials in marine

environments - Part II: Geomaterials

A. Mauricio* & A.M.G. Pacheco^

*Lab. Mineralogia e Petrologia and *Dept. EngenhariaQuimica, Institute Superior Tecnico (Technical University ofLisbon), Av. Rovisco Pais 7, 1096 Lisboa Codex, Portugal;Email: pcd 2045@alfa. ist. utl.pt

Abstract

This two-part paper addresses the specific hazards that most materials are facedwith in coastal areas, particularly in their atmosphere. Pretty common featureslike high relative humidity and airborne salts, which are inherent in such anenvironment, may turn into a nightmare for conservationists, architects andmaterials scientists, that is for everyone involved with old (historic) or newinfrastructure. Two cases are presented and discussed herein. Neither of themwas designed or singled out especially for the occasion: both were taken fromextended programs of metal-corrosion and stone-decay monitoring in the open.The first case (Part I) deals with the implication of saline contamination for thetime of wetness (TOW) of a metallic surface. The results show that standardprocedures for assessing TOW from weather data can severely underestimate theduration of surface wetness and, in the final analysis, lead to somemisclassification of atmospheric corrosivity. The second case (Part II) followsthe evolution of salt efflorescence at an ancient building as a function of local(microclimatic) conditions, in order to get the time probability associated withdeliquescence- crystallisation transitions at a given location. By doing this, it waspossible to identify more-or-less risky areas in the stone monument, which couldthen be subjected to differential surveillance and/or care. Both studies seempertinent to illustrating the need for establishing risk thresholds for materialsselection and infrastructure maintenance that can really hold in marineenvironments.

Transactions on Ecology and the Environment vol 18, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541

88 Environmental Coastal Regions

1 S** Marij a Ta'Cwerra case study

Evaporite minerals are particularly significant in the Mediterranean basinbecause most cultural (historic-architectonic) heritage is concentrated incoastal areas where they can be exposed to marine spray or salt-risingdamp. Their presence contributes significantly to the weathering of

building stones because of their response to cycles of relative humidity.

Since the critical reative humidity points of dissolution or change of stateof hydration of the minerals are usually within the typical ranges of relativehumidity (RH) observed in most temperate climates, they can oscillate

frequently between solution and crystal phases. They can also oscillatefrom a crystalline phase to another [1]. The present case study is aimed atestimating when and where different pure salts may crystallise from

solutions, evaluating the probability of the salt system being crystallised

or deliquescent. The assessment of the salt weathering potential on thesurface of the stone, by means of crystallisation/dissolution and transition

probability estimations (TPE) along the year will also be considered, in a

given monitored site.

1.1 Diagnosis and data collection

To evaluate some effects of coastal environments on the weathering ofhistoric buildings, an extensive study has been carried out at four pilotmonuments along the east-west axis of the Mediterranean basin -Cathedrals of Cadiz (Spain) and Bari (Italy), and Church of S" MarijaTa'Cwerra (Malta). This was done with a specific interest in the action ofmarine salts and air pollution. The various locations of the monumentsreflect dissimilar conditions of salinity, extent of marine and atmosphericpollution, and topographical aspects of the area, leading to different typesand grades of weathering and decay patterns [2].

The church of S'* Marija Ta' Cwerra is located in the village Siggiwi,in the south west of the Malta island, at a distance 3 km far from the sea. Itis a free standing building from the XVII century, less than 10 x 10 nf planview. The church is built entirely of Globigerina limestone. This limestonehas a total porosity of 35% with mainly small pores (2-5 |nm). Thechemical composition of the stone is dominated by calcium carbonate (88to 97 %). The four external walls show severe deterioration, for about two-third of their height, the lower courses are cemented. The middle coursesare deteriorated in the form of alveolar weathering as well as powdering of

several areas. Most of the mortar has been lost from the joints in this area.The uppermost courses are better preserved. At the inside of the building,the plaster has fallen away in several areas revealing powdering and flaking


Environmental Coastal Regions 89

stone underneath and even some of the carvings have almost completely

disappeared [2] In the outside walls, granular disintegration and relief by

rounding and notching are the prevailing weathering forms. The intensityof salt weathering is basically controlled by stone characteristics, especiallyporous matrix properties, and degree of salt accumulation [3]. It can beseen that stone samples from the outside show clear enrichments in Na*and Cl" and a bit in SO/ On the inside, efflorescences are enriched in Na*and Cl', indicating mainly the influence of sea as a cause of chemicaldeterioration of the stone. Anthropogenic chemical emission impacts on thestone are of negligible importance in this church [2].

The evaporite minerals found on the monument are nitrocalcitenitromagnesite, nitratite, halite, thenardite gypsum, mirabilite and niter

[4] Nitrates are almost always dissolved owing to their low

deliquescence humidities when compared to the environmental relativehumidity range usually found inside and outside the church. Regardingcrystallisation pressures, halite is the most dangerous salt. This can beeasily understood comparing crystallisation pressures (atm) of differentsalts, under thermodynamic conditions found in some environments: 554for halite; 282 for gypsum; 292 for thenardite; 72 for mirabilite.

Considering the molar volume (cnrVmole): 220 for mirabilite, 28 forhalite, 55 for gypsum, 53 for thenardite, then mirabilite can be considered

the most dangerous salt. Thenardite and mirabilite are sometimes foundtogether showing that phase transitions between anhydrous and hydrate

forms easily occurs in the stone. They produce also hydration pressure inthe stone porous matrix that is particularly effective because of therapidity of the change. The transition of thenardite to mirabilite is morerapid than hydration of other salts, taking about 20 minutes at 39 C [5].

The environmental data were collected on a hourly basis at differentsites (one outdoors and four indoor), from April 1994 to June 1995, bymeans of a monitoring station. The system comprises a network of

sensors: contact thermometers attached to the surface of the stone andthermohygrometric sensors located 5cm far from the stone surface atdifferent heights from the ground. There are four indoor contactthermometers and four indoor thermohygrometric sensors. They arepositioned in Local 1, Local 2, Local 3 and Local 4 (respectively, inSouth wall - 3,5 m; North wall - 3,5 m; South wall - 0,5 m; North wall -0,5 m). There is also one outdoor thermohygrometric sensor facingsouth/south-west, attached to the dome.

In the search for interactions between atmosphere, salt-inducedweathering and stone condition, accurate temperature and relative humiditymeasurements were carried out in the atmospheric layer close (5 cm) to the



surface (Atm condition). Stone surface temperatures measurements (Surf

condition) were also monitored.

1.2 Data processing

In order to evaluate the potential damage on porous-stone materialsresulting from pure-salt crystallisation, it is essential to become aware of

their deliquescence thresholds (boundary conditions), RHeq The

evaluation should be made for a given salt on the range of temperature

and relative humidity existing on temperate climates. These functions arevery important since they enable to establish phase diagrams for each

pure salt, as well as to follow the evolution of deliquescence humidityalong time as a function of ambient temperature.

To deal with such an issue, a computer program was conceived,based on a few underlying hypotheses [4] in order:i) to estimate atmosphere boundary layer (thermohygrometric)

conditions in equilibrium with the stone-surface;ii) to estimate the phase diagram corresponding to each pure-salt

system;

iii) to study the expected behaviour of each salt system upon its phase

diagram, by means of scatter plots corresponding to the actualatmospheric (Atm) or to the estimated atmosphere boundary layer

conditions on stone-surface (Surf);iv) to estimate the equilibrium relative humidity (RHeq) of some pure-

salt systems as a function of time;v) to look into the probable time-course evolution of each system,

using estimates of deliquescence humidity (RHeq), monitoredtemperatures and relative humidities corresponding to Atmconditions, or estimated relative humidities corresponding to Surf

conditions;vi) to estimate the probability of a given salt system being crystallised

(f (RH < RHeq)) or deliquescent (f (RH > RHeq)), and the probabilityof a crystal/solution transition (TPE).This can be done for any given set of thermohygrometric data during

monitoring time. The following definitions apply here: TPE - is thepercentage ratio of the number of crystal/solution transitions across RHeqto the total number of data points. Both concepts refer to an intervalwhich T and RH chronograms are available.

The program can process data from different sites in a monument.However, it should be noticed that the relative humidity of theatmosphere close to a stone surface is an estimate. It is based on the



assumption of thermal equilibrium between the stone-air boundary layer

and the stone itself. An empirical approximation by Tetens to the

Clausius-Clapeyron equation enables such computations to be performed

[6].The RHeq phase diagrams of each salt system were estimated through

Lagrange interpolation method, from tabulated (experimental) data.Tabulated deliquescence humidities for practically all common salts that

could be relevant for building materials are available from the literature.

1.3 Results

1.3.1 Deliquescence humidity estimation

As an example of an output of the computer program for a monitored site,Figures 1-a and 1-b are shown, concerning data processing for Atmmonitoring conditions. Halite behaviour can be followed outside thechurch from April to October 1994. In Figure 1-a, the estimated phasediagram and outdoor-environment data (scatter plot) is derived. Theseresults allow estimating deliquescence humidity along time as a functionof varying local ambient temperature as it is shown in Figure 1-b.

In Figures 1-b, chronograms of exterior monitored temperature andrelative humidity are shown. Estimations of the equilibrium relativehumidity (RHeq) of halite along monitoring time, as a function ofinstantaneous local temperature can be seen as well.

1.3.2 Transition Probability Estimations

The classification of each monument as to its saline-risk potential can bemade on the basis of TPE values. This is because such values dependsimultaneously on: i) all salts present, ii) all monitoring sites and iii) allmeasurement conditions (Surf, Atm) [7-9]. From the set of all possiblerelationships that can be derived between TPE values, it is shownquantitatively the transition behaviour likely to be expected of some saltsystems (ex: nitratite, halite and niter) by means of TPE values. Thedifference between TPE estimations for Surf and Atm conditions can alsobe easily evaluated (Figures 2-a, 2-b). It should be noticed that niter isexpected to be always crystallised since RHeq is always aboveenvironment RH (not presented in this paper). So, there is no need topresent graphically its TPE values.

A summary of the computation results obtained for the three salts ispresented in Table 1.



Slo MARUA TA" CWERRA-MALTA - EXTERIOR

500 1000 1500 2000 23OO 3OOO 35OO 4OOO

K 9O

><*>^ 70I 60u 50

2O1O

• '% *- _ " " r!V::7 -vt":~ V lr - 8lCJ:!y2z«-J: . Boundary Condition — HALITE

'•"'"""" •'/•%'.:" lx!y% . :';'/' 7; :%i%%.":-y. .' '.V:J."''':;-i:"v* :.;"; :;V''-\"v;'s:-;:-C". X\. a _

. f(RJI < R1U,) =69.4% " "'.;..•: . rVy/t /y"/'' :)-:.'-' f(Rii > RIU,) =30.6% . ...-•':'• •;.:'• .-':'*••' .'•! "•"'-••••"•-. _

. -''" t r ' .i fe s: %xi! ',:;•23- oC

Figure 1. Chronograms and phase diagrams corresponding to sensorlocation "Exterior", a: Phase diagram of Halite (calculated) andscatter plots of air temperature and relative humidity (monitoreddata); b: Chronograms of Halite deliquescent conditions (calculated),air temperature and relative humidity (monitored data).

1.4 Discussion of results

A computer model was conceived and presented elsewere [4]. Anexample was given herein for some salt systems likely to be found in the

Church of S* Marija Ta'Cwerra (Figures l-a,b) and (Table 1). The

behaviour of pure-salts can thus be forecast on the basis of indoor andoutdoor varying environment conditions if it is assumed that the kineticsof salt transitions is fast enough. However, it should be emphasised thatsuch thermodynamically based results are merely indicative of whatmight happen at the stone surface of monuments under surveillance.

Contamination by a single salt is very uncommon, if not at all: a

mixture of salts is present in (almost) every situation, owing to air pollutionand/or rising damp. Recently, Price and Brimblecombe [10], as well asSteiger [11] dealt with the thermodynamics of the much more complexcase of salt mixtures, for temperature conditions of 15, 20 and 25 C They



Table 1 - A summary of the computation results partiallypresented on Figures 2-a,b.

\SaltLocm*---^

Level 1

Level 2

Level 3

Level 4

Atm.

Surf.

Surf-Atm

Atm.

Surf.

Surf-Atm

Atm.

Surf.

Surf-Atm

Atm.

Surf.

Surf-Atm

Nitratite

M

H

MD+

V

V

LD+

M

H

VD+

V

V

LD-

Halite

M

H

MD+

V

V

LD+

M

H

VD+

V

V

HD-

Niter

0

0

0

0

0

0

0

0

0

0

0

0

In the Table:

L low values of TPEM

0 < L < 1 %;1< M < 2 %;2<H<4%;V>4%;

mean values of TPEH high values of TPE

V very high values of TPED difference;

+,-,0 positive, negative, or no differences between TPE;LD, MD HD, VD low, mean, high, or very high differencesbetween TPE;

|LD| < 0.5 % 0.5 < |MD| < 1 % 1< |HD| < 1.5 %.

used the approach made by Pitzer to calculate the relative humidity inequilibrium with any mixed-salt solution. Pitzer approach can in principlebe extended to any situations of varying temperature. Unfortunately, thenature of the data available does not enable us to use such an approach inthis paper.

An index of the environmental-weathering potential should be thenext step beyond, in the near future. Such an index could turn into animportant tool for assessing monuments as to stone decay, provided thatmineralogical, texture, porosity and interfacial (chemical and physical)characteristics could be considered and quantitatively modelled.

The kinetics of deliquescence/crystallisation and crystallisation/



Malta - Nitratite

Local 4

Local 3

Local 2

Local 1

-1,40 8,60

Malta - Halite

Local 4

Local 3

Local 2

Local 1

-5.0O O.OO 5.0OTPE %

10,00

Figure 2. Characteristic TPE values of two pure salt systems inside thechurch, a: nitratite, b: halite.

hydration transitions as well as the processes of salt-solution transport formixed-salt systems inside the porous stone should be modelled too. Thisallows a view to a deeper understanding of their time-course evolutionand to a more accurate simulation and forecast of stone-decay patterns.Given this, an optimal choice of the sampling rate to the environmentfactors acting on a stone monument or any other historic-architectonicartefact, could be envisaged as well [8,9].

2 Conclusions

The results presented above show that the behaviour of the pure-saltsystems conditioned to varying thermohygrometric conditions can be



significantly different when open-atmosphere or stone-surface conditions

are considered. It is not enough to measure thermohygrometric variables

some 5cm far from the stone surface and then extrapolates the results ofRHeq estimations made thereby directly for that surface. At least,thermohygrometry of the atmosphere nearby the surface as well assurface temperature must be monitored.

Considering transition-probability estimations (TPE), it is possible toascertain which salts should be considered potentially more dangerous in

a given context (monument plus environment). The projection of

monitored data on the estimated phase diagram, and the monitored andestimated deliquescence humidity chronogram allows visualisingimmediately when and where phase transitions are likely to occur in thesalt system. The state of the salt system (deliquescent or crystallised) arealso easily ascertained. Some qualitative conclusions can also be made onthe overall appearance of the chronograms, describing local environmentbehaviour along time.

Further research should account for a very important aspect of salt-induced damage on historic buildings: actual salts are seldom pure. Aneffort must be put on adapting the presented methodology for the real

situation, that is: the joint occurrence of several saline species, inside anatural, multiphase and heterogeneous porous matrix, whose behaviour isconditioned by a varying environment on every site under monitoring.

To study different monuments simultaneously, it is very importantthat monitoring and surveillance programs are set up and carried out on auniform (standard) basis and synchronised, for accurate data and resultscomparison. This should be done in what concerns either salts (origin,composition, extent) or buildings (sampling sites, material properties,etc.).

Acknowledgements

Research contracts PBICT/C/CTA/2127/95 and PBIC/C/QUI/2381/95(JNICT-Portugal) assisted in meeting the production costs of the presentpaper (Parts I and II).

References

[1] Livingstone, R, Influence of evaporite minerals on gypsum crustsand alveolar weathering, Proc. of the III Int. Symp. on theConservation of Monuments in the Mediterranean Basin, eds. V.Fassina, H Ott and F. Zezza, Venice, pp. 101-107, 1994.



[2] Torfs, K, Van Grieken, R. & Cassar, J, Environmental effects on

deterioration of monuments: case study of S" Marija Ta'Cwerra,Malta, Proc. Protection and Conservation of the European CulturalHeritage: Research Report N° 4 (European Commission ResearchWorkshop), ed. F Zezza, Bari, pp. 441-451, 1996.

[3] Fitzner, B , Henrichs, K. & Volker, ML, Model for salt weathering at

maltese globigerina limestones, Proc. Protection and Conservation ofthe European Cultural Heritage: Research Report N° 4 (European

Commission Research Workshop), ed. F. Zezza, Bari, pp. 331-344,1996.

[4] Aires-Barros, L, & Mauricio, A., Chronology, probabilityestimations and salt efflorescence occurrence forecasts on monumentbuilding stone surfaces, Proc. 8 Int. Cong, on Deterioration andConservation of Stone, ed. J. Riederer, Berlin, pp. 497-511, 1996.

[5] Fassina, V., Neoformation decay products on the monument's surfacedue to marine spray and polluted atmosphere in relation to indoor andoutdoor climate, Proc. Protection and Conservation of the EuropeanCultural Heritage: Research Report N° 4 (European CommissionResearch Workshop), ed. F. Zezza, Bari, pp. 37-53, 1996.

[6] Monteith, J L. & Unsworth, M.H., Environmental Physics, EdwardArnold, London, pp.20-30, 1990.

[7] Aires-Barros, L. & Mauricio, A., Transition frequencies of evaporiticminerals on monument stone decay, Proc. of the 4 Int. Symp. on theConservation of Monuments in the Mediterranean Basin, eds. A.Moropoulou, F. Zezza, E Kollias and I. Papachristodoulou, Rhodes,Vol. 1, pp. 33-51, 1997.

[8] Mauricio, A. & Aires-Barros, L., Salt systems and monument stonedecay in coastal marine environment, Chemistry, Energy and theEnvironment, Royal Society of Chemistry, Cambridge (in the press).

[9] Mauricio, A., Aires-Barros & Pacheco, A,M,G, Forecast of saltoccurrences on monument stone surfaces, Chemistry, Energy and theEnvironment, Royal Society of Chemistry, Cambridge (in the press).

[10] Price, C , & Brimblecombe, P., Preventing salt damage in porousmaterials, Preventing Conservation: Practice, Theory and Research,International Institute for Conservation, London, pp. 90-93, 1994.

[llJSteiger, M., Crystallisation properties of mixed salt systemscontaining chloride and nitrate, Proceedings of the EuropeanCommission Research Workshop on the Conservation of BrickMasonry Monuments, Leuven (Belgium), pp. 1-9, 1994.


Section 2:

Coastal Erosion


induced damage to materials in marine environments - part ... · *lab. mineralogia e petrologia and...

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