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MORTARS FOR RESTAURATION. DECAY DUE TO SALT CRYSTALLIZATION

PALOMO, A, PUERTAS, F. AND BLANCO, M.T.

insN> Eduardo Torroja de Ciencias de la Conslrucci6n (CSIC), PO Bax 19002 Madrid (SPAIN)

SUMMARY Investigations carried out on the problem of mortars decay due to salt crystalization are presented. Three different types of mortars, considered appropiated for restauration purposes have been studied: a lime mortar, a portland cement mortar and a lime-portland cement mortar. 3.5 cm side cubes of all mortars were submitted to several "Humidity-Drying" cycles with three different liquids: deionized water, NaCl dissolution and Na2S04 dissolution. In order to follow the evolution of the cube shaped specimens, in function of the number of cycles applied, a control of the mechanical strengths was done. These mechanical properties were correlated to other characteristics of the mortars such as porosity or mineral composition. Among other interesting conclusions, it is worth to emphasize the excellent behaviour of portland cement mortar when salt crystallization occurs.

1. INTRODUCTION

The role played by mortars in the general context of the degradation of a historical building has been hardly studied when compared with the great number of investigations accomplished in connection with other building materials (bricks, stones, etc). However, independently of a given application (rendering, bedding, etc), mortars have been used from remote times (1,2) and consequently their presence in the constructions is a constant throughout the history. In any case, even at present the investigations on mortars are, quantitatively speaking, very inferior to the accomplished around concrete (rt might be due to the fact tha concrete constitutes the basic element of most of modem constructions). Among the papers gathered from the bibliography with specific incidence in the mortars study (in their aspect of material belonging to cultural heritage) references 3 to 7 should be emphasized. On the other hand, the salt crystallization constitutes one of the most frequent and effective phenomena contributing to the deterioration of materials from the heritage. The deterioration due to salt crystallization is a complex process that can be produced by different mechanisms(S-13).lt is not always a simple task to establish which of them is the prevailing one and consequently controlling the degradation process. Among the most frecuently accepted theories is the one establishing that deterioration is produced when the internal tensions surpass the traction strength of the material (the crystallization continues until the tension reaches a given magnitude, directly proportional to the degree of oversaturation and inversely proportional the solubility of the salt) (8). The deterioration of materials as a consequence of salts crystallization, besides being a various mechanisms combination, tends to be a cyclical process. Several crystallization - dissolution cycles are normally needed for observing the deterioration of the material (12, 14). It is for this reason that the simulation in the laboratory (through the application of accelerated aging tests) of the phenomena observed at real level constitutes a preoccupation of the researchers. Thus for example, Binda and Baronio (15) by mean of some experiments carried out on bricks demonstrated that the deterioration of the samples as a consequence of the salt crystallization depended on the drying time. In the present work, it was outlined as fundamental objective, to investigate the effect of the crystallization of two types of salts (NaCl and Na2S04) on three classes of mortar: lime mortar, portland cement (OPC) mortar and a portland cement plus lime, mortar. When planning the mentioned objective it was taken in consideration that buildings or monuments restoration requires the utilization of repair materials compatible with the existing in order to

guarantee the structural integrity of the monument.

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2. EXPERIMENTAL

Raw Materials Characterization. All the materials used for the preparation of the mortars were

characterized from the chemical and mineralogical point of view. The chemical analysis of the materials is

shown in Table I.

TABLE I

LIME CEMENT SAND

L. ON IGN. 24.45 1.18 0.05

INSOL. 0.02 0.27 0.40

SiO.Z 0.39 20.67 98.92

Al~3 1.10 4.31 0.18

F~ 0.20 3.39 0.06

Cao 73.82 65.10 0.00

MgO 0.0 2.70 0.28

SOJ 0.0 2.89 0.00

Free Lime - 1.39 -

With respect to its mineral composition , mainly determined through X-Ray Diffraction, it must be

mentioned the quartz-nature of the sand as well as the utilization of a type I OPC basically constituted by

tricalcium silicate, bicalcium silicate and interstitial phase (calcium aluminates plus calcium ferrites). The

lime was constituted by an only and pure phase: [Ca(OHh].

Dosing and conditions of curing. Table II shows the amounts of raw materials (expressed in %wt) defined for dosing the mortars, as well as the "aggragate-binder" ratio and the "water-binder" ratio. A

superplasticiser was added to all the mixtures together with the water, in a proportion of 0.4% with respect to the weight of the water.

TABLE II

MORTAR1 MORTAR2 MORTAR3

Lime 20.30 10.54

Cement 11 .96 10.54

Sand 60.91 71 .79 63.24

Water 18.78 16.23 15.67

Sand/Binder 3/1 6/1 3/1

H20/Binder 0.92 1.35 0.75

The curing conditions of each of the mortars is specified below (in all the cases mortars were molded in the form of 3.5 cm side cubes):

MORTAR 1 (Lime Mortar). The cubes were submitted to an accelerated carbonation process through the

contact of these with a C02 fJU>Ce twice a day. The carbonation chamber was maintained with a relative

humidity around 50% (the presence in the chamber of a saturated solution of K2C03 allowed to keep

constant this relative humidity) in order to be close to the conditions of maximum carbonation rate. Between

the two daily treatments with C02, the specimens were introduced in an oven at 400C for 1 hour. The total duration of curing step was 8 days.

I I

I I

I I

I I

I I

I I

I I

I I I

I I I I

I I ' ' : I

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MORTAR 2 (Cement Mortar). The cubes were kept at the curing chamber at 21°C and 95% of relative humidity for 28 days.

MORTAR 3 (Lime and Cement Mortar). The curing of mortar 3 cubes was a combination of the previous cases: accelerated carbonation during 8 days and curing in wet chamber for 16 extra days.

Humidity-Drying cycles with aggressive dissolutions.

The samples of the three types of mortar, once cured, were totally submerged in containers having the work dissolutions: A) Deionized Water, 8) Saturated Dissolution of NaCl and C) Na2S04.1 OH20 Dissolution (7%wt). Thereinafter the development of the Humidity-Drying cycles were carried out as follows:

First Day.- Extraction of the cubes from the dissolutions (during 30 minutes they were left to drain) and introduction of them in an oven to 105°C for 4 hours. Then cubes were slowly let to reach ambient temperature, until the following day.

Second Day.-lmmersion of cubes in the aggressive dissolutions.

Third Day.- The cubes continue submerged in the dissolutions.

Fourth Day.- The cycle starts to be repeated with the first day operation.

Table Ill shows the times at which cubes were definitely extracted from dissolutions for being studied from the physical ,mechanical, mineral6gical and microestructural points of view.

Deionized Water

Dissolutionof NaCl (saturated)

N1t2504 (70/o) Dissoluti6n Cycles

N1t2504 (70/o) Dissolution No Cycles

TABLE 111

MORTAR1

5 weeks and 3,4,5 and6 months.

2.Sl and 9 weeks and 3 an 4 month

1,2.S and 3 weeks

Sweeks and 3,4,\and 6 mont s

MORTAR2

Sweeks and 3,4,5 and 6 months.

5 weeks and 3,4,S and 6 months

1,2.5 and 3 weeks

5weeks and 3,4,5 and 6 months

Tests of sulfate aggressiveness without Humidity-Drying cycles

MORTAR3

Sweeks and 3,4,5 and 6 months.

Sweeks and 3,4,5 and 6 months

1, 2.Sy3 weeks

5weeks and 3,4,5 and 6 months

Due to the well known destructive power of sodium sulfate when it crystallizes, and consequently the short time that elapses between the beginning of the test and the total destruction of the cubes, it was devised an additional test having the objective of decelerating the deterioration of the mortars in contact with the salt dissolutions. Said test consisted in introducing the materials in the dissolution of Na2S04 maintaining them permanently submerged until the definitive extraction for their study. The extraction times were

identical for each mortar (see Table Ill).

3. RESULTS AND DISCUSSION

The whole of the cubes, once extracted definitely from the aggressive liquids, were submitted to a mechanical , physical, mineralogical and microstructural study in order to compare the evolution of the

three types of mortar in the salt dissolutions.

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Table IV and figures 1, 2, 3 and 4 show some of the results obtained in the mentioned study. From the

results analysis the following observations can be deduced:

Firstly, it must emphasized that the evolution of the mechanical strengths of the three studied mortars

follows a same standard when the liquid at which they are submerged is deionized water; that is to say:

mortar 3 gives always (making the comparisons to the same ages) higher strength than mortar 2 and

mortar 2 higher than mortar 1. This behaviour seems to be logical if it is taken into account that mortar 3

not only possesses the greater percentage of cementitious material of the three studied mortars but its content in portland cement is similar to the one of the mortar 2 (mortar 1 is a lime mortar), and portland

cement is the hydraulic binder of better mechanical characteristics nowadays existing. The

aggregate/binder and water/binder ratios (shown in Table II) are also determinant to explain the

mechanical strengths of the three mortars follows this order: 3>2>1

Secondly, the effect of the cycles provoking the phenomenon of salt crystallizati6n is enormously

destructive in all the studied mortars. In figure 4 an example on the evolution of the cubes with time is

shown.

Wrth respect to mortar 1 (lime mortar), it should be emphasized that it maintaines a level of acceptable

strenghts until 4 months of submission to the crystallization cycles. Tests of longer duration provoke

irreversible alterations in the cubes.

The data provided by XRD indicate the abundant presence in these mortars of halite crystals when cubes

have been submitted to the "humidity- drying" cycles with the dissolution of NaCl and of temardite in the

mortars submitted to the aggression of the cycles with dissolution of Na2S04. The not presence of other

crystalline phases (except calcite and quartz that constitute the original components of the mortar) evidence

that halite in the first case and temardite in the other case, act as responsible elements of the deterioration

of the cubes. In this sense one must take into account that NaCl is a very soluble and hygrosc6pic

compound and when in dissolution it presents a great mobility being able of penetrating profoundly.

Additionally, as it was indicated by Lewin (16), a deterioration by salt crystallization is produced if the salt

has a trend to the oversaturation (a solute deposited under equilibrium conditions does not release energy

and it does not exercise mechanical work) and this trend is high when the solubility of the salt is also high.

In few words, NaCl is an extremely dangerous salt when the humidity-drying cycles permit the

recry~allization of halite in successive occasions.

On the other hand, the lack of tables taking issue with the strengths evolution of the mortars submitted to

crystallization cycles with sodium sulfate, is very meaningful. The problem is deduced with facility from

figure 4: The whole of the mortars submitted to the aggression of sulfates are totally destroyed after 2.5-3 weeks of cycling.

The explanation of this destructive capacity rests in the fact that sulfates are, as a general rule, less soluble

and mobile that other salts. They precipitate in the hydrated form and thereinafter can be transformed into

the anhydrous phase. If once they are as anhydrous compounds, humidity is not very high, then salts are not dissolved but are hydrated.

Specifically, the sodium sulfate identified in our three mortars was in the form of themardite (see table IV).

This anhydrous phase was formed as a consequence of the dehydration of the decahydrate (mirabilite) due

to the system conditions (temperature < 32,4°C and low relative humidity)(17). When increasing the relative

humidity, themardite evolves toward the formation and precipitation of mirabilite. This transformation

implies a change of volume which is responsible for the large tensions generation in the walls of the pores of the materials (the mortars in this case).

Charola et al (18) attribute the expansion in the case of sodium sulfate not to the "anhydrous-decahidrate"

transformation but to a number of nuclei formed in the intelior of the matelial and to their subsequent

growth. In fact, Binda et al (15) indicate that the deterioration of materials depends so much on the type of

salt as on the growth conditions. In any case it seems to exist practical unanimity among the authors

(included those of present work) for qualifying sodium sulfate as the most destructive salt in terms of crystallization processes.

Concerning the studied mortars, the lime mortar has demonstrated to be the most sensitive to the

described destructive process generated by successive recrystallization of sodium sulfate.

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Mortar 2(portland cement mortar) presents a very different behavior, when compared with lime mortar, with respect to the problem of salt crystallization. Firstly, it should be emphasized its high mechanical strength, what has done of it the hydraulic binder most widely used in the present century. As second key feature it must be mentioned its excellent behavior (When compared with the others studied mortars) when submitted to the tensions generated by successive recrystallization of halite. Among the three studied materials, portland cement mortar is the only one which sustains 24 weeks of NaCl crystallization cycles without having a complete deterioration (see figure 2). The justification to this good behavior must be found not only in the superior mechanical strengths of portland cement with respect to lime, that would allow to the OPC mortar to absorb better the internal tensions. Additionally, as it can be deduced from the X-ray diffaction patterns, part of the chlorides present in the test would interact with the aluminate phase in the OPC for generating the Friedel's salt (Ca.A120 4Cl2 . 10H20). This chlorine fixation in form of a new phase which takes part of the cement matrix will partially counteract the destructive process of the successive halite recrystallizati6n. Wrth respect to the behavior of the portland cement mortar when submitted to the cyclical processes "humidity-drying" with sodium sulfate dissolution it must be said that the destruction of the cubes is total after a short period of time (5 weeks) of submission to such aggression. The mechanism previously cited for explaining the destruction of the lime mortars, is repeated in this case and prevails on any other possible alteration process. Special mention deserves the case of the cubes immersed in the sulfate dissolution but not submitted to the cycles. The presence of moderate amounts of ettringite (more than the existing in the mortar 2 cubes submerged in the other liquids) and also the presence of gypsum, indicate the existence of some processes, on principle, expansive ones), respectively derived from the reactions between calcium aluminates On the OPC) and sulfates and between these same sulfates and portlandite. These reactions are parallely produced to the process of themardite recrystallization. It seems that the effect of this sulfate attack to the hydrated phases of the cement is not so intensely destructive as the effect of the themardite recrystallization. In any case it does not mean that said expansive phenomena must be not taken in account as possible cause of irreversible deterioration of cement mortars; by the contrary wherever the possibility of this type of attack is foreseen, the use of sulfa-resistant cements is recomended. The lime mortar cubes are, among the three tested materials, the weakest (from the mechanical point of view) after the six months of immersion in sodium sulfate and no submission to "humidity-drying" cycles. Gypsum formation and themardite nucleation in the pores of the material are the responsible for its gradual decay. Mortar 3, finally, is the one with highest mechanical strengths (rt has the greatest amount of cementitious materiaQ. Actually, it is an intermediate material (compositionally speaking) between mortars 1 and 2. At present, it is occasionally replaced part of the portland cement of the mortars, by given proportions of lime in order to endow of greater plasticity to the paste and to avoid the stiffnes which is produced in

mortars rich in OPC. Wrth respect to the alteration or degradation of this kind of mortar by effect of the salt crystallization.it can be said that its behavior is intermediate between mortars 1 and 2: it behaves well for periods up to three months of submission to cycles but the longer tests imply the total destruction of the cubes On the case of the cycles with NaCO or a considerable deformation in the case of the sulfate attack with no cycles).

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TABLE IV

Mineral composition of Mortars 1, 2 and 3 [ (*), (-) and (.-) in the table] after being submitted to the attack of the different aggressives during the specified time. Data obtained through X-Ray Diffraction.

M (*) Q c p H G T E F

0 Wat. x xxx 6M.

R NaCl xx xxx xxx 4M

T SS xx xxx x 1W

1 SS xx xxx x x ~n~

M (-)

0 Wat xx xxx & SM

R NaCl x x x xxx x SM

T SS xx x xxx xx & 1W

2 SS xx xx xx x x x ~n~

M (-*)

0 Wat x xxx x 4M

R NaCl xx xxx x xxx x 3M

T SS xx xxx x x 1W

3 SS xx xxx x x ~n~

XXX=Abundant, XX=Moderate, X=Little, &=Traces M=Month , W=Week, Wat=Water, SS= Sodium Sulfate Dissolution, (nc)=No Cycles Q=Quartz, C=Calcite, P=Portlandite, H=Halite, G=Gypsum, T=Temardite E=Ettringite, F=Friedel's Salt

REFERENCES

(1) Lea, F.M. ''The chemistry of cement and concrete". Chemical Publishing. London (1971 ). (2) Goria, C. "Evoluzione storica dei leganti e dei conglomerati: dall'empirismo alla loro conoscenza razionale". Cemento: storia, tecnologia, aplicazioni. Fabbri Ed. Milano (1976). (3) Rossi-Doria, P.R. "Mortars for restoration: basic requierements and quality control". Materiaux et Constructions. Vol 19, N° 114, pp 445-448 (1986). (4)Harrison, W.H. "Durability tests on building mortars. Effect of sand grading". Magazine of concrete research. Vol38, NOJS (1986).

(S) Harrison, W.H. and Bowler, G.K "Aspects of mortar durability". Br.Ceram.Trans.J. 89, pp93-101 (1990). (6) Martinez-Ramirez, S.; Puertas, F. and Blanco-Varela, M.T. "Morteros de reparaci6n basados en cal. Ensayos de envejacimiento acelerado". Mat.de Construcci6n. Vol 4S, N° 238, pp 35-4S (199S).

(?)Puertas, F.; Blanco-Varela, M.T.; Palomo, A ; Ortega, J.J.; Arino, X. and Saiz-Jimenez, C. "Decay of roman and repair mortars of the mosaics of ltalica, Spain". Sci.Total Environ. Vol 1S3, pp 123-131 (1994). (8) Evans, l.S. "Salt crystallization and rock wheathering: a revif!W'. Revue de Geomorphologie dynamique. XIX Annee, N° 4, pp 1S3-177 (1970).

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(9) Winkler, E.M. and Wilhelm, E.J. "Salt burst by hydration pressure in architectural stone in urban atmosphere''.

Geol.Soc.Am.Bull. Vol 81, pp 567-572 (1970). (10) Winkler, E.M. and Singer, S. "Crystallization pressure of salt in stone and concrete". Geol.Soc.Am.Bull. pp 3509-

3514 (1970). (11) Winkler, E.M. "Stone: Properties, durability in man's environmenf'. Springer. Verlag (New York) 230 pp. (1973) (12) Binda, L.; Baronio, G. and Charola, AE. "Deterioration of porous materials due to salt crystallization under different thermohygrometric conditions. I: brick''. Vth Int. Cong. on Det. and Conservation of Stone. pp 279-288

Laussane (1985). (13) Lazzarini, L. and Laurenzi-Tabasso, M. "II restauro della pietra''. CEDAM. Ed. Antonio Milani. 320 pp. Padova

(1986). (14) Amoroso, G.G. and Fassina, V. "Stone decay and conservation. Atmospheric pollution, cleaning consolidation and protection". Mat. Science Monographas. Edit Elsevier. 453 pp (1983). (15) Binda, L and Baronio, G. "Mechanisms of masonry decay due to salt crystallization". Duarbility of Building

Materials, 4, pp 227-240 (1987). (16) Lewin, S. ''The susceptibility of calcareous stones to salt decay''. The conservation of monuments in the

Mediterranean basin.

pp 59-63 Bari (1989). (17) Mcmahon, D.J.; Sardberg, P.; Follard, K. and Mehta, P.K. "Deterioration mechanisms of sodium sulfate" 7th Int. Cong. on deterioration and conservation of stone. Vol 2, pp 705-714 Lisbon (1992). (18) Charola, A.E. and Weber, J. "The hydration-dehydration mechanism of sodium sulfate". 7th Int. Cong. on Deter. and Conservation of Stone. pp 581-589 Lisbon ( 1992).

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MORTARS SUBMITTED TO CYCLES IN DEIONIZED WATER STRENGTH EVOLUTION

FIGURE - ,.,.,,,,,

~ 1 80 JfM <WI?

. Mun.J

~ ::r:: G M'j 130

!;; w ~ w er 80 Q_

:lE

8 30

5 12 16 20 24

TIME (WEEKS)

MORTARS SUBMITTED TO CYCLES IN NaCl DISSOLUTION Strength evolution

FIGURE 2

--g 12 16 20 24

TIME (WEEKS)

MORTARS SUBMITTED TO SULFATE ATTACK (NO CYCLES) Strength evolution

g 12 1C 20 24 TIME (WEEKS)

FIGURE 4

H2 O Cycles

5 WEEKS

Na2 S04 Dis Cycles

~ ---1 WEEK

H2 0 Cycles

No2 S04 Dis Cycles

3 WEEKS

H2 0 Cycles

··---

--~WEEKS

Na2 S04 Dis Cycles

3 WEEKS

- 7

6 MONTHS

2,5WEEKS

6 MONTHS

6 MONTHS

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MORTAR 1 Na Cl Dis Cycles

--·-5 WEEKS 4 MONTHS

Na 2 S04 Dis No Cycles

......... -5WEEKS 6 MONTHS

- - - ............... _____ , ....... __ _ MORTAR 2

No Cl Dis Cycles

6 MONTHS

No2 504 Dis NO Cycles

6 MONTHS

·----·················- -

Na Cl Dis Cycles

__ .. _ .. _ 5 WEEKS 5 MONTHS

Na2 S04 Dis NO Cycles

·--5 WEE KS 6 MONTHS