WATER RESISTANT GYPSUM-LIME MORTARS FOR THE RESTORATION OF HISTORIC BRICK BUILDINGS

Bernhard W. Middendorfl and Dietbert Knõfel2

1. ABSTRACT

The use of gypsum as building material can be traced back over a long period of time. Gypsum-anhydrite-lime mortars as well as pure lime mortars were used for the joints on the outside brick walls of religious buildings. These gypsum mortar mixtures are about 500 years old and still in a good condition. The solubility in water is a disadvantage which makes it very difficuIt to use gypsum externally. In recent times this traditional building material has been rejected more and more in favour of hydraulic binder. An effort is made to produce adequate mortar mixtures with the help of modem binder components , which have the necessary technological properties and can also be used for preservation purposes. Water resistant gypsum mortar mixtures based on /3-hemihydrate and CI'-hemihydrate and added hydrated hydraulic lime are suitable building materiais for externai use as joint mortars. Mixtures of different additives and aggregates are used to optimize the technological properties . Results of analyses of selected mortar mixtures will be presented.

2. INTRODUCTION

The use of gypsum as a building material with its different properties was already known in ancient Egypt 2500 B.C. when the Cheops-pyramid was built (1). The oldest description of the mineral gypsum was given by Theophrastus in 314 B.C. (2) . In Germany the 12th century was the heyday of the use of gypsum for composition tloors and mortar. After the first half of the 19th century the knowledge of the production, preparation and properties of different materiais was lost when hydraulic binder carne

Keywords: Mortar ; Gypsum ; Water Resistant; Brick Buildings.

lMineralogist, Department for Structural Materiais, University ofKassel, 34109 Kassel , Germany.

2Professor and Head , Laboratory for Chemistry of Construction Materiais (BCS), University of Siegen , 57068 Siegen, Germany.

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up (3). Modern gypsum building materiais are almost always intended for internai use. This usage is useful with regard to the speeial properties of this building material. General advantages are: favourable produetion methods with regard to energy, quiek and eontrollable setting behaviour, good adhesion to plaster and the ease of working. However, a high solubility in water (2.6 g/l) and a small wet strength of gypsum does restriet its use to inside parts of a building. Nevertheless, ehemical-mineralogieal investigations of historie joint mortars from briek buildings in Northern Germany have shown that gypsum-anhydrite mortars with a small eontent of lime have been used externally for religious buildings (4). A great part of these joint- or masonry mortars is still well preserved .

Aeeording to Steinbreeher (3) this durability is due to their reeipe eonsisting of binder and aggregates with the same kind of substanee as well as due to its preparation with extremely small quantities of water. A water/binder value of less than 0.4 and a bulk density of 2.0 g/em3 is given for a historie gypsum mortar.

The reproduetion of medieval reeipes is nowadays not possible anymore, because of the faet that it is to a great extent depending on the deposits of natural raw gypsum and the regionally different traditions of eraft. The paper shall eontribute some information on the development of water resistant mortars for the restoration of joints based on gypsum and lime whieh both meets building-physical as well as preservative requirements.

Joint mortars for the use at historieal saered briek buildings should meet the following requirement seheme (apart from water resistanee nothing else was assessed here):

- Based on Gypsum and Lime - Water Resistant - Compatibility with Historie Material - Adapted Colour - Dynamie Modulus of Elastieity (after 28 d) - Compressive Strength (after 28 d) - Low Shrinkage Tendeney - Suffieient Time of Workability - Good Workability Properties - Resistant to Frost - No Effloreseenee

3. MATERIALS USED

~ 10 kN/mm2

10 - 15 kN/mm2

< 0.5 mm/m > 1 h

The produetion of water resistam joint mortar mixtures based on gypsum and lime has been tested. Commereially available binders, aggregates and additives have been used for these tests. The binders used were: iJ-hemihydrate (iJ-HH), produeed with natural gypsum stone, ey-hemihydrate (ey-HH) , a so-called flue-gas gypsum whieh is obtained from the desulfurization of eombustion gases of fossil fuels as well as hydrated hydraulie lime (WaKH), produeed with marled limestone. Powdered lime stone (KStM) with a speeifie surfaee of 4100 em2/g and a quartz sand (H 31) with grain sizes from 0.125 to 1.0 mm have been used as inert aggregate . Tartarie aeid (Ws) has been used to optimize setting behaviour. Tests with hydrophobing additives have also been done , ealcium silieonate (BS47) was used.

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4. EXPERIMENTAL INVESTIGATIONS

All fresh mortar mixtures have been prepared for a spreading area of 16 ± 0.5 em according to DIN 1168, part 2. This consisteney proved to be suitable for practical use. According to DIN 1164, part 7 sample pieces with the size 40*40*160 mm have been formed from the mixtures to be investigated. At the same time sample pieees sized 15*15*60 mm have been produced in order to determine water resistance via leaching tests. For this the sample pieces have been stored in flowing water for several weeks to get results quickly. Moreover, the solubility in water has been determined with leaching tests, and in the laboratory wet-chemical measures have been done via the determination of sulfate resp . calcium contento Measured values in table 2 refer to laboratory tests. In addition, sample pieces have been exposed to outdoor weathering for at least 180 days. After termination of laboratory tests the most suitable mixtures of the investigation programme have been inserted into historie masonry .

Hygric swelling and shrinking has been measured at least up to the 28th day. After this period no substantial changes could be noticed.

The bigger the dynamic modulus of elasticity (E-modulus), the smaller the "elasticity", that is, a high E-modulus does only allow a slight elastic change of form which can cause cracks in the masonry. The values for the dynamic modulus of elasticity and for the compressive strength ({3ST) should be distinctly smaller for mortars than for the stone used in the masonry , as , otherwise, cracks and other damages can be the result (5).

The way of carrying out of tests for the determination of workability time, the freeze-thaw-eyc1ing behaviour, the capillary water-absorption and water emission as well as of liability to efflorescence is shown in "Handbuch für Mortel und Steinerganzungsstoffe" (6).

Moreover , the set mortars have been characterized with the help of a scanning eleetron microscope (SEM) and mercury pressure porosimetry to quantify the eauses of higher water resistance.

Table 1 shows some of the mortar mixtures produced and their abbreviation used below. As all mixtures eontain 0 .1 wt.-% tartaTic acid (Ws), with regard to the hemihydrate used , the Ws has not been Iisted separately among the abbreviations. The values in brackets for the abbreviations give the content ofbinder components in wt. -%.

5 . RESULTS AND DISCUSSION

As can be seen in table 2, gypsum mortars produced with a-hemihydrate have the highest value for the E-modulus and a compressive strength of 43 N/mm2 as well as a water solubility being less by 40 % than for mortars produced of pure {3-hemihydrate. Both forms of hemihydrate crystallize rhomboidally, but show different applieation/technical and energetic properties . The way of their production is also different. While a-H H is gained via an autoclave proeedure, {3-HH is formed with atmospheric pressure and temperatues of about 45-200 · C and during setting it forms needle-shaped, monoc1inie-prismatic dihydrate crystals which become felted . The compact structure between individual gypsum crystals formed with a-HH , being rough

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Table 1: Mortar mixtures produced and their abbreviations.

I basic mixture I abbreviation I -ª-HH/il-HH/WaKH(~O/~O/~O) + Ws aBW(532)

a-HH/ jS-HH/W aKH(20/60/20) + Ws aBW(262)

a -HH/jS-HH/WaKH(50!20/30)+Ws aBW(523)

a -HH/jS-HH/WaKH(20/50/30) + Ws aBW(253)

a -HH/jS-HH/WaKH(50/30!20)+Ws +0.5% BS47 aBW(532)+BS

a -HH/jS-HH/WaKH(20/60/20) + Ws +0.5% BS47 aBW(262)+BS

a-HH/jS-HH/WaKH(50!20/30) + Ws +0.5% BS47 aBW(523)+BS

a-HHljS-HH/WaKH(20/50/30) + Ws +0.5% BS47 aBW (253) + BS

a -HH/jS-HH/WaKH(50!20/30) + Ws + 20% KStM aBW(523)+20K

a -HH/ jS-HH/W aKH(20/50/30) + W s + 20% KStM aBW(253) + 20K

a-HH/ jS-HH/W aKH(50/20/30) + W s + 20% H31 aBW(523) + 20Q

a-HH/jS-HH/WaKH(20/50/30)+Ws+20% H31 aBW(253)+20Q

a-HH/jS-HH/WaKH(50/30/20)+Ws+25% H31 +25% KStM aBW(532)+25Q+25K

a-HH/jS-HH/WaKH(20/60/20)+Ws+25 % H31+25 % KStM aBW(262) + 25Q + 25K

a -HHljS-HH/WaKH(50/20/30)+Ws+25 % H31+25% KStM aBW(523)+25Q+ 25K

a -HH/ jS-HH/W aKH(20/50/30) + Ws + 25 % H31 + 25 % KStM aBW(253)+25Q+25K ....................... ... ......... . ............ -.. -- ---_ ...............................................................................•................. .......••.•........................•............................

a-lllI: a-hemihydrate; tJ-lllI: jS-hemihydrate; WaKH: hydrated hydraulic lime; Ws: L( + )-tartaric acid (0.1 wt.-% to the hemihydrate) ; 8S47: hydrophobic agent; KStM: powdered limestone; 1131: quartz sand (0 .125 - 1.0 mm)

and prismatic or plated can be the reason for the high E-modulus values and mechanical strength values.

Moreover, mortars made of a-H H have a smaller porosity than those based on {J-HH, which also indicates that the structure must be more dense and therefore results in a reduced surface. This is the cause of a smaller water solubility.

The addition ofO.1 wt.-% tartaric acid to {J-HH as a retarder, in order to optimize the workability , causes a slight coarsening of structure, while at the same time the needle-shaped character of the dihydrate crystals is nearly lost. The sole adding of Ws increases the compressive strength , the E-modu\us and reduces the water so\ubility of the mortars. The value of the mortars' swelling behaviour also decreases when Ws is added. A slower setting, indicating an ordered crystal growth and resulting in bigger crystals could be a reason for this. The same effect can also be seen when other carbon acids are added. E-moduli and compressive strengths can be controlled by the combination of both dihydrate forms (a-HH, (J-HH); but as these mixtures have a high water solubility, hydrated hydraulic lime has been added because during preliminary

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tests the latter has turned out to be the most suitable binder with hydraulic properties owing to its improving properties for the set mortar mixtures. A comparison of the samples aJ3W (532) and aJ3W(523) with the mortar made of pure ,B-HH shows that compressive strength and the value for E-modulus decrease while the lime content rises .

The water resistance is about 40 % higher than for mortars based on pure ,B-HH. If the ratio between ex-HH/,B-HH is reduced , this also causes a decrease of mechanical

Table 2: Technological properties and resistance to water of selected gypsum mortar mixtures.

E-modulus {JST after swelling/ basic mixture after 28 d 28d shrinking

[kN/mm2] [N/mm2] [mm/m]

{J-HH 9.26 15.55 +0.27 -0 .15

a-HH 21.35 43.25 +0.60 -0.60

{J-HH+Ws 10.29 22.95 +0.13 -0.20

aBW(532) 11.16 26.09 +0. 12 -0.22

aBW(262) 8.88 17.15 +0.15 -0.23

aBW(523) 9.65 18.46 +0.12 -0 .26

aBW(253) 7.58 14.75 +0.10 -0.27

aBW(532) +BS 11.08 23 .74 +0.12 -0 . 18

aBW (262) + BS 9. 11 20.65 +0.12 -0.21

aBW(523) +BS 9.82 22 .68 +0.10 -0.28

aBW(253) + BS 7.28 17.49 +0.11 -0 .33

aBW(523) + 20K 9.45 17.73 +0.11 -0.23

aBW(253) + 20K 6.79 11.86 +0.1 I -0.24

aJ3W(523)+20Q 12.20 23 . 16 +0.13 -0 . 17

aJ3W(253) +20Q 7.97 13 .91 +0.14 -0. 17

aBW(523)+50Q 17.37 25.77 +0.10 -0 . 15

aJ3W(253) +50Q 12.73 16. 11 +0.12 -0 . 15

aJ3W(532) + 25Q + 25K 15 .13 24.77 +0.07 -0 . 11

aJ3W(262) + 25Q + 25K 11.05 16.64 +0.04 -0 . 11

aBW(523) + 25Q + 25K 13.11 21.05 +0.05 -0 . 13

aJ3W(253) + 25Q + 25K 10.23 13.42 +0.06 -0 .14

resistance to water

standardized to B-IDI [%]

100

60

80

65

65

60

73

79

62

60

60

56

65

54

54

67

82

44

33

40

39 ......... ................ -...................................... ................... -................ ............................ ........................................... ......................... -................. E-modulus: dynamic modulus of elasticity; flSt : average compressive strength abbreviations listed in table 1

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strength and of elasticity. If 0.5 wt.- % of a hydrophobic agent (BS47) are added to the mortar mixture the properties are only negligibly influenced. Owing to the hydrophobic agent the individual dihydrate crystals have a thin "protective coating" which is initially water-repellent (see picture 1). However, as investigations with a scanning electron microscope have shown, this coating has small cracks through which water can reach the crystals , can partly dissolve them and so the protective coating loses its effects , a fact which further explains the relatively bad water resistance.

Picture 1: SEM-photo­graph of set and hardened gypsum mix­ture prepared of (j-HH, 0.1 wt.-% Ws and 2.0 wt.- % of the hydro­phobic agent BS47. Because of BS47, the gypsum crystals are covered with a thin coating which protects the material against water attacks. This protection is only tem-porary, because it has a lot of cracks, see arrows (width of picture 1: 63 iLm) .

An addition of powdered lime stone (compare aBW(523) with aJ3W(523)+20K) to the gypsum mortar mixtures does increase water resistance and slightly decreases mechanical strengths and the E-modulus, as the inert powdered limestone aggregate deposits between the gypsum crystals formed during the setting process and as a consequence influences the microscopic formation of the structure of the binder matrix (see picture 2). Compared with mixtures without aggregates, quartz sand as an aggregate causes a slight increase of mechanical strength values and of the E-modulus. Quartz grains are embedded in an undisturbed binder matrix (see picture 3), which results in the fact that the set mortar has a small porosity, especially concerning capillary porosity, and owing to this offer only a small surface to the solving attack of water, which is also shown by the results for water resistance. The addition of quartz sand also has a positive impact on the shrinking behaviour of mortars so that they can also be described as having small shrinkage. The binder has been weakened with 50 wt.-%, which reduces the water solubility even further. A combination of powdered limestone and quartz sand has been chosen deliberately, to be able to fix the properties of the set mortars according to the requirement scheme. It became obvious that mixtures with a binder content of a-HH, 20 wt.-% have the most favourable properties for use in historic brick walls.

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Pictures 4-7 show the test pieces (15*15*60 mm) of the total series of experiments, which have been stored hanging freely in flowing water for 28 days. These extreme conditions do not simulate the real weathering influences to which a building is generally exposed, but they show results for an effective assessment of binder mixtures after a short period of time. The results of these survey tests could be correlated with the solubility measurements done in the laboratory.

Picture 2: SEM-pboto­grapb of lhe set and bardened Ct­

HH / 6-HH I WaKH(50/301 20) + 0.1 %Ws +20% KStM nllxture. The inen aggregate of powdered lime stone between lhe gypsum crystals can be seen (widlh of picture 2: 47 /LlII).

Picture 3: SEM-pboto­grapb of lhe set and bardened Ct­

HH / 6-HH I WaKH(50/301 20) + 0.1 %Ws +50 % H31 nllxture. The good bonding of lhe quartz grains and lhe binder can be seen . Tbe samp1e stands out as it bas onJy a small poro s it y, wIDcb can be basicall y seen in lhe picture (width af picrure 3: 63 /LlII) .

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Pictures 4-7: Sample pieces (15*15*60 mm) after 28 days of storage in flowing water. As a comparison, picture 4 also shows the sample which was produced of pure IJ-HH (in the picture uppermost). The 4 sample pieces below in picture 7 were still moist when the photo was taken, which explains the dark colour.

~ct~e4: r~====~=~~~~~~~=~~:~~~ii~~~-~'f'~'-'~-'-~-~~~~~=-=-='~-'-Sample pieces Z\Il!t,Yer:;lIll$b: lHllT (15*15*60 mm) after 28 days of storage in flowing water.

1. QoolffiJIHIDIW_KHtSMO/Ul)+ W$

,. «-oUltln.-fUlIWaK}f(46141)/:20)+ Ws

3. ...ltUlfl.HllJWa1<.U{30f50t20) + w~

4. tt-WIfl\.8U/WaKl:I(2ot«ltZO} + Ws

5. (t.Ulttn.-RUfW~KlI(SOIZOOO)+ W~

6. ifoollllln.-mllW.lUf{4Dt'w(30) + Ws

1. ... MIIJl..llHl\VaKU{30/40t30}+ Ws

s. tt-HRl6.nlt/WaKn(1(lfSO!)O)+W~ I I

í L:==-==-:~ ________ --=:,;;-,====~-==-__ .J

~cture 5: Sample pieces (15*15*60 mm) after 28 days of storage in flowing water.

,-=--_ ... _---

12 ..... nH/IJ-lillfWaKJt(20fW21l) +Ws + BS47

13. lI'-Rl{/tJ.lIHtWaKH(50t2otJOh' WS+ BS47

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---1

I

On the pictures 4-7 one can see that the mortar pieces have slightly dissolved , but they have a distinctly higher water resistance than the sample based on pure {J-HH (picture 4, uppermost sample) . The samples' colour nearly corresponds to historic joint mortars (samples 29-32 on picture 7 have been moist when the picture was taken) so that one can talk about an optical adaptation.

Picture 6: r Sample pieces 'i (15*15*60 mm) after 28 days of SlOrage in flowing water. !

Picture 7: Sample pieces (15*15*60 mm) after 28 day s of storage in flowing water.

!

21. ",.UlltlhBlL'WaKH(51l!JMO)+Wi+ 1lI%H31

z. .. ~. .. .. m /ll-HU!WaKH(5Il!20l30l+ Ws+ 24%H:Jl

......... ___ . ___ J

Ali mortars investigated were highly resistant to freeze-thaw-cycling and did not show anyeftlorescence. The workability time lay within the range of an hour , but it can be lengthened with an increased addition of tartaric acid.

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6. SUMMARY

Investigations have shown that it is possible to produee water resistant joint mortars based on gypsum and lime whose properties are eompatible with historie briek walls. They ean be produeed with eommereially available binders. Thus, the outside walls of briek buildings having been built with a gypsum mortar ean be restored with a gypsum based mortar. Damages sueh as formation of ettringite when C3A-eontaining eement mortar is used ean be avoided. Properties of set mortars ean be adapted to a desired requirement seheme owing to a eombination of individual mortar components used.

The mixtures: a-HH/I1-HH/WaKH (20/50/30)+0.1 %Ws+20%KStM a -HH/I1-HH/WaKH (20/50/30)+0.1 %Ws+20%H31 a-HH/I1-HH/WaKH (20/50/30)+0.1 %Ws+25%KStM+25%H31

turned out to be most suitable. Test walls were built with these mortars and subsequently they were inserted into the Southern facade of St. Wilhadikirehe in Stade (Northern Germany) . These test surfaees are eontrolled continuously. After half a year they still do not show any damages.

7. ACKNOWLEDGEMENTS

We wish to thank Mrs. I. Hommel and DipI.-Labchem. A. Zoller for their support during experimental investigations and Dipl.-Ing. K.G . Bottger for criticai and helpful comments. Moreover, we are indebted to the Umweltbundesamt in Berlin for kindly giving financiai support to this research project.

8. REFERENCES

1. Livingston, R., Wolde-Tinsae, A. , and Chaturbahai, A., "The Use of Gypsum Mortar in Historie Buildings", Struetural Repair and Maintenance of Historie Buildings 11, Eds. C.A. Brebbia, J. Dominguez & F. Escrig, Computational Mechanies Publications, Southampton, UK, VoI. 1, 1991, pp. 157-165.

2. Schwiete, H. E., and Knauf, A. N., "Alte und neue Erkenntnisse in der Herstellung und Anwendung der Gipse", Merziger Druckerei und Verlags GmbH, 1969.

3. Steinbrecher, M. , "Gipsestrich und -mortel: Alte Techniken wiederbeleben", Bausubstanz, 10, 1992, pp. 59-61.

4. Middendorf, B., and Knofel, D., "Characterization of Mortars from Historie German Brick Buildings and Requirements for Restoration Material" , Proceedings of the 3rd International Masonry Conference, London, UK, 1992, in press.

5. Knõfel, D., and Schubert, P., "Zur Beurteilung von Morteln für die Instandsetzung von Mauerwerk," Bautenschutz und Bausanierung, 13, 1990, pp. 10-14 + 15-20.

6. Knofel, D., and Schubert, P., "Handbuch Mortel und Steinergãnzungsstoffe in der Denkmalpflege," Sonderheft aus der Publikationsreihe der BMFT­Verbundforschung zur Denkmalpflege, Verlag Ernst & Sohn, Berlin, 1993.

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