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ANALYSIS BY X-RAY DIFRACTION AND LIGHT MICROSCOPY OF RAMMED EARTH WALLS

(GRANADA. SPAIN).

ONTIVEROS ORTEGA, ESTHER

lnslitUIO Andaluz clel Patrimonio. Isla de la Cartuja n• 1, Sevilla.

SEBASTIAN PARDO, EDUARDO

Facultad de Cienclas, Universidad de Granada. Fuentenueva sn, Granada.

VALVERDE ESPINOSA, IGNACIO

e.u Arqunectura Tecnica Universidad de Granada. Fuentenueva sn, Granada.

SUMMARY The rammed earth walls, in different varieties, is the constructive form utilized to build Granada city walls. To know the mineralogic composition of any material used in construction is very important, because it determines extensively its resistance and durability. This have been the first aim of this work; the quarry of extraction of the material has been identificated too. The used of the X-Ray Diffraction and optical Microscopy have allowed to know the initial composition and to make the existence of secondary minerals clear. On the other hand, the optical microscopy is a technique with high resolution for the study of mortars of lime. As a conclusion, whith the mineralogic identification it is possible to know: resistance of the material to external chemical and phisical agents and manufacture tecnique.

1. INTRODUCCION.

The old city of Granada analysed here are of Arabic origin, erectes fron the VlllTH to the XIVTH. The walls were built of rammed earth, using the "Tapial" method as construction technique. Lime mortars are fundamental to this type of wall, acting as an exterior protective covering or as an essential component of the inside of wall. These walls are an important element of the historical heritage of the city. They have lasted more than eigth centuries, a clear indication of their resistence and durability (Photograph 1).

Photograph 1. Wall of Don Gonzalo.

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Knowledge regarding the materials used can supply us with infonnation on the techniques employed and,

consequently, provide solutions for appropriate conservation steps.

2. METHODS AND TECHNIQUES.

The two main techniques have been used in the mortars analysis were x-ray diffraction an light microscopy. These techniques aided in the detennination of the composition of the material, the nature of the aggregate and the binder, the techniques used in the fabrication, and their state of conservation. The different sections of walls studied have been grouped together accordins to their period of construction: Alcazaba Cadima (prior to the XITH century), Alcazaba Gidida (Xlr~ . and the Albayzfn Arrabal. A mineralogical study of source quarry has also been carried out with the aim of contrasting data and clarifying the possible variations the material may have undergone from the time it was put into place in the

walls to the present.

Sample preparation.

Mineralogical study of the different sample was carried out after sifting into three fractions: particles larger than 31 .7 mm, held by a 1/4 ASTM screen); particles from 31 .7 mm to 0.074 mm, held by a n° 200 ASTM screen and particles under 0.074 mm passing throngh the n° 200 ASTM screen. Extraction of the clay fraction was carried by eliminating the carbonates in an initial stage. Afterwards, the samples wrere washed to eliminate chlorite and acetates residue. Finally, the clay was separated by

centrifugation.

- The powder method for the bulk sample was used to obtain the overall composition of various sample.

- The oriented aggregate method was employed to establish the mineralogical composition of the clay and

lime fractions.

Quantitative mineralogical analysis, studied by x-ray diffraction, can be affected by such factors as grain size, crystallinity, chemical structure and composition of the clay and lime fraction. Quantitative mineralogical analysis, studied by x-ray diffraction, can be affected by such factors as grain size, crystalinity, chemical structure and composition, isomorphic substitution, sample homogeity,

instrument condition, etc. Light microscopy analysis has centred mainly on the textural analysis of the mortars used in the construction of the walls, including porosity, aggregate and matrix composition, size and amount of the aggregate, and types of deterioration in the aggregate. In order to differentiate the calcite from the dolomite, the thin sections were stained with alizarine.

3. RESULTS

The results of the average bulk mineral composition from the different areas studied obtained by XRD are gathered in table 1. Quarry data are given in Figure 1 .

Table I. Mineralogical composition in walls.

Walls Calcite Dolomite Quartz Phyllosil.

COG 35 10 24 10

MSM 43 31 25 <5

MSA 31 38 25 5

Alcz. Cadima 40 34 25 5

Alcz Gidlda 31 45 24 0

COG. Wall of Don Gonzalo MSM. Wall of San Miguel MSA. Wall of San Antonio

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Nivel 1

Nivel 2

cueno ,. ...

Coto"""•

""' Nivel 3 Cuano

sa ...

Figure 1. Bulk composition in quarry

Analysis of the above results has led us to the following conclusions:

Fll01 ....

- The composition of the material from the walls and from the quearry is similar, consisting basically of calcite, dolomite, quartz, and traces of plyllosilicates and feldspars.

-In the walls, the results obtained show no significant variations from one stretch of wall to another. In the quarry, however, the phylisilicates content is higher, particular1y with respect to levels 2 and 3.

There are certain variation observed in the quarry: In level 1 there is a higher dolomite content and a significant drop in phyllosilicate. In level 2 the quartz content is high and there is a sharp fall in calcite with respect to the other two levels. In general, there is wide compositional range among the sample studied, both in the quarry (regardless of the leveO and in the walls. This fact is easily explained if we keep in mind that we are dealing with a conglomerate composed of rocks of very diverse nature and origin.

Mineralogical composition by fractions.

Given the compositional hererogeneity of the materials studied from both the quarry and the walls, we carried out a compositional analysis of the fractions. Taking into account that the aggregates comprising these mortars are more than 90% particles from 6.35 mm to 0.074 mm. it was thought appopriate to make a comparative study of this fraction. The aim was to calculate the amount of lime added to the mortars and concretes during the fractional composition of quarry and wall materials.

Ai:azaba Gid1da.

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· ~es - ~ c: 4' ,

Cerca D. Gonzalo

.:...; : .

Figure 2. Fractional mineralogical composition of walls material (particles larger than 31.7 mm).

Calcita 39'\.

Ca!c:'.3

37"1:

Cuarzo 21% Calcita

33%

Oolomrta 36%

Alz. Gidida

Cuarzo 35%

23~

Muralla S. Antonio

Feld +Filo 4%

Cuarzo 27%

eld +Filo . 40:~

Dolomita 36%

Muralla S. Miguel.

Cuarzo 43%

Cerca Don Gonzalo

Feld+= : •..

Figure 2. Fractional mineralogical composition of walls material (particles from 31.7mm to 0.074mm).

C&k:ll• ,,..

c.1cr1. ,. ..

(a:. ::

Alz. Gidida

Feldespa! :-s 4 0":

Muralla S. Miguel

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Cuarzo 17•,

Calcita · · i1/0;;1/r,:3!VS 60C:c-

5'-a

Feloes;iaics 5' c

t 1~ :z·: : 7"

Cerca Don Gonzalo

~·.

Muralla S. Antonio

C Ja·z: 15 ~'

. ; : .o~ :.:.::-: co

Figure 2. Fractional mineralogical composition of wall materials (particles under 0.074 mm)

C.a110

"" Nivcl I c:-no c-m ,.., ,. ..

Fv+F•O. C• io&a ... C &IC•I• ,_.,...

o--..~ ....

Nivcl 2

r .•. :.,s ....

O o.U"""11t•

o. ...... ..... Oolomtta - .....

c ...... o -.. Nivcl 3 ....

•loa • F,lds r ~ . : • • • ,.. . ... (.41:1:11 ,.. ..

0.01-· ,,...

Figure 3. Fractional mineralogical composition of quarry material (left: particles larger than 31.7 mm, middle: particles from31 .7 to 0.074 mm, right: particles under 0.074 mm)

Fll +F.d• . ...

, + F•lds ...

·~· • F•IOs , ..

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If may be noted that, in the quany, the composition of the coarse fraction is very similar in all three levels. The major components are carbonated pebbles (calcite and/or dolomite). The feldespars and phyllosilicates occur in very small amounts, and the latter consist primarity of micas, not clays. This fraction usually surpasses 30% in levels 1 and 2 while in some sample from level 1 it can surpass 60%.

- The intermediate fraction is very reduced in carbonates. - The fine fraction shows an increase in phyllosilicates in levels 2 and 3.

In the walls it can be surmised that, when preparing the mortars, the fine fraction was not included as there is no clay in the mortar. Analysis of the fraction under 0.074 mm reveals that it mainly comprises calcite (close to 60%). Although we assume that this fraction was omitted from the mortar preparation, the mineralogical data indicate there to be significant amount of quartz and dolomite in the walls, coming from the aggregate. We should also note the existence of traces of gypsum (secondary mineral), even though it does not appear in the graphs. The mineralogical composition of these patinas is very homogeneous the different stretches of wall analysed (tablell)

Tablell. ineralogical composition of the superficial patinas.

Patinas Calcite Dolomite Quartz Oxalates

Average values 74 5 20 traces

Approximately 90% of the aggegates comprising the mortars in these materials consist of particles from 6.35 mm to 0.074 mm. A comparative study consist has been carried out on the quany and the wall establishing the possible contribution of lime. The results indicate that regardless of the construction period, lime comprises approximately 20% of the mortars.

4. MINERALOGICAL AND PETROGRAPHIC ANALYSIS BY LIGHT MICROSCOPY.

Light microscopy analysis reveals that these mortars and/or "lime concretes" basically consist of aggregates of direrent type (rocks fragments), evenly distributed throughout the putty and the binder, which is calcitic, as demonstrated by staining of thin sections with alizarine. The binder is either micrite or spar. Calcite frequently recrystallises into druses, forming elongated neelelike crystals tha grow perpendiculary from the walls of pores or fractures. The crystals are very birefrigent with typical carbonate iridescence. In other cases, the sparitic cement is granular and grows randomly, filling pores or fisures, and in some cases small amounts of clay can be detected. In the binder matrix we have observed small spheres of micritic calcite with very well-defined borders not having the same caracterics as a normal aggregate (Photo 1). Two possible origins could account for these nucleii or "inclusions": 1) They could originate from the lime, being uncooked remains of carbonate, or 2) they could be either non-carbonate porttlandite or calcium oxide nucleii that were not appropriately put out. The contact of these aggregate with the binding is even at times, but it is also common to find edges with crystal growth and reactive processes (cationic interchange). This occurrence is apparent in the increase of fracturing and porosity at the edges of these crystals and/or rocks (Photo 2). The precipitation of minerals as a product of weathering along fissures and in pores is quite frequent. Most of the fissuring is retractive and is localised in zones with less aggregate. The nature of the weathering minerals cannot be clear1y established under light microscopy due to their crytocrystalline nature. Photograph 3 shows gypsum crystals growing perpendicularly from the surface of a crack, but it is also common to find them forming part of a microcrytalline putty nex to calcite.

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Photo 1. Small spheras of micritic calcite with very well-definid borders (Crosser polars).

Photo 2. Edges with crystal growth and reactive processes (crosser polars).

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Photo 3. Gypsum crystals growing perpendicular1y from the surface of crack (crosser polars).

5. CONCLUSIONS.

The XRD compositional date from the quarry and walls reveal that aproximately 20% of the mortars comprised added lime, which was unevenly distributed according to the types of wall which it was used. The material used for making the mortats and the "concretes" was screened to eliminate the under 0.074 mm fraction. Light microscopy analysis has served to corroborate the resistence and durability of the materials due to the following characteristic: - In most cases the binder has carbonated perfectly, showing significant development of calcite crystals

filling pores and sealing fissures. - The aggreates have been well preserved, as can be noted by the fact that reaction processes, where they

occur, affect very superficial areas and do not substantially affect the quality and resistance of the material.

- The lime production was not particulary careful, as indicated by the presence of lumps of micritic calcite, which are indicative of inadequate quenching or problems in obtaining homogeneous heating possibly due to a polarised heat source.

- Finally, the appearance of secondary minerals linked to weathering processes is in the initial stages only. Nervertheless, this occurrence should be taken as an indication of the urgent need to control any phenomena that may provoke or aid wearthering.

Environmental pollution and the use of inadequate conservation materials such as cement mortars are two factors in particular need of attention.

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