Binder characterisation of mortars used at different ages in the San Lorenzo church in Milan

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

Available online at www.sciencedirect.com

www.elsevier.com/locate/matchar

Binder characterisation of mortars used at different ages in the San Lorenzo church in Milan Luca Bertolini⁎, Maddalena Carsana1 , Matteo Gastaldi2 , Federica Lollini3 , Elena Redaelli4 Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, via Mancinelli 7, 20131 Milano, Italy

AR TIC LE D ATA

ABSTR ACT

Article history:

The paper describes a study on the mortars of the basilica of San Lorenzo in Milan, which

Received 16 November 2012

was carried out to support an archaeological study aimed at dating and documenting the

Received in revised form

construction techniques used throughout the centuries. The church, which was founded

2 January 2013

between the 4th and 5th century, at the end of the period when Milan was the capital of the

Accepted 21 March 2013

Roman Empire, was subjected in time to extensions, collapses and reconstructions that lasted until the Renaissance period and even later on. Thanks to the good state of

Keywords:

conservation, San Lorenzo church is a collection of materials and construction techniques

Binder

throughout a period of more than a millennium.

Magnesian lime

Mortars were investigated in order to compare the binders used for structural elements

Cocciopesto

built in different historical ages. From an archaeological study, samples of mortars

Gypsum

attributed to the late Roman period, the Middle Ages and the Renaissance were available.

Ancient mortars

The binder of each sample was separated by the aggregates and it was characterised on the basis of X-ray diffraction analysis, thermogravimetric analysis and scanning electron microscopy. Constituents of the binder were identified and their origin is discussed in order to investigate if they could be attributed to the original composition of the binder or to possible alteration in time due to atmospheric pollution. Results show that, even though the binder is mainly based on magnesian lime, there are significant differences in the microstructure of the binding matrix used in mortars ascribed to the different historical periods. In the Roman period, in correspondence of the structural elements that required higher strength, also hydraulic cocciopesto mortars were detected. Gypsum was found in most samples, which was maybe added intentionally. © 2013 Elsevier Inc. All rights reserved.

1.

Introduction

Characterisation of masonry and mortar is an essential step in the study of the sequence of the phases that have led to the construction of a historical building and to understand the different construction techniques used in time [1–7]. In

buildings that have undergone several restoration works during the centuries, materials, construction practices and technologies may vary according to the construction period and local uses. By studying mortars, materials used in different restoration or reconstruction works may be distinguished and the knowledge on the construction technologies

⁎ Corresponding author. Tel.: + 39 02 2399 3138; fax: + 39 02 2399 3180. E-mail addresses: [email protected] (L. Bertolini), [email protected] (M. Carsana), [email protected] (M. Gastaldi), [email protected] (F. Lollini), [email protected] (E. Redaelli). 1 Tel.: + 39 02 2399 3142; fax: + 39 02 2399 3180. 2 Tel.: + 39 02 2399 3114; fax: + 39 02 2399 3180. 3 Tel.: + 39 02 2399 3144; fax: + 39 02 2399 3180. 4 Tel.: + 39 02 2399 3115; fax: + 39 02 2399 3180. 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.03.008

10

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

for masonry could be extended [8–10]. Furthermore, the study of ancient mortars has a major role in relation to the assessment of the state of conservation of masonry and the design of restoration works [11–14]. Binders used for masonry or plaster mortars are usually divided in air-hardening and hydraulic binders. Gypsum and lime are classified among the former, while hydraulic limes, which can harden in moist environments, belong to the latter (cements used nowadays have been developed only since the 19th century) [15]. Nevertheless, the distinction between the two classes is not straightforward. As a matter of facts, many types of binder were used in different ages in order to obtain mortars with specific properties and, specifically, the stability in wet environments (this is an essential requisite to allow long-time resistance to outside exposed materials). In the literature, several examples of materials with “hydraulic” properties are reported, which were obtained, for example, by burning at specific temperatures limestone mixed with clay minerals (either deliberately or unintentionally added) or by mixing slaked lime with substances able to provide hydraulic properties to the mortar, such as pozzolana [16] or ground bricks (cocciopesto) [6,14,17–19]. The study of binders of ancient mortars is complex, in that the analysis is carried out on a material that has undergone transformations of various nature throughout the centuries. In addition to those due to the phenomena of setting and hardening (e.g., the chemical reactions of hydration of gypsum or carbonation of lime), further alteration and transformation in time are promoted by the environment as well as pollution [20–22]. Different types of analysis and observations can be made on mortar samples taken from historic buildings that can provide useful data for the characterisation of the source material and the study of alterations or degradation [23–28]. However, individual analysis can only provide partial information related to specific properties of the binder (for example, the presence of certain chemical compounds or the porosity). The identification of the original features of the binder and its alteration, should be based on the evaluations that gather information of various kinds (the “history” of the building, knowledge of building technologies in different periods, etc.) and therefore requires a multidisciplinary approach, which should integrate the study of materials with archaeological and historical studies. This paper deals with the mortars of the church of San Lorenzo in Milan [29–32]. The basilica has Roman origins; some sources attribute its construction to the second half of the 4th century AD. Since then it was subjected to many reconstructions of various extents, especially in the 11th and 12th centuries, when it suffered several fires and collapses, and in the 16th century. Several restoration works were made in later periods, documented in reference [29], which was written during the restoration carried out from 1937 to 1944 by Chierici. The church, thanks to its good state of preservation, is a “treasure” that collects the materials and construction techniques used in the Milan area from the times of the Romans until the nineteenth century. It is, therefore, also a record of the binders used in mortars in different historical periods. Few publications deal with binders used in the basilica of San Lorenzo. Chierici et al. [29] sometimes used vague terms such as “lime”, “strong lime” (which should draw the attention

to some hydraulic properties of the mortar), “lime and cocciopesto”, etc. They also cited a previous study, which reported some visual observations on the mortars and evaluations on the content of lime and silicate (without, however, indicating the procedure of analysis). Later, in the 1980s, samples of mortar were analysed for chemical and petrographic composition, primarily aimed at evaluating the adhesion between mortar and brick [33]. Studies on some samples of mortar collected from the church were also conducted by other authors [34,35]. Nevertheless, no comprehensive studies have been carried out aimed at the complete characterisation of the binders used in different ages and construction phases. Results of tests carried out in the framework of an extensive study aimed at the characterisation and dating of the various buildings of the church are discussed. Based on archaeological and historical studies, hypotheses were formulated on the dating of the various portions of the building walls [36]. In this context, samples were taken for masonry mortars of various phases of construction [37]. Samples of mortar were collected from portions of masonry attributed to the late Roman period (up to the 4th century AD), the Middle Ages and the Renaissance. The characterisation of the binders found in the mortars of different historical periods is discussed.

2.

Materials and Methods

Small fragments of mortars were initially observed with the naked eye and with a stereo-microscope, documenting their size, the colour of the binder, the size and colour of the aggregates. The cohesion of the mortar was then empirically evaluated, following a procedure described in an Italian Recommendation (Normal 12/83), which provides a qualitative assessment of the behaviour of mortar following the application of a gentle pressure among fingers, suggesting the classification: very tough when the specimen does not break; tough when the specimen breaks without crumbling, friable when the sample crumbles; incoherent when the sample is inconsistent to the touch. Subsequently the samples were gently disrupted, obtaining both fragments of mortar, used for observations at the scanning electron microscope, and powders, used for thermal analysis and X-ray diffraction. The powders were obtained by separating manually the binder matrix from the aggregate particles. Initially coarse particles of the aggregates, which were visible to the naked eye, were separated, then the sample was further crushed and fine aggregates were separated with the help of a stereo-microscope. The remaining powder was ground manually; from the fraction retained by a sieve of 75 μm, further fractions of fine aggregate were separated and then the residual fraction was ground to pass completely through a sieve of aperture of 75 μm. X-ray diffraction (XRD) analyses were carried out on the powders using a diffractometer with CuKα radiation and a scan rate of 2.4°/min. The identification of crystalline compounds was performed by a software (Philips X'Pert SW) and then verified manually with the JCPDS database. Thermogravimetric analyses (TG) were performed on the same powders used for XRD analysis. The samples were initially brought to 70 °C and maintained for 20 min with a helium flow of 20 l/min.

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

Subsequently, a temperature scan with a rate of 10 °C/min was started, maintaining the flow of helium, up to 1200 °C. Scanning electron microscope (SEM) observations were carried out on gold spattered fragments of mortar. The microscope was equipped with an EDS X-ray spectrometer.

3.

Results and Discussion

The sampling of mortars was carried out in combination with the evaluation of the texture of walls and construction activities. The samples taken in different parts of the church and its chapels were grouped according to different stages of construction, which had previously identified with a stratigraphic analysis [36]. 19 samples representative of different areas related to the initial phases of construction of the basilica (in the Roman period, until the 5th century AD), 9 samples of the Medieval period and 13 samples of the Renaissance period were identified. Each sample was named with an acronym consisting of two letters that identify the body of the building of the church from which the sample was taken, followed by a sequential number. Fig. 1 depicts the position of the different samples. Table 1 lists the samples, with a brief description.

3.1.

Visual Observation

The samples were constituted by fragments of mortar of a few tens of mm, as shown for example in Fig. 2a. Only in one case (sample NO2, Fig. 2b) a core of diameter 40 mm and length 100 mm was available, which had been taken from the walls of the North-West tower during the restoration work by Chierici [29] and had been preserved in the church. Prior to crushing, the samples were observed visually and at the

11

stereo-microscope. The last columns of Table 1 summarise the main characteristics of each sample. Some of the mortars attributed to the Roman period contained fragments of ground brick (cocciopesto), which reached a size of 5–6 mm; Fig. 3 shows, for example, the observation at the stereomicroscope of a polished surface of the mortar of sample NO2. The binder of the mortars was typically white in colour; only in some samples with fragments of ground brick (NO2, S14 and SS3) also the binder had a pink colour, probably due to the fact that, in these materials, the brick was mixed with the binder in the form of fine particles. A grey surface deposit was observed on several samples. For these specimens fragments containing the deposit were analysed separately from the inner layers. In the case of sample SI4 (taken from a foundation) instead of the deposit, a surface layer of several mm in which the mortar is grey and friable was observed. Most of the samples of the Roman period were classified as tough (Table 1). The medieval samples included greater quantities of powder than samples from the Roman period. This feature is related to their lower cohesion; in fact, the Medieval samples were classified mainly as friable and one (CC2) was even incoherent to the touch. Only three samples (SE2, SO3 and TC4) were evaluated as tough (Table 1). The medieval samples are darker in colour than the Roman ones, and only sample TC2 is a white mortar, whilst in other cases the colour varies between grey, brown and yellow. No sample is classified as cocciopesto, and this is consistent with the abandoned use of cocciopesto mortars during the Middle Ages. Four samples (CC2, SA5, SE1 and TC3) had a grey surface deposit; their location (Fig. 1) does not provide useful information to explain such deposit, as three were taken outside, while sample SA5 was indoor. The Renaissance samples were more similar to those of the Roman period compared to medieval ones, especially as

Fig. 1 – Map of the basilica and approximate location of samples attributed to the Roman (italics), Middle Age (underlined) and Renaissance (bold) periods.

12

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

Table 1 – Samples and summary of results of visual observation and cohesion classification. Location Cittadini Chapel

Church (San Lorenzo)

Period Roman Middle Ages Renaissance Roman Middle Ages

Renaissance

North-East tower

Roman Renaissance

North-West tower

Roman

South-East tower

Renaissance Middle Ages

South-West tower

Middle Ages

Sant'Aquilino Chapel

Roman

Sant'Ippolito Chapel

Middle Ages Renaissance Roman

San Sisto Chapel

Roman

Renaissance

Sample

Cocciopesto

Matrix colour

Surface deposit

Cohesion

CC1 CC2 CC3 INT1 TC1 TC2 TC3 TC4 TC5 TC6 TC7 TC8 TC9 TC10 NE1 NE2 NE3 NE4 NO1 NO2 NO3 SE1 SE2 SO1 SO2 SO3 SA1 SA2 SA3 SA4 SA5 SA6 SI1 SI2 SI3 SI4 SS1 SS2 SS3 SS4 SS5

No No No No No No No No No No No No No No No Yes No No Yes Yes No No No No No No No No No Yes No No No No No Yes No No Yes No No

White Brown Grey White White White Brown Brown Grey White White White White Grey White White Grey Grey White Pink Grey Grey White-Grey Yellow Yellow Grey White White White White Brown Brown White White White Pink White White Pink White Brown

No Yes Yes Yes No No Yes No No No No No No No Yes Yes Yes Yes Yes No Yes Yes No No No No No No No Yes Yes No No No No Yes No No Yes No No

Tough Incoherent Tough Tough – Friable Friable Tough Friable Tough Tough Very tough Very tough Friable Very tough Tough Very tough Very tough Very tough Very tough Very tough Friable Tough Friable Friable Tough Friable Brittle Friable Tough Friable Friable Tough Friable Friable Friable Brittle Tough Tough Tough Friable

regards colour and cohesion. The colour of the binder matrix was predominantly white or grey, whereas the cohesion could be classified as tough or very tough, except for three samples, which were evaluated as friable. Four samples, three of which were taken outside and one inside, had a grey surface deposit.

Fig. 2 – Samples SA3 (a) and NO2 (b).

Fig. 3 – Stereo-microscope image of a polished surface of cocciopesto mortar of sample NO2.

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

3.2.

X-ray Diffraction Analyses

X-ray diffraction (XRD) analyses were carried out on the powders, in order to identify crystalline compounds in the binding matrix. Fig. 4 shows, as an example, the results obtained on a sample of white mortar (SA3) and on one of pink cocciopesto mortar (NO2), both of the Roman period, as well as a Medieval sample (SO2). In sample SA3 three main compounds were identified: calcium carbonate with the crystalline structure of the calcite (C), silica with the structure of the quartz (Q) and gypsum (G). Several peaks of lower intensity showed the likely presence of a silico-aluminate of potassium (muscovite, M). The calcium carbonate has to be attributed to the carbonation of calcium hydroxide, i.e. to the reaction with carbon dioxide which is responsible for the hardening of lime mortars; after about 15 centuries from the date hypothesised for the mixing of the mortar, the original crystalline slaked lime, Ca(OH)2, cannot be found [38]. Fig. 4 also clearly shows the presence of gypsum in sample SA3. This may have been originally added to the mixture of the mortar; the practice of mixing lime and gypsum, however, is poorly documented in the bedding mortars for masonry, especially in the region of Milan. Gypsum may also have been introduced in the mortar as an aggregate. Finally, it may be the result of alteration of calcium compounds due to the effect of sulphur-based pollutants (definitely present, at least since the last century, in the centre of Milan). Quartz and muscovite are probably contained in small fractions of the aggregate that remained in the powder after manual separation. Sample NO2, consisting of a cocciopesto mortar in which the binder is pink in colour, shows a diffractogram different from the previous one (Fig. 4). Peaks of calcite and quartz are still evident. The latter, in this case, can be attributed to the presence of minute fragments of bricks. No other crystalline compounds were detected. Compared to sample SA3, however, a slight increase in the background can be observed, especially in the range of 2θ between 15° and 35°, which suggests the presence of non-crystalline compounds. Medieval mortars are distinguished from those of the Roman and Renaissance periods for the presence of a marked raise in the background in the range of 2θ below 15°, which identifies the presence of an amorphous compound. For

Fig. 4 – XRD analyses of samples SA3, NO2 and SO2.

13

example, Fig. 4 shows the results obtained on sample SO2. This feature is similar to that observed in the study of magnesian mortars reported by Atzeni et al. [2], who attributed the raising of the XRD pattern to amorphous compounds of magnesium. Table 2 summarises the results of XRD analyses performed on different samples, indicating “yes” for the compounds whose characteristic peaks were clearly identified and “?” for those whose presence was likely, but either the intensity of the peaks was low or not all the peaks could be clearly detected. Calcite (C) and quartz (Q) were detected in all the samples, and their peaks were always those of greater amplitude. Gypsum (G) was found in many samples, especially of the Roman and Medieval periods. There are often silico-aluminates in the form of muscovite (M) and albite (F), especially in the Medieval and Renaissance samples. In several samples, especially of the Renaissance period, dolomite (D, CaCO3·MgCO3) was found. This compound indicates the use of magnesian limestone. Only in some cases peaks that identify other types of carbonates, i.e. magnesite (MgCO3) and aragonite (CaCO3 with a rombic crystal structure), or other crystalline compounds were detected (Table 2). Some of these crystalline compounds might also be attributed to the presence of residual fragments of aggregates in the powders analysed and not to the fraction of binder (in particular, quartz, muscovite, albite and dolomite).

3.3.

Thermal Analyses

Fig. 5 shows an example of thermogravimetric analysis (sample SA3), showing the loss of mass of the sample as a function of temperature (TG) and its derivative (dTG). Considering the whole set of analysed samples, four relevant temperature ranges can be observed in the TG curves, in correspondence of which mass losses were evaluated. The two outer intervals, that is, for temperatures below 150 °C and above 1000 °C, were associated with gypsum. The first interval corresponds to the loss of water for the formation of hemihydrate and subsequently anhydrite (collectively: CaSO4·2H2O → CaSO4 + 2H2O), the second is associated to the decomposition of anhydrite (CaSO4 → CaO + SO3). Through the loss of mass in these two intervals the percentage of gypsum present in the sample was calculated. For example, in sample SA3 (Fig. 5) from the loss of 2.8% by mass observed around 100 °C a content of 13.4% of gypsum can be calculated, while from the loss of 6.9% at temperatures above 1000 °C a content of gypsum of 14.8% can be estimated. Therefore, in this sample, the content of gypsum can be estimated at about 14% (in Table 2 the average between the values estimated from the mass loss in the two temperature intervals is reported). It should be observed that the decomposition of calcium sulphate took place in the range of temperature of 1000–1200 °C in the tests carried out with helium flow (i.e. at temperature lower than those expected in air). Specific control tests showed that, even with standard samples of gypsum, the transformation occurred at higher temperatures in the presence of a flow of air. This phenomenon is not reported in the literature, because the TG analysis on ancient mortars generally does not exceed 1000 °C. In all the samples a marked mass loss in the range 560– 700 °C was observed. This was attributed to the decomposition

14

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

Table 2 – Summary of results of X-ray diffraction and thermogravimetric analyses. Period

Roman

Middle Ages

Renaissance

Sample

CC1 INT1 NE1 NE2 NO1 NO2 SA1 SA2 SA3 SA4 SI1 SI2 SI3 SI4 SS1 SS2 SS3 SS4 TC1 CC2 SA5 SE1 SE2 SO1 SO2 SO3 TC2 TC3 TC4 TC5 CC3 NE3 NE4 NO3 SA6 SS5 TC6 TC7 TC8 TC9 TC10

XRD

a

TGA

C

Q

G

D

M

F

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Yes ? – Yes Yes – Yes Yes Yes Yes ? Yes – ? Yes Yes Yes – ? Yes Yes Yes – Yes Yes – – Yes Yes Yes – ? – Yes – Yes – – – – –

? ? Yes ? – – – ? – Yes – – – ? Yes Yes Yes Yes Yes Yes Yes – – – – Yes – – – Yes Yes Yes Yes Yes – Yes – Yes Yes Yes Yes

? – ? – – – ? ? ? Yes – – – ? ? Yes – – Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes – Yes Yes Yes Yes Yes – – Yes – –

Yes Yes Yes Yes – – – ? – Yes – ? ? Yes Yes Yes Yes – Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

b

Others

% CaCO3

% Gypsum

– N Di – – – – A – – – – – – – Di – – – B Di, Di B – Di, L Di Di, Di, – N L L L Di, L L L L – –

44.7 62.4 39.5 39 61.3 32.7 59.2 49.5 53 48.8 – – – 32.9 57.4 29.7 41.3 – 62.9 12.3 27.9 28.6 36.8 31.8 24.5 18.8 22.9 22.2 22.5 26.8 17.3 28.4 38.6 18.6 26.3 4.1 35.2 37.9 26.1 52.0 41.8

9.6 3.5 n.d. 17.5 3.2 3.0 10.9 4.9 14.1 2.5 – – – n.d. 4.4 12.6 17.0 – n.d. 19.2 10.8 10.7 n.d. 5.9 2.6 n.d. n.d. 6.5 5.7 6.6 8.3 n.d. 1.8 8.3 4.8 12.2 n.d. n.d. n.d. n.d. n.d.

L

B

R L, R

L

a C = calcite (CaCO3), Q = quartz (SiO2), G = gypsum (CaSO4·2H2O), D = dolomite (CaMg(CO3)2), M = muscovite (KAl2(Si3Al)O10(OH)2), F = albite (NaAlSi3O8), N = magnesite (MgCO3), A = aragonite (CaCO3), Di = diabantite (Mg2.8Fe2 + 2.2Fe3 + 0.8Al1.1)O10.8(OH)7.7), L = leucite (K(AlSi2O6)), B = birunite (8.5CaSiO3 + 5.5CaCO⋅3CaSO⋅415H2O), R = roedderite ((Na,K2.5(Mg,Fe)4.9(AlSi)12O30). b n.d. = not detected: – = sample not analysed.

of calcium carbonate (CaCO3 → CaO + CO2), for example in sample SA3 a mass loss of 23.3% was measured in this temperature range, which allowed to calculate a calcium carbonate content of about 53%. Finally, in all the samples a progressive loss of mass in the range 150–560 °C was measured. This variation may not be attributed to any specific transformation. In this range the sample may experience various phenomena such as the decomposition of various magnesium compounds, the loss of bound water of silicates, the transformation of organic substances, etc. The loss of mass of the various samples in this interval varied between about 4% and 12% and no significant correlations could be observed with other characteristics of the samples. In the majority of the curves of the samples of the Renaissance period, a mass loss

varying between 3.4% and 8.4% was observed in a restricted range of temperature of 450–520 °C, which is clearly shown by a peak in the dTG curve (as in the example of Fig. 6 reporting results obtained on specimen NO3). Mass loss in this temperature range is attributed to the decomposition of magnesium carbonate [39]. The last two columns of Table 2 summarise the content of calcium carbonate and gypsum detected in the samples. Calcium carbonate (in the form of calcite as shown by XRD analyses) is the compound present in greater quantities. Its content varies between 30% and 63% by mass in the Roman samples, from 12% to 37% in Medieval ones and from 17% to 52% in Renaissance samples (with the exception of sample SS5, in which only 4% was observed). The lower content of

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

Fig. 5 – Thermogravimetric analysis of sample SA3.

calcium carbonate that characterises, in general, the Medieval samples, associated with the presence of the raise in the background of the XRD diffractograms, could suggest the use of limes with a higher magnesium component with respect to those used in the Roman period. Nevertheless, TG analyses did not provide clear evidence on the presence of magnesium compounds; only in sample TC5 a net loss of mass in the range 400–500 °C was observed that can be attributed to the decomposition of magnesium carbonate. The presence of gypsum was found in most samples, particularly in the Roman and Medieval periods. The content ranged between 2.5% and 19% by mass; only in samples in which no gypsum peaks were detected in the XRD analysis the mass loss corresponding to the dissociation of gypsum was accordingly absent in the TG curve. A correspondence between the presence of gypsum and the classification of a friable cohesion can be observed. Particularly, Medieval sample CC2 in which a percentage of 19% by mass of gypsum was detected, was even classified as incoherent. In the Renaissance samples, gypsum was found in only five samples, with a percentage ranging between 1.8% and 12.2% and not always the presence of this compound is in agreement with the results of XRD. Samples CC3, NE4 and SA6 contained gypsum according to TG, but not for XRD, vice versa in sample NE3 gypsum was detected by XRD analysis, but not by TG.

Fig. 6 – Thermogravimetric analysis of sample NO3.

15

By comparing Tables 1 and 2 it can be observed that, for these samples, there is no correlation between the cohesion classification (Table 1) of the sample and the content of gypsum detected by thermogravimetric analysis (Table 2). Results of TG analysis do not help to clarify the origin of gypsum. For example, the fact that it was not found in samples NE1 and SI4, exposed to indoor atmosphere, may support the hypothesis that it formed after the construction as a result of reaction with pollutants (from which the internal samples would have been more protected). However, it should be noted that sample NE1, though it was collected inside the North East tower, has to be considered in contact with the external environment, since the tower has no frames. In addition, the exposure conditions are not different from those of sample NE2, which had the highest content of gypsum. In mortar NE2 analyses were carried out on the binder removed both from the core of the sample and from the surface covered by the grey deposit and there were no appreciable differences. Gypsum, finally, was found in other samples placed indoor (for example, NO1) and especially in the core NO2 taken from the pillar of the tower North-West (in this case a sample collected in depth in the wall was analysed). Furthermore, samples of different historical periods were compared to verify if gypsum was present in the same part of the building, but samples both with and without gypsum were found in all the structural elements of the church.

3.4.

Scanning Electron Microscopy

The observations with the scanning electron microscope, which were carried out on fragments of mortar that also contained aggregates, allowed us to observe the morphology of the binding matrix and, by means of elemental microanalysis EDS, to identify the chemical elements of its components. Fig. 7a, for example, shows the matrix around an aggregate of sample SA3. This is formed by particles of size lower than 10 μm, constituted predominantly of calcium carbonate, as evidenced by the presence of calcium, oxygen and carbon in the EDS microanalysis of Fig. 7b. The particle indicated by spot 2 in Fig. 7a is a siliceous aggregate (Fig. 7c). The most common morphology of the particles of calcium carbonate is shown in Fig. 7a. This is compatible with the crystalline structure of hexagonal calcite, in which calcium carbonate produced by carbonation of lime is normally found. The presence of calcite is also in agreement with the XRD analysis, which indicates the presence of this crystalline compound in all samples. Only in some cases particles of calcium carbonate with shape that resemble the rombic structure of aragonite could be observed, as for example in Fig. 8a. EDS analyses, as in the example of Fig. 7b, show the presence of magnesium in the binding matrix. It can therefore be assumed that a binder obtained by burning limestone that in addition to calcium carbonate contained magnesium carbonate was used. This type of limestone is widespread in the region around Milan [3] and therefore its use for the mortar is likely. Also the presence of dolomite observed in the XRD analysis of some samples is in agreement with the hypothesis that magnesium-containing minerals were used in the manufacturing of the mortars.

16

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

Fig. 7 – SEM image of a binder made of calcite (CaCO3) particles embedding a siliceous aggregate in sample SA3 (a) and EDS analysis in spots 1 (b) and 2 (c).

When magnesium concentration increases at the EDS analysis, the morphology of the particles of the hydration products (which later had been subjected to carbonation) changes and particles no longer have a well-defined shape, as shown in Fig. 8b. It is not easy to identify the type of compound corresponding to particles rich in magnesium. The XRD analysis shows the presence of dolomite and diabantite (Table 2), but no other compounds of magnesium are present. Several authors have shown that magnesium can be found in the binder of ancient mortars in the form of magnesium hydroxide (Mg(OH)2), magnesite (MgCO3) or hydromagnesite (4MgCO3·Mg (OH)2·4H2O) [2,8,10]. The presence of magnesium hydroxide is unlikely because of the long time passed since the production of the mortar; XRD analysis, however, did not show even the peaks of either magnesite or hydromagnesite. It may, therefore, be reasonable to assume that the particles rich in magnesium shown in Fig. 8b are predominantly formed by a mixture of carbonates of calcium and magnesium with a low degree of crystallinity. The binder in the area rich in magnesium tends to show a more compact structure; this observation is in agreement with the higher strength and better durability that is generally attributed to magnesian lime mortars [2]. Observation at the scanning electron microscope also allowed the identification of particles of gypsum inside the binding matrix. These, in general, are localised in certain areas of the binder, for example Fig. 8c shows, on the left part,

a particle consisting of gypsum crystals (as shown by EDS analysis) and on the right part a compact area consisting of calcium and magnesium carbonates. Only in the Medieval sample SE1 there was a widespread distribution of lamellar particles of gypsum (Fig. 8d). In some cases the high amount and morphology of the gypsum particles may suggest that the presence of gypsum is not voluntary, but it is the result of chemical changes due to atmospheric pollutants. However, the distribution of the zones with gypsum particles in the various samples appears to be random and no greater concentration in the surface areas was observed (as it might be expected in the case that gypsum was produced by chemical alterations due to pollutants in the atmosphere). This still leaves doubts on its origin. The presence of silicon and aluminium is an interesting aspect that emerged from EDS analysis of the areas in which there are particles of calcareous binder, as shown in Fig. 7b. Since the analyses are made on spots and, therefore, directly on the particles that constitute the binder, it should be assumed that these elements have been incorporated in the reaction products. It can therefore be assumed the presence of silico-aluminates and, therefore, of compounds able to confer certain hydraulic properties to the binder. These compounds typically are non-crystalline and, therefore, cannot be detected with X-ray diffraction analysis. In cocciopesto mortars, which contain small particles of ground bricks, the presence of silico-aluminates of calcium can be attributed to the pozzolanic

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

17

Fig. 8 – SEM image of different microstructural features observed in the binder of various samples: (a) rombic particles of CaCO3 in sample NO2, (b) particles rich in magnesium in sample NE1, (c) particles rich in gypsum in sample SS2, (d) lamellar particles of gypsum in sample SE1.

reaction of lime with the silico-alluminate compounds that constitute the burnt clay of the brick. This reaction can take place in the mass of the binder provided the burnt clay is introduced in the mix in the form of very small particles. In fact, the presence of fragments of brick with size of the order of millimetres (such as those shown in Fig. 3) can lead to the formation of reaction products with lime only in correspondence of their surface. In this case, the brick may have the function of “good aggregate” since it enhances the adhesion to the binder, but the reactions with the binding matrix are limited to a neighbourhood of a few μm from its surface [6,12,14]. It was observed that in samples of cocciopesto mortar with a pink matrix the ground brick was introduced in the binding matrix also in the form of very fine particles, probably as a result of grinding. For example, Fig. 9 shows the polished surface of a portion of sample NO2; EDS microprobe maps of Ca, Si and Al allow a clear identification of burnt clay particles with size of the order of μm in the regions poor in calcium and rich in silicon and aluminium. The same maps show the presence of silicon and aluminium in the matrix around these particles, suggesting that reaction occurred between lime and burnt clay particles leading to the formation of the silico-calcium-aluminate hydrates. The observation of a fracture surface of sample NO2 even clearly showed the presence of silico-aluminates with a fibrous structure (Fig. 10) similar to that which can be observed in the hydration products of modern Portland cement. These results show that, at least in the case of mortars with a pinkish matrix, the addition of ground bricks (in place of pozzolana, which was not available in the region around Milan)

resulted in the production of a binder with hydraulic characteristics. The use of this binder in the parts of masonry that are most loaded (for example, the NO2 sample was extracted from a pillar of the North-West tower), suggests that their use was intentional. The presence of silicon and aluminium, however, is not limited to “pink” mortars, but it is evident in all samples, even those which do not appear as cocciopesto (i.e. do not show fragments of brick visible to the naked eye). Unfortunately, it was not possible to give a clear explanation for the presence of these elements. Silico-aluminates may have been inadvertently introduced in the raw materials, e.g. using marly lime. However, the literature on lime binders in the area around Milan shows that magnesian-lime binders are used, but excludes the presence of hydraulic compounds and thus the presence of silico-aluminates [7,9]. Alternatively, it can be assumed that even for the mortars which have a whitish colour a minimum addition of any substance with pozzolanic activity was made, such as particles of very finely ground cocciopesto (which later may have been completely incorporated in the reaction products of the binder) or of clays (e.g. burnt kaolin, the use of which is documented in the region around Genoa by Boato and Mannoni [1]).

4.

Conclusions

Analyses carried out on mortars collected from San Lorenzo church and attributed to different historical periods (Roman, Middle Ages and Renaissance) showed the presence of

18

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

Fig. 9 – Polished surface of cocciopesto mortar of sample NO2 (a) and EDS maps of calcium (b), silicon (c) and aluminium (d).

Fig. 10 – Example of silico-aluminates with fibrous structure in the binder of sample NO2: SEM micrograph (a) and EDS analysis in spots 1 (b) and 2 (c).

MAT E RI A LS CH A R A CTE R IZ A TI O N 8 0 (2 0 1 3) 9–2 0

magnesian-lime binder. At present, the binding matrix consists mainly of calcite (CaCO3), although also the presence of magnesium compounds was confirmed. Whilst within the same historical period samples showed similar features, significant differences were observed in the microstructure of the binding matrix used in mortars ascribed to the different historical periods. The binders of the mortars of the Roman period, which were predominantly whitish, had a high amount of calcite and showed good cohesion. The Medieval samples were characterised by darker colour and lower cohesion; the possible presence of amorphous compounds of magnesium was detected. Samples of the Renaissance period, which had a colour between white and grey, in general had good cohesion and showed the presence of magnesium carbonate in the binder. Only among Roman samples detected in the most loaded parts of the structures, pinkish cocciopesto mortars were recognised. The presence of calcium silico-aluminates was detected in these samples, due to the addition of ground bricks, which reacted with lime to produce silico-aluminates and, therefore, compounds with hydraulic properties. Nevertheless, silicon and aluminium were detected in the binder matrix of all the Roman, Medieval and Renaissance samples, showing the presence of silico-aluminates even where the binder was whitish and was composed predominantly of calcium carbonate, which could be either intentionally added to give a certain hydraulic behaviour to the mortar or present as impurities in the raw materials used to produce the lime. The binders of most of the mortars contained a substantial percentage of gypsum whose particles were randomly distributed in the binding matrix and their concentration did not increase in correspondence of the surface layers or in samples exposed directly to the outside atmosphere. This allowed deducing that gypsum, rather than being the product of the transformation of the binder for the effect of sulphur-based pollutants, was introduced during the production of the mortar.

Acknowledgement This research was financed by Fondazione Banca Popolare di Milano.

REFERENCES [1] Boato A, Mannoni T. Materiali e tecniche nella Genova portuale: i calcestruzzi alla pozzolana dall’età moderna alla rivoluzione industriale. Sci Beni Cult 1993;IX:12–20. [2] Atzeni C, Massidda L, Sanna U. Magnesian lime. Experimental contribution to interpreting historical data. Sci Technol Cult Herit 1996;5:29–36. [3] Bruni S, Cariati F, Fermo P, Pozzi A, Toniolo L. Characterization of ancient magnesian mortars coming from Northern Italy. Thermochim Acta 1998;321:161–5. [4] Biscontin G, Pellizzon Birelli M, Zendri E. Characterization of binders employed in the manufactured of Venetian historical mortars. J Cult Herit 2002;3:31–7. [5] Silva DA, Wenk HR, Monteiro PJM. Comparative investigation of mortars from Roman Colosseum and cistern. Thermochim Acta 2005;438:35–40.

19

[6] Boke H, Akkurt S, Ipekoglu B, Ugurlu E. Characteristics of brick used as aggregate in historic brick–lime mortars and plasters. Cem Concr Res 2006;36:1115–22. [7] Caro’ F, Riccardi MP, Mazzilli Savini MT. Characterization of plasters and mortars as a tool in archaeological studies: the case of Lardirago castle in Pavia, Northern Italy. Archaeometry 2008;50:85–100. [8] Moropoulou A, Bakolas A, Bisbiku K. Characterization of ancient, Byzantine and later historic mortars by thermal and X-ray diffraction techniques. Thermochim Acta 1995;269: 779–95. [9] Benedetti D, Valetti S, Bontempi E, Piccioli C, Depero LE. Study of ancient mortars from Roman Villa of Pollio Felice in Sorrento (Naples). Appl Phys A 2004;79:341–5. [10] Genestar C, Pons C, Màs A. Analytical characterisation of ancient mortars from the archeological Roman city of Pollentia (Balearic Island, Spain). Anal Chim Acta 2006;557:373–9. [11] Bertolini L, Carsana M, Marra E. Degradation of mortars and steel inserts from the ciborium of the medieval abbey of San Pietro al Monte. In: Acierno D, D'Amore A, Caputo D, Cioffi R, editors. Special topics on materials science and technology: an Italian panorama. Leiden: Brill; 2009. p. 45–54. [12] Gleize PJP, Motta EV, Silva DA, Roman HR. Characterization of historical mortars from Santa Catarina (Brazil). Cem Concr Comp 2009;31:342–6. [13] Bertolini L, Coppola L, Gastaldi M, Redaelli E. Electroosmotic transport in porous construction materials and dehumidification of masonry. Constr Build Mater 2009;23:254–63. [14] Miriello D, Barca D, Bloise A, Ciarallo A, Crisci GM, De Rose T, et al. Characterisation of archaeological mortars from Pompeii (Campania, Italy) and identification of construction phases by compositional data analysis. J Archaeol Sci 2010;37: 2207–23. [15] Klemm WA. Cementitious materials: historical notes. In: Skalny J, editor. Materials science of concrete I. The American Ceramic Society; 1989. p. 1–26. [16] Jackson M, Deocampo D, Marra F, Scheetz B. Mid-Pleistocene pozzolanic volcanic ash in ancient Roman concretes. Geoarchaeology 2010;25:36–74. [17] Bakolas A, Biscontin G, Moropoulou A, Zendri E. Characterization of structural byzantine mortars by thermogravimetric analysis. Thermochim Acta 1998;321:151–60. [18] Zendri E, Lucchini V, Biscontin G, Morabito ZM. Interaction between clay and lime in “cocciopesto” mortars: a study by 29Si MAS spectroscopy. Appl Clay Sci 2004;25:1–7. [19] Kramara S, Zalar V, Urosevic M, Körner W, Mauko A, Mirtič B, et al. Mineralogical and microstructural studies of mortars from the bath complex of the Roman villa rustica near Mošnje (Slovenia). Mater Charact 2011;62:1042–57. [20] Riontino C, Sabbioni C, Ghedini N, Zappia G, Gobbi G, Favoni O. Evaluation of atmospheric deposition on historic buildings by combined thermal analysis and combustion techniques. Thermochim Acta 1998;321:215–22. [21] Zappia G, Sabbioni C, Riontino C, Gobbi G, Favoni O. Exposure tests of building materials in urban atmosphere. Sci Total Environ 1998;224:235–44. [22] Sabbioni C, Zappa G, Riontino C, Blanco–Varela MT, Aguilera J, Puertas F, et al. Atmospheric deterioration of ancient and modern hydraulic mortars. Atmos Environ 2011;35:539–48. [23] Chiari G, Santarelli ML, Torraca G. Caratterizzazione delle malte antiche mediante l'analisi di campioni non frazionati. Mater Struct 1992;3:111–37. [24] Riccardi MP, Duminuco P, Tomasi C, Ferloni P. Thermal, microscopic and X-ray diffraction studies on some ancient mortars. Thermochim Acta 1998;321:207–14. [25] Alvarez JI, Martin A, Garcìa Casado PJ, Navarro I, Zornoza A. Methodology and validation of an hot hydrocloridic acid attack for the characterization of ancient mortars. Cem Concr Res 1999;29:1061–5.

20

MAT ER I A LS CH A R A CTE R IZ A TI O N 8 0 ( 2 0 13 ) 9–2 0

[26] Alvarez JI, Navarro I, Garcia Casado PJ. Thermal, mineralogical and chemical studies of the mortars used in the cathedral of Pamplona (Spain). Thermochim Acta 2000;365:177–87. [27] Franzini M, Leoni L, Lezzerini M. A procedure for determining the chemical composition of binder and aggregate in ancient mortars: its application to mortars from some medieval buildings in Pisa. J Cult Herit 2000;1:365–73. [28] Miranda J, Carvalho AP, Pires J. Assessment of the binder in historical mortars by various techniques. Archaeometry 2012;54:267–77. [29] Chierici G, Calderini A, Cecchelli C. La Basilica di San Lorenzo Maggiore a Milano. Milan: Treccani; 1951. [30] Kinney D. The evidence for the dating of S. Lorenzo in Milan. J Soc Archit Hist 1972;31:92–107. [31] Lewis S. San Lorenzo revisited: a Theodosian palace church at Milan. J Soc Archit Hist 1973;32:197–222. [32] Kleinbauer WE. ‘Aedita in Turribus’: the superstructure of the early Christian church of S. Lorenzo in Milan. Gesta 1976;15:1–9. [33] Baronio G, Binda L. Characterization of mortars and plasters from ancient monuments of Milan (Italy). Mason Soc J 1988;7: T23–9. [34] Rampazzi L, Pozzi A, Sansonetti A, Toniolo L, Giussani B. A chemometric approach to the characterisation of historical mortars. Cem Concr Res 2006;36:1108–14.

[35] Riccardi MP, Lezzerini M, Caro’ F, Franzini M, Messiga B. Microtextural and microchemical studies of hydraulic ancient mortars: two analytical approaches to understand pre-industrial technology processes. J Cult Herit 2007;8: 350–60. [36] Fieni L. La basilica di San Lorenzo Maggiore a Milano: analisi stratigrafica e datazione del complesso tardoantico. In: Sandri MG, editor. L'eredità di Monneret de Villard a Milano. Florence: All'insegna del Giglio; 2002. p. 179–206. [37] Bertolini L, Manera M, Fieni L. I leganti nelle malte romane della basilica di San Lorenzo a Milano. In: Mascolo G, editor. Proc. III Conference “Restauro e conservazione dei beni culturali: materiali e tecniche”, Cassino, 3–4 October 2003; 2003. p. 153–62. [38] Adams J, Dollimore D, Griffiths DL. Thermal analytical investigation of unaltered Ca(OH)2 in dated mortars and plasters. Thermochim Acta 1998;324:67–76. [39] Khan N, Dollimore D, Alexander K, Wilburn FW. The origin of the exothermic peak in the thermal decomposition of basic magnesium carbonate. Thermochim Acta 2001;367–368:321–33.