Consfruction and Building Materials, Vol. 11, No. I, pp. 3340, 1997 C 1997 Elsevier Science Ltd. All tights reserved

Printed in Great Britain 095@0618/97 $17.00+0.04l

PII: S0950-0618(96)00031-1

The role of brick pebbles and dust in conglomerates based on hydrated lime and crushed bricks*

G. Baronio, L. Bindat and N. Lombardini

Department of Structural Engineering, Politecnico of Milan, Piazza Leonardo da Vinci, 32, 20733 Milan, ha/y

Received 30 May 1996; revised and accepted 28 October 1996

Phoenicians were probably the first ones to use mortars based on hydrated lime and crushed or dust bricks, followed by all other people who were in contact with them. The Romans employed those mortars any time they needed a rendering exposed to severe environment or a floor at a very humid ground level, in foundations where the water table was high or for thermal baths. The use of brick dust in a lime mortar is, nowadays, generally interpreted as an alternative to pozzolana or to other pozzolanic materials. The authors themselves have found, some years ago, clear reaction layers at the contact between the binder and the brick pebbles. Those bricks were found to be mostly pozzolanic under a pozzolanicity test. In some cases, the test gave negative results; nevertheless, the mortars were compact, strong, better than the normal putty lime mortars even when very thick joints were realized, as in the case of the Byzantine buildings. The authors are exploring the possibility of other roles played by the bricks, especially in the case of very thick joints, perhaps producing a good bond while also serving as aggregates and giving overall good physical and mechanical performances to the mot-tars. Some mortar specimens made from hydrated lime and crushed bricks with chosen size and distribution have been prepared together with a specimen made from large bricks and thick mortar joints. The carbonation process and the strength and deformability at different curing times is explored experimentally. 0 1997 Elsevier Science Ltd.

Keywords: conglomerates based on lime and brick dust; pozzolanic reaction; role of joint thickness

Introduction

The use of mortars based on hydrated lime and brick dust

dates back to the most ancient times. The Romans used this type of mortar in every part of their empires whenever pozzolanic materials were not available and a mortar

insoluble in water was needed. This was the case of renderings exposed to severe climate, and floors where capillary rise of water was expected, or structures of thermal baths. But they also used the same material in foundations and in load-bearing masonry walls. They certainly were ignorant of the chemistry of mortars, but

they knew by experience that brick dust or pebble played a very important role in the mortar consistency and strength. The use of crushed bricks, particularly brick dust, in the preparation of mortars based on putty lime has been interpreted in the present times as an alternative use to other pozzolanic materials.

In the latest times of the Roman Empire, and especially during the Byzantine times, lime mortars with crushed bricks became very popular and were used also in the joints of the load-bearing facing walls. Gradually, the thickness of the joints was increased together with the dimension of the

*An earlier version of this paper was presented at and published in the

Proceedings of the North American Masonry Conference, 2-5 June 1996

+Corresponding author

aggregates and of the included crushed bricks; in these cases, the resulting material should be better called ‘conglomerate’ rather than mortar. In some cases, the brick dust that gave the mortar a pink colour disappeared and only the large sized pebbles were included.

Reactions between the lime and any pozzolanic material are well known to take place easily; the pozzolanic material is usually recommended to be very finely ground in order to realize a large, specific surface, and hence a large contact between the two materials. When the joint thickness becomes greater than 40 mm (70 mm in the case of Hagia

Sophia in Instanbul), the size of the brick pebbles increases even up to 25 mm and the pozzolanic reaction, if any, can take place only along the external border of the pebble. In this case, it is still unknown whether the crushed brick has another role besides being pozzolanic. The reason for using such thick joints in masonry and the role of those joints in the structural performance of the masonry itself are also

unknown. The authors intend to explore these roles and the influence of thick joints on the strength and deformability of the masonry at very early stages and after hardening. Therefore, an exhaustive experimental investigation on the composition and characteristics of mortar based on lime and crushed bricks sampled from Byzantine masonry walls has been carried out. Mortars and masonry specimens have been prepared from mortar joints having compositions

33

34 Role of brick pebbles and dust in conglomerates: G. Baronio et al.

similar to the old ones and the mechanical behaviour of the specimens under slowly increasing and/or stepped loading

of constant loads is being detected.

Historical background

The use of mortars based on hydrated lime and brick dust dates back to the most ancient times: according to some

authors, King Solomon employed these mortars to render

the cisterns used to collect water and the Phoenicians knew the use of brick dust gave hydraulicity to the air lime

mortars. The first reliable document, which mentions the

use of crushed bricks for the preparation of mortars for renderings and floors, goes back to the Roman times (Catone. De Re Rustica, 2nd cent. BC). The ancient

treatises, among which Vitruvius (De Architectura Libri Decem, AD 3240) is the most important, describe the use

of crushed bricks in the lime mortars; these materials were

indicated with several names. among which the name 0~~1s trstaceum seems to be the most significant. The use was

spread by the Romans throughout their empire in Europe, northern Africa and west Asia (Turkey). This material was employed for several purposes. Dust bricks were mainly

used for rendering and for the upper layers of tloors, but

crushed bricks with large grain size were recommended not only for masonry walls, arches and foundations where high

humidity or water were present. but also to improve the performance of mortars and conglomerates in normal

conditions. Figures I4 show. as examples. mortars from the renderings of a tower of the church of S. LorenLo in

Milan (AD 5th cent. ). a joint of the barrel vault covering the main swimming pool in the Roman baths in Bath. LJK (AD I st cent. ). the Statthalterpalast m .i\ugsburg. I AD ?nd--5th

cent.). a joint of 9s. Sergius and Baccua 111 Istanbul. ‘Turkey

(AD 6th cent.), respectively. During the latest time of the Roman Empire, the use 01

crushed bricks in the joints of load-bearing walls became more frequent, while the thickness of the horizontal joints was gradually increased from IO-15 mm up to 60-70 mm.

This habit was continued during the Byzantine period until the joints reached a thickness of 70 mm in Hagia Sophia in

Figure 1 Rendering of S. LorenLo in Milan, Italy (AD 5th cent.)

(photograph taken with compound microscope)

Figure 2 Joint of the main barrel vault of Roman baths, Balh, UK (AD

!\t cent.) (photograph taken with compound microscope)

Figure 4 Mortar from a Joint of 3s. Sergus and Bacchus. Istanbul,

Turkey (AD 6th cent. J

lnstanbul. The use of crushed bricks stayed for the duration

of the imperial times and seems to be less dealt with during the medieval times.

This lower diffusion can be related to the poor economy and production for building construction, which includes a low production of bricks. The renaissance treatises seem to

Role of brick pebbles and dust in conglomerates: G. Baronio et al.

propose again the lesson learned from Vitruvius: in Italy, L.B. Alberti (De Re Aedificatoria, Florence, 1485), F. di

Giorgio Martini (Trattati di Architettura, Ingegneria e Arte militare-after the 1482) and A. Palladio (I Quattro Libri

dell’Architettura, Venice, 1570) still mention in their treatises, the use of crushed bricks as they were proposed by Vitruvius without any modification. The illuministic and

neoclassic literatures of the 18th cent., always concerned about the construction techniques of the past times, proposed again the ancient technology. Nevertheless, F.

Milizia (Principi di Architettura civile, 1781) tried to give a scientific explanation of the pozzolanic effect of fired clay

(tiles) on lime by comparing it with the pozzolana.

_ 16

3 2 I4

,E_ l2 g 10

P

e 5 E

z

8 6

0 4

a 2

During the 19th cent., the time of the discovery of cement, the manuals (e.g. Rondelet, Breyman) still suggest

the use of traditional materials, but only after going into the details and explaining to the professionals about their right use and composition, like in G. Pegoretti (Manuale pratico

per l’estimazione dei lavori architettonici, stradali idraulici e di fortificazione, Milano, 1843).

0 40 50

Hydr%l ion g rnole~ 90 100

Figure 5 Results of pozzolanicity test on several bricks from Ravenna

Byzantine buildings

bricks seemed to develop a pozzolanic action only after 30 days or even more (Figure 5).

Characterization of ancient mortars based on lime and crushed bricks

Since 1984, the authors have studied some mortars and plasters sampled from ancient and old historic buildings in Milan. In particular, they examined a rendering dated AD 5th cent., sampled from one of the towers of the church of S. Lorenzo2*6; this rendering was made, according to the Roman technique and composition, with hydrated lime; sand and crushed/powdered bricks. From a microscopic examination of portions of the rendering, it was possible to observe the binder/brick interface reactions with conse- quent new formations. The bonding materials had pene-

trated the brick at the mortar/brick-fragment interface, as demonstrated by the partial alteration of quartz pebbles present in the brick itself. The alteration was due to the alkaline environment created by the presence of calcium hydrate in the bonding agent. Reaction products were found

in the existing cavities, thus eliminating any breaks in continuity between the mortar and the brick. Small

fragments of brick also seemed to have, in turn, penetrated the adjacent mortar. Other portions of this rendering were observed under SEM and analysed with the electronic microprobe and a real bond layer was detected. The

chemical composition of the layer shows reaction products consisting chiefly of silica and calcium. Samples from the floors of Roman and Byzantine buildings and from the joints of Byzantine buildings in Ravenna and elsewhere were also examined and analyzed by the authors and the results are presented in Ref. 4. In both cases, reaction layers were found between the brick pebbles and the binder, no matter how large was the dimension of the pebble. XRD analyses also allowed to make the hypothesis that the firing temperature of the bricks was not higher than 950°C.

From the research experience, three main questions can be formulated: (i) if the bricks used within those mortars would have developed there in a rather long time (e.g. more than 30 days), would it ever be still possible for them to fix the calcium hydrate in time, before the total carbonation of the mortar could take place; (ii) why so thick mortar joints, constituted with a conglomerate rather than with a mortar, were used during the imperial times; (iii) could the brick

pebble have another role in the mortar, beyond the pozzolanic one?

Characterization of a Byzantine conglomerate

The authors are carrying out an experimental and numerical investigation on the structure of S. Vitale in Ravenna7. The

aim is to detect the state of conservation of this Byzantine construction with a complicated structure where the traditional materials were used in a very interesting way. Concerning the construction technology of the masonry walls and piers, investigation by coring and by the use of a horoscope seems to show that they are solid, thick, one leaf walls (900-2000 mm thickness). The particular feature of the masonry, similar to other Byzantine buildings, is represented by the thickness of the joint, which has a ratio

of 1 : 1 to the thickness of the bricks (Figure 6). A vast research programme on the Byzantine masonry

materials is being carried out at the University of Thessaloniki (Greece) by Prof. 1. Papayanni, within a NATO contract, and very good similarities can be found between the Byzantine buildings in Greece and Italy.

As a first step of the present research, the material coming from the remaining of the original walls of S. Michele in Africisco, a former church in Ravenna contemporary to S. Vitale (6th cent.), was examined. The sampling was possible because restoration works were being carried out on the building including the remains.

Nevertheless, in several cases, no reaction was detected This is in order to spare sampling of the S. Vitale walls,

between the binder and the brick pebbles. Therefore, a by studying a similar masonry. The sampling of the

pozzolanicity test was carried out” on bricks used for the material was carried out ‘respecting’ the building in a

masonry where the mortars were sampled; the test gave more representative way and satisfactorily, quantity-wise,

positive results after 8 or even 15 days; in some cases, the possible, to carry on with reliable chemical, petrographi-

36 Role of brick pebbles and dust in conglomerates: G. Baronio et al.

Figure 6 Derad of the masonry walls of S Vltale m Ravenna (AD 6th

cent.)

Figure 7 Details of a joint (SMAm6) of S. Michele m Afr~civx (AD 6th

cent. 1

cal-mineralogical and SEM analyses. The authors3.’ and

other researchers have stated that in order to detect the grain size and distribution of the aggregate, a quantity of

material, at least 300-500 g, has to sampled. The sampled material, coming from the joints (Figure 7)

and from the internal parts of the wall can be considered as

a conglomerate, due to the max dimension of the brick

pebbles, which is 16 mm.

Chemical analyses. The chemical analyses were

performed according to the ASTM 114, both on the conglomerate (SMAm3) and on the fraction of the same material passing through the 0.075 mm sieve (SMAm123). The mentioned fraction was obtained during the separation by sieving of the different fractions of the material while

detecting the grain size and distribution. The treatment used for the separation will be described in a following section. The results of the chemical analysis, reported in Table 1,

show the presence of an aggregate, which is partially siliceous and partially calcareous, and a binder based on air lime.

The soluble silica in SMAm3 is 0.56%, while that in SMAm123 was 6.34%. In the cases where calcareous aggregates are present in large quantity, microscopic analysis is necessary in order to detect the ratio of binder to aggregate.

Table 1 Chemical analyses of the sample SMAm3 and the material passing through the 0.075 mm sieve (SMAm 123)

_

SMAm3 (%) SAm123<0.075 mm (7~)

SiOz 40.83 IS.43 AlzO~+Fe20q 7.48 4.33 CaO 26.45 s 1.77

MgO I .oo 0.74 NazO 0.58 0.78 KZO 1.2s 1.34 SO3 0.26 0.46 Loss on ign. 22.12 24.44 CO? 18.28 24.17 Sol. Sil. OSh 6.34 Ins. res. 43.75 9.42

Figure 8 Reaction layer in a pebble (photograph taken with a compound

microscope)

Optical analyses. The optical analysis carried out by the metallographic microscope shows a 1 : 3 binder to

aggregate ratio. Furthermore, clear reactions could be seen between the brick pebbles and the binder and also

between pebbles and the binder (Figure 8). The petrographical-mineralogical analyses of thin sec-

tions of the conglomerate show that the aggregate is

composed of crushed bricks, calcareous and siliceous pebbles. The brick pebbles belong to two types of bricks,

red and yellow ones. The siliceous part of the aggregate

contains a high percentage of flintstones, sometimes of fossiliferous nature, with the typical radial fibrous structure

of chalcedony’. The calcareous aggregate is present in various sizes; a low percentage of mono- and poly- crystalline quartz is also present. Also, the petrographical analyses confirm the presence of new formation products between the binder and the brick pebbles (Figure 9) and

between the binder and the flintstone; sometimes these products have filled voids and cracks (Figure 10).

SEM observations also confirm the presence of the reaction products between the binder and the brick pebbles (Figure 11), and the analysis using EDS shows the presence of calcium and silica in the reaction products (Figure 12).

The presence of these elements supports the hypothesis of a pozzolanic reaction.

Grain size and distribution of the aggregates. Two samples of conglomerate were considered, called SMAm3

Role of brick pebbles and dust in conglomerates: G. Baronio et al. 37

Figure 9 Reaction between the binder and a brick pebble in thin section

Figure 10 New formation products in cracks and around a flinstone

pebble

Figure 11 Reaction layers detected by SEM

and SMAm89, having weights 484.14 and 475.28 g, respectively. The separation of the aggregate from the binder was obtained by thermic treatment3. Figure 13 reports the grain size and distribution curves. As is clearly shown, the maximum diameter of the aggregate is >16 mm. The largest fraction is mainly composed of crushed brick as can be seen from Figure 7. In Table 2, a detailed

presentation of the percentage and the type of aggregates

31 -Au9- 1995 1020.26 Preset = 60 lets

Vert = 2767 counts Disp= 1 Elapsed= 60 sets

:

i!iJ Al

M9 Na

1

i- 0.000

Ca

SMA m 6 (5)

6

, 9

Range = 10.230 KeV 10.110 -b Integral 0 = 169292

Figure 12 EDS analysis of the reaction layers

0.01

grain diameter [mm]

Figure 13 Grain size distribution of conglomerates SMAm3 and

SMAm89

Table 2 Percentage of the various type of aggregates in different size intervals

Particle size (mm) Amount (%) Calc. + Sil. (%) Brick (%)

16-6 10 5.0 5.0 6-3 17 8.5 8.5 3-0.07s 73 65.7 7.3

100 79.2 20.8

in various size intervals are given. The results show that there is a dominant presence of brick (50%) having the

largest and the medium sizes, while the percentage of brick dust is decreasing for the smallest sizes. The values of Table 2 were calculated by macroscopic observation of the size range 16-3 mm, stereo-microscopy, magnetic separation and modal analysis in the size range 3.0- 0.075 mm. The material passing through the 0.075 mm sieve (SMAm123) was subject to chemical analysis (see

38 Role of brick pebbles and dust in conglomerates: G. Baronio et al.

Table 1); nevertheless, the quantification of the binder and aggregate, brick dust and the other powdered aggregate could be made only by microscopic observation. Three main components were found: binder, brick dust and red

flintstone. The high percentage of soluble silica found in the

chemical analysis of the material passing through the

0.075mm sieve, even if partly due to the presence of dust brick and flintstone, is certainly mostly due to the observed

binder/brick pozzolanic reaction in the pebbles.

Physical characteristics. Bricks and conglomerate of S. Michele in Africisco were also subject to physical tests.

The most relevant parameters obtained are given in the following: the total absorption average value for bricks was

21.9%, the bulk densities were 1600-1745 kg mm~3 and 1635 kg m-3 for the bricks and conglomerate, respectively. Apparently, the density of the conglomerate was lower or

very similar to those of the bricks, while in several other

cases of ancient buildings, the mortar bulk density was an

average of around 1700 kg mP3.

The use of thick joints in Byzantine masonry walls and

arches

As mentioned above, during the latest times of the Roman Empire and during the Byzantine times, the use of thick

joints for masonry brick- and stone-walls became popular. The thickness of the joints became equal or even greater

than the thickness of the bricks; in the meantime, the

dimension of the bricks was usually very large (around 3500-3800 mm). The reason for this use is not very clear,

since it is known that masonries with thick joints tend to be weaker than masonries with thin joints’. On the other hand,

the size of the brick pebbles, as of the other aggregates,

tend to increase as the thickness of the joint increases and totally different grain size distributions are found for renderings rather than for the thick mortar joints.

Other roles of crushed bricks in the joint conglomerate

In the case of large sized brick pebbles, even if reaction layers can be detected along the contact surface between

the binder and the pebbles; nevertheless, the reaction

cannot penetrate very far into the pebble. Hence, the reaction can only realize a better adhesion between the

binder and the aggregate, a modification of the external surface of the pebble, but does not effect in a complete modification of the pebble itself. Different hypotheses can be made on the other possible roles of the crushed bricks into the mortar and/or the conglomerate: influence of their

suction rate on the bond with the binder, increment of the overall deformability of the mortar, influence on the weight of the mortar, etc. Therefore, there must be a special role played by the brick pebbles, which is not based on their pozzolanicity only. The understanding of this role is considered very important by the authors, who have decided to experimentally investigate into the character- istics of these conglomerates. Therefore, the subsequent step of the research was to reproduce a similar material and to test it at different ages of curing alone and within a masonry prism.

Preparation of a conglomerate based on lime and crushed bricks

Following the results of the experimental study carried on the conglomerate joints of S. Michele in Africisco,

the decision was taken by the authors to reproduce a similar material. Crushed bricks from a new production -

for restoration purposes, pozzolanic after 30 days r were

proposed before using the ancient bricks, which could

be found in a very low quantity, and will be used at the

very end of the research in order to enable a comparison

with the original material. The aggregate quantities and the grain size and distribution were calculated on the

basis of the original material following the grain size and distribution as in Figure 13 and the percentages

presented in Table 2.

The ratio of binder to aggregate of the mix was 1 : 3. The workability test was carried out according to the Italian

code, UNI/8020’, and the values were 60%. 37 specimens

of dimensions 40 mmx40 mm x 160 mm and six other

cubes with 100 mm side were prepared from this mix. All

the specimens were prepared and cured in a climatic chamber at 20°C and 65% RH.

A first set of three specimens, BYZ13-15, was submitted to the flexural and compressional test after 28 days. The average values were 0.28 and 0.597 N mm-*,

respectively. Successively, other sets will be tested at

60, 90, 180, 360 days in order to detect the dependency of strength on time, as it was done in Ref. 9. After 35

days, the shrinkage of three conglomerate specimens of dimensions 40 mm x40 mm x 160 mm was also measured.

The average value of the shrinkage was 0.5%. A carbonation test with an alcoholic solution of phenolphtha-

lein was carried out on the fractured surfaces of the

remaining portions after testing. Figure 14 shows an

example of the results: the carbonation depth was only about 5-6 mm and this can explain the reason for a very low strength of this mortar at an early age. Furthermore, if the carbonation of the mortar is so slow, then there might be

the possibility of pozzolanic reactions between the binder and the brick pebbles even after 30 days or more.

Figure 14 Carbonation depth of the conglomerate after 28 days

Role of brick pebbles and dust in conglomerates: G. Baronio et al. 39

Preparation and testing of stack bond prisms with thick joints

In order to study the behaviour of a masonry built with thick joints and its influence on the strength and deformability of the material, a set of stack bond prisms is being built and will be tested at a very early age with slowly increasing loads. In fact, it is well known that the strength of the masonry is dependent on the joint thickness and, typically, there is an optimal thickness of about 15 mm, above which the masonry strength decreases. The results of this experimental research should explain how the conglomerate is developing its strength and how the carbonation is taking place with time.

The bricks were produced, on purpose, with ferrous clays adding quartz sand fired at 950°C having a bulk density of 1620-1695 kg me3 and a total absorption of 20%-23%.

A first stack bond prism was built containing four soft- mud brick courses and three conglomerate joints. The dimensions of the bricks were 310 mmx510 mmx40 mm, the thickness of the joints being 45 mm. The height of the specimen, 5 min after it was built, was 293 mm.

Three timber moulds were used to contain the joints. After 6.5 h, the moulds were removed and the height of the specimen measured was 289 mm. The specimen was cured at 20°C and 65% RI-I. After 24 h, the height was still diminishing to 287 mm. Subsequently, the specimen was loaded every day with three bricks (Figure 15); the loading history is presented in Figure 16.

In Figure 17, the response of the material as stress- deformation-time is presented. After 35 days and, when the load was 5.626 kN corresponding to 0.0349 N mme2, the total loss of height was 3.162 mm. In a real structural element, this contraction would cause a rather appreciable loss in height: 50 mm for every 4 m of height. This high deformation was probably typical of masonries with very thick joints as can be seen in Figure 18. The test will now be continued until the value of the stress calculated for the piers of S.Vitale in Ravenna is reached. Afterward, the prism will be kept under constant load for two months and then the bricks and the joints will be recovered and the

Figure 15 The stack bond prism (BYZPl) under the first loading

condition (three bricks)

9

0 ’ I I I 1 I I

1 2 3 4 5

Time (min) XI o 4

Figure 16 The loading history of BYZPl for the first month

o (N mm-’ ) XlO‘2 3.5 -j

3.0 _I

2.5 4

2.0 -/

8 I 9

10

Figure 17 Stress-strain-time curves for BYZPI

carbonation and bonding phenomena will be studied in the laboratory.

Conclusions and further developments

At this stage of the work, some concluding remarks, possible on the research, the original materials and the new specimens, can be drawn.

From the study of the ancient material, particularly of the conglomerate from S. Michele in Africisco, the following information was obtained: (i) the ratio of binder to aggregate in the conglomerate is 1 : 3; (ii) the aggregate

40 Role of brick pebbles and dust in conglomerates: G. Baronio et al.

Brick I !$-_-?_

‘i Voids _-e i

--y--.._l

Figure 18 Part of a section of one of the original wall of S. Michele in

Africisco (AD 6th cent.)

is composed of crushed and dust brick, and siliceous and

calcareous pebbles; (iii) the brick pebbles are prevalent in the largest sizes, with diameter >I6 mm; (iv) these bricks can develop a very slow pozzolanic action; (v) the carbonation of the mortar is very slow, as is well known; (v) the largest sized brick pebbles may have other roles beside the pozzolanic one, those of influencing deform- ability and weight; (vi) the role of thick joints is still not

clear. From the first of the results of the research on the new

materials, the following remarks can be made: (i) the rate of carbonation of the conglomarate is very low, (ii) the early deformation of the masonry, due also to creep behaviour and the shrinkage of the conglomerate, is very high at an early age; but after 28 days, tends to become more stable.

The further developments in the research will consist in mechanical tests on other masonry prisms. trying to understand the mechanical behaviour of masonry with

thick mortar joints; in the meantime, the carbonation of the

binder and pozzolanic action of the brick pebbles at different ages will be continuously studied.

Acknowledgements

The authors wish to thank Arch. A. M. Iannucci. Soprintendente dei Beni Ambientali e Architettonici of Ravenna, Italy; Mr. S. Clews, curator of the Roman baths,

Bath, U.K.; the Roemisches Museum of Augsburg, Germany; and Prof. F. Akoz, Yildiz Technical University, Faculty of Civil Eng., Istanbul, Turkey, for being so friendly to send samples of the original material. The

authors also extend their thanks to Mr. P. Perolari, Mr M. Iscandri, Eng. H. Falter, C. Tedeschi, G. D’Uva. The

research is supported by MURST.

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2

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