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Physico-chemical adhesion and cohesion bonds in joint mortars imparting durability to the historic structures
Construction and Building Materials 14 Ž2000. 35]46
Physico-chemical adhesion and cohesion bonds in joint mortars imparting durability to the historic structures A. MoropoulouU , A. Bakolas, K. Bisbikou Department of Chemical Engineering, Section of Materials Science and Engineering, National Technical Uni¨ ersity of Athens, 9 Iroon Polytechniou, 15780 Athens, Greece Received 14 April 1999; received in revised form 2 August 1999; accepted 25 August 1999
Abstract It is well known that even though historic mortars present low strength and elastic moduli they confer durability to the structures surviving today. The present work investigates the durability of historic mortars in relation to the production technologies employed. Thermal analysis allows for classification of historic mortars in both lime and hydraulic types. Mineralogical data, concerning fabrication and texture, along with thermal analysis provide criteria on specific classification, for: typical lime, crushed brick]lime, cementitious, rubble masonry, hot lime technology and gypsum mortars. The correlation of the measured tensile strength Žfmt,k . with the estimated CO 2rstructurally bound water ratio, indicates direct proportionality to the levels of the hydraulicity. Physico-chemical adhesion and cohesion bonds, studied by SEM-TEMrEDX, developed at the matrix and at the binderraggregate interface, respectively, becomes the key factor in interpreting the considerable durability that the historic mortars confer to the structures as bearing elements. Q 2000 Elsevier Science Ltd All rights reserved. Keywords: Historic mortars; Adhesion and cohesion bonds; Durability of historic structures
1. Introduction Although mortars seem to be simple systems, in reality they are very complicated multi-phase composites, comprised of crystalline and amorphous phases and containing inert as well as reactive aggregates. The presence of reactive additives can cause various chemical reactions which make it difficult to differentiate newly formed compounds from the older ones w1x. Historic composites concern ‘disturbed’ systems, as in ‘service’ for decades of centuries under severe me-
U
Corresponding author. National Technical University, Chemical Eng. Dept., Mater. Sci. & Section, Zograou Campus, 157 80 Zograou, Athens, Greece. Tel.: q30-1-772-3276; fax: q30-1-772-3215. E-mail address: [email protected] ŽA. Moropoulou.
chanical and environmental loadings. Therefore, the characterization of such materials can be achieved by integrating properly the results of the various analyses, in order to understand the procedures employed to produce the final composites and the nature of the physico-chemical bonds developed among its constituents w2]4x. It is obvious that the relatively low strength of old mortars does not explain their longevity. Historic structures can be considered as early concrete structures, when cohesion and adhesion phenomena among binding material and active aggregates or mortar and building units are established w3,5]8x. On the contrary, it has been found that weak adhesion bonds affect negatively the durability of the masonry, especially when physico-chemical phenomena, like dissolution, transport and recrystallization of salts leading to salt decay, occur w6x.
0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 4 5 - 8
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Hence, the physico-chemical nature of cohesion and adhesion bonds should be investigated in order to elucidate the durability of the historic mortars. Some authors, aiming to classify several mortar types according to the DTA-TG results, suggest the supplementary use of XRD and mineralogical results as well and proceed to the analyses without performing any physical separation w1x. DTA can be used to identify various component materials and evaluate the reactions associated with controlled heating of the mortar. This method reveals thermal transformations, like dehydration, dehydroxylation, oxidation and decomposition w9,10x. In addition, crystalline transitions can be observed, which are exothermic or endothermic in nature. In combination with thermogravimetric analysis ŽTGA., the mass of the sample is monitored Žweight loss. as a function of temperature. The determination of mechanical characteristics of historic mortars has been proved to be difficult due to the large dimensions of the samples required for the classical strength test. Therefore, mechanical tests are usually performed on new mortar samples imitating historic mortar syntheses w11x. Up to now experimental correlation between material and mechanical properties has been restricted to the relation between binder content and both open porosity and strength of lime mortars w12x. However, these experiments do not enable us to search out the effective relationships between composite materials properties and their mechanical performance in the historic structure. In order to achieve direct strength measurements on historic samples the fragment test method has been employed. In the present work ancient Greek, Hellenistic, Roman, Byzantine, post-Byzantine and later historic mortars are sampled from Fortifications, Monasteries, Churches and Historical Buildings in Rhodes, Crete, Corfu, Mount Athos and Constantinople. Mortar samples from Versaille Palace are also used as characteristic examples of gypsum mortars. The samples were investigated by thermal and mineralogical analytical techniques and the information was validated by the results of mechanical tests.
cohesion within the matrix and adhesion bonds, developed at the binderraggregate interface, were derived from the study of the characteristic micrographs concerning the specific classification of mortars according to the production technology employed. 2.1. Mineralogical and physico-chemical analysis For the mineralogical and physico-chemical analysis the following instrumental techniques were used: v
v
v
v
v
2. Experimental procedure Forty samples of historic mortars were investigated by mineralogical and physico-chemical analysis in order to classify the mortars according to the levels of hydraulicity. Twelve representative samples Žwith the same binderraggregate ratio. were used for undergoing mechanical tests as well and the correlation between tensile strength measurements and the inverse ratio of hydraulicity were performed accordingly. Insights on
v
X-ray diffraction analysis ŽXRD. of finely pulverized samples were performed on a Siemens D-500 X-ray diffractometer, based on an automatic adjustment and analysis system with a Diffract-EVA quality analysis software, in order to identify the mineral constituents of the mortars. To facilitate the direct observation of various spectra, a diffraction interval between 2 u-5 and 2 u-60, with a step of 0.02 was used. Optical microscopy: thin sections of mortar samples were examined under a polarizing Zeiss microscope. The microphotographs taken allowed for the petrographical]mineralogical characterization of the mortar constituents, microscopic observations of the different mineral phases in the matrix as well as the qualitative distribution between binder and aggregate. Scanning electron microscopy ŽSEM.renergy dispersive analysis ŽEDX.: microscopic observations of the mortar samples were performed, under scanning electron microscopy ŽPhilips 515 unit., with secondary electron emission, in order to examine the microstructure and the texture of the mortars and the adhesion binderraggregate. Energy dispersive X-ray microanalysis Želementary semi-quantitative analysis . was also performed, based on the Super Quantitative EDAX software, in order to determine the composition of the aggregate and binding materials and their interface as well. Transmission electron microscopy ŽTEM.renergy dispersive analysis ŽEDX. ŽPhilips M20rST. was used to identify the amorphous phases. This procedure was used to study the nature and the presence of calcium silico-aluminate gels in the matrix. This was followed by the physical separation by means of granulometric analysis, by sieving through a series of sieves ŽISO 565. of diameters Ž m m.: 63, 125, 250, 500, 1000, 2000, 4000 and weighing, which allowed the separation of the mortar into its components, in which the finer fraction Ž- 63 m m. was richer in the binder. Thermogravimetric analyses ŽTG-DTG.rdifferential thermal analyses ŽDTA.: thermal analyses were performed by a Mettler TG 50, thermo balance, thermal analyzer system. The weight loss of a mass
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of a 20]50-mg sample was monitored through a heating cycle from 30 to 10008C. The heating process reveals thermal transformations such as dehydration, dehydroxylation, oxidation, and decomposition. In addition, crystalline transitions are observed. Differential thermal analyses ŽDTA. were performed on a Perkin Elmer Thermo analyzer DTA 1700, in order to elucidate the nature of the thermal transformations indicated by the TG-DTG plots.
2.2. Mechanical properties The tensile strength Žfmt,k . measurements were performed according to the fragments test method w13,14x. In particular, small historical mortar fragments Žgravel size. were taken from the joints and arranged in a special mould within a strong matrix Žan epoxy resin or a much stronger mortar..
3. Classification of historic mortars according to the levels of hydraulicity Previous works have already proved that DTA along with TG, X-ray diffraction techniques and electron microscopy analysis are a useful tool for the identification of mortars and for the determination of the degree of hydration and carbonation of lime binding materials and mortar composites w1,3,7x. In the characterization of historic mortars, thermal analysis could serve as a tool in revealing the production process w15x. The temperature ranges and the relative weight losses, under heating, were found to be a reliable measure for the characterization of these materials. In particular, the temperature ranges are relative to the weight loss due to adsorbed water Ž1208C., when there are no particular hydrated salts, to the loss of chemically bound water Ž200]6008C., when there are no other compounds that undergo weight loss in this temperature range and the loss of CO 2 Ž) 6008C. due to the decomposition of carbonates. Weight losses at reaction temperatures transferred more or less near to 7508C, render the loss of CO 2 , not from primary calcite, but from re-carbonated lime w10x. The dehydroxylated clays acted as a ‘pozzolan’ which imparts early strength to the mortar w3x. However, the phenomenon becomes more complex and occurs when hydraulic reactions take place at the aggregatermatrix interface as well. The DTA and TG-DTG analyses identified the dehydration of calcium silico-aluminate phases, giving clear evidence of the hydraulic phases at the mortar matrix, rather than that of pure lime.
Fig. 1. Structurally bound water Ž%. in function with CO 2 Ž%..
From the results of thermal analysis it was found that in general, the CO 2 bound to carbonates and the water bound to hydraulic components Žin weight loss%. discern two groups of mortars, the typical lime and the hydraulic types, respectively. Fig. 1 shows in weight loss% the water bound to hydraulic components in relation to CO 2%. Two areas of mortars are discerned: 1. The typical lime mortars, showing, however, CO 2% Žweight loss%, ) 6008C. over 32% and chemically bound water lower than 3% Žweight loss between 200 and 6008C.. 2. The so-called ‘ pozzolanic’ mortars, including all the categories of crushed brick, cementitious, hot lime, portlandite mortars or modern cement. In the ‘pozzolanic’ area two subgroups could be distinguished: one with over 10% hydraulic water content, where the more condensed and higher strength
Fig. 2. The inverse ratio of hydraulicity in relation to CO 2 Ž%..
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fmt,k, whereas at high values of the ratio Ži.e. practically for lime mortars totally carbonated. they correspond with low values of the fmt,k and at intermediate values of the ratio they correspond with intermediate values of the fmt,k. Assuming that the binderraggregate ratio is more or less the same for all the mortars under investigation and that aggregates of a calcitic nature prevail, due to which the CO 2rH 2 O ratio attains its maximum, one may deduce that the measured tensile strength of the mortars is proportional to their level of hydraulicity. However, in order to generalize, a correction should be assumed, as far as mortars with different gradation will be concerned, since in that case the CO 2rstructurally bound water values for the binder Žfraction - 63 m m. will be diversified. Fig. 3. Correlation between fmt,k values and estimated ratio CO 2rstructurally bound water.
mortars are identified, and the other subgroup with less than 5% structurally bound water content. Fig. 2 shows the ratio of CO 2rH 2 O Žstructurally bound water. of the inversely expressed hydraulic character of the mortars in relation to the CO 2 . The inverse trend of hydraulicity of the mortar samples is shown to augment exponentially with CO 2 , diversifying the various mortars per production technology. The cementitious mortars are concentrated at the bottom, the crushed brick and hot lime mortars in the middle of the curve and the typical lime mortars at the upper right corresponding at ratios of ) 10% and CO 2 ) 32%.
4. Mechanical properties in correlation to the levels of hydraulicity The tensile strength in terms of fmt,k measurements are presented in Fig. 3 in correlation with the estimated ratio of CO 2rstructurally bound water, with a regression coefficient of 0.776. Following the classification deriving from the results of the thermal analysis, one may observe a sequence of fmt,k values like:
5. Specific classification according to the cohesion and adhesion bonds developed
The specific classification of mortars in groups of characteristic transformations, indicated by weight loss against temperature, allows to distinguish among typical lime, cementitious, with crushed brick, with portlandite, with gypsum, with modern cement or hot lime technology, mortars ŽFig. 4.. Mineralogical data, as well as observations concerning fabrication and texture, SEM and TEM provide further evaluation criteria, as they discern mortars according to the cohesion of the matrix and the adhesion bonds occurring at the binderraggregates interfaces. The characteristic micrographs present several types of mortars stemming various historic periods, with absolutely different behavior, even though with more or less the same granulometric gradation. Coordinating the results of mortar classification second to cohesion and adhesion bonds, with the results of classification according to the integrated thermal-mineralogical analysis, it was found that the investigated Mediterranean mortars are discerned into the following categories. 5.1. Typical lime mortars
v v v v
for for for for
lime mortars 0.35) fmt,k; cementitious 0.55) fmt,k ) 0.375; crushed brick mortars fmt,k ) 0.55; and portlandite mortar fmt,k ) 0.6.
Inverse proportionality between the ratio of CO 2rstructurally bound water and the fmt,k is observed. At low values of the ratio CO 2rstructurally bound water Ži.e. for mortars presenting high hydraulicity. correspond with high and augmenting values of the
Thermal analysis shows the absence of any important weight loss before the calcite decomposition releases over 30% CO 2 Žequivalent to over 68% CaCO 3 . ŽFig. 4a.. The mortars do not present any hygroscopic behavior, since they contain approximately 1% adsorbed water. The mineralogical results, along with gradation curves, show mortars consisting mainly of calcite Ž; 80%. and quartz. The binding material is fine crystal-
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Fig. 4. Characteristic TG-DTG curves discerning mortars in: Ža. typical lime; Žb. with portlandite; Žc. crushed brick]lime; Žd. hot lime technology; Že. cementitious; Žf. modern cement; Žg. raw material; and Žh. gypsum.
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lized calcite ŽFig. 5a., totally carbonated. The aggregates are mainly calcitic consisting of fossil fragments and clastic quartz grains ŽFig. 6a.. A large group of mortars are typical lime sand mortars of binder to inert ratio 1:2]1:3. The presence of finely crystallized calcite indicates a rather high, almost total, conversion of calcium hydroxide wCaŽOH. 2 x into calcium carbonate ŽCaCO 3 . by atmospheric CO 2 , and agrees with the good tensile strength measured. This can be compared to the Corfu
mortars which have a higher quartz content and are more coarsely crystallized. The rather high rate of carbonation is attributed to a construction method that uses thin layers of joints partially covering the porous stone. The humid environment facilitates CO 2 diffusion Žover 65% of relative humidity.. Only when the permanent humidity is extremely high Žover 80%., as for example, in the Caretto Tower ŽRhodes. } due to the Roman sanitation works } then the carbonation is inhibited and portlandite crystals are observed ŽFig.
Fig. 5. Scanning electron micrographs: Ža. matrix of fine crystalline calcitic matrix Ž680 = .; Žb. portlandite crystal Ž1420 = .; Žc. reaction rims at the ceramic fragment-binder interface of a crushed brick]lime mortar Ž190 = .; Žd. rubble masonry mortar with two distinct phases of hydration]carbonation, embedded by quartz and calcite grains Ž200 = .; Že. fine grained cementitious matrix of Hellenistic mortar Ž930 = .; and Žf. compact hydraulic matrix mortar of a hot lime technology mortar Ž655 = ..
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5b.. In the cases of mortars with portlandite crystals, where carbonation is inhibited, structures of a higher density and strength result ŽFig. 6b.. However, in that case, two main peaks are observed, the one of portlandite Ž4608C. and the other of calcite Ž7808C. ŽFig. 4b.. 5.2. Crushed brick]lime mortars Among the great variety of traditional mortars,
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crushed brick mortars are of specific interest due to their elevated bearing capacity. According to the results and the interpretation of the various analytical techniques and instrumental examinations concerning the crushed brickrlime mortars of Rhodes, Crete and Hagia Sophia in Istanbul, the following concluding considerations could be reached. The examined mortars from the several characteristic historic periods in the Mediterranean Basin present various binderraggregate ratios estimated per weight
Fig. 6. Optical microscopy: Ža. calcitic matrix of a typical lime mortar with shaped quartz grains Ž40 = .; Žb. compact calcite]portlandite matrix with ferrous muscovite grains Ž40 = .; Žc. crushed brick]lime mortar Žreaction rims at the ceramic]binder interface . Ž25 = .; Žd. rubble masonry matrix with plagioclase and quartz grains Ž25 = .; Že. cementitious mortar with small angular quartz grains and calcitic rounded ones with many fossils Ž40 = .; and Žf. compact hydraulic matrix of a hot lime mortar with quartz fragments Ž40 = ..
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from 1:4 to 1:2. Most probably the weathering, i.e. the washing out of calcite, accounts for ‘low’ ratios like 1:4, not necessarily implying on purpose lime ‘poor’ Byzantine process technologies. From the XRD results, it is evident that the binding material of the samples is mainly calcitic with the presence of calcium silico-aluminate hydrates that confirm the hydraulic character of the matrix. However, the presence of quartz, plagioclase of various types and clay minerals, due to the different aggregate fragments, was detected. Thermal analysis data show the presence of adsorbed water over 1%, 3.5% up to 6.5% of water bound to the several silico-aluminates hydrates and 10]30% of CO 2 due to the decomposition of calcium carbonate ŽFig. 4c.. The binder which comprises the compact mortar matrix ŽFig. 6c. presents a hydraulic character attributed partially to raw materials, like marly limestones or limestone]clay mixtures employed to produce lime, and partially to hydraulic compounds which derive from the interactions between lime]ceramic fragments or lime]ceramic powder. The specific ‘pozzolanic’ character of the crushed brickrlime mortar is attributed to the adhesion reactions occurring at the ceramic]matrix interface ŽFig. 5c, Fig. 6c., their nature depending both on the dimensions and type of ceramic Žraw materials, clays and firing temperature. and the calcium hydrate content of the mortar. The grain and fragment size of the crushed brick directly influences its hydraulic reactivity, as well as its physico-mechanical properties. The observed reactions could probably be attributed to calcium silicate formations at the interface along the brick fragment, acting as the silicate source and membrane and the lime, which makes the interfacial surface alkaline and causes chemical reaction. The penetration of lime into the ceramic and the consequent reaction transforms the microstructure of the ceramic by shifting the pore radii into smaller pores, and augmenting the apparent density w8x. The transformation of the pore size distribution matches with the hydraulic character of the mortar matrix, imparting to the mortar high physico-chemical resistance to polluted and marine atmosphere, as well as high strength w6x. 5.3. Hot lime technology mortars From our previous results, concerning Mount Athos Monasteries, the Arsenal mortars of Symonospetra present high tensile strengths in comparison with other traditional mortars and could be classified above the usual levels presented by hydraulic lime mortars, attaining higher values even from those presented by crushed brick lime mortars Ž0.85]1.5 MPa. w5,16x.
The mechanical strength measured could respond effectively to a dynamic stress, as simulated in the case of the monuments in the greater area of the Serbomacedonian mass. The mineralogical and physico-chemical investigations identify the raw materials and production technology, which are used to impart a high strength to the mortars. According to the mineralogical analysis, the mortars consist mainly of calcite with embedded quartz clastic grains, feldspars and phyllosilicate minerals. Typically the same magnesium silico-aluminate minerals are also found in the binder. Optical ŽFig. 6f. and scanning electron microscopy ŽFig. 5f. investigations give a clear evidence of a compact cementitious matrix, while secondary calcite is recrystallized within the pores. The mortars from thermal analysis ŽFig. 4d. show approximately 1% adsorbed water, 3]6% structural water, which was attributed to water bound to silicoaluminate hydrates, and 18]30% of CO 2 due to the decomposition of carbonates Žcalcite, dolomite.. The XRD results showed that the mortars mainly consisted of calcite with embedded quartz clastic grains, feldspars, dolomite and phyllosilicate minerals. The hydraulic character of the Arsenal mortars could be attributed to hot lime technology. Fine grinded magnesium silico-aluminate dust of the montmorillonitic clays in the area, could have been mixed and reacted with the in situ slaked lime on the process of the so-called ‘pozzolanic’ reactions. However, for a given and stable elemental structure of the clays considered, with the characteristic presence of magnesium, the evolution of the hydraulic character of the mortar seems to be exponential. A ratio of 2:w1:5x-limer‘pozzolanic’ or active clay admixturesrinert aggregates would render analytically the general estimated mixture of 1:3 limeraggregates. Hence, Arsenal mortars present an intermediary between Roman and modern marine structures produced by hot lime technology w5x. 5.4. Cementitious mortars Vitruvius w17x describes ‘Opus Caementicium’ or the so-called cementitious mortars as ‘an artificial conglomerate of gravel with sand and lime cement’. The view that basic silicates are formed by burning and then hydrolyzed by water yielding lime and hydrated silicates, was propounded by A. Winkler and has since been fully established w18x. Raw clays consist essentially of a group of hydrated silico-aluminates, though alumina may be replaced to varying extents by ferric oxide, and to a lesser extent by bases such as MgO, Na 2 O and CaO w19x ŽTG of raw material, Fig. 4g..
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In the case under study, a wide range of mortars from Rhodes ŽHellenistic cement., from Corfu ŽVenetian and English period. and from Byzantine and postByzantine monasteries of Crete, present these features. XRD results show the presence of various silicoaluminate hydrates, chlorite, portlandite and illite. The thermal analyses ŽFig. 4e. of these mortars show 1]4% adsorbed water, water bound to silico-aluminate hydrates 3]16% and CO 2% varies between 10 and 20%. The binderraggregate ratio is estimated to be approximately 1:3. The sequence of small]medium fragments along with the hydraulic components by optical microscopy ŽFig. 6e. explains the adherence and coherence observed by scanning electron microscopy ŽFig. 5e.. In distinction, the modern cement plaster from Hagia Sophia ŽFig. 4f. shows the characteristic peaks of transformation concerning gypsum and adsorbed water, calcium silicate hydrate and calcium carbonate. 5.5. Mortars with gypsum For the sake of classification the Versailles Palace mortar sample ŽFig. 4h. was analyzed to show the characteristic peak of gypsum dehydration Ž130]1608C., followed by the peak of CaCO 3 decomposition Ž7308C.. 5.6. Rubble masonry mortars Various types of rubble masonry mortars are evidenced w20x, all of them of hydraulic lime, presenting a very coherent, cementitious compact matrix, well adhered to the medium grained aggregates ŽFig. 6d.. XRD analysis shows the presence of calcite and quartz as the main components of the mortars. It is a common characteristic between all mortars that dolomite and other minerals rich in magnesium, like serpentine, chlorite and anthophilite are present. Components like portlandite, montmorillonite and calcium silico-aluminate hydrates are detected almost in all samples. Thermal analysis results reveal adsorbed water over 1%, structurally bound water approximately 6% and CO 2% varies between 19 and 29%, due to the decomposition of carbonates Žcalcite, dolomite.. From the bottom to the top of the masonry, different textural and microstructural characteristics indicate different limeraggregates ratios and lime stratification, which points out various carbonation and drying levels. The successive supply of the rubble stone masonry by lime, interprets the stratification of the more or less condensed zones, as the aggregates sediment in the lime putty creates a white band of a hardened matrix ŽFig. 5d.. In situ slaking of lime, as an exothermic reaction, gives rise to relatively high temperatures, which in the
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high alkaline environment offer the energy required to activate reactions for the production either of hydraulic neo-formations or of crystalline compounds, like portlandite w21x. This technology interprets the different carbonation and drying levels, i.e. the higher carbonation at the outer and top of the masonry and the more cementitious matrix in the mortar nucleus near the ground, due to successive watering and the higher temperatures evolved and retained in the bulk. Hence, crystalline portlandite and hydraulic components render the mortar nucleus to dense pore structures imparting high strength and adhesion to the rubble masonry.
6. Physico-chemical bonds inferring durability to the mortars and structures The main function of the mortar is to bind the building units together in order to assure a durable masonry. The historical masonry is, thus, a composite system whose durability is related both to the nature of the single constituents and to the particular interaction between mortar and stone or mortar and brick. In all these complex systems, the adhesion between the various building units varies, depending on the interaction between load bearing binder and building unit elements under various conditions within the structure w4x. The mechanism of interaction can, probably, be complicated by the coexistence of two factors. The interaction is mainly diversified by the composition of materials either rich in silica or rich in carbonates. The adhesion and the durability of the masonry are influenced by workmanship and by the environmental conditions. For example, it is well known that the use of the hot lime technique was appreciated because of the consequent increase of adhesion between load bearing mortar and aggregate fragments. The environmental conditions, humidity in particular, can play an important role by favoring the reactivity between the constituents, which induces the disaggregation of the different mixture, or modifies the kinetics of the reaction. The purpose of the investigation of the interfaces between the load bearing binder and the building unit element is to understand the fundamental mechanism of bonding and the chemistry at the aggregatermatrix interface. In that case the formation of compounds due to the interaction between brick and mortar has been evidenced by scanning electron microscopy ŽSEM. and by transmission electron microscopy ŽTEM.. Fig. 7 shows the brick matrix interface building up the adhesive bonds of a granoblastic compact crushed brick]lime mortar ŽFig. 7d.. Fig. 7a]c present the electron probe microanalysis results in mapping. The
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interface between mortarrbrick shows that carbonates ŽFig. 7a. is substituted for calcium silicate ŽFig. 7b. with simultaneous compaction of the calcite content in the boundary. The brickrmortar interface investigated in depth in a range of 120 m m, considering as the transient area the range 30]50 m m, revealed the following significant features: Ca element decreases, while Si species increase from the mortar matrix to the brick. The boundary surface is evidenced, where the physicochemical penetration of CaŽOH. 2 to the zones immediately adjacent to the brick determines the alkaline environment. In this environment amorphous silicates, among these providing acid surface could be activated and filling the discontinuities of the matrix ŽFig. 6c. should develop the reaction product. The binding materials of the examined sample show a composition presenting high percentages of Ca and lower percentages of Si and Fe. The presence of Fe is characteristic of the ceramic fragments ŽFig. 7c.. These fragments usually present the texture of an initial vitrification stage attributed to low firing temperature w22x. The analysis of the brick]mortar interaction compounds can represent a fundamental step in understanding the mechanism of adhesion between mortar and porous materials like stones, ceramics and ‘vitrified’ bricks. The evaluation of the hydraulic formations is rather difficult due to the physico-chemical conditions and the
discontinuity at the crushed brick]lime interface. However, transmission electron microscopy provides, for the first time, valuable information on a Byzantine crushed brick]lime mortar ŽRhodes., concerning the development of an amorphous calcium silicate and silicoaluminate hydrate gel between the crystalline phases of the calcite and the dispersed ceramic fragments, in a tube structure or sheet formation accordingly, which in some points gives rise to quasi crystalline phases ŽFig. 8a,b.. The presence of the gel phase considers a matrix formation of the hydraulic model, which allows for greater energy absorption and explains the good performance of the historic composites in resisting earthquakes w8x.
7. Conclusions The thermal analysis integrated with the results of the mineralogical analysis permits the classification of the mortars into: lime, cementitious, mortars with crushed brick, and portlandite or produced by the technology of hot lime or rubble masonry mortars. A correlation between the fmt,k measured values and the estimated ratio CO 2rstructurally bound water is established. Assuming that the binderraggregate ratio is more or less the same for all the mortars under investigation and that aggregates of calcitic nature are
Fig. 7. SEM mapping: distribution of the elements ŽCa, Si and Fe. at the brick]mortar interface ŽByzantine period Agia Sofia crushed brick and lime mortar sample..
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explain the distinction between the various types of mortars and discern them in these categories according to the mechanical strengths measured. However, the fact that amorphous hydraulic phases confer elasticity to the mortar and the structures’ constituents provides insights to the durability of the historic mortars and the structures and contributes to revealing physico-chemical mechanisms inferring mechanical strength.
Acknowledgements Acknowledgements are attributed by the authors to: Professor Theodosios Tassios, Mr M. Chronopoulos and Mr. Chr. Spanos for the fragment tests performed at the NTUA Lab of Reinforced Concrete; Professors Ahmet Cakmak ŽP.U.-C.E.O.R.., I. Aksay ŽP.U.-Ch. Eng.. and Mr C. Chun for the TEM analysis performed at the Princeton Materials Institute and Professor Guido Biscontin for the SEM mapping analysis performed at the Department of Environmental Sciences of the University of Venice, as well as for their valuable comments and kind collaboration. References
Fig. 8. TEM micrographs: calcium silicate hydrate and calcium silico-aluminate hydrate gelrcrystalline interface of the mortar matrix with brick fragments ŽRhodes-Byzantine crushed brick and lime mortar sample., magnification Ža. 30 000 = , and Žb. 72 000 = .
prevailing, due to which the CO 2rH 2 O structurally bound water ratio attains its maximum, one may deduce that the measured tensile strength of the mortars is directly proportional to their level of hydraulicity. The longevity of the examined historic composites is strictly related to the mortar production process and to the physico-chemical bonds which are developed accordingly between binderraggregates, allowing for continuous stresses and strains. The structural Byzantine mortars display considerable mechanical strength along with longevity and may be considered as early examples of reinforced concrete. Allowing the structure to absorb energy without affecting its material properties irreversibly, which is not encountered in most modern masonry or concrete structures. The technical and technological peculiarities related to the specific cohesion and adhesion bonds developed
w1x Chiari G, Santarelli ML, Toracca G. Charatterizzazione delle malte antiche mediante l’ analisi di campioni non frazionati. Mater Strutt 1992;3:111]37. w2x Moropoulou A, Biscontin G, Bisbikou K et al. Physico-chemical study of adhesion mechanisms among binding material and brick fragments in ‘cocciopesto’. Sci Beni Cultural 1993;IX: 415]29. w3x Livingston AR, Stuzman EP, Mark R, Erdik M. Preliminary analysis of the masonry of Hagia Sophia Basilica. Materials issues in art and archaeology III. Mat Res Soc, Pittsburgh, 1992:721]736. w4x Bakolas A, Bertoncello R, Biscontin G et al. Chemico-physical interactions among the constituents of historical walls in Venice. Materials issues in art and archaeology IV. Mat Res Soc, Pittsburgh, 1995:771]777. w5x Moropoulou A, Tsiourva Th, Bisbikou K, Bakolas A. Hot lime technology imparting high strength to historic mortars. Constr Build Mater 1996;10Ž2.:151]9. w6x Moropoulou A, Biscontin G, Theoulakis P et al. Study of mortars in the Medieval City of Rhodes. Proceedings of the International Congress on the Conservation of Stone and Other Materials, Unesco-Rilem, Paris, 1993:394]401. w7x Binda L, Baronio G. Indagine sull’aderenza tra legante e laterizio in malte ed intonaci di ‘cocciopesto’. Bolettino d’Arte 1986;35]36:109]15. w8x Moropoulou A, Cakmak A, Biscontin G. Crushed brick lime mortars of Justinian’s Hagia Sophia. Mater Issues Art Archaeol V, 1997:in press. w9x Mackenzie RC. Differential thermal analysis 2, 1st ed. London: Academic Press, 1970. w10x Adams JE, Kneller AW. Thermal analysis of medieval mortars from gothic cathedrals in France. Engineering geology of ancient works. Balkema, Rotterdam: Engineering Geology of Ancient Works, 1988:1019]26.
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w11x Papayianni I. Durability lessons from the study of old mortars and concretes. P.L. Mehta Symposium on Durability of Concrete, May 1994, Nice, France. w12x Schafer J, Hilsdorf HK. Ancient and new lime mortars } the correlation between their composition structure and properties. Proceedings of the International Congress on the Conservation of Stone and Other Materials, Unesco-Rilem, Paris, 1993: 605]613. w13x Katsaragakis ES. A new tensile test for concrete. Mater Struct 1987:20Ž120.. w14x Tassios Th, Vachliotis C, Spanos C. In situ strength measurements of masonry mortars. Proceedings of the International Conference on Repair and Strengthening of Stone Masonries, Iccrom, Athens, 1989:53]61. w15x Moropoulou A, Bakolas A, Bisbikou K. Characterization of ancient, byzantine and later historic mortars by thermal analysis and X-ray diffraction techniques. Thermochim Acta 1995;269r270:779]95. w16x Christaras B, Moropoulou A. Environmental effects on the
w17x w18x w19x
w20x
w21x
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Monasteries of Mount Athos; the case of Symonos Petra Monastery. J Soil Dynam Earthquake Eng 1995;14Ž16.:307]12. Vitruvius. The ten books on architecture, M.H. Morgan Trans. New York: Dover Publications 1960. Winkler A. Prakt Chem 1956;67:444. Moropoulou A, Biscontin G, Bisbikou K, Bakolas A, Zendri E. ‘Opus Caementicium’ mortars and plasters in a heavily polluted marine atmosphere. Il Cemento 1995;92:261]78. Doglioni F, Bellina A, Bona A et al. Ricerca sulle tecnologie storiche di costruzione e manutenzione del Duomo di S. Andrea Ap. a Venzone ŽUD.: Le malte di sacco murario. Sci Beni Culturali 1996;2:571]95. Moropoulou A, Biscontin G, Bakolas A, Bisbikou K. Technology and behavior of rubble masonry mortars. Constr Build Mater 1997;11Ž2.:119]29. Maniatis Y, Tite SM. Technological examination of neolithicbronze age pottery from central and southeast Europe and from the near east. J Archaeol Sci 1981;8:59]76.
Physico-chemical adhesion and cohesion bonds in joint mortars imparting durability to the historic structures A. MoropoulouU , A. Bakolas, K. Bisbikou Department of Chemical Engineering, Section of Materials Science and Engineering, National Technical Uni¨ ersity of Athens, 9 Iroon Polytechniou, 15780 Athens, Greece Received 14 April 1999; received in revised form 2 August 1999; accepted 25 August 1999
Abstract It is well known that even though historic mortars present low strength and elastic moduli they confer durability to the structures surviving today. The present work investigates the durability of historic mortars in relation to the production technologies employed. Thermal analysis allows for classification of historic mortars in both lime and hydraulic types. Mineralogical data, concerning fabrication and texture, along with thermal analysis provide criteria on specific classification, for: typical lime, crushed brick]lime, cementitious, rubble masonry, hot lime technology and gypsum mortars. The correlation of the measured tensile strength Žfmt,k . with the estimated CO 2rstructurally bound water ratio, indicates direct proportionality to the levels of the hydraulicity. Physico-chemical adhesion and cohesion bonds, studied by SEM-TEMrEDX, developed at the matrix and at the binderraggregate interface, respectively, becomes the key factor in interpreting the considerable durability that the historic mortars confer to the structures as bearing elements. Q 2000 Elsevier Science Ltd All rights reserved. Keywords: Historic mortars; Adhesion and cohesion bonds; Durability of historic structures
1. Introduction Although mortars seem to be simple systems, in reality they are very complicated multi-phase composites, comprised of crystalline and amorphous phases and containing inert as well as reactive aggregates. The presence of reactive additives can cause various chemical reactions which make it difficult to differentiate newly formed compounds from the older ones w1x. Historic composites concern ‘disturbed’ systems, as in ‘service’ for decades of centuries under severe me-
U
Corresponding author. National Technical University, Chemical Eng. Dept., Mater. Sci. & Section, Zograou Campus, 157 80 Zograou, Athens, Greece. Tel.: q30-1-772-3276; fax: q30-1-772-3215. E-mail address: [email protected] ŽA. Moropoulou.
chanical and environmental loadings. Therefore, the characterization of such materials can be achieved by integrating properly the results of the various analyses, in order to understand the procedures employed to produce the final composites and the nature of the physico-chemical bonds developed among its constituents w2]4x. It is obvious that the relatively low strength of old mortars does not explain their longevity. Historic structures can be considered as early concrete structures, when cohesion and adhesion phenomena among binding material and active aggregates or mortar and building units are established w3,5]8x. On the contrary, it has been found that weak adhesion bonds affect negatively the durability of the masonry, especially when physico-chemical phenomena, like dissolution, transport and recrystallization of salts leading to salt decay, occur w6x.
0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 4 5 - 8
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Hence, the physico-chemical nature of cohesion and adhesion bonds should be investigated in order to elucidate the durability of the historic mortars. Some authors, aiming to classify several mortar types according to the DTA-TG results, suggest the supplementary use of XRD and mineralogical results as well and proceed to the analyses without performing any physical separation w1x. DTA can be used to identify various component materials and evaluate the reactions associated with controlled heating of the mortar. This method reveals thermal transformations, like dehydration, dehydroxylation, oxidation and decomposition w9,10x. In addition, crystalline transitions can be observed, which are exothermic or endothermic in nature. In combination with thermogravimetric analysis ŽTGA., the mass of the sample is monitored Žweight loss. as a function of temperature. The determination of mechanical characteristics of historic mortars has been proved to be difficult due to the large dimensions of the samples required for the classical strength test. Therefore, mechanical tests are usually performed on new mortar samples imitating historic mortar syntheses w11x. Up to now experimental correlation between material and mechanical properties has been restricted to the relation between binder content and both open porosity and strength of lime mortars w12x. However, these experiments do not enable us to search out the effective relationships between composite materials properties and their mechanical performance in the historic structure. In order to achieve direct strength measurements on historic samples the fragment test method has been employed. In the present work ancient Greek, Hellenistic, Roman, Byzantine, post-Byzantine and later historic mortars are sampled from Fortifications, Monasteries, Churches and Historical Buildings in Rhodes, Crete, Corfu, Mount Athos and Constantinople. Mortar samples from Versaille Palace are also used as characteristic examples of gypsum mortars. The samples were investigated by thermal and mineralogical analytical techniques and the information was validated by the results of mechanical tests.
cohesion within the matrix and adhesion bonds, developed at the binderraggregate interface, were derived from the study of the characteristic micrographs concerning the specific classification of mortars according to the production technology employed. 2.1. Mineralogical and physico-chemical analysis For the mineralogical and physico-chemical analysis the following instrumental techniques were used: v
v
v
v
v
2. Experimental procedure Forty samples of historic mortars were investigated by mineralogical and physico-chemical analysis in order to classify the mortars according to the levels of hydraulicity. Twelve representative samples Žwith the same binderraggregate ratio. were used for undergoing mechanical tests as well and the correlation between tensile strength measurements and the inverse ratio of hydraulicity were performed accordingly. Insights on
v
X-ray diffraction analysis ŽXRD. of finely pulverized samples were performed on a Siemens D-500 X-ray diffractometer, based on an automatic adjustment and analysis system with a Diffract-EVA quality analysis software, in order to identify the mineral constituents of the mortars. To facilitate the direct observation of various spectra, a diffraction interval between 2 u-5 and 2 u-60, with a step of 0.02 was used. Optical microscopy: thin sections of mortar samples were examined under a polarizing Zeiss microscope. The microphotographs taken allowed for the petrographical]mineralogical characterization of the mortar constituents, microscopic observations of the different mineral phases in the matrix as well as the qualitative distribution between binder and aggregate. Scanning electron microscopy ŽSEM.renergy dispersive analysis ŽEDX.: microscopic observations of the mortar samples were performed, under scanning electron microscopy ŽPhilips 515 unit., with secondary electron emission, in order to examine the microstructure and the texture of the mortars and the adhesion binderraggregate. Energy dispersive X-ray microanalysis Želementary semi-quantitative analysis . was also performed, based on the Super Quantitative EDAX software, in order to determine the composition of the aggregate and binding materials and their interface as well. Transmission electron microscopy ŽTEM.renergy dispersive analysis ŽEDX. ŽPhilips M20rST. was used to identify the amorphous phases. This procedure was used to study the nature and the presence of calcium silico-aluminate gels in the matrix. This was followed by the physical separation by means of granulometric analysis, by sieving through a series of sieves ŽISO 565. of diameters Ž m m.: 63, 125, 250, 500, 1000, 2000, 4000 and weighing, which allowed the separation of the mortar into its components, in which the finer fraction Ž- 63 m m. was richer in the binder. Thermogravimetric analyses ŽTG-DTG.rdifferential thermal analyses ŽDTA.: thermal analyses were performed by a Mettler TG 50, thermo balance, thermal analyzer system. The weight loss of a mass
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of a 20]50-mg sample was monitored through a heating cycle from 30 to 10008C. The heating process reveals thermal transformations such as dehydration, dehydroxylation, oxidation, and decomposition. In addition, crystalline transitions are observed. Differential thermal analyses ŽDTA. were performed on a Perkin Elmer Thermo analyzer DTA 1700, in order to elucidate the nature of the thermal transformations indicated by the TG-DTG plots.
2.2. Mechanical properties The tensile strength Žfmt,k . measurements were performed according to the fragments test method w13,14x. In particular, small historical mortar fragments Žgravel size. were taken from the joints and arranged in a special mould within a strong matrix Žan epoxy resin or a much stronger mortar..
3. Classification of historic mortars according to the levels of hydraulicity Previous works have already proved that DTA along with TG, X-ray diffraction techniques and electron microscopy analysis are a useful tool for the identification of mortars and for the determination of the degree of hydration and carbonation of lime binding materials and mortar composites w1,3,7x. In the characterization of historic mortars, thermal analysis could serve as a tool in revealing the production process w15x. The temperature ranges and the relative weight losses, under heating, were found to be a reliable measure for the characterization of these materials. In particular, the temperature ranges are relative to the weight loss due to adsorbed water Ž1208C., when there are no particular hydrated salts, to the loss of chemically bound water Ž200]6008C., when there are no other compounds that undergo weight loss in this temperature range and the loss of CO 2 Ž) 6008C. due to the decomposition of carbonates. Weight losses at reaction temperatures transferred more or less near to 7508C, render the loss of CO 2 , not from primary calcite, but from re-carbonated lime w10x. The dehydroxylated clays acted as a ‘pozzolan’ which imparts early strength to the mortar w3x. However, the phenomenon becomes more complex and occurs when hydraulic reactions take place at the aggregatermatrix interface as well. The DTA and TG-DTG analyses identified the dehydration of calcium silico-aluminate phases, giving clear evidence of the hydraulic phases at the mortar matrix, rather than that of pure lime.
Fig. 1. Structurally bound water Ž%. in function with CO 2 Ž%..
From the results of thermal analysis it was found that in general, the CO 2 bound to carbonates and the water bound to hydraulic components Žin weight loss%. discern two groups of mortars, the typical lime and the hydraulic types, respectively. Fig. 1 shows in weight loss% the water bound to hydraulic components in relation to CO 2%. Two areas of mortars are discerned: 1. The typical lime mortars, showing, however, CO 2% Žweight loss%, ) 6008C. over 32% and chemically bound water lower than 3% Žweight loss between 200 and 6008C.. 2. The so-called ‘ pozzolanic’ mortars, including all the categories of crushed brick, cementitious, hot lime, portlandite mortars or modern cement. In the ‘pozzolanic’ area two subgroups could be distinguished: one with over 10% hydraulic water content, where the more condensed and higher strength
Fig. 2. The inverse ratio of hydraulicity in relation to CO 2 Ž%..
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fmt,k, whereas at high values of the ratio Ži.e. practically for lime mortars totally carbonated. they correspond with low values of the fmt,k and at intermediate values of the ratio they correspond with intermediate values of the fmt,k. Assuming that the binderraggregate ratio is more or less the same for all the mortars under investigation and that aggregates of a calcitic nature prevail, due to which the CO 2rH 2 O ratio attains its maximum, one may deduce that the measured tensile strength of the mortars is proportional to their level of hydraulicity. However, in order to generalize, a correction should be assumed, as far as mortars with different gradation will be concerned, since in that case the CO 2rstructurally bound water values for the binder Žfraction - 63 m m. will be diversified. Fig. 3. Correlation between fmt,k values and estimated ratio CO 2rstructurally bound water.
mortars are identified, and the other subgroup with less than 5% structurally bound water content. Fig. 2 shows the ratio of CO 2rH 2 O Žstructurally bound water. of the inversely expressed hydraulic character of the mortars in relation to the CO 2 . The inverse trend of hydraulicity of the mortar samples is shown to augment exponentially with CO 2 , diversifying the various mortars per production technology. The cementitious mortars are concentrated at the bottom, the crushed brick and hot lime mortars in the middle of the curve and the typical lime mortars at the upper right corresponding at ratios of ) 10% and CO 2 ) 32%.
4. Mechanical properties in correlation to the levels of hydraulicity The tensile strength in terms of fmt,k measurements are presented in Fig. 3 in correlation with the estimated ratio of CO 2rstructurally bound water, with a regression coefficient of 0.776. Following the classification deriving from the results of the thermal analysis, one may observe a sequence of fmt,k values like:
5. Specific classification according to the cohesion and adhesion bonds developed
The specific classification of mortars in groups of characteristic transformations, indicated by weight loss against temperature, allows to distinguish among typical lime, cementitious, with crushed brick, with portlandite, with gypsum, with modern cement or hot lime technology, mortars ŽFig. 4.. Mineralogical data, as well as observations concerning fabrication and texture, SEM and TEM provide further evaluation criteria, as they discern mortars according to the cohesion of the matrix and the adhesion bonds occurring at the binderraggregates interfaces. The characteristic micrographs present several types of mortars stemming various historic periods, with absolutely different behavior, even though with more or less the same granulometric gradation. Coordinating the results of mortar classification second to cohesion and adhesion bonds, with the results of classification according to the integrated thermal-mineralogical analysis, it was found that the investigated Mediterranean mortars are discerned into the following categories. 5.1. Typical lime mortars
v v v v
for for for for
lime mortars 0.35) fmt,k; cementitious 0.55) fmt,k ) 0.375; crushed brick mortars fmt,k ) 0.55; and portlandite mortar fmt,k ) 0.6.
Inverse proportionality between the ratio of CO 2rstructurally bound water and the fmt,k is observed. At low values of the ratio CO 2rstructurally bound water Ži.e. for mortars presenting high hydraulicity. correspond with high and augmenting values of the
Thermal analysis shows the absence of any important weight loss before the calcite decomposition releases over 30% CO 2 Žequivalent to over 68% CaCO 3 . ŽFig. 4a.. The mortars do not present any hygroscopic behavior, since they contain approximately 1% adsorbed water. The mineralogical results, along with gradation curves, show mortars consisting mainly of calcite Ž; 80%. and quartz. The binding material is fine crystal-
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Fig. 4. Characteristic TG-DTG curves discerning mortars in: Ža. typical lime; Žb. with portlandite; Žc. crushed brick]lime; Žd. hot lime technology; Že. cementitious; Žf. modern cement; Žg. raw material; and Žh. gypsum.
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lized calcite ŽFig. 5a., totally carbonated. The aggregates are mainly calcitic consisting of fossil fragments and clastic quartz grains ŽFig. 6a.. A large group of mortars are typical lime sand mortars of binder to inert ratio 1:2]1:3. The presence of finely crystallized calcite indicates a rather high, almost total, conversion of calcium hydroxide wCaŽOH. 2 x into calcium carbonate ŽCaCO 3 . by atmospheric CO 2 , and agrees with the good tensile strength measured. This can be compared to the Corfu
mortars which have a higher quartz content and are more coarsely crystallized. The rather high rate of carbonation is attributed to a construction method that uses thin layers of joints partially covering the porous stone. The humid environment facilitates CO 2 diffusion Žover 65% of relative humidity.. Only when the permanent humidity is extremely high Žover 80%., as for example, in the Caretto Tower ŽRhodes. } due to the Roman sanitation works } then the carbonation is inhibited and portlandite crystals are observed ŽFig.
Fig. 5. Scanning electron micrographs: Ža. matrix of fine crystalline calcitic matrix Ž680 = .; Žb. portlandite crystal Ž1420 = .; Žc. reaction rims at the ceramic fragment-binder interface of a crushed brick]lime mortar Ž190 = .; Žd. rubble masonry mortar with two distinct phases of hydration]carbonation, embedded by quartz and calcite grains Ž200 = .; Že. fine grained cementitious matrix of Hellenistic mortar Ž930 = .; and Žf. compact hydraulic matrix mortar of a hot lime technology mortar Ž655 = ..
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5b.. In the cases of mortars with portlandite crystals, where carbonation is inhibited, structures of a higher density and strength result ŽFig. 6b.. However, in that case, two main peaks are observed, the one of portlandite Ž4608C. and the other of calcite Ž7808C. ŽFig. 4b.. 5.2. Crushed brick]lime mortars Among the great variety of traditional mortars,
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crushed brick mortars are of specific interest due to their elevated bearing capacity. According to the results and the interpretation of the various analytical techniques and instrumental examinations concerning the crushed brickrlime mortars of Rhodes, Crete and Hagia Sophia in Istanbul, the following concluding considerations could be reached. The examined mortars from the several characteristic historic periods in the Mediterranean Basin present various binderraggregate ratios estimated per weight
Fig. 6. Optical microscopy: Ža. calcitic matrix of a typical lime mortar with shaped quartz grains Ž40 = .; Žb. compact calcite]portlandite matrix with ferrous muscovite grains Ž40 = .; Žc. crushed brick]lime mortar Žreaction rims at the ceramic]binder interface . Ž25 = .; Žd. rubble masonry matrix with plagioclase and quartz grains Ž25 = .; Že. cementitious mortar with small angular quartz grains and calcitic rounded ones with many fossils Ž40 = .; and Žf. compact hydraulic matrix of a hot lime mortar with quartz fragments Ž40 = ..
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from 1:4 to 1:2. Most probably the weathering, i.e. the washing out of calcite, accounts for ‘low’ ratios like 1:4, not necessarily implying on purpose lime ‘poor’ Byzantine process technologies. From the XRD results, it is evident that the binding material of the samples is mainly calcitic with the presence of calcium silico-aluminate hydrates that confirm the hydraulic character of the matrix. However, the presence of quartz, plagioclase of various types and clay minerals, due to the different aggregate fragments, was detected. Thermal analysis data show the presence of adsorbed water over 1%, 3.5% up to 6.5% of water bound to the several silico-aluminates hydrates and 10]30% of CO 2 due to the decomposition of calcium carbonate ŽFig. 4c.. The binder which comprises the compact mortar matrix ŽFig. 6c. presents a hydraulic character attributed partially to raw materials, like marly limestones or limestone]clay mixtures employed to produce lime, and partially to hydraulic compounds which derive from the interactions between lime]ceramic fragments or lime]ceramic powder. The specific ‘pozzolanic’ character of the crushed brickrlime mortar is attributed to the adhesion reactions occurring at the ceramic]matrix interface ŽFig. 5c, Fig. 6c., their nature depending both on the dimensions and type of ceramic Žraw materials, clays and firing temperature. and the calcium hydrate content of the mortar. The grain and fragment size of the crushed brick directly influences its hydraulic reactivity, as well as its physico-mechanical properties. The observed reactions could probably be attributed to calcium silicate formations at the interface along the brick fragment, acting as the silicate source and membrane and the lime, which makes the interfacial surface alkaline and causes chemical reaction. The penetration of lime into the ceramic and the consequent reaction transforms the microstructure of the ceramic by shifting the pore radii into smaller pores, and augmenting the apparent density w8x. The transformation of the pore size distribution matches with the hydraulic character of the mortar matrix, imparting to the mortar high physico-chemical resistance to polluted and marine atmosphere, as well as high strength w6x. 5.3. Hot lime technology mortars From our previous results, concerning Mount Athos Monasteries, the Arsenal mortars of Symonospetra present high tensile strengths in comparison with other traditional mortars and could be classified above the usual levels presented by hydraulic lime mortars, attaining higher values even from those presented by crushed brick lime mortars Ž0.85]1.5 MPa. w5,16x.
The mechanical strength measured could respond effectively to a dynamic stress, as simulated in the case of the monuments in the greater area of the Serbomacedonian mass. The mineralogical and physico-chemical investigations identify the raw materials and production technology, which are used to impart a high strength to the mortars. According to the mineralogical analysis, the mortars consist mainly of calcite with embedded quartz clastic grains, feldspars and phyllosilicate minerals. Typically the same magnesium silico-aluminate minerals are also found in the binder. Optical ŽFig. 6f. and scanning electron microscopy ŽFig. 5f. investigations give a clear evidence of a compact cementitious matrix, while secondary calcite is recrystallized within the pores. The mortars from thermal analysis ŽFig. 4d. show approximately 1% adsorbed water, 3]6% structural water, which was attributed to water bound to silicoaluminate hydrates, and 18]30% of CO 2 due to the decomposition of carbonates Žcalcite, dolomite.. The XRD results showed that the mortars mainly consisted of calcite with embedded quartz clastic grains, feldspars, dolomite and phyllosilicate minerals. The hydraulic character of the Arsenal mortars could be attributed to hot lime technology. Fine grinded magnesium silico-aluminate dust of the montmorillonitic clays in the area, could have been mixed and reacted with the in situ slaked lime on the process of the so-called ‘pozzolanic’ reactions. However, for a given and stable elemental structure of the clays considered, with the characteristic presence of magnesium, the evolution of the hydraulic character of the mortar seems to be exponential. A ratio of 2:w1:5x-limer‘pozzolanic’ or active clay admixturesrinert aggregates would render analytically the general estimated mixture of 1:3 limeraggregates. Hence, Arsenal mortars present an intermediary between Roman and modern marine structures produced by hot lime technology w5x. 5.4. Cementitious mortars Vitruvius w17x describes ‘Opus Caementicium’ or the so-called cementitious mortars as ‘an artificial conglomerate of gravel with sand and lime cement’. The view that basic silicates are formed by burning and then hydrolyzed by water yielding lime and hydrated silicates, was propounded by A. Winkler and has since been fully established w18x. Raw clays consist essentially of a group of hydrated silico-aluminates, though alumina may be replaced to varying extents by ferric oxide, and to a lesser extent by bases such as MgO, Na 2 O and CaO w19x ŽTG of raw material, Fig. 4g..
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In the case under study, a wide range of mortars from Rhodes ŽHellenistic cement., from Corfu ŽVenetian and English period. and from Byzantine and postByzantine monasteries of Crete, present these features. XRD results show the presence of various silicoaluminate hydrates, chlorite, portlandite and illite. The thermal analyses ŽFig. 4e. of these mortars show 1]4% adsorbed water, water bound to silico-aluminate hydrates 3]16% and CO 2% varies between 10 and 20%. The binderraggregate ratio is estimated to be approximately 1:3. The sequence of small]medium fragments along with the hydraulic components by optical microscopy ŽFig. 6e. explains the adherence and coherence observed by scanning electron microscopy ŽFig. 5e.. In distinction, the modern cement plaster from Hagia Sophia ŽFig. 4f. shows the characteristic peaks of transformation concerning gypsum and adsorbed water, calcium silicate hydrate and calcium carbonate. 5.5. Mortars with gypsum For the sake of classification the Versailles Palace mortar sample ŽFig. 4h. was analyzed to show the characteristic peak of gypsum dehydration Ž130]1608C., followed by the peak of CaCO 3 decomposition Ž7308C.. 5.6. Rubble masonry mortars Various types of rubble masonry mortars are evidenced w20x, all of them of hydraulic lime, presenting a very coherent, cementitious compact matrix, well adhered to the medium grained aggregates ŽFig. 6d.. XRD analysis shows the presence of calcite and quartz as the main components of the mortars. It is a common characteristic between all mortars that dolomite and other minerals rich in magnesium, like serpentine, chlorite and anthophilite are present. Components like portlandite, montmorillonite and calcium silico-aluminate hydrates are detected almost in all samples. Thermal analysis results reveal adsorbed water over 1%, structurally bound water approximately 6% and CO 2% varies between 19 and 29%, due to the decomposition of carbonates Žcalcite, dolomite.. From the bottom to the top of the masonry, different textural and microstructural characteristics indicate different limeraggregates ratios and lime stratification, which points out various carbonation and drying levels. The successive supply of the rubble stone masonry by lime, interprets the stratification of the more or less condensed zones, as the aggregates sediment in the lime putty creates a white band of a hardened matrix ŽFig. 5d.. In situ slaking of lime, as an exothermic reaction, gives rise to relatively high temperatures, which in the
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high alkaline environment offer the energy required to activate reactions for the production either of hydraulic neo-formations or of crystalline compounds, like portlandite w21x. This technology interprets the different carbonation and drying levels, i.e. the higher carbonation at the outer and top of the masonry and the more cementitious matrix in the mortar nucleus near the ground, due to successive watering and the higher temperatures evolved and retained in the bulk. Hence, crystalline portlandite and hydraulic components render the mortar nucleus to dense pore structures imparting high strength and adhesion to the rubble masonry.
6. Physico-chemical bonds inferring durability to the mortars and structures The main function of the mortar is to bind the building units together in order to assure a durable masonry. The historical masonry is, thus, a composite system whose durability is related both to the nature of the single constituents and to the particular interaction between mortar and stone or mortar and brick. In all these complex systems, the adhesion between the various building units varies, depending on the interaction between load bearing binder and building unit elements under various conditions within the structure w4x. The mechanism of interaction can, probably, be complicated by the coexistence of two factors. The interaction is mainly diversified by the composition of materials either rich in silica or rich in carbonates. The adhesion and the durability of the masonry are influenced by workmanship and by the environmental conditions. For example, it is well known that the use of the hot lime technique was appreciated because of the consequent increase of adhesion between load bearing mortar and aggregate fragments. The environmental conditions, humidity in particular, can play an important role by favoring the reactivity between the constituents, which induces the disaggregation of the different mixture, or modifies the kinetics of the reaction. The purpose of the investigation of the interfaces between the load bearing binder and the building unit element is to understand the fundamental mechanism of bonding and the chemistry at the aggregatermatrix interface. In that case the formation of compounds due to the interaction between brick and mortar has been evidenced by scanning electron microscopy ŽSEM. and by transmission electron microscopy ŽTEM.. Fig. 7 shows the brick matrix interface building up the adhesive bonds of a granoblastic compact crushed brick]lime mortar ŽFig. 7d.. Fig. 7a]c present the electron probe microanalysis results in mapping. The
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interface between mortarrbrick shows that carbonates ŽFig. 7a. is substituted for calcium silicate ŽFig. 7b. with simultaneous compaction of the calcite content in the boundary. The brickrmortar interface investigated in depth in a range of 120 m m, considering as the transient area the range 30]50 m m, revealed the following significant features: Ca element decreases, while Si species increase from the mortar matrix to the brick. The boundary surface is evidenced, where the physicochemical penetration of CaŽOH. 2 to the zones immediately adjacent to the brick determines the alkaline environment. In this environment amorphous silicates, among these providing acid surface could be activated and filling the discontinuities of the matrix ŽFig. 6c. should develop the reaction product. The binding materials of the examined sample show a composition presenting high percentages of Ca and lower percentages of Si and Fe. The presence of Fe is characteristic of the ceramic fragments ŽFig. 7c.. These fragments usually present the texture of an initial vitrification stage attributed to low firing temperature w22x. The analysis of the brick]mortar interaction compounds can represent a fundamental step in understanding the mechanism of adhesion between mortar and porous materials like stones, ceramics and ‘vitrified’ bricks. The evaluation of the hydraulic formations is rather difficult due to the physico-chemical conditions and the
discontinuity at the crushed brick]lime interface. However, transmission electron microscopy provides, for the first time, valuable information on a Byzantine crushed brick]lime mortar ŽRhodes., concerning the development of an amorphous calcium silicate and silicoaluminate hydrate gel between the crystalline phases of the calcite and the dispersed ceramic fragments, in a tube structure or sheet formation accordingly, which in some points gives rise to quasi crystalline phases ŽFig. 8a,b.. The presence of the gel phase considers a matrix formation of the hydraulic model, which allows for greater energy absorption and explains the good performance of the historic composites in resisting earthquakes w8x.
7. Conclusions The thermal analysis integrated with the results of the mineralogical analysis permits the classification of the mortars into: lime, cementitious, mortars with crushed brick, and portlandite or produced by the technology of hot lime or rubble masonry mortars. A correlation between the fmt,k measured values and the estimated ratio CO 2rstructurally bound water is established. Assuming that the binderraggregate ratio is more or less the same for all the mortars under investigation and that aggregates of calcitic nature are
Fig. 7. SEM mapping: distribution of the elements ŽCa, Si and Fe. at the brick]mortar interface ŽByzantine period Agia Sofia crushed brick and lime mortar sample..
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explain the distinction between the various types of mortars and discern them in these categories according to the mechanical strengths measured. However, the fact that amorphous hydraulic phases confer elasticity to the mortar and the structures’ constituents provides insights to the durability of the historic mortars and the structures and contributes to revealing physico-chemical mechanisms inferring mechanical strength.
Acknowledgements Acknowledgements are attributed by the authors to: Professor Theodosios Tassios, Mr M. Chronopoulos and Mr. Chr. Spanos for the fragment tests performed at the NTUA Lab of Reinforced Concrete; Professors Ahmet Cakmak ŽP.U.-C.E.O.R.., I. Aksay ŽP.U.-Ch. Eng.. and Mr C. Chun for the TEM analysis performed at the Princeton Materials Institute and Professor Guido Biscontin for the SEM mapping analysis performed at the Department of Environmental Sciences of the University of Venice, as well as for their valuable comments and kind collaboration. References
Fig. 8. TEM micrographs: calcium silicate hydrate and calcium silico-aluminate hydrate gelrcrystalline interface of the mortar matrix with brick fragments ŽRhodes-Byzantine crushed brick and lime mortar sample., magnification Ža. 30 000 = , and Žb. 72 000 = .
prevailing, due to which the CO 2rH 2 O structurally bound water ratio attains its maximum, one may deduce that the measured tensile strength of the mortars is directly proportional to their level of hydraulicity. The longevity of the examined historic composites is strictly related to the mortar production process and to the physico-chemical bonds which are developed accordingly between binderraggregates, allowing for continuous stresses and strains. The structural Byzantine mortars display considerable mechanical strength along with longevity and may be considered as early examples of reinforced concrete. Allowing the structure to absorb energy without affecting its material properties irreversibly, which is not encountered in most modern masonry or concrete structures. The technical and technological peculiarities related to the specific cohesion and adhesion bonds developed
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