The use of dolomitic lime in historical buildings: History, technology and science The use of dolomitic limestone in lime production is historically well known in the north and south ofTtaly but it is also documented in France, Germany and United Kingdom, especially between the XVI and the XVII century. This contribution intends to consider such material from three different points of view: the historical one, through the study of archi ve documents, treatises and manuals; the technological one, through the analysis of the still existing buildings as well as the production cycJe; the scientific one, carrying out a research into the parameters that allow to expound the excellent result for ages of the mortar produced with the employment of such lime. These three aspects, one to each other strictly connected, aid to reach a more complete knowledge about one of the most used material s in ancient buildings. Archive documents and ancient manuals offer some information concerning typical usages in different areas, giving a general, even if incomplete, picture of production techniques that often were handed on orally and left to the craftsmen. The early technical-scientific publications dealing with this subject date back to the beginning of the nineteen century, when the scientific study of the building materials carried out methodically imposed itself. Such contributions are particularly interesting because the authors still keep in their minds the data provided from the experience. Stopping for a moment to consider the composition of the primary product, Rita Vecchiattini we can notice that an apparent discordance exists between practice in the pre-industrial era, result of centuries of empirics based improvement, handed on by word of mouth or at most by manual s and treatises of that time, and technical knowledge of the industrial era, based on behavioural observations and subject of specific analytic publications. In fact while in the past they considered useful, if not even necessary, the presence of suitable quantities of magnesium in the mixtures of mortars, instead in the last century the utilisation of such binder has been dropped because considered of inferior quality. But the resistance and durability that magnesic mortars, in their different utilisation, have offered against physical and chemical agents of decay are the silent evidences of the performance they can offer, enabling to doubt all theories about binders from Louis Vicat on. At this point science become absolutely necessary to give answer to questions that take root in the history of masonry. On this subject some of the early encouraging results of the scientific research will be enunciate. INTRODUCTlON Lime is undoubtedly one ofthe materials most used in the history of building, both in bedding and lining mortars and in concrete imitating stone. It was widely used from the 3rd century BCI to the start of the Proceedings of the First International Congress on Construction History, Madrid, 20th-24th January 2003, ed. S. Huerta, Madrid: I. Juan de Herrera, SEdHC, ETSAM, A. E. Benvenuto, COAM, F. Dragados, 2003.
Transcript
The use of dolomitic lime in historical buildings:History, technology and science
The use of dolomitic limestone in lime production ishistorically well known in the north and south ofTtaly
but it is also documented in France, Germany andUnited Kingdom, especially between the XVI and theXVII century.
This contribution intends to consider such materialfrom three different points of view: the historical one,through the study of archi ve documents, treatises andmanuals; the technological one, through the analysis
of the still existing buildings as well as the productioncycJe; the scientific one, carrying out a research into
the parameters that allow to expound the excellentresult for ages of the mortar produced with theemployment of such lime.
These three aspects, one to each other strictlyconnected, aid to reach a more complete knowledge
about one of the most used material s in ancientbuildings.
Archive documents and ancient manuals offersome information concerning typical usages indifferent areas, giving a general, even if incomplete,picture of production techniques that often werehanded on orally and left to the craftsmen. The earlytechnical-scientific publications dealing with thissubject date back to the beginning of the nineteencentury, when the scientific study of the building
materials carried out methodically imposed itself.Such contributions are particularly interestingbecause the authors still keep in their minds the dataprovided from the experience. Stopping for a moment
to consider the composition of the primary product,
Rita Vecchiattini
we can notice that an apparent discordance existsbetween practice in the pre-industrial era, result ofcenturies of empirics based improvement, handed onby word of mouth or at most by manual s and treatises
of that time, and technical knowledge of the industrialera, based on behavioural observations and subject of
specific analytic publications. In fact while in the pastthey considered useful, if not even necessary, thepresence of suitable quantities of magnesium in the
mixtures of mortars, instead in the last century theutilisation of such binder has been dropped becauseconsidered of inferior quality.
But the resistance and durability that magnesicmortars, in their different utilisation, have offeredagainst physical and chemical agents of decay are the
silent evidences of the performance they can offer,enabling to doubt all theories about binders fromLouis Vicat on.
At this point science become absolutely necessaryto give answer to questions that take root in thehistory of masonry. On this subject some of the earlyencouraging results of the scientific research will be
enunciate.
INTRODUCTlON
Lime is undoubtedly one ofthe materials most used inthe history of building, both in bedding and liningmortars and in concrete imitating stone. It was widely
used from the 3rd century BCI to the start of the
Proceedings of the First International Congress on Construction History, Madrid, 20th-24th January 2003, ed. S. Huerta, Madrid: I. Juan de Herrera, SEdHC, ETSAM, A. E. Benvenuto, COAM, F. Dragados, 2003.
2066 R. Vecchiattini
twentieth century, after which lime was graduaJIyreplaced by cement, in the conviction that the «new»
material was better than the old in every respec1.Actually, restoration experience has shown that,apart from aesthetic problems, specific physical
characteristics of cement2 do not make it compatiblewith lime-based structures and linings.
Today lime is once again present on the market, tosatisfy the requirements of people working in the
restoration field, who are continually in search ofproducts respecting the theorised criteria ofcompatibility and reversibility. However, it does notseem that the lime produced and cast today is able toprovide objects that are as efficient and resistant as
the majority of the old ones were. During the decades
it took for people to become certain that in pre-industrial construction it was not advisable to usecement, except for structural purposes, know-howreferring to the production and employment of lime
was irremediably 10s1. Hence architects and restorersare now concentrating their attention more and moreon historic mortars, both to copy their recipes in orderto solve design problems, and to study andcharacterise them simply in order to know more aboutthem.
The present paper fits into this broad panoramaof studies, since it looks in some depth at aparticular type of lime, dolomitic, deriving from the
decomposition of dolomite limestone, a rockconstituted by a double carbonate of caIcium and
magnesium' CaMg(CO,)2' It appears interesting toanalyse it since the use of this rock for the production
of lime, now abandoned -although dolomiticlimestone constitutes more than 50% of thecarbonates on earth- was historically very commonin the North and South of Italy but is al so documentedin Spain, France, Germany and England (Newton1987; Manzano et al. 2000), above all from the 16thto
the 17thcenturies. Indeed, numerous analyses, aimingabove all at characterising mortars present in historiebuildings, bear witness to the use of dolomitic
lime even where this was not the only materialavailable, nor even the most convenient to use. Thisobservation, together with the finding of archive
documents (Vecchiattini 1998; Fieni 2000)specifically requesting lime from dolomitic limestonequarries, and the empirical datum that dolomitic
limestone-based mortars do very weJI in the «test oftime», makes it possible to consider the choice of this
material one made in full awareness and not bychance, that is to say one due to convenience or toincapacity of operators to make the right selection.
lnteresting and revealing, regarding the capacity tochoose binders, is the case of the celebrated PortaBozzolo villa at Casalzuigno (Varese province)(Bassani e Cassani 1994; Fieni 1995, 64--68), in
which the lining mortars were done with care andattention varying with the importance of the rooms,but always with magnesic limestone from Valcuvia.What is particularly striking is that the variation in thequantity of magnesium in the binders used indicates
the employment of limestone coming from thedecomposition of both magnesic calcareous rock anddolomite. SpecificaJIy, the magnesium content ishigher in the outer layers, which are more exposed,
than in the underlying ones, which are protected.So why was such a resistant and widely used
material considered inferior, starting from the firstdecades of the twentieth century, and graduallyforgotten? It was in order to answer this question thatan ongoing study on this theme was begun, with theaim not only of providing as much knowledge aspossible on dolomitic limestone, but al so ofevaluating the possibility of bringing it back intobuilding practice.
It is now clear that there is an apparent discordancebetween know-how in the pre-industrial epoch, the
outcome of centuries of honing based on empiricalcriteria handed down oraJIy or at most through themanuals of the day, and the theoretical knowledge ofthe industrial era, based on behavioural observationsand dealt with in specific analytic publications. Theart of producing limestone greatly changed during thelast century, which was profoundly marked by ageneralised technological renewal that was translatedinto the employment of raw materials which weremore and more carefully chosen and of hi-techequipment able to satisfy the needs of a big number ofconsumers, who were often not very qualified and
were further and further away from the know-how ofthe pas1. Recovering the ways in which, in the past,traditional binders were produced and cast means re-examining and interpreting old «know-how» and alsoreconstructing, with the help of science, the route
taken in the past to produce and cast material s that, asthe facts demonstrate, gave better results thanpresent -day ones. Hence in the framework of researchapplied to the conservation of the cultural heritage, it
The use 01'dolomitic lime in historical buildings 2067
is necessary to blend history and science, and toapproach the theme from different points of view.
THE CONTRIBUTlON OF HISTORY
The study of the written sources is fundamental toknowledge of some aspects of the production andemployment of lime, which otherwise it would be
impossible to recreate: archive documents and old
manual s furnish notices on the specific customs of thevarious geographical areas, helping to build up ageneral though incomplete picture of operati vepractice, mostly left in the hands of craftsmen and
handed down orally.In the pre-scientific era the selection of materials
was based on data from practical experience. After theworks of Pliny the Second ([ 1469] 1982) and
Vitruvius Pollio ([1486] 1992), nothing reallyimportant was added to knowledge of limestone down
to the treatises of the modern epoch and laboratoryexperiments in the nineteenth and twentieth centuries.
None of the authors mentioned above directly namesdolomitic limestone, since classification of rocks issomething comparatively recent, but each of themgives more or less cursory descriptions which, ifcorrectly interpreted, furnish precious information.
In general the treatises may contain errors due tothe authors being distant from building sites, butVitruvius' text appears fairly reliable regardingmaterials and working techniques (Decri 2002).Vitruvius is aware that not all stones with whichlime can be processed are the same and that not allof them give the same quality of lime, in termsof performance and use, when he defines somestones for making lime as optima:; moreover, hisexplanations as a whole demonstrate that he realises
that the quality of the binder is also int1uenced by thetechniques used for baking, extinguishing andmixing. He distinguishes two types of rocks that are
suited to making lime: light-coloured stone, de albosaxo, and hard stone, silice (Vitruvius Pollio ([1486]1992, II- V -l). In effect, pure limestone, which isalmost white, could be identified with light-colouredstone, while in Latium the word sil ice was used to
refer to a tenacious basalt used to pave roads. It isquite evident that lime cannot be made from basalt,
but in colour, hardness and fracture morphology this
stone resemb1es do1omite 1imestone (Decri 2002).
Going further into the subject of lime production,
the author indicates that «quella che verra prodottadalla (pietra) densa e piu dura, sara utile nellemurature, invece quella che verra prodotta dalla(pietra) porosa, negli intonaci . . . » (Vitruvius Pollio[1486] 1992, II-V-I). This observation allows us to
establish not only a clear relationship between theappearance of the stone for lime and the respectiverock, dolomite limestone or calcareous rock, but also
the correspondence between the quality of theproduct derived and the various uses, dolomite
limestone being used to make bedding mortars andcalcareous rock for lining mortars.
The theme of the careful choice of the raw material
is also present in the work of Andrea Palladio,according to whom, in order to get good lime, thestones, whether from a quarry or a river, must bechosen with care and attention: «ogni pietra de' montie buona, che sia secca, di humori purgata, e frale, echenon habbia in se altra materia, che consumata dalfuoco, lasci la pietra minore: onde sara miglior quella,
che sara fatta di pietra durissima, soda, e bianca, echecotta rimarra il terzo piu leggiera della sua pietra.Sono ancho certe sorti di pietre spugnose, la calcedelle quali sara molto buona all'intonacature de' muri.. . . Ogni pietra cavata a far la calce e migliore dellaraccolta, e di ombrosa, & humida cava piu tosto che disecca, e di bianca meglio si adopra, che di bruna. Lepietre che si pigliano dai fiumi, e torrenti, cioe iciottoli, o cuocoli; fanno calce bonissima, che fa moltobianco, e polito lavoro: onde per lo piu si usa nelle
intonacature de' muri» (Palladio [1570] 1994',8).One of the first writers in whom we find a more
modern scientific attitude is Francesco Milizia, whoin 1781 wrote: « . . . tutte le pietre su le quali l' acquaforte4 agisce e produce effervescenza, sono proprie a
far calce: le piu dure e le piu pesanti sono le migliori. . . » (Milizia [1781] 18472). Hence Milizia tooindicates as the best one hard and heavy stone, which,from the cursory descriptions, sounds very much likedolomite limestone. Indeed, evaluating the degree ofhardness in accordance with the Mohs5 scale, and thespecific weight of calcite and dolomite, one noticesthat calcite has a hardness of 3 and a specific weightbetween 2.65 and 2.80 g/cm' while dolomite has ahardness of 3.5-4 and a higher specific weight,between 2.85 and 2.95 g/cm'. Hence the adjectives«dense», "hard» and «heavy» are well suited to
d010mitic rack.
2068 R. Vecchiattini
The first publications of a scientific-technicalcharacter on the theme being discussed here date fromthe start of the nineteenth century, when the scientificand systematic study of building material s wasasserted. The contributions from this period areparticularly interesting because the authors are stilJ
aware of the data of traditional experience, dailyverifiable in the know-how of manufacturers andmason.
Laboratory experiments began with Vicat (ISIS),inspector-general des Ponts et Chaussées in France,
who mostly dealt with hydraulic limes, whose mainconstituents he identified: calcium, silica, aluminiumand magnesium6 The author mentions a substantialdifference between magnesic lime and dolomitic limeand, during an experimental discussion of thecalcination of the raw material on the basis of thebaking times and temperature, indicates that a
calcareous rock containing 20-25% magnesiumcarbonate, 10~14% clay and 65-66% calciumcarbonate gives rise to excelJent hydraulic limeknown as magnesic lime; he also emphasises that thelatter is not to be confused with dolomitic lime,deriving from the baking of dolomitic limestonewhich, containing neither silica not aluminium, has
no hydraulic properties.7Vicat also mentions an interesting observation
made by the Piedmont engineer M. Signorile,according to whom the nature of the fuel has aparticular intluence on the characteristics of artificiaJ
limes deriving from the baking of a mixture of clayand dolomitic limestonc. If wood is used as the fuelfor caJcinations, top-quality hydraulic lime isobtained, while if the fuel is coal, which containssuJphides, the rcsult is lime that sets in a couple ofdays but crumbles altogether by the fifth. Theexplanation of this behaviour, according to Signorile,
is that in the second case magnesium sulphate andcalcium sulphate (chalk) are formed, depriving themortar of resistance in time. Such a negative outcome
seems to be due precisely to the presence ofmagnesium. Indeed, baking of a clayey calcareous
rock without magnesium, with coal, which issaturated with calcium sulphate, gives a productwhich exhibits no swelling.8
Greater sensitivity of dolomitic limes to impuritiesin the air containing sulphur oxides is also noted bythe scholars Leo Wilhelm Berens and EberhartSchiele, who highlight the fact that mortars
containing hydroxide dedving from dolomitic lime
are particularly affected by degradation processes,with evident sandiness or pulverisation phenomena(Berens e Schiele 1976,437).
It was precisely observations like these thatprompted specific research aiming at determining thepossible differences, in terms of performance andresistance, between mortars made with dolomitic
binders resuIting from baking with wood-based fueland those made with binders resulting from bakingwith coal-based fue!.
While Vicat maintained that 6-12% magnesium
enhances the quality of the hydraulic binder,9 makingit indestructible, Winkler (1856), again dealing withhydraulic binders, even considered magnesium oxide
a detrimental component since, Iike calcium silicate,it remains unchanged in water. From that moment on,contradictory theses followed one another down tothe start of the twentieth century, when Gallo (190S)published the results of an in-depth study carried out
in the laboratory of chemistry applied to buildingmaterials at the Royal School of Engineers: hedeclared that magnesium was dangerous in lime-based mortars. According to the scholar, <da presenzadelJa magnesia nella calce. . . desee in particolar
modo dannosa perché si spegne molto lentamente, eriduce moIto la quantita di grassello che una buonacalce grassa pua dare collo spegnimento; e per questo
che si prescrive ordinariamente che una calce grassa,per essere impiegata con profitto, non debba
contenere pii1 del 5% di magnesia» (Gallo 1908, 146).The intrinsic incapacity of magnesium earbonate toaggregate in eompact struetures during evaporation of
the mix water is the main reason for the exclusion ofmagnesic and dolomitic limes from good-qualitybinders. Indeed, Gallo indieates as fundamental, dueto the consequences that can be deduced from it inrelation to its use in building, «la grande differenza disolubilita propria dell'ossido di calcio e de]]'ossido dimagnesio . . . Mentre la calce e solubile inproporzione di gr 1,27 per litro d'acqua a 16° cirea, la
magnesia invece non e solubile che in rapporto di 1per 50 mila litri di aequa». And he adds that <da
piccolissima solubi ]ith delJa magnesia, fa SI che siaminima la quantita di essa che pua disciogliersi nellamassa d' acqua presente, e d' altra parte e lontanissimo
l' assorbimento del]' anidride carbonica, da parte dellasoluzione stessa. Quindi una calce magra pcrmagnesia, fa presa pii1 lentamente, e si disgrega
The use of dolomitic lime in historical buildings 2069
piu facilmente perché non av viene una rapida
trasformazione di que sta in carbonato, e le varie parti
restano disgregate per la interposizione di particelleinattive di magnesia; ne deriva inoltre che il solventesi evapora molto prima che la magnesia si siatrasformata in carbonato, e la massa resta friabile edincoerente. . . »(Gallo 1908, 149; 154~155).
Although by that time many were against the use of
dolomitic limes, in the work of enlarging Genoaharbour in the early years of the twentieth century,
after a big debate and numerous experimental trials,concretelO was used which was made up of a mortar
based on dolomitic limestone from Sestri Ponente 11
and pozzolana. Observation of the varyingcompression and tensile strength of mortars prepared
in difIerent periods of the year led to someexperimentation taking various factors into account,including the period when the mortar was mixed, thedampness of the pozzolana and the magnesiumcontent in the limestone. Tn this connection in 1932Salvatore Levi noted that «J'elevata percentuale diossido di magnesio non favorisce certo la resistenzadei calcestruzzi. (tuttavia) massi posti in opera nel1882 per la costruzione del ponte A. Doria erecentemente salpati non presentano tracce dicorrosione pur essendo stati confezionati con talicalci» (Levi 1932, 7).
Actually, we know of no in-depth studies on thesubject, and even the regulations for the acceptance oflimes,12 published in 1940, only give vague
indications: alongside the subdivision between richlime in lumps,13 poor lime in lumpsl4 and hydratedlime in powder,15 the possible use of air-settingmagnesic limes containing more than 20% ofmagnesium oxide is also contemplated.
There are a lot of manuals that give theclassification of binders and in these, magnesic limesare mainly defined as rich limes: in 1912 LicurgoBertelli maintains that « . . . un tenore di magnesia delJ0% e gia suf'ficiente per dare ad un calcare ilcarattere della magrezza, e quando la magnesiaraggiunge il 25 o il 30% non pua piu essere impiegato
. . . Le calci magre contengono magnesia e questa . . .e la causa della loro deficienza di rendimento; inoltre
l' ossido di magnesio si idrata molto piu lentamente e,per la sua debole funzione basica, compie piu
difficilmente le reazioni che producono l' indurimentodelle calci grasse. Queste calci vengono generalmente
rifiutate dai costruttori» (Bertelli 1912, 82 and 93)
and in 1922 Luigi Mazzocchi indicates as « . . . "calcimagre" quelle provenienti dalla cottura di calcarimagnesiaci (dolo mi e). Esse possono contenere sino al50% di magnesia, ma basta anche il 10% di magnesiaper rendere «magra» una calce. . . il lororendimentol6 in grassello e tanto minore quanto piu e
elevata la percentuale di magnesia» (Mazzocchi19326, 11).
[t is only in more recent times that different
opinions have been expressed. For example, according
to Bolis, « . . . le calci con piu del 20% di magnesia(MgO) prendono il nome di «magnesiache»; una
piccola percentuale di magnesia smagrisce la calce,
una del 40-50% fornisce invece un'ottima calceparagonabile alla grassa . . . » (Bolis 1961) and, likehim, Piepoli maintains that « . . . le calci grasse,
ottenibili da calcari con non oltre il 10% di impurita(fra silice, magnesia, allumina), oppure anche da
dolomie con quantita pressocché equimolecolari dicalce e magnesia, si spengono rapidamente e conforte sviluppo di calore, dando un grassello bianco,omogeneo, dolcissimo al tatto e all'incirca il triplo,
sia in peso che in volume, della calce viva adoperata. . . » (Piepoli 1980).
TECHNOLOGY AND SCIENCE
It is curious to notice that in quite a short period of time,just over fifty years, scientific thought regarding the
presence of magnesium in lime changed radically. So itseems natural to wonder what changed from the secondhalf of the nineteenth century to the first half of thetwentieth. The historical-archaeological analysis thathas been going ahead for years in the Ligurian territoryand more in general in the regions of Northern ltalyshows that the change in the quality of products wenthand-in-hand with one of the most importantrevolutions in the building field: the introduction of thecementitious binder and the revolution in productiontechnologies. There was a transition from intermittentkilnsl7 of a pre-industrial type, with bundles of wood
sticks as the fuel, to continuous kilns of an industrialtype, burning first coal and then mineral oils and natural
gases. The influence of the type of fuel used on the
oxide product of dolomitic limestone decomposition isalready reported on, alongside other indications, byvarious authors, though without ever being analysed
fram the scientific point of view. Deville, in a study on
co, H,o so, N, Total Vn
INm'/kg] [Nm'/kg] lNm'/kg] [Nm'/kg] [Nm'/kg]
WOOd'l 0.926 0.675 - 0.001 5.378
Coal 1.122 0.542 0.006 0.007 6.259
PC02 PH20 PS02 P"[mmHg] lmmHgl ImmHgl lmmHg]
Wood 130.805 95.381-
0.113
Coal 136.183 65.752 0.696 0.836
2070 R. Vecchiattini
cement made with magnesium oxide, obtained bymeans of calcinations of magnesium chloride, mixedwith calcium carbonate, reports that «Per poterpreparare il cemento Deville in proporzione industriale,
si cerco di utilizzare la dolomite che, riscaldata atemperatura inferiore al rosso perde l' acido carbonico
che e combinato aUa magnesia e non quello combinatoalla calce» (Ghersi 1903). Vicat too indicates thathydraulíc limes are baked with wood fuels to set in softwater, because the heat produced is adequate,
moderated in such a way as to arrive at shaded red andsubsequently at red heat for enough time to expel alJ the
carbonic acid.18 BertelJi, quoting a study by Maede(1909), maintains that it is possible to eliminate the
drawbacks connected with the use of calcareous stoneswith too high a magnesium content for making Portlandcement, indicating that the disgregation phenomenon «
. . . e dovuto essenzialmente alla cottura alla quale estato sottoposto il cemento. Conferma questa opinioneil fatto che nei cementi cotti a temperatura bassa lamagnesia non e nociva: tali i cementi romani; e ancora
il fatto che proporzioni anche notevoli di magnesia,ottenuta per decomposizione del carbonato atemperatura normale, aggiunta ad un cemento, non ne
modificano le proprieta» (Bertelli 1912, 498).Once again there seems to be a link between the
performance of the final product and the bakingmode, though the explanations offered are vague andmostly based on empirical observations. These
indications, linking the properties of dolomiticmortars to the temperature at which they are baked, inpoint of fact also indicate a dependence on the type of
fuel used: wood or coa!.Rence it was considered interesting to analyse20 the
influence of both the temperature and the type of fuelon the decomposition product, starting from the studyof gaseous atmospheres.
The study of the atmospheres present in kilns firedwith different fuels has made it possible to identifysignificant differences between the contributions of
the different components of the fumes.
The composition of the theoretical fumes clearlyshows the difference between the quantities of carbondioxide and steam: during combustion, wood fuelproduces less carbon dioxide but more steam thancoa!.
Rowever, the most significant datum emerges fromthe calculation of the partíal pressures, linked to thetotal quantity oftheoretical fumes,22 which shows thatthe partial pressure of the steam, in the wood-firedkiln, is about twice as high as that present in the kiln
fired with coa!.
Neglecting the partial pressures of carbon dioxide
and nitrogen, which are decidedly low and hencehave no int1uence on the dolomite limestone mass, ananalysis was made of the effect of the carbon dioxide
and the steam on the microstructure of the grains ofcalcium oxide and magnesium oxide produced duringdolomite decomposition.
In the scientific literature, the effect of carbondioxide on the microstructure of calcium oxide,produced by the decomposition of the caJciumcarbonate, was already well known. At low partialcarbon dioxide pressure the oxide still hasrecollection of the initial crystalline form, while highcarbon dioxide pressures favour sintering of thegrains, so that bigger grains with a different shape are
formed. The same effect has been observed betweensteam and magnesium oxide: the steam catalyses thedecomposition of the magnesium carbonate, above aIl
at low temperatures, rendering the oxide productunstable and provoking localised collapses in themicrostructure. The sintering of magnesium oxideinside grains that maintain their global volumeincreases the total porosity of the sample and theaverage size of the pores.
Rence in an atmosphere of carbon dioxide alone,
the zones of the oxide product prove to be made up ofbig grains of calcium oxide and small grains of
magnesium oxide, not sintered. In an atmosphere of
carbon dioxide and steam, both the calcium oxide and
The use of dolomitie lime in historical buildings 2071
the magnesium oxide zones are made up of biggrains, with consequent higher porosity.
Decompositions carried out in wood-fired kilns
and hence in an atmosphere rich in steam producemagnesium oxide with a large grain and pores that aremore accessible to reagents. Towards liquid phasesthis microstructure should have greater reactivity23that, in the case of extinction, is translated into betterhydration because of the formation of lime paste.
Only now does it appear possible to understand
indications like the one given by Misuraca (190O,179), namely that « . . . Sembra che una corrente di
vapore acqueo, durante il periodo di demolizione per
ca1cinazione del ca1care, agevoli la decomposizionemedesima. Era ritenuta percio buona norma quella dimettere un recipiente con acqua sulla soglia dellabocca da fuoco, durante l'operazione di cottura,perché il vapore acqueo potesse facilmente esseretrasportato nella massa incandescente. . . »
NOTES
1. Utilised in Egypt. perhaps even before the Ptolemaic
periodo it was known to the Minoans. the Myceneans
and the Greeks in the archaic epoch, who used it above
all in linings. but it was also used in a systematic and
widespread way in the Roman epoch. towards the end of
the 3'" century BC (Davey 1965, p . . . .),<11cemento e in realta un legan te in grado di produrre
malte e calcestruzzi a bassissima poro sita. molto rigidi
e ad alta resistenza alla compressione. con un'elevata
dilatazione termica, vicina a quella del ferro, ma con
una conducibilita assai piu bassa. Queste earatteristiche
fanno si che con il cemento armato si possano costruire
strutture foggiabili a volonta, e in grado di sostituirequalsiasi altro materiale portante, anche in presenza di
acqua. In ambiente atmosferico, tuttavia, il cemento pub
dare luogo nel tempo a degradi fisici e chimici legatiproprio al1e sue caratteristiche intrinseche (per esempio:
lesioni di spigolo per sbalzi termici, creazione di solfatidannosi).
Per un certo tempo si e creduto, inoltre, che
l' impermeabilita del cemento ne faecsse un ottimo
materiale per i rivestimenti esterni: oggi si e capito che
son o piu igienici i muri idrorepellenti all' es terno, ma ingrado di permettere una traspirazione dell'umidita
dall'interno verso l'esterno». (Mannoni 2000, 7).Do10mite limestone is a rock of sedimentary origincontaining a large quantity of the mineral do10mite.
Ideal dolomite is made up of a crystal1ine reticule
oí' aiternate strata oí' caicium ions and magnesium
2.
3.
4.5.
ions, separated by strata of CO] and is typical1y
represented by a stoichiometric chemica1 composition
of CaMg(C03), in whieh calcium and magnesium arepresent in equal proportions.
Le. hydroch10ric acid (HC1).
A seale of hardness, deftned as resistance to scratching,
made up of ten mineral substances empirical1y orderedby mineralogists in such a way that each one incises the
one immediate1y below and is incised by the one
immediately above. Resistance to scratching is
measured, for comparison purposes, by the incision
produced by one of the mineral s belonging to the Mohs
12. Norme per l'accettazione delle ca]ci, Roya] decree of
16 November 1939,2331, Milan 1940, 12-31.] 3.
". . . di colore pressocché bianco, e i] prodotto della
cottura di calcari [con un] contenuto in CaO + MgO =94% in peso [ .. ] deve avere un rendimento in
grassello = 2,5 !TI'/!» Norme per l' accettazione delle
calci, Roya] decree of 16 November 1939, 2231, Milan
]940,13.
14. ". e il prodotto della cottura di calcari lcon un]
contenuto in CaO + MgO = 94% in peso [ . . . ] devc
avere un rendimento in grassello = 1,5 mc/t» Norme per1'accettazione delle calci, Royal decree of 16 November1939, 223], Milan ] 940, 13.
15. "... e il prodotto dello spegnimento completo dclle calcipredette, fatto dallo stabilimento produttore in modo da
ottenerla in polvere tIna e secca. l .. 1 Questa calcecomprende due categorie di prodotti, per i quali devono
essere soddisfatti i seguenti requisiti: fiore di calce:
contenuto in umidita = 3%; contcnuto in carboni e
impurita = 6%; contenuto in idrati di calcio e magnesio =91 %; deve dare un residuo a] vaglio da 900 maglie/cmq
= l '7e;deve dare un residuo a] vaglio da 4900 maglie/cmq
= 5%; deve rispondere alla prova di stabilita di vo]ume;
calce idrata da costruzione: contenuto in umidita = 3%;
contenuto in carboni e impurita = 6%; contenuto in idrati
di calcio c magnesio = 82%; deve dare un residuo a]
vaglio da 900 maglielcmq =2%; deve dare un residuo al
vaglio da 4900 maglic/cmq = 15%; deve rispondere alla
prova di stabilita di volume»] 6. <di rendimento di una calce [el il volume assunto dalla
pasta per rispetto all'originario volume di calce viva
sottoposto allo spegnimcnto. E questo straordinario
rendimento, [per la calce dolcc o grassa]da 3 sino a 3
volte e mezzo il volume primitivo, il vero motivo per
cui in talune fabhrichc la calce dolcc e tuttora dai
costruttori prcferita alle calci idrauliche» (Mazzocchi
1932',8).
17. Kilns are defined as intermittent whcn they reguire
distinct and successive phases to work: loading, baking,cooling and unloading (AA VV ] 839, 512; AA VV
1878, 61-62). The typical layout of an intermittent
production unit, fired with wood, is madc up of a vase
with a circular plan that deve]ops vcrtically to create acylindrical structure, ending in a pseudo-vault and
surmounted by a chimney. Inside you can a]ways see awrapping ring going even two-thirds oI the way up and
marking the impost of the apertures above. This
structure has the task of facilitating loading operations,
in that the upper level can easi1y be reached. The insidediameter of ki]ns, measured at the height of thewrapping ring, varies from a minimum of four to a
maximum of six metres; the inside height, measured in
line with the central chimney, varies independently of
the width, and oscillates between eight and twelve
metres. The main entrance is the only aperture at the
base of the ki]n and is often done with a double splayed
stone or brick arch. There are generally three upper
apertures, placed at the vertices of an imaginary
isosceles triangle inscribed in the base circumference.Above, other minor apertures and air vcnts vary in
number, shape and position in the curvature of the
pseudo-vault, in different ki]ns. A charactcristic elemcnt
of every lime ki]n is the chimncy at the top oI tbepscudo-vau]t, whose shape is always different.
18. "On remarque gu'a composition égale, les chaux
hydrauligues sont. pour 1'emploi en cau douce,
meilleures cuites au bois gu'au charbon; cela peut tenir,principalement, au degré d'intensité de ]a chaleur
produite; ]a meilleure chaux, pour ]' emploi spécifié,
serait celle pour laguelle une chaleur modérée,
correspondant au rouge gui succcde au rouge sombre,
aurait été soutenue assez ]ongtemps pour en expulser
tout l' acide carbonigue; cette observation serait en
défaut pour l' eau de mer» (Vicat 1856, 8).] 9. This denomination comprises all hydraulic products
from clayey calcareous stones with a high clay content,
baked at a moderate lemperature, below the fusion limit,
and pulverised by grinding with mechanica] means. Theproduct of the grinding is yellowish wilh red-brown
nuances. Such cements set rapidly, in 5-]0 minutes,
with very elevated hardening. (Bertelli 1912, 352-358).
20. ]n the framework of the Postgraduate Course in
Materials Engineering, 15t!. cycle of the Milan
Polytcchnic, held by the present writer at the
Department of Building, Urbanistic Studies and
Materials l,ngineering of Genoa University.
21. As regards the composition of the wood, an average was
calculated belween the trees that we know from
historical sources to havc been used (beech, pine, aldcr,juniper and sessi1c oak).
22. The quantity of theoretical fumes produced by wood(5,378 NmJ/kg) is lower than that produced by coal
(6,259 NmJ/kg).
23. The next stage of the research, at present beingorganised, consists precise]y in evaluating the reactivity
of the oxide product in the presence of steam.
The use of dolomitic lime in historical buildings 2073
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