ROMAN CEMENT – KEY HISTORIC MATERIAL TO COVER THE EXTERIORS OF BUILDINGS Prepared by G. Adamski1, L. Bratasz1, N. Mayr2, D. Mucha1, R. Kozlowski1, M. Stilhammerova3 and J. Weber2

(1) Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Poland (2) Institute of Conservation Sciences and Restoration Technology, University of Applied Arts Vienna, Austria (3) The Monuments Board of Slovak Republic, Slovakia 1. Introduction Roman cements were natural cement binders fired from marls (limestone containing a significant amount of clay) below their sintering temperatures, i.e. between 800 and 1,100°C and grinding burnt stones to a required fineness. Their main characteristics were a short time of setting of around 15 minutes and warm yellow-to-brown colour. They were first produced in England in 1796 when James Parker patented a cement known as Parker’s or Roman cement. The material was obtained by firing clay-bearing calcareous nodules found in the London clay beds on the Isle of Sheppey, England. The success of the cement synthesis resulted from the natural intimate mixture of lime and clay (source of silica, alumina and iron oxide) in the marl, which could not be attained in any man-made mixture. Despite implied links to the Roman binders, the term ‘Roman cement’ was unknown before this date and the material was very different from the hydraulic binders used by the Romans. The technology was rapidly transferred to mainland Europe and the USA. The short setting time, warm colour, little shrinkage on setting and good durability to atmospheric influences and salt crystallization made Roman cement a favoured material for economic and easy manufacture of ornaments and renders for the exterior of buildings in the period of European Historicism and Art Nouveau (19th/early 20th century). This period of rapid urban growth still plays a major role in the culture and aesthetic appearance of central areas in most European cities. Roman cement mortar was highly recommended in contemporary technical literature and textbooks for stuccoists as ideal for plastering applications (particularly run mouldings and castings) and it is now often referred to as the exterior equivalent of gypsum plaster as it offered the same speed of set and manipulation as gypsum yet could withstand exterior conditions very effectively.

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It is known from contemporary sources that - e.g. in the Austro-Hungarian Empire of 1887 - the amount of Roman cement produced was five times higher than that of either Portland cement or hydraulic lime, respectively [1]. The dominance of the newer Portland cement on the market and modern functional architecture, with its total absence of ornament, brought a quick decline in the production and use of Roman cement in the years after World War I. Also the earlier craft techniques were discontinued. As a consequence, there is currently an absence of any broader information on the material characteristics, ageing behaviour and adequate technologies for protection and restoration. Only recently, with growing interest in European art of the late 19th/early 20th centuries, have attempts been undertaken to investigate these historic renders and to develop strategies and adequate measures for their conservation. The current lack of appropriate binding materials - matching those available to the craftsmen of the 19th century - deprives architects and object conservators of original historic technology to restore and conserve the concerned objects. As a result the historic Roman cement decorations are replaced or repaired with modern cement products, which generally do not match the physical and mechanical properties, durability, structure and appearance of the original decorations. Hence, the preservation and restoration of the large preserved built heritage of European Historicism and Art Nouveau is still unsatisfactory. As a consequence an European Union 5th FP project entitled “ROCEM” (2003-2006) is seeking to reintroduce Roman cement to market so that restorers and conservators will be able to meet the fundamental principle of modern conservation - that the historic buildings should be repaired by using materials which are compatible with the historic substance [2]. 2. Investigations of historic buildings More than 20 historic buildings and objects rendered and decorated with typical Roman cement stuccoes were documented and sampled within the project. The samples were then subject to a range of analytical techniques to understand the material characteristics. The buildings were representative of different European areas in terms of materials, historic periods and geography. They range from Wycombe Abbey (1804), an early example of the use of Roman cement in England not long after it was patented there in 1796, to Municipal Commercial Academy in Cracow, Poland (1904) built shortly before the outbreak of World War I. The information has been collated into a historic building database available at the project website www.heritage.xtd.pl.

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The survey of the Roman cement stuccoes has revealed a considerable diversity of the historic application techniques. The fine flat finished renders varied in thickness between 2 - 50 mm. They could consist of a single render coat applied directly to a solid masonry background or be a sandwich structures in which the render coat was followed by the second coat providing a final level surface. The second coat might in turn be followed by a thin finish layer or set coat. Because of the fine nature of this coat the aggregates were usually sieved before mixing with the cement. It was striking to discover that Roman cement coats were possible to apply much thicker without shrinkage,

Figure 1. Typical stratigraphy of Roman cement render surface – finish layer coveredwith 0,1 mm thick wash. Two paint coatings are later addition.

sometimes up to 50 mm, when compared to lime coats which did not exceed 10-12 mm. Roman cement wash, composed of the cement diluted in water, was a universal technique for finishing stuccoes (Figure 1). The cement wash is believed to be applied to fresh plastered Roman cement stuccoes. The technique was also employed to coat plain or ornamental plasterwork made of lime or Portland cements mortars to make them appear as Roman cement stucco. The grand stuccoes of historicism and Art Nouveau contained always linear or oval mouldings, like cornices, obtained by applying in situ the mortar and passing repeatedly a profile over them. It was usual to run a moulding in two or more coats, the inner coat

being a coarse-grained ‘core’ on which a fine-grained thinner finish layer was applied. Roman cement was a preferred material to execute the in situ run work due to its quick setting which facilitated the progress of the process. The run work could be executed also ex situ in a plaster workshop and the fragments produced were then mounted on the façade. The prefabrication in a workshop was also typically employed for manufacturing ornaments by casting in a mould (Figure 2). As run mouldings, the casts have usually a finer outer layers and a coarse interior core. Exceptionally the ornaments were hand modelled in situ. The Roman

Figure 2. Consoles supporting a cornice –at the façade (right photo), dismounted to show the casting technique and collect samples (left photo).

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cement stuccoes could cover the entire façade but very often they were combined with brick or stone depending on the architectural concept (Figure 3). The survey of the Roman cement stuccoes has revealed generally their excellent durability. It has been recognised that their failure has almost exclusively resulted from an improper maintenance making the stuccoes vulnerable to the chronic excessive dampness and freeze/thaw heaving. In the upper parts of the facades the source of dampness has been damaged or ineffective exterior water handling system leading to rain water leaks (Figure 4). In the area above the ground ineffective drainage and waterproofing of the foundation walls could have led to the intrusion of moisture and destruction of the renders of the façade, mainly due to transmission and crystallization of salts. One of the main conservation problems recognised have been later repair and .

Figure 3. Window – frame combining glazed brick and Roman cement ornaments

restoration measures which involved painting-over or re-plastering, often irreversibly, the original surfaces (Figure 5). The unaltered Roman cement facades, preserving their original colour and architectural surface, are therefore rare in spite of the fact that the technique was used on a massive scale during the period of rapid urban growth in Europe in the nineteenth and early twentieth centuries. The reason is vulnerability of the architectural heritage of the period to redevelopment and renovation projects that fail to understand and to take into account in particular fabric, texture,

Figure 4. Damage of Roman cement renders due to infiltration of improperlydrained rain water. colour and traditional pattern of the facades. These features, carefully evaluated and conserved for monuments of earlier periods, are under-appreciated and viewed as disposable as far as this more modern architecture is concerned.

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3. Microstructure of historic Roman cement mortars Aggregates Aggregates were separated from their binders for a more than 30 mortar samples. This was done separately for each layer if the sample was a sandwich structure with a thin coat layer on a coarser core. The separation was a several-step procedure including mechanical separation and chemical treatments by hydrochloric acid. Grain size distribution of the aggregate was determined by sieve analysis and microscopical methods. The results

Figure 5. Uncovering a Roman cement ornament disfigured by a thick coating – apast attempt to ‘renovate’ a façade.

showed that very fine sand with an unimodal granulometric curve and a maximum at 0,125 or 0,25 mm, respectively, was used as aggregate in majority of mortars. Distinctly bimodal curves were found for samples taken from large cast elements or from a concrete floor. Thus, there was the tendency to use more coarse aggregate for cast elements than for flat renders or in-situ run work. Nevertheless, for more than 90% of the analysed samples the mean grain size for the aggregate was below 1mm – which means that much finer aggregates were used in historical mortars than recommended in contemporary standards for mortars and renders. The total amount of aggregate is lower for cast elements (typically < 20 - 30 %) than for renders (typically 30 – 55 %). The shape of aggregate grains varied from very round to very angular and from low to highly spherical – it seems that the historical practice was to use locally available sands, with no particular requirements for the particle shape. Next to sands, carbonate aggregate

(crushed limestone, grains of slaked lime or lime mortars) or large grains of overburnt cement stones were found.

Binders The microstructure of the Roman cement pastes, as observed in back scatter mode scanning electron microscopy, shows a very fine “groundmass” encapsulating distinct individual grains of the order of 10 µm or larger called “phenograins” [3]. The phenograins were recognised as generated by progressive hydration of large Roman cement grains; the size of the largest grains was found to match the

Figure 6. Phenograin showing a hydratedring around an unhydrated core.

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ca. 0,2 mm grinding fineness of Roman cement prescribed in the contemporary standards. The process is incomplete even at the advanced ages of the historic samples, leading to grains with a composite structure – usually a hydrated rim around an unhydrated core. Grains of silica containing low amounts of calcium and other elements have also been revealed. The phenograins are believed to be of significant importance for the mortar properties, as they act just like aggregates, and their amount may occasionally sum up to a total of as much

as 25 %. A representative micrograph containing individual examples of partly hydrated phenograins embedded in the groundmass is provided as Figure 6.

Figure 7. Unhydrated Roman cement clinker containing predominantly belite.

The mineralogical composition of the Roman cement binders was established using X-ray diffraction analysis. The following components were identified: – Crystalline calcium carbonates (calcite and vaterite), products of carbonation of

calcium hydroxide and of C-S-H gel formed in the hardened binder. – Crystalline calcium sulphates (gypsum and ettringite), products of sulphation

reaction due to deposition of atmospheric SO2. – Crystalline unhydrated components of primary Roman cements - calcium silicates

(belite – C2S, wollastonite - CS and gehlenite – C2AS) which survived in unreacted grains of Roman cements or encapsulated in mortars because of their low rate of hydration. These phases can be labelled as Roman cement fingerprint phases. Their presence could be also confirmed through scanning electron microscopy (SEM-EDX), where they are visible in most of the phenograins (Figure 7).

– Amorphous component consisting predominantly of aged, hydrated cementitious phases, frequently containing little calcium due to extensive carbonation reaction of the initial C-S-H gel with a corollary formation of crystalline calcium carbonates.

As confirmed by the above results, the mineralogical composition of historic Roman cements was quite different from that of Portland cement as their major silicate phase was belite (C2S) and not alite (C3S). The minimisation of the alite phase was result of the relatively low temperature of calcination, below around 1200 OC required by phase equilibria for the alite formation. The coarse grained belite and frequent occurrence of well crystallised gehlenite in many of the historic mortars, point also to the prolonged residence times acting in a traditional kiln production.

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4. Porosity structure of historic Roman cement mortars Mercury porosimetry was used to determine porosity structure of historic mortars. Three categories of pores could be distinguished. The finest pores with the pore diameter below 0,2 µm are present within the hardened aged Roman cement matrix. This is especially evident if a pore volume distribution curves of a mortar and pure historic Roman cement hardened under water are compared (Figure 8). The larger pores with the

diameters between 0,2 – 5 µm are present only in mortars hardening on air. These pores can be due to evaporation of water during the hydration process and their volume maybe indicative of water/cement ratio used originally. Pores larger than 5 µm are rare and due to a secondary porosity generated by mortar weathering and related damage of the mortar fabric.

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Figure 8. Pore volume distribution curves for a pure Romant cement hardened under water and a mortar.

The analysis of total porosities for a representative set of mortars, subdivided into the three pore categories described above, shows that no important differences between geographical locations nor date of the objects was established. This is spectacularly illustrated in Figure 9 where the pore structure of the oldest sample (1804, England) is compared with the youngest (1904, Poland). Results for both are very similar. In contrast, the variability of the pore structure within mortars used on the same object can be considerable.

5. Compatibility between new/repair mortars and historic ones

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The essential objective of the current ROCEM project consists in bringing back to the market the historic Roman cement to address properly the conservation needs of the vast built heritage of the nineteenth and beginning of twentieth centuries. The approach has been to imitate the historic manufacturing process but also to optimise it using a range of laboratory studies and advanced optimisation techniques. Sources of raw materials in

Figure 9. Pore volume distribution curves for a pure hardened Roman cement and two mortars.

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Austria and Poland were surveyed and marls of chemical and mineralogical composition which would produce required hydraulic nature and appearance of the final burnt material were selected. Optimal calcination parameters - time, temperature and heating rate – have been determined in laboratory using as criteria setting time, soundness and strength development of the cement pastes. Larger quantities of the materials have been obtained in a pilot scale process. Initial results of the optimisation programme for Roman cement manufacturing are described elsewhere [4].

Figure 10. Failure of incompatible Portland cement repair mortar.

New, repair Roman cement mortars will be produced from the cements developed

ny new binder or mortar recommended for use in building conservation should fulfil

our groups of parameters relevant for the compatibility of mortars have been selected

mechanical parameters to assess that the repair material is less strong and rigid than

ompressive and flexural strength, dynamic elasticity modulus and pull-off strength, the

according to the mortar recipes based on several sources of information - data obtained from the study of historic mortars, historic literature, archival sources and standards - specially the detailed Standard for Roman Cements elaborated and published in 1880 by the Viennese Society of Engineers and Architects [1], contemporary standards. Athe criterion of maximum compatibility with the original fabric of a historic object. This requirement has evolved from the negative past experiences when use of inappropriate mortars led to a pronounced accelerated degradation (Figure 10). The project has recognised the broadest possible meaning of the compatibility as a capacity of the repair mortar to interact with the original historic materials without inducing (directly or indirectly) any damage. Furthermore, the repair mortars must offer the range of colours of the original Roman cements - the very property making the binders ideal for façade decoration techniques. Fand are systematically determined for historic and new repair mortars. They comprise: -

the historic one

clatter parameter determined for historic mortars well adhering to the substrate (brick masonry) to give an estimate of the maximum adhesion of the render attainable on thehistoric facades

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- porosity and related properties to ensure proper water and water vapour transport through the support/repair system

dry bulk density, total porosity and pore size distribution by mercury porosimetry and argon adsorption, capillary moisture content (total water accessible porosity) and initial rate of water absorption (IRA) – called also water absorption coefficient (w-value). Furthermore, NMR imaging will be used to assess moisture and salt transport across the combinations of substrate/repair mortars to detect possible zone of deterioration at the substrate/repair mortars interface. - response of the mortars to moisture and temperature to ensure that their movement

on the masonry will not cause failure due to cracking and decohesion equilibrium moisture content for varying relative humidity, hygric and hydric expansion and shrinkage, thermal expansion and shrinkage within -20 to 50oC - colour and texture of mortars to ensure aesthetic compatibility. Precautions will be undertaken that: - high quality of craftsmanship – execution of the repair work will be maintained in

mortar processing and application in order not to introduce an unintentional interface between the original and the repair, and make otherwise sound repair material incompatible,

- recognizing that substrate can influence the structure and properties of the repair

layer due to varying fresh mortar - substrate interaction during the hardening, the properties of the repair mortars will be studied for samples prepared on the historic substrates – bricks and Roman cement renders.

Aesthetic component of the compatibility will be assessed separately by experienced conservation experts by means of comparative analysis. Mechanical, physical or structural parameters will be measured for a representative number of well preserved historic mortars. Allowable ranges of values obtained for each parameter will be assumed as tolerance limits within which respective parameters of the repair mortars should fall to deem them compatible with the historic original. The tolerance limits can be object, geographical area or time period specific. Field experience has shown that salt crystallization and freeze-thaw cycles are the most recurrent causes of damage to the Roman cement renders. Therefore, these two decay processes will be used to estimate and compare decay functions – cause-effect relationships between degradation factor and damage - for the historic materials and the best available new repair materials. Tests on the combinations of substrate/repair mortars will be included. If the decay functions will be similar, the compatibility assessment will

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be regarded as complete and reliable. If significant differences in the decay progress appear, additional parameters accounting for the difference in susceptibility of the historic versus repair material to damage will be looked for. Recognizing that withdrawal of samples from historic renders can inevitably imply some mutilation to the object, a testing technique of a low level of destructivity has been developed. It is based on acoustic emission to detect microcracking and stresses in the resistance tests, even at early stages when no visual change of the sample can be observed. Furthermore, the technique is fast and able to localize the damage progress spatially in the tested sample, e.g. at the interface between the historic and repair renders. 6. Acknowledgements Substantial part of this research was carried out within the ROCEM project, supported financially by the European Commission 5th Framework Programme, Thematic Priority: Environment and Sustainable Development, Key Action 4: City of Tomorrow and Cultural Heritage. The authors would like to thank conservation experts of the ROCEM project - Simon Swann for a detailed glossary of technical terms describing the nineteenth century stuccowork and Christian Gurtner for providing some of the illustrations. Thanks are also due to Rob van Hees of TNO Building and Construction Research in Delft, for his support in defining the project’s work on compatibility of mortars. 7. References 1. Tarnawski, A., ‘Kalk, Gyps, Cementkalk und Portland-Cement’, (1887). 2. ‘Roman cement to restore built heritage effectively’, European Commission EVK4-

CT-2002-00084 ROCEM project, www.heritage.xtd.pl 3. Diamond, S. and Bonen, D., ‘Microstructure of Hardened Cement Paste – A New

Interpretation’, J.Am.Ceram.Soc., 76 (1993) 2993-99. 4. Jaglin, D. and Hughes, D.C., ‘Production and characterization of a Roman cement

produced from a marl originating Lilienfeld, Austria’, in ‘Cement and Concrete Science’, Proceedings of a Conference, University of Warwick, September, 2004 (published by Institute of Materials).