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THE MECHANISMS and CAUSES of PORTLAND LIMESTONE DECAY -A CASE STUDY DUFFY, A. P. SUMMARY This Paper Describes a Programme Of

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THE MECHANISMS and CAUSES of PORTLAND LIMESTONE DECAY -A CASE STUDY DUFFY, A. P. SUMMARY This Paper Describes a Programme Of 135 THE MECHANISMS AND CAUSES OF PORTLAND LIMESTONE DECAY -A CASE STUDY DUFFY, A. P. Carrig Conservation Engineering Limited, Dublin, Ireland; PERRY, S. H. Department of Civil, Structural and Environmental Engineering, Trinity College, Dublin, Ireland. SUMMARY This paper describes a programme of sampling and analysis of Portland limestone which was carried on a building at Trinity College, Dublin, Ireland with a view to determining the causes of decay of the stone. Petrographic analysis and soluble salt examination of the samples were used to determine the most likely mechanisms of decay. The visual condition and surface environment of the stone in each sample area was noted and related to the mechanism of decay. Three possible mechanisms of Portland limestone decay are described and the practical implications of the findings are discussed. 1. INTRODUCTION The University of Dublin, Trinity College represents the finest single collection of Georgian buildings in Ireland. Although the university campus has undergone almost continuous development over a period of more than four centuries, its most important buildings date from a short period in the mid- 1Sth century, a period of great political and economic importance for Dublin. In common with the architectural style of many monumental buildings constructed in the city at that time, the facades of these buildings were constructed of Portland limestone imported from the south of England, and of Leinster granite, sourced in the Dublin mountains nearby. Portland stone was employed in areas of detailed and delicate carving, such as in the entablatures, columns and window architraves, whereas the more coarse grained granite was used in walling in the form of either flat or diamond-cut ashlar. The university campus is located in the city-centre of Dublin and, as a result, a polluted environment has surrounded it for a considerable time. Coal burning was, until recently, the primary means of domestic heating in the city, and the stone buildings of the campus have consequently suffered much damage (the enactment of clean air legislation has, however, lead to a large reduction in the levels of air-borne particles). Heavy carbonaceous build-up is obvious on many areas of the buildings, in other sections flaking, granulation and scaling of the stone are apparent. Surface loss of the Portland limestone of up to 25 mm has been recorded in some areas, and loss of Leinster granite in areas of rupture and granulation is much greater. As a result of the poor condition of the stone of many of the historic campus buildings, a decision was taken by the college authorities to commence a conservation programme to minimize the risk of future damage to these buildings. The stone conservation philosophy adopted throughout this programme was based on determining the causes of stone decay in all areas of the buildings and designing a conservation programme on the basis of eliminating these causes of decay. To this end, a programme of building stone sampling and analysis preceded the conservation work. This paper describes in part the results of this programme. It deals specifically with the mechanisms and the causes of decay of Portland limestone at Trinity College, Dublin. 2. PORTLAND LIMESTONE DECAY PROCESSES The processes which lead to the decay of Portland limestone include dissolution loss, mineral alteration, salt-induced decay, freeze/thaw cycling, abrasive weathering and biological growth. Both moisture and thermal cycling of Portland limestone, which cause decay by inducing 136 expansion/contraction responses in stone minerals, are not considered further here. With regard to thermal cycling, temperature fluctuations experienced by stone exposed to the Irish climate are small and thus thermally-induced decay is not a significant factor in its decay. Neither is moisture cycling an important factor in the decay of Portland stone; the absence of any clay-like minerals mean that little moisture absorption and expansion occurs. Of the other decay processes described above, biological growth and abrasive weathering, also, will not be considered in this instance. This is because there was relatively little biological growth on the Portland of the buildings, probably as a result of the high ambient pollution levels, and thus the impact of biological growth can be considered minimal. Moreover, in urban environments, biological stone decay mechanisms may not be as important relative to other decay processes. For example, the rate of solution loss of limestone as a result of lichen colonization has been found to be very low compared to other solution loss mechanisms [1]. Abrasive weathering is not a significant factor in Dublin, it is more important in exposed rural or desert areas. 2.1 Dissolution The process of dissolution is important for Portland due to the stone's high solubility relative to many other building stones. Dissolution occurs in areas where water movement occurs and the process is the result of a combination of effects. Portland is dissolved as a result of 'Karst', which refers to the natural solubility of limestone in natural rainwater - increased water acidity leads to increased stone loss. The dissolution of soluble salts formed between rain events may account for large amounts of stone dissolution loss [2]. The dissolution loss of the binder of the stone results in particulate loss, where the stone grain becomes detached from the stone matrix. Dissolution of Portland results in the removal of the binder and grain, leaving only hard shelly material standing proud. 2.2 Mineral Alteration In Portland limestone, the ooliths and binder of the stone, which are predominantly calcite, may be converted to gypsum in the presence of sulphur dioxide (S02). This process occurs when S02. in the dry phase, is adsorbed into the stone's surface where it oxidizes, due to the presence of surface catalytic impurities such as Fe203, to form S03. This in turn reacts with the calcite of the grain and binder to form calcium sulphite dihydrate (CaS03.2H20) which is then oxidized to form gypsum (CaS04.2H20) [3]. 2.3 Salt-Induced Decay Salt-induced stone decay processes are caused when salts exert internal pressures on stone through crystallization, hydration and thermal expansion. When these stresses exceed the compressive strength of the stone, disruption and decay occur [4]. Salts crystallize out of either under-saturated or super-saturated salt solutions which may be present in the pore spaces of a stone. Precipitation of salts from under-saturated solutions, as in the case of the evaporation of the solvent, does not result in stresses being exerted on the stone but may, however, result in pore blockage and a reduction in stone permeability. The precipitation of salts from super-saturated solutions, caused either by a decrease in solvent temperature or evaporation, leads to a release of energy: stresses are exerted on the walls of pores and cracks and this may result in stone decay [4]. Salt hydration occurs when a salt adsorbs water as a result of surface wetting or increases in relative humidity. This change in the hydrated state of the salt is accompanied by a volume increase and results in pressure being exerted on the stone matrix, which may cause decay. The thermal expansion of salts occurs when a salt is subject to an increase in temperature. The expansion coefficients of most salts are, however, small and this fact, coupled with the low temperature fluctuations for most buildings, means that the stresses exerted are relatively low in comparison with those caused by crystallization and hydration. Salts are derived from both the environment and building materials. For example, mortar dissolution processes result in the availability of calcium (5), whereas the surface deposition and oxidization of 137 sulphur dioxide leads to the availability of sulphate ions; these ions crystallize to form salts both on and in the stone. 2.4 Freeze/Thaw Freeze/thaw decay refers to the damage to stone resulting from the freezing and expansion of water. The risk of stone damage in this way depends on the saturation coefficient of the stone which is defined as the volume of pores filled with water divided by the total pore volume. Stones with saturation coefficients of greater than 0.9 are most likely to suffer damage, stones with coefficients of less than 0.8 are not at risk [6]. This risk of high saturation is negatively proportional to the porosity of the stone. Therefore, Portland limestone, which is relatively porous, is unlikely to have a high saturation coefficient and to suffer damage by freeze/thaw cycling. 3. SAMPLING AND ANALYSIS The east wing of the Public Theatre, located in the Front Square of Trinity college and known as East Theatre, was chosen as the building for the study of the mechanisms and causes of Portland limestone decay. A map of the building's location is shown Figure 1. The building was chosen for the study for three reasons: 1. The Portland limestone decay types were representative of other buildings nearby listed for conservation; 2. these decay types were visually pronounced; 3. the building offered easy access for inspection and sampling. fl Jfl, . " ~ 11cr ;:/ JI .[ o '=' ~ Coll~~eGreen ~fI ~ ~ Parliment Square ) c, East Theatre L; north face --~---' Figure 1: The location of East Theatre within the college campus. The north face of East Theatre, which is illustrated in Figure 2, is made of Portland limestone balustrades, cornices, window architraves and string courses; the diamond-cut ashlar at ground floor level and flat ashlar at higher levels is of Leinster granite. The Portland limestone exhibited a number of different visual decay types of which three predominated: 1. Heavy black crust build-up was common in sheltered, wet areas which were not subject to surface water flow - these areas included cornice soffits and ornamented areas; 2.
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