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. loss of Portland, as indicated by protruding shelly material or the loss of carved detail, was common in exposed, washed areas; 3. delamination and blistering of the stone was common in very sheltered areas which were not subject to direct wetting either by rainfall or surface washing. These decay types were found, for 138

example, in comers under cornice overhangs and were associated with a very light surface build­ up of what appeared to be carbonaceous material. The surface delamination in these areas was typically 1-2 mm thick and revealed a white or creamy-yellow, powdery substrate behind the laminate. I

north elevation Figure 2: The locations of Portland limestone and Leinster granite in East Theatre. Sample locations 1, 2 and 3 are also shown.

3.1 Sample locations Samples of each of these three predominant visual decay types were taken. The locations of these samples are illustrated in Figures 2. Sample 1 was of Portland limestone with a heavy carbonaceous build-up 3 to 5 mm thick and was taken from the soffit of the upper cornice. The area was not subject to direct rainfall or surface washing, but was frequently wetted as a result capillary transport of moisture from directly wetted areas of the stone. Sample 2 was taken from the window sill of the attic window. This area was subject to direct wetting by rainfall and surface washing. Protruding shelly material indicated that in the order of 1-2 mm of stone had been lost from the sill. Sample 3 was taken from directly under the cornice at the upper right hand corner of the building. The stone was lightly soiled and suffered blistering and delamination. This area was not subject to direct wetting by either rain or surface flow as indicated by the lack of clean washed stone.

3.2 Sampling Procedure Drilled and cored samples were taken from each of the sampling areas. Drilled samples were taken using a drilling machine with a hardened steel drill bit. In each location three 8 mm diameter drilling samples were taken at three 3 mm depth intervals into the stone; the drilling sampling technique is illustrated in Figure 3.

0-3 mm sam le 1 -====- ,_4' ----:3-'-6'-'m"""m"-'--"s~am~l~e 11 ' •· 6-9 mm sam le

wall __J ' Figure 3: Three drilling samples were taken at three 3 mm depth intervals into the stone at each sampling location. 139

After the drilled samples were taken, the location of each was consolidated using an epoxy resin. The surface of each sample was saturated in situ with the resin which was then allowed to cure for 48 hours. This consolidation was carried out to prevent the break up of the weathered surface of the stone. A 15 mm diameter cored sample was subsequently taken from each location. The hardened steel core was diamond tipped and water was used as a coolant.

3.3 Analysis The drilling samples were powdered and eluted in deionized water, filtered and analysed for chloride (Ci-), nitrate (No32-). sulphate (Soi-). calcium (Ca2+). magnesium (Mg2+) and sodium (Na2+). A Dionex 2010i ion chromatograph was used to analyse 5 ml of the filtered sample for c1-. No32- and soi- concentrations. A further 1O ml of each filtered sample, containing 0.4 ml of a 10% lanthanum solution, was analysed for ca2+, Mg2+ and Na2+ using a Parkin-Elmer 3100 atomic absorption spectrophotometer. The core samples were used to make 40 µm petrographic sections. Particular care was taken not to damage the surface areas during fabrication of the thin petrographic sections as this was the area of greatest interest and most easily damaged by the fabrication process. The petrographic sections were analysed under cross and parallel nicols using a Leitz Wetzlor petrographic microscope with a photographic attachment.

4. RESULTS

4.1 Soluble Salt Results The concentrations of all samples from all depth ranges were summed to give total concentrations for each ion; these are shown in Figure 4. It can be seen that soluble calcium and sulphate were found to be the predominant ions present in the eluted samples, with total concentrations of 4443 and 3913 mmol.g-1 respectively. These concentrations were nearly two orders of magnitude greater than the next highest concentrations, which were of the magnesium and sodium ions at 47 and 46 mmol.g-1 respectively. Total soluble chloride and nitrate concentrations were 20 and 8 mmol.g-1 .

4500 - 4000 - ' i. ~ 3500 - I 0 I E 3000 - E I I ~ 2500 - .!:! ~ 2000 - c: 1500 l I. ! . ..u c: 0 1000 ; () .- 500 - I 0 Ca2+ Mg2+ Na2+ Cl- N03- 5042- Ion

Figure 4: The concentrations of all samples from all depth ranges were summed to give total concentrations for each ion. 140

The concentrations of soluble calcium for the three individual samples are shown in Figure 5. It can be seen that samples 1 and 3 had the highest concentrations of this ion and that concentrations for both of these samples were highest in the 0-3 mm range, which was the sample closest to the surface of the stone. Sample 2 had slightly lower total concentrations, but the depth concentrations differed from 1 and 3 in that ca2+ concentrations were lowest at the surface and highest in the 3-6 mm range.

2000 1800

~ 1600 c5 1400 E 0-3mm .[ 1200 0 c .g 1000 O ~mm I! c.. 800 • 6-9mm u 600 ~ 0 400 200 0 Sample1 Sample2 Sample3

Figure 5: The concentrations of soluble calcium (Ca2+) for the three individual samples.

In all cases, individual values of so42- were almost identical to those of ca2+. Again, samples 1 and 3 had the highest total concentrations and the highest values were recorded in the 0-3 mm range for these samples. In sample 2, the total soi- concentrations were lower than for samples 1 and 2, and the highest concentration was in the 3-6 mm range. Figure 6 shows the sample soluble Na2+ results. It can be seen that sample 3 contained the highest total concentrations of this ion followed by samples 2 and 1. However, sample 1 and, even more so, sample 3, exhibited a trend of increasing concentration of Na2+ towards the surface, while the opposite was the case for sample 2. The concentration of Mg2+ was slightly higher for sample 1 than for the other samples, and concentrations were similar for all depths. The concentrations of No32-, although very low, were significantly higher in the 0-3 mm range of sample 3 than for any other sample.

20 - 18 -

~ 16 - 0 14 - E .§. 12 - G-3mm c = E 10 - "§ :J ~mm 8 - "E • S-9mm ..u c: 6 - 0 0 4 -

Sample1 Sample2 Sample3

Figure 6: The concentrations of soluble sodium (Na2+) for the three individual samples. 141

4.2 Petrographic Results

No variation in porosity was recorded throughout the depth of sample 1. There was, however, some evidence of slight dissolution of the sparry calcite binder at the surface of the stone. Small amounts of reprecipitated calcite were observed in the surface pores of the sample. In sample 2, porosity was higher near the surface of the stone. This was as a result of dissolution of the sparry calcite binder. There was no evidence of dissolution of ooliths or any other form of damage. In the petrographic section of sample 3, a delaminated layer, about 1 mm thick, was separated for the sample's surface. This layer was comprised of fragmented ooliths, gypsum and reprecipitated calcite. The surface of the stone behind this laminate was made up of a 1 mm surface layer where ooliths were fractured and separated from the stone substrate. In the top 2 mm of this substrate, gypsum and reprecipitated calcite were observed and some ooliths had been dissolved and calcite reprecipitated in their centres. Deposits of amorphous iron oxides were also observed in pores near the surface of the substrate. No altered features were observed more than 2 mm into the surface of the stone substrate.

5. DISCUSSION

5.1Sample1 The heavy crust present on this sample was probably the direct result of the sampling site's static wet surface environment. Wet surfaces result in the accelerated deposition of aerosols (7) and this, combined with the fact that the area was not subject to surface water flow (as a result of direct wetting by either rain or surface flow from adjacent areas), resulted in the heavy crust build-up. Soluble salt analysis indicated high levels of ca2+ and soi- as well as small amounts of Na2+ and Mg2+. It is therefore likely that the predominant soluble salt present was CaS04, with small amounts of Na2S04, Mg2S04, Na2C03, Mg2C03 and NaCl. Concentrations of the soluble ions were, without exception, highest in the 0-3 mm range. This sample depth represents the crust on the stone's surface, rather than the stone itself, indicating the carbonaceous build-up is rich in gypsum. Indeed, gypsum has been found to be the predominant component of black crusts, accounting, in some cases, for over 65% of the crust [8]. Petrographic analysis of sample 1 showed no variation in porosity form the exterior to the interior of the sample, and only slight evidence of binder dissolution at the surface of the sample was observed. This suggests that the presence of the surface crust is not causing any damage to the stone substrate. However, water retention by the crust after a rain event may have resulted in small amounts of surface dissolution of the stone.

The most likely mechanism of decay for this sample is illustrated in Figure 7. During rainfall, surface water movement results in the dissolution of some Portland limestone and salts. In areas subject to direct wetting, the resultant ions are transported both over and through the stone. After the rain has stopped and the stone's surface is still wet, gaseous and particulate deposition of S02 occurs leading to the formation of salts at the surface of the stone. The process continues and salt build up occurs only in areas which are not subject to direct wetting. The surface build up of the salt does not lead to greatly increased rates of stone decay. 142

/§Q2- ' II

2 so - ~ _.__-~ -~·~- (1) \j (2)

(3) (4)

Figure 7: A crust build up mechanism of decay. During rainfall, water moves over some areas of the stone and dissolves some salts and stone (1). After the rain has stopped and the stone's surface is still wet, gaseous and particulate deposition of S02 occurs (2) leading to the formation of salts at the surface of the stone (3). The process continues and salt build up occurs only in areas which are not subject to direct wetting (4).

5.2 Sample 2 The clean and washed appearance of Sample 2 were the result of its exposure to direct rainfall. The assumption that the sample area was frequently washed is supported by the results of the soluble salt analysis which showed that the sample had lower soluble salt concentrations of all salts towards the surface of the sample. The salts at the surface of the stone were regularly dissolved and carried away in solution either to an adjacent area or further into the pore structure of the stone. The petrographic analysis of the sample revealed that its porosity was higher towards the surface of the sample and that this increase in surface porosity was the result of sparry calcite binder loss. This suggests that rainwater movement in the surface pores of the stone has led to the dissolution of the binder. The mechanism of decay of this sample is shown in Figure 8. The Portland limestone is initially wetted by rainwater which results in the dissolution of the sparry calcite binder. The ion-rich water moves deeper into the pores of the stone where salts crystallize. After the rainfall has ceased, pollutants are deposited onto the wet surface of the stone and lead to acidic surface conditions. This results in the attack of the binder and the formation of salts in the surface pores of the material. These salts are dissolved during the next rain event. When the binder around a stone grain has been dissolved, the grain is lost. 143

Pollutants l l l

(2)

~(3) (4) Figure 8: The dissolution decay mechanism: here the Portland limestone is wetted by rainwater which results in the dissolution of the sparry calcite binder (1). The salt-rich water moves deeper into the pores of the stone where the salts crystallize. After the rainfall has stopped, pollutants are deposited onto the wet surface of the stone, leading to acidic surface conditions (2). This results in the attack of the binder and the formation of salts in the surface pores of the material (3). These salts are dissolved during the next rain event; when the binder around a stone grain has finally been dissolved, the gain is lost (4). 5.3 Sample 3 Sample 3, which was removed from a dry area of the building, was slightly soiled and exhibited surface delamination. The soluble salt content of the sample was found to be high and in the same range as Sample 1. However, unlike sample 1, Sample 3 did not have a surface crust, and the high concentrations of soluble salts were recorded within the stone matrix. As in the case of both of the other samples, soluble calcium and sulphate ions were recorded in the highest concentrations, thus suggesting the presence of Caso4. These concentrations were greatest in the 0-3 mm range. Petrographic analysis of the sample showed the delaminated layer to be approximately 1 mm thick and to be composed of fragmented ooliths, gypsum and reprecipitated calcite; the surface of the underlying layer was of a similar composition. This area of delamination coincided with the high soluble salt concentrations recorded in the 0-3 mm range of the sample. The decay mechanism is illustrated in Figure 9. Gaseous and particulate pollutants are deposited onto the surface of the stone. These dissolve in small amounts of surface and internal moisture - the result of surface wetting by mists or surface condensation. The resultant acidic conditions lead to the dissolution of small amounts of the sparry calcite binder. Upon evaporation of the solute, calcite and gypsum are deposited in the surface pores of the stone resulting in a reduction of the permeability of the surface of the stone. Small amounts of moisture become trapped behind this less permeable surface. Gases continue to be adsorbed by the stone and the sparry calcite behind the surface is altered to gypsum. Finally, differential movement occurs between the impermeable surface and the altered substrate behind. Water absorption by the gypsum and surface thermal gradients are responsible for this movement which eventually leads to separation of the surface from the substrate. This manifests itself as a blister on the surface of the stone which then breaks to reveal a laminate. 144

""1~. • . =o":'_1_i 'J ,.___..,

( 1) (2) (3) (4)

Figure 9: The delamination decay mechanism: pollutant deposition, surface stone alteration to gypsum and calcite reprecipitation results in the blockage of the surface pores of the Portland limestone (1). Small amounts of moisture become trapped behind this less permeable surface and lead to dissolution and mineral alteration in the sub-surface zone (2). Differential movement occurs between the impermeable surface and the altered substrate behind leading to blistering (3) on the surface of the stone which then breaks to form a laminate (4).

6. CONCLUSIONS

6.1 General The study relates three forms of decay to the surface conditions which lead to their occurrence. The loss of stone by solution occurs, not surprisingly, in areas exposed to direct rainfall. These areas have lower soluble salt contents towards the surface of the stone and higher salt concentrations further into the stone as the result of salt dissolution and transport processes. Crust build up occurs in areas subject to long durations of static surface wetness. Delamination and blistering of Portland occur in areas which are dry and subject to dry deposition of gaseous and particulate pollutants. The decay mechanisms themselves are described. Surface loss of Portland is caused when water dissolves the sparry calcite binder and loosens the grain of the stone, thus leading to its loss. Crust build up is caused when soluble calcium, which is transported from adjacent dissolution areas, combines with environmentally-derived sulphate to form surface gypsum. Delamination is a complex mechanism resulting from surface pore blockage which leads to accelerated sub-surface decay. In the samples examined, crust build up was not found to be associated with the same severity of damage to the Portland limestone as relief and delamination. It resulted only in small amounts of sparry calcite dissolution at the stone-crust interface.

6.2Practical Implication of the Findings The mechanism of Portland limestone dissolution results not only in the decay of the particular stone in question, but also in the transport of calcium ion-rich water to adjacent stonework. Here it is either available for deposition as reprecipitated calcite or for reaction with sulphate to form gypsum. Therefore, a reduction in the rate of dissolution of Portland limestone in areas exposed to rainfall would have the double benefit of reducing the rate of dissolution of the limestone itself and of reducing the amount of calcium ion available for reaction elsewhere on the building. This reduction could be achieved either by the use of a lead flashing, or by the application of a water repellent. 145

It is important to prevent the blockage of the surface pores of limestone in dry areas as this may lead to delamination. This could be achieved by simple maintenance in the form of a pressure water wash of the stone every year or so. This would dissolve and remove gypsum and calcite salts. Crust build up was not found to be very damaging to the Portland stone in the samples examined. The build up is, however, unsightly.

ACKNOWLEDGEMENTS The authors would like to thank the Commission of the European Union for the funding which made this work possible, the Stone Decay Research Team at Trinity College, Dublin, the Irish State Laboratory and the Buildings' Office, Trinity College, Dublin. The authors also wish to thank Erner Bell, Jim McDermott and Paul O'Brien.

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