19TH CENTURY PROTEST HELPS SAVE PARLIAMENT HOUSE FOR THE 21ST CENTURY TESTING MORTARS FOR REPOINTING SANDSTONE

Transcription

1 TH CENTURY PROTEST HELPS SAVE PARLIAMENT HOUSE FOR THE 21ST CENTURY TESTING MORTARS FOR REPOINTING SANDSTONE BEAUCHAMP I DAVI 0 eeauchcrnp Consulting Engineers pty Ltd zsembery, STEPHEN Brick & Mortar Research Laboratory SUMMARY Damage to the sandstone of Melbourne's Parliament House has occurred when cement rich mortars were used to repoint the stonework. A simple method of testing has been used to evaluate the compatibility of different mortars and sandstone to try and avoid this damage occurring when future repairs are made to the building 1. INTRODUCTION One of Melbourne's most important historic buildings is Parliament House. The eastern wing of the building was commenced in 1859 using Darley sandstone to clad the brick carcase of the building. The stone is a soft, argillaceous, sandstone of low durability. and problems with it soon occurred. Prior to the building the second stage of the of Parliament House, in , a wide search was undertaken to find a more durable sandstone. Eventually Stawell sandstone, which is a fine-grained, hard, siliceous sandstone, was chosen after a Board of Enquiry had examined and reported on suitable stones. Parliament House At one stage it appeared that sandstone from either New South Wales or Tasmania might be used instead of a local stone. As a protest against this possibility and to show the enduring qualities of local stone, the Member of Parliament for western Victoria, the Honourable John Woods, had a pillar of Stawell sandstone erected in the grounds of the Exhibition Buildings. This pillar is still standing and shows few signs of decay. From 1901, when the six colonial states of Australia joined together to form the Commonwealth of Australia, until 1930, when Canberra was built as the capital of Australia, the building served as the Commonwealth Houses of Parliament. As a mark of gratitude the Federal Government, in 1930, paid for the building of the north-east wing of Parliament House, which was also clad with Stawell sandstone. At about the same time repairs were made to the rest of the building and many of the joints in the sandstone were re-pointed with an orange coloured, cement rich mortar.

2 DAMAGE TO THE STONEWORK In 1994 Beauchamp Consulting Engineers were employed to survey the stonework prior to it being cleaned. During the survey it was found that damage was occurring at the joints. Testing the products of decay showed that at the worst decayed areas there were high concentrations of water soluble sulphates present, up to mg/kg. X-ray diffraction testing identified this material as bassanite, CaS04 x0.5h20. Typical Damage To Stonework 3. HISTORIC CLUE TO A SUITABLE MORTAR Because the pillar of Stawell sandstone erected at the Exhibition Buildings showed little decay, unlike the same stone at Parliament House, it was decided to test the mortar used for its erection. The mortar when tested gave off an odour of hydrogen sulfide. From this it was concluded that the cement used in the mortar was a blended cement containing blast furnace slag. The composition of the mortar was found to be 1 part cement, 0.0 to 0.6 parts lime and 0.7 to 2.7 parts sand. Cement was not manufactured in Australia in the 19th century and hence any cement would have to have been imported, probably from the United Kingdom. There are several references to the use of blast furnace slag in cement manufactured in the United Kingdom in the 1890's. Blast furnace slag is a pozzolanic material and it was thought its presence in the mortar might be a reason why decay at the joints had not occurred to the pillar of sandstone at the Exhibition Buildings.

3 TESTING PROCEDURE To check this assumption an experiment was designed to evaluate and compare the performance of four mortar mixes. Tests were carried out for a number of the physical properties of the wet and hardened mortars. The flexural tensile (bond) strength of the mortars and the sandstone were determined. An adaptation of a test developed by Ritchie 1 for the study of efflorescence in brickwork was used to test how much efflorescence each of the mortars produced. At the end of the experiment, both the type and quantity of the salts deposited on the surface of the stone wicks was determined and the effect of the respective mortars on the integrity of the stone was recorded. 5. MATERIALS The following materials were used:- 1) Cement A General Purpose (GP) Cement. 2) Cement B Builders Cement (GB), containing 20% fly ash. 3) Hydrated lime Commercially available brand (Limil). 4) Sand Fine 'double washed' sand from the work site. 5) Fired clay A cream burning local brick clay fired in a laboratory kiln at 9000 C and crushed to pass a 300 mm sieve. 6) Sandstone a) For masonry specimens: cut to size from Stawell sandstone from the Parliament House site. (200 x 100 x 60 mm). b) Efflorescing wicks: Stawell sandstone cut to 75 x 40 x 6 mm. Stawell sandstone has the following physical properties:- Compressive strength 72 MPa Water absorption 6.0 % Bulk density 2.30 Saturation coefficient 0.48 Clay content 5 % (mica and kaolinite) Average grain size 0.25 mm Composition Durability class 95 % quartz, well sorted A (0.3 % loss in 12 cycles of salt crystallisation) Binder Geological data Silica (quartz) Silurian-Devonian Western Trough sandstone 6. MORT AR MIXES All mortar mixes were batched as parts by volume. The bulk densities used for the proportioning of the mortar ingredients were based on published values and are given below : General Purpose Cement (C) and Blended Builders Cement (BC) 1500 kgtm 3 Hydrated Lime (L) 560 kg/m 3 Sand (S) 1675 kg/m~ Fired Clay (BO) 1500 kg/m Mix 1. - C 1 : L 1 : 56 Mix 3. - C1 : L2 : S9 Mix 2. - BC1 : L2 : S9 Mix 4. - BD1 : L 1 : S3 7. TESTS AND TEST METHODS 7.1 Physical Properties of the Mortars Testing of the physical properties of the mortars was done in accordance with the following Australian standards :- 1) Water retention of the mortar 2) Compressive strength of the mortar 3) Shrinkage of mortar 4) Liability to efflorescence (unused stone) 5) Flexural tensile (bond) strength of masonry AS AS AS AS AS

4 1580 For the flexural tensile strength of the masonry a counter-balanced bond wrench was used. The load being applied by a tensile testing machine and measured by means of a load cell and digital recorder. the method of constructing the test specimens was in accordance with the method given in Appendix A, AS The workability of each mortar mix was adjusted to provide a consistency judged to be acceptable to the stonemason. Each group of test specimens consisted of three stack bonded couplets. With the exception of the fired clay/ lime mortar, the test specimens were tested after 7 days curing under polythene wrapping at ambient temperatures. 7.2 Efflorescing Wick Test Test Specimens Freshly mixed mortar was placed in a plastic tube, 75 mm long with an internal diameter of 50 mm. One end of each wick was embedded in the mortar to a depth of 15 mm. Two specimens were cast of each mortar mix. The test specimens in their moulds were stored in the laboratory air for seven days before commencement of the experiment. Test Specimens Experiment After the initial curing period, the specimens were subjected to cycles of wetting and drying, by partially immersing each, still in its mould, in 30 mm of distilled water in separate containers. They were left to soak for 7 day, followed by 7 days air drying in the laboratory. This.process was repeated seven times, after which the specimens were allowed to dry, still in their moulds, for 28 days. At the end of the final drying period each of the stone wicks was examined and photographed, then carefully removed from its mortar bed. Any mortar still adhering to the portion of the wick previously embedded in mortar was removed by gentle scraping. The mass loss due to salt attack was determined by drying the specimens at 115 C for 24 hours in a ventilated drying oven, followed by cooling in a desiccator and weighing in an analytical balance. The specimens were then totally immersed in 200 ml of distilled water in a beaker and left to soak for 24 hours. The solution was then decanted and put aside and the extraction repeated in another 200 ml of water for 24 hours. At the end of this period, the two lots of 200 ml of solutions were combined and filtered through a No. 524 fine filter paper. The wick was washed five more times over the filter paper using jets of distilled water from a wash bottle. The wicks were returned to the drying oven for 24 hours of drying, followed by cooling and weighing. The mass of residue on each filter paper was also determined and the mass loss from each wick was calculated. The ph of each solution of efflorescing salts was determined with a portable ph meter. The mass of efflorescing salts on each wick was determined by evaporating the collected filtrates to dryness on a hot plate. The residue was cooled in a dessicator and we.:.:_ ig~h:_::e~d~ ~-

5 1581 The composition of the residue was determined by X-ray diffraction technique, identifying the salts present and their relative proportions. Sandstone Wicks at Completion of Testing Supplementary Tests After initial observations of the behaviour of the test specimens, an additional efflorescing wick was made from each mortar mix. After curing these specimens were partially immersed in a weak solution (0.2 g/l) of a mixture of hydrochloric and nitric acid. The concentration of the mixed acids chosen was to try and represent the long term effects of acid rain in the Melbourne metropolitan region. 7.3 SUMMARY OF RESULTS Properties of Fresh and Set Mortars and Bond Strength of Masonry Couplets Table 1 Mortar Mix Retentivity Average Compressive Strength of Av. Flexural Shrinkage (parts by Mortar(MPa) Tensile % volume) Strength of 7D~s 28D~s Maso'!!l'..J..MPal C1 :L1 :S BC1 :L2:S C1 :L2:S B/D1:L1 :S Not done C = General Purpose Cement BC = Geelong Builders Cement L - Hydrated Lime S - Sand BID - Brick Dust Results From Efflorescing Wicks and Other Materials Table 2 Specimen Sample ph of Particle (%) Salt Salts Found Observations Number Extract Loss (%) in Extract 1 Brick dust (1) None Not damaged BD1 :L 1:S3 identified 2 Brick dust (2) Not tested Not damaged BD1:L1 :S3 3 GP Cement (1) None Damaged C1 :L 1:S6 identified 4 GP Cement (2) Not tested Damaged C1:L1 :S6

6 GB Cement (1) Trona & BC1 :L2:S9 halite++ Damaged 6 GB Cement (2) Not tested BC1 :L2:S9 Damaged 7 GP Cement ( 1) Trena & halite Damaged C1 :L2:S9 8 GP Cement (2) Not tested Damaged C1 :L2:S9 9 Unused Stone Trena & halite x 10 GP Cement Trena & halite Damaged 11 Unused Stone not done Trena & halite 12 Mortar only GP 11.9 x 0.24 Calcitefr' & C1 :L 1:S6 x C1:L1:S6 _.9YE!.SUmlRJ 13 Mortar only 11.6 x 0.19 Calcite & x C1 :L2:S9 _.9YE!.SUm 14 Mortar only 11.8 x 0.17 Calcite & x 15 BC1 :L2:S9 bassanite """""" Mortar only 10.3 x 0.16 Calcite x B/D1:L1:S3 *immersed in weak acid ** Stone wick alone, previously soaked in water, not embedded in mortar *** Previously unused stone wick, not embedded in mortar + Trona = NaH(C03)2x2H20 ++ Halite = NaCl +++ Bassanite = CaS04xO.SH20 #Gypsum= CaS04x2H20 ##Calcite= CaC03 Note : Liability of the unused stone wicks to effloresce was found to be nil. 8. DISCUSSION 8.1 General Discussion While no laboratory tests are capable of exactly reproducing the conditions to which the stone is likely to be exposed during its lifetime, the results of such tests can be useful tools for the selection process. The test results of the flow, strength and bonding properties of the mortars evaluated are given in Table 1. In this investigation the bond and compressive strengths of masonry built with 'strong' and 'weak' mortars mixed with the cement types and a pozzolanic material, together with the potential of these mortars to promote efflorescence on the surface of the stone were compared. It was expected that the mortars made with the selected blended cement would have lower soluble salt contents for the following reasons : 1) Soluble salts in the mortar originate from the free alkali content of the Portland cement. Fly ash is expected to be free from salts and therefore a blend of cement with 20% of fly ash is expected to have less water soluble material in it. 2) According to the manufacturer of the blended cement, fly ash is able to react with some of the soluble material in the mortar. Therefore it was concluded that the use of a blended cement should result in less efflorescence on the surface of the stone. The results in Table 1 show no significant difference in the relative strength of the mortars made from the two cement types. The bond strength test results of the masonry samples made with cement based mortars exceeded the minimum requirements of the Masonry Code, AS The strength of the lime and brick dust mortar was disappointing. The improvement in strength, if any, gained by the addition of crushed brick dust to the strength of traditional lime mortar, is not worth the additional cost. The reduction in strength between 7 and 28 days curing was puzzling and may have been caused by unsuitable curing conditions. The compatibility of the mortars with sandstone, in this investigation, was assessed by both qualitative and quantitative means. The performance criteria of the experiment were that, at the end of the efflorescing wick tests, the mortars should not cause physical and/or visual disfigurement to the embedded stone's surface and that the selected mortar should have sufficient durability to resist weathering and be capable of achieving an adequate bond with the stone.

7 1583 The durability of the mortar also had to be balanced against the requirement that, if efflorescence was to take place, it should occur on the surface of the stone. As shown in Table 2, all three composition mortars damaged and disfigured the stone's surface, whilst the lime/brick dust mortar lacked strength and the ability to provide sufficient bond between the stone units. 8.2 Qualitative Assessment of the Visual Damage Stripes of grey/brown discolouration were found in the body of the stone specimens when they were soaked in water. It was observed that when a specimen was totally immersed in clean water, after only a short period of soaking, the water in the container turned brown. When only one end of the specimen was immersed in water, only the area above the immersed section of the stone showed dark discolouration, indicating the water soluble nature of the stain in the stone. Therefore during assessment of the performance of the mortars, the occurrence of the stain on the surface of the efflorescing wicks was attributed to the stone alone. In addition to the brown discolouration of the wicks above the surface of the mortar, the presence of white efflorescing salts was also observed, indicating that the mortar was capable of causing efflorescence. The extent of efflorescence increased with each cycle of wetting and drying and after five cycles, signs of deterioration were observed at the edges of the stone, adjacent to the level at which the efflorescing salts were deposited. As the effect of the mortar on the stone's surface became conclusive, the experiment was terminated after seven cycles. The test specimens soaked in the mildly acidic solution showed similar behaviour, except that the deterioration of the edges and the appearance of efflorescence on the surface occurred earlier. 8.3 Quantitative Assessment The results of the quantitative assessment are given in Table 2. There were a number of unexpected results obtained from this experiment and owing to the limited nature of the investigation, only qualified conclusions can be drawn from them. The variability of the results from one pair to the other with the same mortar was found to be larger than expected, but it is evident that the mortars made with either of the two cements caused reasonably large mass losses after completion of seven cycles of wetting and drying. The performance of the blended cement did not live up to expectation since, as the results show, the mass losses from the stone specimens in this mortar were on the average some 40% higher than that observed with General Purpose cement (formerly known as Type A Portland cement) mortars. The nearly 20% higher mass losses from the stone specimens embedded in the lower cement content C1 :L2:S9 mortars was also unexpected. However, owing to the small number of results and the observed high variability between pairs of results, this difference may not be as significant as it appears. The same qualification applies to the quantity of efflorescing salts collected from the efflorescing wicks. 8.4 X-Ray Diffraction Tests XRD patterns were obtained from 9 samples. In most cases the crystalline phases could be identified satisfactorily, however in the case of samples 1 and 3 the patterns were too poor for positive identifications. The leachants tended to fall into two groups, either sodium or calcium salts. In the former category two main phases were identified: Trona = Halite = NaH(C03)x2H20 NaCl In the latter category the following were identified: Bassanite = Gypsum= Calcite = CaC0 3 CaS0 4 x0.5h20 CaS0 4 x2h20

8 1584 The mineral names were used for convenience. It should be noted that the phases identified did not necessarily exist in the mortar, but may have formed by the chemical processes after leaching. However they do identify mobile ions in the original materials. Bassanite, in particular, was almost certainly from gypsum when the leachant was dried in an oven. Similarly, calcite was probably formed from the carbonation of calcium hydroxide leached from the sample. The discrepancy, as shown in Table 2, between the composition of the salts identified in the X-ray diffraction tests from the efflorescing wicks and from the water soluble extracts from the mortars on their own was thought to have been due to the different curing conditions for the test specimens from which the leachant were derived. As such, different chemical reactions have occurred in the specimens. The samples for the 'mortar alone' results were the crushed pieces after compression strength testing. These samples were air cured for at least 4 months before the determination of their salt content, while the efflorescing wicks had undergone cyclic wetting and drying. 9.0 CONCLUSIONS The results of this investigation clearly demonstrated the need for care in the selection of a suitable mortar to use in sandstone masonry. The past performance of the sandstone in Parliament House is consistent with the results of this investigation, which showed that mortars containing Portland cement have the potential to damage the stone masonry. None of the mortars evaluated met the requirement for the task they were nominated for. While the lime/brick dust (pozzolan) mortar was found not to damage the stone's surface, its strength and bonding properties were found to be inadequate. The composition mortars, containing both cement and lime had adequate strength but were found to damage the stone's surface and therefore were deemed unsuitable for use. Further research is needed, possibly testing mortars with lower cement content and evaluating mortar additives which claim to reduce efflorescence from the mortar. REFERENCES 1. Ritchie T, October 1995, 'Study of Efflorescence Produced on Ceramic Wicks by Masonry Mortar', in Journal of The Ceramic Society. Vol. 38, No. 1 O