Mortars for use with granite masonry: a feasibility study.

Final Report

KTTBE award 2007/8

Prepared by:

John Hughes

University of the West of Scotland, Paisley Campus, Paisley PA1 2BE, Scotland, john.hughes@uws.ac.uk

With

Steven LaingLaing Traditional Masonry, The Stables, Castle Fraser, Sauchen, Aberdeenshire AB51 7LD, Scotland, steven@traditionalmasonry.co.uk

Maureen E. YoungHistoric Scotland, Technical Conservation, Research and Education Group, South Gyle Crescent, Edinburgh EH12 9EB, Scotland, maureen.young@scotland.gsi.gov.uk

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Part 1: Project findings

General SummaryDamage to masonry and interior dampness, related to water ingress and retention highlights the correct choice of mortar when pointing walls. Compatible mortar composition is important where masonry units are impermeable, as in the case of load-bearing granite, because the mortar controls the ingress and egress of moisture. It is common to find water penetration which can be traced to incorrect specification of mortar; inappropriate mixes trap moisture inside walls, causing problems with frost heave and bedding mortar decay. The development of appropriate mortars for structural granite walls is an area which has received little investigation.

Previous studies have emphasised the use of cement-based mortars for repointing of granite masonry, as do standard specifications for masonry walling. Some studies have acknowledged that deformability (to absorb movement) is a desirable property, and have attempted to minimise the content of soluble calcium as a source of damaging salts. This resulted in the formulation of very lean cement mortars (1:10). The experience of several practitioners and testing laboratories has also been to focus on the use of cement in mortars in combination with hydrated lime, and the single use of hydraulic lime binders to ensure early set and adapt the properties to match that of granite, which is a very hard stone. The quality of the bond between mortar and stone, particularly in the fresh state and its later influence on adhesion in addition to shrinkage of mortars leading to de-bonding, is repeatedly raised by practitioners questioned for this study.

In the UK, the use of cement is frowned upon for repointing, repair and maintenance of older and historic stone buildings, even for granite. Traditional methods of repair demand the use of lime-based mortars to ensure authenticity and compatibility. Current practice is increasingly to use hot-lime mortar mixes, those that use quicklime mixed directly with sand and water, which slakes intimately with the aggregate releasing a considerable amount of heat. These mortars are anecdotally suggested to improve the properties of mortar, and historic mortars exhibit evidence for hot-mixing in almost all examples, further reason to demand hot-mixing for repair mortars used on historic buildings. This traditional practice is being rediscovered currently, but there have been very few studies into the properties of mortar made this way. In this study we aimed to reflect real practice, and study and compare the properties of mortars mixed hot with quicklime, and consider what properties that could be measured to best evaluate performance for use with granite masonry, to improve the moisture handling characteristics of mortar.

Four mortar mixes, three containing different types and proportions of lime and one cement-quicklime mix, were prepared using current site methods (volume mixing) and prepared into standard specimens for later testing, as well as being applied to small blocks of granite to asses ease of working and the bond between the mortar and stone. Fresh properties of workability and water retention were measured just after mixing. After a period of curing (setting and hardening) the samples were subjected to compressive and flexural strength testing and capillarity measurements (capillarity refers to the ability of the mortar to draw water into its structure). The mixing and curing process was relatively unconstrained, but was faithful to site practice.

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The results reveal that the cement-lime mortar has the greatest strength and lowest capillarity (ability to absorb water). The lime-based mixes have lower strength, and a higher capillarity (up to 4 times that for cement). The bond properties appear satisfactory under qualitative visual inspection, and shrinkage was lowest for the cement-lime mix, the other mixes showing a shrinkage of a maximum of 1.2%. There is a predictable inverse relationship between strength and capillarity.

There is much still to resolve- for example the drying behaviour may be critical in the performance of these mortars, and counter the tendency to view the lower absorption cement mortars as preferable to prevent water ingress. Further porosity, water vapour permeability, bond and durability testing would be desirable to understand the effects of mortar composition on compatibility and moisture handling properties of granite masonry.

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The problemMoisture penetration and interior dampness in solid wall granite masonry buildings has been recognised as a significant problem in Aberdeen and the North East of Scotland (Fig 1). A study of the characteristics of the problem, its causes and the perceptions of practitioners and possible solutions was led by Aberdeen Council (through the Aberdeen Property Care Initiative) and carried out by the Robert Gordon University The results of this study are summarised in Young (2007).

Figure 1: Example of dampness in tenement gable in Aberdeen (Photos: Maureen Young). On the right cement-based mortar retains moisture, drying more slowly than the surface of the granite, mainly due to the higher capillary water retention of the mortar.

The low porosity and impermeability of granite1 means moisture movements are necessarily concentrated within mortar joints. The majority of dampness problems occur in 19th century granite buildings, typically tenements with solid wall construction. Ashlar walls present a minimum area of exposed mortar joints for moisture to penetrate, whereas rubble walls, common on the rear and gable facades of traditional stone buildings in Scotland, present a relatively larger area of mortar to the environment. In the study by Young, the majority of problems were encountered in rubble walls, suggesting a role for absorbent mortar in moisture ingress.

Traditional construction methods utilised lime-based mortars to bed and point masonry, and often on rubble facades mortar was applied as a harl, or thrown render coat, which served to better protect more vulnerable construction from environmental forces. Changes in knowledge and availability of materials in the 20th century led to repair and reconstruction of stone masonry using cement-based mortars. These are mostly harder and more inflexible than the original lime-based mortars that they replace. Mechanical incompatibility often leads to cracking and the debonding of mortar from masonry units, and the development of routes for moisture ingress. Cement mortar also presents a pore-size distribution in contrast to lime based mortars, possessing generally finer pores and lower moisture and water vapour permeability. In effect this means that when cement absorbs moisture through a higher capillary suction force, it cannot dry as quickly as lime. In addition, if moisture enters masonry

1 Or any impermeable unit; the same issues discussed here are applicable to low or zero suction units, such as engineering bricks and glass bricks. Granite typically has a porosity of less than 2%.

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through cracks, the lower porosity and permeability of cement pointing slows or prevents effective drying.

Granite is a very hard rock, and, in principle, cement is broadly compatible with it physically and mechanically. General specifications for mortar composition based on unit type and design for durability place cement and cement-lime mortars as suitable for use with hard masonry units (e.g. BS5628: 2001). However, evidence from case studies of granite buildings with moisture ingress problems in Aberdeen indicate that problems often closely follow repointing and repair measures where cement mortar is utilised. The materials and properties of repointing mortars are clearly implicated in causal relationships with moisture ingress. Maintenance of the building rainwater goods and other features that control water shedding from the building is, of course, an important factor in this. The role of mortar is significant in permitting ingress and preventing egress of water.

The work by Young (2007) suggests that some mortars may be too permeable, allowing the ingress of moisture. The indicated range of permeability some of these mortars is consistent with lime-based material. Also, the practice of removal of older mortars, especially when mechanical power tools are used, is suggested to dislodge material that bridges void spaces, particularly between the internal face of the solid masonry wall and lath and plaster. Combined with larger areas of mortar exposed on rubble masonry walls there is a tendency for worsened water ingress in these situations.

The views of practitioners dealing with dampness in granite masonry buildings indicated that the most commonly perceived causes of moisture penetration problems occurs due to defects and failures of roofing or rainwater goods. A minority reported that hard mortars contributed to dampness problems and that problems began after repointing had taken place. In relation to the role of mortars in dampness most respondents agreed that mortars are commonly incorrectly specified, often being too hard. For repair, the most effective interventions were repairs to slating, guttering and measures to effectively manage water shedding from buildings. However, repointing and even re-rendering (harling) of buildings was successful in 50% or more of cases reported.

Previous workThere is not a great deal of published work that refers specifically to the application of mortars for the construction or repointing of granite masonry. A brief overview of some selected literature is given in this section.

Duffy et al. (1993) describe their work in selecting a repair mortar for the repointing of the facades at Trinity College in Dublin, a building constructed of a combination of Dublin Granite and Portland Limestone. They established general requirements for the selection of a pointing mortar, it should:

prevent the ingress of moisture be weaker than the substrate, to allow removal later without damage be deformable to allow movement and prevent cracking and debonding that

allows ingress of moisture

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not be a source of soluble calcium, that can react with pollution and form damaging gypsum crusts

avoid shrinkage; strong mortars often have this property, resulting in a loss of bond

posses “good” workability, that improves application and therefore appearance.

They tested a range of mortars, all containing cement, on its own or in combination with lime, PFA, Barium Hydroxide and with and without plasticizers. They found that a very lean cement:sand (1:10) mortar, with plasticizing admixtures to ensure workability for the mason, possessed low strength, and the lowest content of soluble calcium. However, their lime based mortars once aged also had low soluble Ca contents. A slightly modified version of this mortar was applied to the building. In a related study, O’Brien et al (1995) looked at the effects of these mortars on the decay of Granite at Trinity College, and found a relationship between the position of decay and older mortar joints, the latter acting as a source of soluble Ca ions that combine with atmospheric pollutants such as SO2 to form damaging surface crusts. The repair mortar was confirmed to produce less Ca in surface runoff due to the lack of free lime in the cement-based mortar.

Mosquera et al. (2006) considered a range of restoration mortars containing cement from the point of view of how pore structure affects vapour diffusion. They found that binder composition affected pore-size-distribution in the mix in the most profound manner, lime mortars having generally coarser pores than cement-based mortars, something that is well understood. The lean 1:9 cement :sand mortar exhibited mechanical properties similar to a 1:1:6 Cement :lime :sand mortar, but with much lower vapour diffusivity. This is significant as they, and other studies (e.g. Young 2007), acknowledge the importance of mortar joints in impermeable masonry for dealing with moisture movements. They conclude however, that the 9:1 mortar is the most suitable for granite masonry as it possessed the lowest free calcium content- reiterating the conclusion from Duffy (1993) and O’Brien (1995).

The effect of cement on the properties of mortar, in relation to porosity and vapour transmission, in combination with mechanical properties is significant. Colantuono et al (1996) discuss the formulation of mortars to restrict rising damp. As cement content increases the capillary coefficient decreases along with the time for soaking, but also the time for drying. Cement rich mortars also posses lower total porosities compared to lime rich materials, and generally finer pore-size distributions, particularly pores coarser than 1 um (Arandigoyen and Alvarez 2007). The difference in pore sizes will have a significant control on water ingress and drying behaviour of mortars.

As part of this study the opinions of a group of international experts was canvassed (members of the RILEM TC-203RHM “Repair Mortars for Historic Masonry” and the NRC-CNRC Mortars group in Canada). Responses highlighted and generally converged on important issues in the specification of mortars for use with impermeable masonry units. Granite is used as a masonry unit material wherever it occurs, and has often been chosen not just for availability but also durability An interesting point of comparison is that the responses were from northern countries- Canada and Finland but also Portugal, where there is a significant amount of granite construction in the north and west of that country.

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The main points to emerge from the experience of these correspondents, that can be expressed as requirements for the design of a repair mortar were:

Strength: this is not considered to be very important, but that weaker mortars tend to be less durable. This was highlighted by workers in Canada, who are particularily concerned with designing for freeze-thaw resistance.

Shrinkage: cement promotes higher shrinkage, therefore promoting a loss of bond between mortar and stone, which can let water penetrate masonry. It was thought that a slightly expansive mortar was desirable, achievable by using lime.

Bond: this was considered to be perhaps the most important aspect of mortar application on granite. As granite has a very low porosity, it is essentially impermeable, there is no suction on the mortar and therefore less formation of an intimate bond. If mortar has poor water retention then application and bleeding on-site may generate a thin film of water that prevents contact between the stone and binder. See also below for details of a CRN-CNRC study into bond on granite.

Moisture movement: all agreed that the mortar should permit effective drying of masonry through the mortar joints, whilst preventing excessive ingress. Good workmanship and careful detailing plays a significant role in this to prevent pooling of water or excessive joint thicknesses2.

Materials: almost all the experience and practice reported (including the references reviewed briefly above) described using mortars that included Portland Cement. The following Table 1 is a summary of mixes suggested and/or used in practice: all the mixes do contain lime, and the highest cement content is in the 1:1:6 mix, which is the minimum cement content-mix that would be recommended in current masonry codes for hard masonry units (e.g. Eurocode 6).

Table 1: summary of mortar compositions suggested by correspondents to this study.

Cement:lime:sand Lime:Hydraulic Lime:sand Additives?1:2:8-9 1:2:81:1:6 1:2:9.5 well graded

1:2:7.5 n.w. graded1:3:12 1:1:6 (SA lime) lime with air entrain.1:3:9 (SA lime) mud and straw1:3:8

The NRC-CNRC (Canadian National Research Council) recently carried out a short study of bond strength of mortar mixes for the restoration of the Supreme Court of Canada building in Ottawa (Trischuk K 2007), a building constructed from granite. The mortars were a mixture of standard cement-lime mixes, proprietary mixes and hydraulic-lime based mixes. Fresh properties were recorded along with compressive strength derived from mortar cubes and flexural bond strength derived from stack-bonded masonry prisms. Bond strength was highest for mixtures containing cement,

2 There is a suggestion that traditional practice in Scotland was to deliberately widen joints out to cover adjacent masonry. This may have the, currently unproven, effect of providing a greater area of mortar from which moisture can evaporate, which using a traditional lime mortar, the pore characteristics would permit more effective evaporation compared to cement (A. Forster pers comm.)

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which also had the highest density compressive strength, and for the standard 1:1:6 mix the lowest air content (consistent with the highest density). Interestingly, wetter mixes gave higher bond strength values. The highest bond strength achieved was from a proprietary mix similar to a 1:1:6, with unknown additives that may have promoted a good bond. The strongest mix in compression was unsurprisingly the rather dry 1:1:6 cement mix, which if made wetter and more workable may have achieved a higher bond strength.

AimsGiven the issues outlined in the previous sections, the question arises of what materials, what methods of preparation and application produces a mortar for repair (and reconstruction) of granite masonry, that is compatible with the masonry and minimises both water ingress and facilitates effective drying. This study aims to establish the feasibility of practical approaches to the development of adaptable and compatible repair mortars for granite masonry. The current practice of the industry partner in this study, Laing Traditional Masonry (LTM) focuses on the use and rediscovery of traditional hot-lime mortars, a topic that has not received a great deal of academic research to date. Hot-lime mortars are increasingly in vogue with conservation practitioners, as they appear to provide performance benefits, but have never to date been the subject of a technical study to clarify their performance in relation to other mortars. This feasibility study aims to narrow the focus on the development of the variety of adaptable hot-lime mixes for use with granite masonry. and to understand the type of performance indicators that can be used to increase the sophistication and compatibility of specification.

Mix design, sample preparation and tests.The mortar mixes for this feasibility study were chosen mainly from the viewpoint of the practitioner, with an emphasis on lime as a binder, notwithstanding the discussion above as cement is not generally viewed favourably as a component of conservation and repair mortar mixes for stone masonry in the UK. It was decided that as a point of comparison that a mortar containing cement should be considered, with one important difference, that all of the mortars were, as reflects current trends in conservation mortar practice in Scotland, mixed hot (the mortars discussed above in the review all contain hydrated non hydraulic and hydraulic lime or cement).

Hot mixing is when the mortar is mixed using quicklime, added to the mix that hydrates and exothermically reacts to form hydrated lime intimately in the mix during preparation. This is now understood to be correct method of traditional lime-based mortar production, though it had fallen out of favour since the early 20th Century. The cement gauged mortar was also mixed hot with quicklime. Details of the mixes are given in Table 2.

The binders used were: Mix 1 :NHL 3.5 Singleton Birch lump quicklime (Fig. 2) Mix 2 : Singleton Birch non hydraulic quicklime (6mm chips Fig.2) Mix 3: Socli Roundtower NHL 5 Hydrated lime gauged with Singleton Birch

non-hydraulic quicklime Mix 4: Blue Circle OPC (42.5) gauged with Singleton Birch non-hydraulic

quicklime

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Table 2: Details of mortar mixes prepared. Proportions are in measures of volume. Absolute weights of materials given in italic below volume proportions.

Mix designation 1 2 3 4LimeQL NHL 3.5 1Wt kg 9.3QL Non Hydraulic 1 1 1Wt kg 9.6 9.6 4.8NHL5 Hydrated lime 1Wt kg 7.9OPC 1Wt kg 7.3Sand 4 4 6 6Wt kg 67.4 67.4 101.1 56.45

Total water 15 10.8 15.6 10.61

Water demandWt % water/binder 1.61 1.125 0.89 0.88% water 16.4 12.3 12.3 13.6Water retention % 88.6 92.3 87.9 93

Flow cm 1 16.3 14 14.4 13.2@ 20 mins 15.2 13.2 13.5 11

Wt (g) 0.5 l 962.2 984.8 992.7 971.2Density 1.92 1.97 1.98 1.94

The selection of mixes was based on current practice of Laing Traditional Masonry (LTM), with the addition of a comparison cement-lime mortar. Mix 1 is currently being used by LTM in a new-build project using granite. Mix 2 is a typical non-hydraulic hot lime mortar commonly used for restoration work in the UK, and represents a compositional end member in the lime-cement binder family. Mix 3 is a gauged mix of non-hydraulic hot lime and hydraulic hydrated lime (an NHL 5), which is a widely available material, increasingly used in restoration and new build in the UK and further afield.

Mix 1 is more unique in a wider context, as hydraulic quicklime is not commonly used in hot-lime mortar. All of the mortars represent a novel approach to formulating mortar for the repair of granite masonry buildings. This study was intended to establish the feasibility of this approach of using hot-lime mortars and to highlight the properties of such mortars that will enable an improvement in site practice and an increased sophistication in design for compatibility in granite masonry.

The mixes were proportioned by volume, the weights also recorded and then mixed in a rotating drum mixer (Fig. 3 - not ideal for lime based mortars, but not that uncommon in practice), prepared at the workshops of Laing Traditional Masonry at Castle Fraser. The consistency of the mortars was set by the masons and workability tested according to BS4551 : Part 1: 1998, on a flow table (Fig.5). Water retention was determined using the filter paper method according to EN459-2:2001 (Fig 5).

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The density of the fresh mortar was determined by weighing 500ml of mortar tamped into a specially truncated measuring cylinder.

The aggregate used was Tom’s Forest concrete sand, with a grain size up to 7mm (Grain Size Distribution information not available at the time of writing). Standard mortar prisms (160 x 40 x 40 mm) were produced using aluminium molds coated with release oil, according to the methodology on EN 1015-11, six prisms made for each of the four mixes. Small 2-layer stack-bonded prisms (Fig. 4) were made using 50x50mm square cuts of granite, at least three per mix. Larger mortar “sandwiches” were also produced using longitudinally cut granite prisms (Fig 4). These specimens were intended for testing of bond strength by direct tension (still to be performed at time of writing) and qualitative estimation of bonding characteristics of the mortar with granite. This also served to indicate the ease of application of the fresh mortars. Discussions with the masons from Laing Traditional Masonry revealed that they felt the smooth cut surfaces on the granite blocks were much smoother than would used in practice. They had concerns that the mortar would not bond in the wet state. As a result some were bonded on smooth cut surfaces and others on rougher surfaces that were often available on the reverse of the blocks away from the side originally intended to be used for bonding. This should give a qualitative indication of the effect of surface roughness in keying mortar, and promoting bond.

Figure 2: Left- Singleton Birch NHL 3.5 lump quicklime used in mix 1. Right: Singleton Birch 6mm chip non-hydraulic quicklime used in mixes 2, 3 & 4.

Figure 3: Left; Andrew adding water to achieve desired workability. Right: mix 2, chips of quicklime still visible in mix- lending a “lime inclusion’ texture typical of hot lime mixes.

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Figure 4. Left. Fresh prisms in mould. Seven prisms to right are mix 1, and the contrasting mix to the left is mix 4, containing cement, causing the dark grey colour. Right: Small stack bonded prisms of granite, and longer sandwiches, in this case with mix 4 cement-lime mortar in joint.

Figure 5: Left: Measuring workability using a flow table, the spread of the mortar is measured after controlled falls of the flow table. Right: Water retention test. A fixed volume of mortar is placed on a filter paper with known suction for a set time. The water loss from the mortar is calculated as a water retention %.

Figure 6: Left: arrangement of flexural test- prism approx 160mm length, with flexural crack visible at point of failure. Right: arrangement of compressive test. The broken ends of the prism that were split during the flexural test are used for this test. Steel plates, 40mm in width are used to compress the sample, to achieve a standard cross section of 40x 40mm.

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The specimens were cured indoors at Laing Traditional Masonry’s workshop at Castle Fraser. Initially they were kept under Hessian to prevent excessive drying, as per on-site practice. The prisms were removed from the moulds at 14 days. An earlier extraction was considered, but even at 14 days the mortars were still soft. After that time the prisms and stone ‘sandwiches’ were kept in temperatures estimated between 10-20C and over 50% RH. No direct continuous recording of temperature or relative humidity was carried out.

Flexural and compressive strength testing was performed according to EN 1015 part 11, using a Dartec universal test apparatus, under displacement control of 1mm/minute for both flexure and compression (Fig. 6). Capillarity testing was performed on mortar prisms according to EN 1015 part 18. Three prisms per mix were tested and four each of the broken halves of the prisms subjected to compressive testing (Fig. 9 below), and two half prisms per mix for capillarity.

Shrinkage was assessed through the deviation of the mortar prisms from the mould dimensions, along the length of the prisms. At this stage bond with stone was assessed qualitatively and visually. Direct tensile testing is planned for the near future, once an experimental set up can be devised to mount and grip the test specimens effectively.

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ResultsThe method of mixing of mortars was deliberately realistic, as described above, reflecting on-site practice and following the methods used by Laing Traditional Masonry. This makes direct quantitative comparisons between the mixes more difficult than might otherwise be in a laboratory study. However, the results show some general trends worth noting. Table 3 contains a summary of the test results.

Table 3: Summary of results

Mix Flex Stren.

Comp. Stren.

Capillarity Coef.

W/B Flow Water retention

At 33dN/mm2

At 33dN/mm2

90minKg/(m2/min2)

24 hrKg/m2

cm %

1 0.38 0.94 1.55 20.22 1.61 16.3 88.62 0.39 0.62 1.65 18.44 1.125 14.0 92.33 0.44 1.04 1.51 20.06 0.89 14.4 87.94 3.06 9.84 0.38 17.50 0.88 13.2 93.0

Fresh PropertiesAll the mixes preformed well in the fresh state, and there were no issues of bleeding once in contact with the granite specimens. The masons generally felt that the cement based mortar (mix 4) was much stiffer than would be used in practice. The mortars were hot, and were evaluated at approximately 20-30 minutes from the start of mixing. There are known, but largely unconstrained issues with the properties of hot-lime mortars, especially the control of proportioning and their fresh properties (e.g. Forster 2004, Hughes and Taylor 2008). The presence of visible white lime inclusions in all the mixes (see Figure 7) indicate that lime was, in all probability, still hydrating and taking water from the mix and therefore reducing workability during the tests. The cement mortar is notable for the lowest water/binder content, accounting in large part for the lack of workability. The lime based mixes had a greater workability but with a much larger water content, due in fact to the necessity of water addition to hydrate quicklime, to produce a workable binder phase. The water content of mix 3, that contains 50% hydrated lime in the binder, confirms that there is a reduction in water demand for an already hydrated lime binder.

Figure 7: Lime inclusion texture (the white spots are unmixed quicklime) of the fresh cement mortar (specimen produced for water retention test).

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Hardened PropertiesThe flexural and compressive strengths of the mixes confirm (see Fig. 8), of course, that the addition of Portland cement increases strength considerably. The lime rich mortars have a higher ratio of flexural strength to compressive strength than the cement-lime mortar. The values for the lime mortars are below that expected of the classifications of materials involved, except in the case of the non-hydraulic lime mortar. However, for the cement mortar the strength is larger than that given in, for example BS5628: 1992, that gives strength values for a 1:1:6 cement lime mortar between 2.5-3.6 N/mm2. Duffy et al (1993) give their values for 1:1:6 28 day strength at approximately 7.5 N/mm2, and Mosquera et al at 6.2 N/mm2. The value obtained here of 9.84N/mm2, is also perhaps surprisingly large as it was mixed by volume with quicklime, which on hydration effectively doubles in volume, and would therefore be expected to produce a more lime rich mix that that typically prescribed in standards such as BS5628.

0

2

4

6

8

10

12

1 2 3 4

Mix No.

N/m

m2

Flexural Strength

Compressive Strength

Figure 8: Flexural and compressive strengths of the test mixes.

Capillarity The capillary coefficient (See fig 9 below) of the mixes, both for 0-90 minutes and for 24 hours absorption is inversely correlated with strength (Figures 10 & 11). This is interpreted to be clearly associated with binder type, the effect of cement addition producing the greatest difference. This will relate to the microstructure obtained from the addition of cement, which is widely recognised to change the pore-size distribution, towards finer pores, and in some cases a reduction of overall porosity. This is consistent with the findings of Mosquera et al (2002).

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Figure 9: Capillarity testing. Mortar prisms have their sides coated in wax, are then split in two and the broken surface immersed in up to 1cm depth of water. The absorption of water into the mortar by capillary suction is then measured by weighing the samples at set intervals.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1 2 3 4 5 6 7 8 9 10 11

Compressive Strength N/mm2

Cap

illar

y C

oeff

icie

nt K

g/m

2/m

in2

Figure 10: Variation of Capillary coefficient (0-90mins absorption) with compressive strength.

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17.5

18

18.5

19

19.5

20

20.5

0 1 2 3 4 5 6 7 8 9 10 11

Compressive Strength N/mm2

24ho

ur C

apill

arity

Coe

ffic

ient

Figure 11: Variation of Capillary coefficient (24 hours absorption) with compressive strength.

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Mix 4

Mix 4

2

2

1

1

3

3

It is clear from these elementary tests presented here that there is a significant difference in pore-related moisture absorption between the lime-based and the cement-lime mortar. It is not possible to distinguish the lime based mortars particularly from each other on the short-term capillarity. It is notable, however, that the non-hydraulic hot lime mortar (mix 2) has the lowest 24 hour capillarity amongst the lime-based mortars (Figure 11).

Shrinkage, Adhesion and bondThe shrinkage of the mortars was measured simply, looking at the lengths of prisms at the test dates. The lime-based mortars (mixes 1 to 3) showed a slight shrinkage of 1mm and at most 2mm, over the 160mm length of the prism, a percentage shrinkage of 0.6 -1.2%. The cement-lime mortar did not show any shrinkage at all. This implies a probable slight expansion, perhaps due to hydrating quicklime in the cement-lime mortar. Cement mortars are expected in general to exhibit shrinkage, so this possible expansion may be balancing the inherent tendency of the cement mortar. The cement-lime mortar also had the lowest water content of the mortars, and as wetter mixes are known to exhibit shrinkage, so the lack of shrinkage may also be related to the low water content of the mortar.

All the stone-mortar sandwiches appeared to be well bonded, each could be lifted by the top layer of stone without disruption. As mentioned above, it is hoped to obtain a measure of direct tensile strength in the near future, once an experimental setup can be devised.

AppearanceThe feasibility of designing adaptable mortar mixes for use in repointing granite masonry is in part dependant on aesthetic properties of the mortars, especially as the mortars used for repointing can constitute a high proportion of the surface area of a structure. The lime-based mortars, mixes 1 to 3, all posses a generally light-cream- beige appearance, controlled largely by the colour of the lime. This can be altered by the addition of pigments of course, but also fundamentally by altering the sand type, texture and colour. Figure 12 below shows the 4 mixes side-by-side. The dimensions of the prisms are 160mm in length. The cement mortar on mixing was a dark grey colour, that lightened on setting. However it retains a very grey colouration even once dried.

All the mixes exhibited the characteristic white spotting of a hot-mixed mortar (see Figure 7 above), and that is very common in historic mortars in older buildings (Hughes et al. 2001). The white spots are caused by the failure of some particles of quicklime to hydrate and mix smoothly into the mortar. This can cause problems, as the quicklime that does not initially hydrate can do so later, and its expansion, to nearly double volume, can cause unsoundness in the mortars. Previous experience of hot-lime mortars has highlighted steps that can be taken to minimise the effect (Hughes and Taylor, 2008), such as longer mixing or pre-hydration. None of the mortars however suffered catastrophic disruption, and so were judged to be usable in practice.

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Figure12: Comparison of appearance of the 4 mixes, from left to right mixes 1 to 4.

Conclusions and outstanding issuesOverall, the approach to applying hot-lime mixes for use with granite appears, in practice to be feasible. The results are consistent with general trends of material behaviour evident from the literature, particularly that cement increases strength, and reduces capillarity in the mortars.

The mortars can all be applied to stone acceptably, in the fresh state, and the lime based mortars offer apparently adequate bond and strength properties. No deleterious expansion was encountered in the prism samples, and perhaps more significantly in the stone “sandwich” samples, there was no delamination due to late lime hydration.

The project aimed at delivering an outline of appropriate testing. The measurement of strength properties was determined partly by ease of measurement, but also for establishing a base of comparability with other studies (it is very commonly tested) and for quality assurance. It is also a possible indicator of other properties, as strength is related to porosity, for example, and also properties such as bond. We also demonstrate a relationship with capillarity, and important moisture transport property.

However, several questions remain, and this study usefully points to issues to be resolved in the future.

Curing: The samples were produced in a realistic manner and cured in mostly “real”, variable conditions, though indoors. Future characterisation studies

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would benefit from a more consistent approach to curing to allow better comparisons between mixes, devoid of environmental affects.

Proportioning: the mixes were proportioned by volume, as is practiced on-site, but this makes experimental testing and comparison between mixes more difficult that those proportioned by mass. Also the use of quicklime that increases in volume in the mixing process makes it much harder to understand the real proportions of a given mix once hardened.

The lime mixes presented here clustered around a narrow range of values of strength and capillarity, relative to the difference between them and the cement mortar. The cement mortar is useful as an experimental control, but not acceptable generally for real conservation and repair works on masonry. Understanding the variations in properties of lime-based mortars, mixed hot, is perhaps a higher priority for the future.

Capillarity data suggest that the use of lime-based mortars would result in a significant ingress of moisture into masonry joints. This needs to be balanced against the drying behaviour of the mortars. This should be measured in the future alongside vapour permeability, a major determinant of the egress of moisture from within the microstructure of the mortar.

Mechanical properties: apart from compressive and flexural strength, the significant property that should be measured is the strength of bond between the mortar and the stone. Bond is a key to the likelihood of fracturing along the interface and subsequent moisture ingress. Another key property, in relation to bonding, is the elastic modulus, a measure of the elasticity of the material, or coarsely put, its deformability. The lower the elastic modulus of the material the more ability it has to accommodate movements in the masonry, that could cause cracking, de-bonding of mortar from stone and moisture ingress. There is a general principal that elastic modulus should be designed to be within certain limits, imposed by the masonry unit’s modulus (e.g. Knöfel and Schubert 1993). More information is clearly required to understand this especially for hot-lime mortars.

Ageing and durability: how do all the properties discussed above change with time, and how does that affect the performance of masonry, in regard to moisture movements. The effect of environmental exposure should be assessed through accelerated weathering, that would throw up and exaggerate any significant durability issues dependant on mortar type.

In conclusion a further phase of mix design and testing should focus not only on the general performance of the mortars as determined through strength testing, but focus on the porosity and permeability characteristics, elastic modulus in relation to stone substrate, bond with the stone and durability. It may not be necessary to consider the properties of cement mortar alongside the lime-based mortars. Cement is not considered as appropriate in this context of the repair of historic buildings in any case. A focus on hot-limes and the effects of various combinations of hydraulic lime and hydrated lime could form the basis of a significant research project to optimise mortar design and behaviour .

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References

Arandigoyen M. and Alvarez J.I., “Pore structure and mechanical properties of cement-lime mortars”, Cement and Concrete Research, 37, 2007, 767-775

British Standards Institution, 2001, EN 459 EN 459-2 Building Lime: Part 2. Testmethods.

British Standards Institution, 1999, BS EN l0l5-ll, Methods of test for mortarfor masonry- Determination of flexural and compressive strength of hardenedmortar.

British Standards Institution, 1985-2001, BS 5628, Code of practice for use ofMasonry

British Standards Institution, 2003, BS EN 998-2, Specification for mortar formasonry - Part2: Masonry mortar

British Standards Institution, 1998, BS 4551, Methods of testing mortars, screeds and plasters, part 1. Physical testing

Colantuono A., Dal Vecchio S., Marino O., Mascolo G. And Vitale, A., “Cement-lime mortars joining porous stones of masonries able to stop the capillary rise of water”, Cement and Concrete Research, 26, 1996, 861-868

Duffy A.P., Cooper T.P. and Perry S.H., “Repointing mortars for conservation of a historic stone building in Trinity College Dublin”, Materials and Structures, 26, 1993, 302-306

Forster, A., 2004, Hot-Lime Mortars: A Current Perspective, J. Architectural Conservation, 10, 7-27.

Hughes, J.J., Leslie, A.B. and Callebaut, K. “The petrography of lime inclusions in historic lime based mortars,” Annales Geologiques des pays Helleniques, Edition Speciale, Volume XXXIX, ISSN: 1105-004, 2001 p359-364.

Hughes J.J., and Taylor A.K., “Compressive and flexural strength testing of brick masonry panels constructed with two contrasting traditionally produced lime mortars.”, proceedings of the RILEM International Workshop “Repair Mortars for Historic Masonry”, TU Delft, January 2005, RILEM publications, in press, 2008.

Knöfel, D. and Schubert, P. Handbuch, P., “Handbuch, Mörtel und Steinergänzungsstoffe in der Denkmalpflege“ (Handbook, Mortars and Stone Replacement Materials in Restoration) Ernst & Sohn, Berlin, 1993

Mosquera, M.J., Benitez, D. and Perry, S.H., “Pore structure in mortars applied to restoration Effect on properties relevant to decay of granite buildings” (sic), Cement and Concrete Research, 32, 2002, 1883-1888

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O’Brien P.F., Bell E., Pavia Santamaria S., Boyland P. and Cooper T.P. “Role of mortars in the decay of granite”, Science of the Total Environment, 167, 1995, 103-110.

Trischuk K., Durability of mortars for the Supreme Court of Canada – Phase 1, NRC-CNRC Report B1372.1, 2007 (Draft)

Young, M. E., “Dampness penetration problems in granite buildings in Aberdeen, UK: causes and remedies”, Construction and Building Materials, 21, (2007), 1846-1859

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