Influences Affecting Compressive Strength of Modern Non-Hydraulic Lime Mortars Used in Masonry Conservation

Influences affecting compressive strength of modern non-hydraulic lime mortars used in masonry conservation

J. Valek & P.J.M. Bartos Advanced Concrete and Masonry Centre, University of Paisley, Scotland

Abstract

Non-hydraulic lime based mortars are currently more and more often used for the repair of historic masonry. They are presumed to be compatible with the original mortarlmasonry and their properties are not always evaluated, mainly due to a lack of research into mortars produced in a traditional manner. Influences of curing conditions, quality of mixing and water content on compressive strength of a non-hydraulic lime mortar mix made of lime putty were reviewed and examined. The mortar specimens were prepared with various water contents, cured in outdoor and indoor conditions and tested after 1, 2, 4 and 6 months.

Phenolphthalein pH indictor was used to measure the carbonation depth. The strongest mortar specimens were from the indoor ageing condition although the mortar was less carbonated. The addition of water into the fresh mix increased porosity and lowered the compressive strength, however there were other influences which affected this relationship.

1 Introduction

In conservation practice there are two main ways to design a new repair mortar.

The first one is based on the traditional conservation approach when the new mortar is a copy reproducing the original mortars as close as possible. The second one is more scientifically based, involving a detailed testing of properties of new and original mortars in order to ensure their compatibility. This approach is a more universal and advanced way of designing a compatible mortar

572 Structural Studies, Repairs and Maintenance ofHistorica1 Buildings

as the first one is based merely on the presumption that the copied mortar will perform in the same manner as the original one. Although this presumption may not always be correct, as the historic mortars often undergo certain changes during their ageing [l], in the case of non-hydraulic lime mortars such application is probably close enough in terms of compatibility needs. However, the lack of research into the properties of lime-based mortars produced in the traditional ways inhibits accurate compatible design. The biggest hmdrance to this research is the complexity of influences on the mortar performance.

To assess the performance of non-hydraulic lime mortars or their compatibility a number of tests are usually carried out [2]. Compressive strength testing remains a key factor in this assessment, however it should not be used as a sole measure of quality. In fact, for determination of compatibility of non-hydraulic lime mortars the value of the compressive strength alone may not be needed at all.

However, in research of the mechanical and physical properties of lime mortars, compressive strength testing is a standard way to assess their hardening, setting and strength, and is related to carbonation, porosity and other physical properties [3].

Figure 1: Stone and lime mortar interaction and influencing factors after Bartos and Lawson [6].

2 Influences on lime mortars performance

Non-hydraulic lime based mortars are expected to have a relatively low compressive strength, approximately 0.5-3MPa and are expected to adjust to seasonal and minor structural movement without damage (4, S]. Durability of lime based mortars can vary depending on conditions and ageing but surviving examples of historic mortars well over 600 years old are available world-wide. Durability and performance of non-hydraulic lime mortars are limited not only by the material itself but also by workmanship, ageing and curing condtions and stone and mortar interaction, as presented in figure 1 [6]. Especially in the case of historic masonry, proper workmanship was determinative for the quality of work and therefore the performance of lime tnortars. Swiving masonry of historic structures is prove the quality of such practical and empirical knowledge of the past, however exact assessment is tnore difficult as the masonry and mortar have undergone durability testing over the centuries.

Structural Studies, Repairs and Maintenance of Historical Buildings 5 7 3

2.1 Influence of workmanship and masonry construction

The performance of modem non-hydraulic lime mortars made of lime putty is considered to be strongly dependent on workmanship 111. The workmanship comprises not only the application of mortar but also particular constructional details, adequate workability, final surface finish, in-situ protection and curing.

The workmanship should reflect the actual state of masonry and environmental conditions. Moisture suction parameters of masonry units together with often dBerent ageing conditions affect the bond between mortar and masonry. Properties of mortars cured in steel moulds are therefore dBerent from those of mortar cast between masonry 171.

2.2 Influence of water content

It has been suggested by Schafer et al. [S] that a correlation between porosity of an ancient and new lime mortar could be used as a method to estimate the compressive strength. The amount of water added determines porosity of a hardened mortar. Together with a degree of compaction they characterise a volume of voids in the mortar. A higher porosity means a lower strength. The mechanical properties of lime mortars are improved if the amount of water is reduced [9]. Lime putty usually contains enough water for mortar (bedding andtor pointing mortar) to be prepared without a firther addition of water 141.

2.3 Influence of workability

Type of aggregate, its grading and lime puttylaggregate proportions control the amount of water needed to provide a good workability. The optimal waterhinder proportions differs depending on construction and application techniques. Good compaction of lime mortar is vital for its performance. Good workable Lime mortar possesses a greater degree of plasticity; it is often described as near to a modelling clay.

2.4 Influence of curing and ageing

The curinglageing conditions for non-hydraulic lime mortars are to promote a combination of drying out (initial setting) and carbonation at such a rate that minimises shrinkage. The ideal environment has a temperature around 20°C and relative humid@ between 50-70% [10]. The strength development of the lime based mortars due to carbonation is inherently a very long-term process, depending on the curinglageing conditions.

Curing described by British Standard [l11 for mortar specimens is not appropriate for non-hydraulic lime mortars. Moist curing in a container can possibly be used, however the container should not be airtight. According to literature [l21 and considering the nature of the hardening process of lime mortar such humid conditions can retard the carbonation and do not represent ambient conditions encountered in practice.

574 Structural Studies, Repairs and Maintenance of Historical Buildings

2.5 Influence of mixing and lime putty maturing

The mixing and production methods of non-hydraulic lime mortars can also have a very strong influence on their performance. Maturing of some lime putties reduces their particle size and improves their water retentivity [13]. Sand [4]. carrying capacity should improve with reduction of particle size Hand or mechanical mixing can produced mortars of different quality. From practice it is known that the mortar plasticity can be improved by way of mixing. Traditional techruques of mixing 'by hand' involved beating, chopping and ramming on a wooden board until the mix was sticky and workable [4]. Ready-mixed mortar

should be re-mixed before use.

3 Experimental

The above review of influences on performance of lime mortars presented some factors that were considered before the experimental part was carried out. The objective of this research limited itself to examine only the influence of curing conditions on strength development with ageing and the effect of water addition

during mixing. In order to eliminate the other influences and simpllfy the testing, one type of mortar, the same mixing method and 5Ox50x50mm cube specimens were used. The specimens were cured in laboratory conditions and not in realistic conditions withm masonry. Cube specimens were more practical and the experiment was seen as a first step in order to examine the basic principles.

3.1 Material and curing conditions

A non-hydraulic ready-mixed lime mortar commonly used in conservation for repair works on stone masonry was used to cast sixty 50mm cubes. The lime

mortar was a 1:3 mix, consisting of a minimum six-~nonth-oldnon-hydraulic Shapfell lime putty and Gowrie concrete sand. Density of the fresh mortar was determined as 2080 kg/m3.

The specimens were cured in three different conditions marked as a, b and c.

The curing condition a was the most stable. The temperature was, on average, around 20°C. The relative humidity (RH) was between 50% to 60%.

The curing condition b was an indoor condition. The room was not heated, and therefore a whole range of temperatures above zero from 1°C to 23°C was

monitored. On average, the temperatures were around 13°C to 16°C and RH from 58% to 65%.

The condition c was a natural weather condition, where the specimens were

exposed to temperatures below zero during winter. The exposure site was relatively sheltered and the temperatures and relative humidity were similar to typical average weather conditions in the west of Scotland. The lowest temperature was in winter (-S°C), and the hottest was during summer (+26"C).

Structural Studies, Repam and Matntenance of Historical Buildi~gs 575

Relative humidity varied from 9% to 94% but on average the RH was quite high over all seasons between 75 - 85%.

3.2 Sample preparation and testing methods

For the determination of the influence of curing conditions the ready-mixed mortar was re-mixed in a pan mixer before casting the specimens. No water, or minimum water was added and the amount was recorded. All cubes were prepared according to the British Standard [l11 method. The cubes were demoulded after hvo days and stored in the particular a: b or c curing condition for 1,2,4 and 6 months prior testing.

The influence of water addition was combined with the influence of curing in conditions a and c. In th~scase. the lime mortar was mixed in a laboratory mixer to control the gaugng. Lime putty, sand and their proportions were the same as for the ready mixed mortar, only the amount of water was increased for each set of specimens. The cubes were cured in indoor condition (a) with temperature around 20°C and relative humidity 55% for the first month, then half of the cubes 8-14°C were placed outdoors (c) with an average temperature range of and relative humidity range of 60-85% for next five months. After that period, the porosity, carbonation depth and strength were determined. This experiment was a part of another project which assessed porosity, permeability and carbonation of non-hydraulic lime mortar mix [14].

The workability of mortars was measured on a flow table following the standard test described in British Standard [l l].

The depth of carbonation was determined by a Phenolphthalein pH indicator. The principle of the method is that carbonation reduces the pH value of 12.5 of Calcium Hydroxide (Ca(OH)2) to pH values below 9 of Calcium Carbonate (CaC03). When the sample is sprayed with the pH indicator it changes its colour from colourless to magenta in a region where the pH value is higher than 9. According to [l51 the method does not recognise a partially carbonated mortar; only fully carbonated mortar can be determined.

The porosity of the specimens was determined by immersing the specimens into water under vacuum for four hours. The mass of water absorbed in a specimen was a difference of the oven dried mass (at 105kS°C) and the vacuum saturated mass. More precisely, the porosity is here defined as a percentage of a volume of a mortar's pore space in total volume of the mortar. The porosity was accompanied with a saturation coefficient. It is a measure of the extent to which the pores are filled when the mortar is exposed and allowed to absorb water. The sample was immersed into water for 24 hours. The average value of ths coefficient was 0.71. These simple methods are recommended for assessment of durability of building stones [16].

576 Structural Studies, Repairs and Maintenance of Historical Buildings

4 Results and discussion

The mortar for the specimens cured in the a, b and c conditions was mixed to have the same workability, between 130 to 140mm (measured on flow table).

When the required workability was not achieved a small amount of water was added, remixed and the flow measured again, see figure 2. In the case of the second experiment water was intentionally added to the mix to provide different mortar consistency. The first six cubes, marked 01, were made without the addition of water. The amount of water was increased gradually with the number of the cube (100ml per number) but only cubes 06. 01 l and 016 were cast and used further in th~sexperiment. The actual content of the water present in the fresh mix was determined by drying the mortar in an oven at 105f5"C. The moisture content is the mass of water in a sample over the mass of dry sample.

The results in figure 2 shows that a significantly higher moisture content means a lugher flow measured on the flow table. However, additional effects were also observed. The amount of water added to the mix was not completely found in the moisture content of the fresh mortar; there was a loss of the moisture during the mixing. Therefore moisture content of the fresh mortar mix cannot be deduced from the flow table value or from the amount of added water. Mortars with the same workability and mixed for the same time can vary in moisture content. Conversely, it can be said that the mixing process determined the workability of the mortars. A scatter or range of workability of mortar with one moisture content was controlled by the quality of the mixing. The amount of water added to the mortar had a secondary effect. It was also noticed that a high flow value measured on the flow table does not mean in general the best workability, however for one and the same moisture content the higher flow means better workability.

go Oabc

15.0 16.0 17.0 18.0 19.0 20.0 21.0 Moisture content [%]

Figure 2: Flow table values as a function of the moisture content.

The compressive strength of cubes cured in a and b conditions improved with ageing. The strength of cubes from a condition stopped growing after 4 months.

Carbonation depth on these cubes was just on the surface. The mortar was either not carbonated or the measurement by the Phenolphthalein pH indicator did not allow assessment of partially carbonated regions. The second hypothesis was proved as correct later by the measurement of the weight changes due to carbonation 1141. It was also noted that the cubes from the dryer conditions reached higher compressive strengths. This suggested that the pore structure and strength obtained by drying out can have a sigmficant effect during early age of mortars. The c condition was extreme as severe frost damage occurred just after the first fourteen days of curing. The cubes never dned out and the carbonation depth was also just on the surface during the first and second months. The strength improved with the change of weather conditions however, comparing to the others. the improvement was negligible. The maximum nominal stress after ageing of one full year was 0.14MPa with a coefficient of variability of 2 1.20%. Although carbonation took place the average strength of the mortar degraded by the frost damage &d not improve.

It is well known fact, that non-hydraulic lime mortar should be protected from frost attack and therefore, the c condition cannot be seen as a standard condition.

Therefore the experiment was compared to other cubes cured in the c condition [6]. The set of cubes cc was tested at age of 1 and 18 months with the depth of carbonation 1 and 16mm respectively. The compressive strength improved with ageing as the carbonation progressed. However, the compressive strength was lower than the one of the cubes cured in the indoor conditions.

Age when tested [month]

3: in b Figure Compressive strength of lime mortar cubes stored conditions a, and c and tested at the age of 1,2,1 and 6 months.

The most consistent results were obtained from the condition a, see table 1. Tlus curing condition was also the most stable, hence there may be a relationship between the variability of exposure conditions and the variability of strength.

578 Structural Studies, Repairs and Maintenance ofHistorica1 Buildings

Table 1. Comparison of variability of compressive strength

Curing Coefficients of variability [%l conditions l month / 2 months 1 4 months 1 6 months

Addition of water did not provide a linear correlation between amount of water added, moisture content and porosity. As it was mentioned above a loss of water occurred during mixing and moisture content of mortar used for 06 cubes was in fact lower than that of the mortar used for 01 cubes. In terms of porosity, it can

be concluded that a general relationship applies the higher moisture content than the higher total porosity, see figure 4. The cubes cured outdoor (c condition) were more carbonated than the cubes cured indoor in the a condition. The carbonation caused an increase of volume that filled the pores [E].Therefore it

was expected for the cubes cured outdoor to possess a lower porosity than their twin-cubes cured indoors, see figure 4.

Moisture content [%l

Figure 4: Different total porosity of mortar specimens cured in i-indoor condition (a) and o-outdoor condition (c) for different amount water present in the mortar.

The maximum strength was higher for the cubes cured indoor than for the cubes

cured outdoor. However, the fill carbonation depth determined by Phenolphthalein was on surface and 12.5mm for indoor and outdoor condition respectively. The cubes cured indoors were approximately 30% less carbonated than the outdoor aged cubes [I41 but possessed a higher strength at age of 6 months, see figure 5. The increase of the amount of added water did not have any

obvious influence on the strength of the mortar samples 01, 06 and 01 l. However, the strength of the last mortar sample 016, with the highest water content, was considerably lower in both curing conditions.

5 cm olim olli - 2.5 - m, 06i o14im Q m 060 .5 2.0 - z 0110 * 010 ' 2 0140 n 1 .S

0 26.50 27:00 2750 28b0 28:so z9:oo 29.50 B Total porosity [%]

Figure 5: Compressive strength measured as a function of the total porosity of non-hydraulic lime mortar.

5 Conclusions

From the testing carried out the following conclusions can be drawn:

The curing conditions applied affected the mortar strength development. The strongest mortar specimens were obtained from dry indoor conditions, where they also acquired the strength faster than the ones cured outdoor. The drying out process appeared to be more significant for the strength

development in early stages (first six months) than the degree of carbonation. Less variable curing condtions resulted in a less variable strength.

Initial curing conditions during the hardening process were the most important for the strength development. Extreme outdoor conditions (i.e. temperatures below zero) during the first month of curing caused complete deterioration of the non-hydraulic lime mortars; such mortars were not able to acquire any significant strength later on.

The amount of the moisture in the fresh mortar was proven to have an effect on strength and porosity. In general, the addition of water decreased the strength of non-hydraulic lime mortars. Ho\vever. there was a certain margin within which the effect of added water on porosity and strength were

relatively small.

Spec~ficationbased merely on composition for a modem non-hydraulic lime mortar to match exactly the strength and porosity of the hlstoric one seems to be very difficult, as there is a strong mfluence of the curing conditions, quality of the mixing and the moisture content. However, the cubes tested were not produced and cured in realistic masonry condition and the influence of these factors should be a matter of further research.

5 80 Structural Studies, Repairs and Maintenance of Historical Buildings

References

Hughes, J.J. et al., Alternation textures in historic Scottish lime-mortars and the implications for practical mortar analysis, Proc. of 7'h Euroseminar on

microscopy applied to building materials, eds. H. Pietersen et al., University of Technology, Delft, 1999. Rossi-Doria, P.R., Mortars for restoration: basic requirements and quality control, Materials and Structures, vol. 19, no. 114, pp. 445-448, 1986. Valek, J., Lime Mortars in Historic Buildings, PhD thesis, University of

Paisley, 2000. Gibbons, P., Preparation and Use of Lime Mortars, Historic Scotland Technical Advice Note 1, Historic Scotland, Edmburgh, 1995. Binda, L. et al., Experimental Study on the Mechanical Role of Tluck

Mortar Joints in Reproduced Byzantine Masonry, Proc. of RILEiLl International Workshop, Historic Mortars: Characteristics and Tests, eds. P.J.M. Bartos et al., RILEM, pp. 228-246, 2000. Bartos, P.J.M. & Lawson, J.A., Stone-lime mortars interaction in Scottish historic buildings, Abstract published in Proc. of The Historic Scotland

International Lime Conference, Historic Scotland, 1996. Lawrence S. and Samarasinghe W., Assessing the Durability of Masorq Mortars. Proc. of the 12th International Brick and Block Masonly Conference, Volume 4, Madnd, pp. 1053-1062, 2000.

Schhfer, J. & Hilsdorf, H.K., Ancient and new lime mortars - the correlation between their composition, structure and properties, Proc. of Conservation of Stone and Other Materials, ed. M.J. Thiel, UNESCO, Paris, pp. 605-6 12, 1993. Torraca, G., Porous Materials Building, Material science for architectural

conservation, ICCROM, third editio~k1988. [l01 Van Balen, K. & Van Gemert, D. Modelling lime mortar carbonation, Materials and Structures, 27, pp. 393-398, 1994. [ll] BS 455 1: Part 1: 1998, Methods of testing mortars, screeds and plasters, Part 1. Physical testing.

[l21 Parrot, L.J., Carbonation, moisture and empty pores, Advances in Cement Research, 4, No. 15, pp. 111-118, 1991192. 1131 Hansen, E.F. et al. Effects of Ageing on Lime Putty, Proc. of RZLEM International Workshop, Historrc Mortars: Characteristics and Tests, eds. P.J.M. Bartos et al., RILEM, pp. 197-206, 2000.

[l41 Valek, J., Hughes, J.J. and Bartos, P.J.M., Gas Permeability, Porosity and Carbonation of Modem Conservation Lime Mortar Mix, Proc. of EUROMT I999 conference, Volume 6, Materials for Buildings and Structures, ed. F.H. Wittmann, WILEY-VCH, pp. 209-215, 2000. [l51 L.J. Parrot & D.C. Killoh, Carbonation in a 36 year old, in-situ concrete,

Cement and concrete research, vol. 19, pp. 649-656, 1989. [l61 Ross, K. & Butlin R.N., Durability tests for building stone, BRE Report BR 14 I, Building Research Establishment. Garston CRC, 1989.