Α Nano Approach to Consolidation of Degraded Historic Lime Mortars

Α Nano Approach to Consolidation of Degraded Historic Lime Mortars

Miloš Drdácký1, a, Zuzana Slížková1,b and Gerald Ziegenbalg2,c 1Institute of Theoretical and Applied Mechanics of the Academy of Sciences of the Czech

Republic, Prosecká 76, 19000 Praha 9, Czech Republic 2Ingenieur Büro Ziegenbalg IBZ-Freiberg, 09599 Freiberg, Germany

[email protected], [email protected], [email protected]

Received: November 14, 2008; revised: January 19, 2009; accepted: January 21, 2009

Keywords: lime water, calcium hydroxide nanosuspension, CaLoSiL, consolidation, lime mortar, calcareous stone.

Abstract. This paper presents a comparative study of the effects of calcium hydroxide based agents on consolidating a lean lime mortar. In the first part, it describes the properties and characteristics of CaLoSiL® – a new stone strengthener based on colloidal suspensions of lime nanoparticles in various solvents. It further summarizes the results of recent tests on the influence of applying nano-agents based on colloidal calcium hydroxide dispersed in ethanol or isopropyl alcohol (CaLoSiL®), and compares them with the effects of lime water and with a traditional barium hydroxide treatment. The tests were carried out on non-standard test specimens fabricated from a lean mortar (1:9) and developed specially for the purpose of testing the relatively low strengthening effects that are typical for lime water consolidation. The comparison presented in this paper is based on the mechanical characteristics of consolidated lime mortar.

Introduction

Nanotechnologies have been studied in recent years in order to explain both how ancient objects were produced and how they have been treated, and also to solve various problems in safeguarding immoveable cultural heritage, e.g. [1], [2].

Conservation of degraded calcareous materials is a field in which progress and benefits can be anticipated from the application of nanomaterials and nanotechnologies, including biotechnologies. However, in consolidation tasks, this modern technology aims to exploit traditional materials due to their chemically natural affinity to the treated degraded original material. Therefore, calcium hydroxide nanoparticles have been developed and tested for consolidating frescoes and limestone [3], [4], [5] together with innovative approaches applying, e.g., calcium alkoxides [6]. Nanotechnology does not always help to solve the difficulties in effectively impregnating porous materials. For example, nanosize particles of silica prepared by basic catalysis of tetraethyl orthosilicate were investigated on Lecce stone [7] with no considerable consolidation effects, because the nanoparticles did not penetrate sufficiently into this porous stone. Nevertheless, there has been increased interest in research on nanotechnologies applicable in conservation, namely for consolidation and protective treatment of thin surface layers of historic materials. Substantially improved cohesion of degraded marble by means of linear and branched calcium metallorganic compounds has been reported [6], [16]. Impregnation of disintegrated marble with a solution of these substances caused the growth of a homogeneous carbonate nanolayer of roundish and platelet-like crystallites from 100 to 1500 nm in thickness [8]. Oxide or metallorganic nanoparticle suspensions can be produced by synthesis in a liquid medium using polyol synthesis, which means that “seeds” of the desired compound are produced in a high boiling medium and their growth is stopped in order to promote nucleation, using a grafting agent [9]. Of course, not only inorganic but also organic and mixed consolidation products in the form of microemulsions and nanodispersions have been developed and are available under various trade names, e.g. �anorep 10 (30% silane /

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25% siloxane nanodispersion with particles of 50-80 nm), �anosil 30 (silica nanodispersions, diameter of particles 20-30 nm), silica acrylic nanoproducts (20-100 nm), and Atomo (acrylic microemulsion, diameter of particles 20-40 nm) [10].

A contemporary approach to compatible consolidation of degraded historic lime mortars has been based in some countries on repeated application of a water solution of calcium or barium hydroxides. This technology requires tens or hundreds of application cycles in order to achieve observable strengthening effects, due to the very low solubility of both hydroxides in water [11]. Therefore new agents taking advantage of a suspension of calcium hydroxide nanoparticles in alcohols have been developed and tested.

�anosuspensions for lime mortar consolidation

A new stone strengthener CaLoSiL®, based on colloidal suspensions of lime nanoparticles, has been developed for consolidating calcareous historic materials – lime mortars and limestone. CaLoSiL® contains calcium hydroxide nanoparticles suspended in various solvents. Typical concentrations are between 5 and 50 g/L, which influences the opaqueness of the suspension accordingly and may result in a light whitish haze on the consolidated surfaces (Fig. 1).

Figure 1: Calcium hydroxide sols (CaLoSiL®) in different concentrations: 5 g/l, 15 g/l, 25 g/l and 50 g/l (from left to right – photo G. Ziegenbalg).

The particles range in size between 50 and 250 nm, depending on the production process. Due to

the nanometer size of the particles, stable sols are formed which are able to penetrate deep into damaged stone or mortar. Treatment of surfaces and also stones with CaLoSiL® results in the formation of solid calcium hydroxide after evaporation of the solvent. In contact with atmospheric carbon dioxide, conversion into CaCO3 (calcite) takes place in a way similar to traditional lime mortars. The solvent evaporates without any residues. No compounds that deteriorate the stone or mortar are formed. Thus, consolidation is achieved by a material originally present in mortar and many natural stones. Fig. 2 shows the structure of CaCO3 bridges precipitated between grains of only slightly compacted pure sand.

14 Journal of Nano Research Vol. 8

Figure 2: Fine structure of CaCO3 crystals in matrix connecting sand grains, (e.g. a spherical grain in the right upper corner), see also Fig.7 (photo G. Ziegenbalg).

The material has a wide variety of applications, e.g. in the same way as described above, the

surfaces can be coated with calcium hydroxide or limestone. Paper restoration and conservation of fresco and wall paintings are examples of other applications.

Experimental programme

A special testing programme has been prepared and executed for comparing and assessing the strengthening effects of the new nano-products.

Test specimens. Specific test specimens in the form of short tubes for compression tests and plates for tension tests were designed (Fig. 3). The specimens were fabricated from a lean lime mixture, cast in stainless formwork, with no separation and well compacted, which enabled them to be pushed out from the formwork immediately after casting. This prevented the origination of shrinkage defects and created no surface barrier against penetration of the agents. The tubular form increased the surface-to-cross-section area ratio and intensified the measurable strengthening effect. It also improved the penetration of the agent and the maturing conditions.

Figure 3. Specimens for compression tests (right) and tension tests (left – photo M. Drdácký).

Journal of Nano Research Vol. 8 15

The specimens were made of lime mortar prepared from powdered lime hydrate and river sand in a ratio of 1:9 by volume, and were left to harden for six months before application of the consolidation substances. Then the agents were applied, and after completion of the application the specimens were left to mature for another 60 days. This was considered sufficient to allow carbonation of the calcium hydroxide. In both cases, the level of carbonation was checked by phenolphthalein testing.

Consolidation of lime mortar. The tubular and flat specimens were penetrated with various agents. In this paper we present results of treatment with distilled water, lime water (permanent wetting & periodic drying), barium hydroxide (“barium water”), CaLoSiL 15 and CaLoSiL 25. Each agent was applied by continually dripping it from a syringe on to tubes fixed in a horizontal position on a rotating shaft, (Fig. 4), or lying on supports in a horizontal position (tension specimens). Two basic technologies were followed – periodic drying, where the tubes were left to dry for twelve hours, and the permanent wet method, where the new dose was applied as soon as the mortar was capable of soaking it but before it dried out. In most of the tests, two schedules were adopted – 40 to 60 cycles and 160 cycles of application of the agent in the case of water and lime water.

Figure 4. Applying the consolidating agents (photo T. Drdácký). Testing the strengthening effects. There is a lack of suitable experimental techniques capable

of evaluating strengthening effects in thin layers after surface treatment on degraded or weak mortars. Most methods require conditions to be fulfilled that are not achievable in practice. For example, a recently improved resistance micro-drilling method requires a long path (min 30 mm) of a drill in composite materials with hard grains, as in the case of quartz sand mortars, for reliable assessment of the material quality [12], and it is not applicable for plaster or thin mortar layers. This paper therefore presents a comparison based on mechanical testing of specific thin-walled specimens described above and tested as bodies evenly consolidated by the selected agents.

The short mortar tubes were loaded along the tube axis in compression. The attained compression strengths were checked against those measured on a set of rectangular specimens. The average compression strength determined from the untreated tubular reference specimens was 0,260 MPa, and the average compressive strength measured on the rectangular specimens was 0,549 MPa (after low slenderness ratio correction) [13], [14]. It follows from the results that the tubular specimens provide lower compression strength - approximately 50% of the values measured on the rectangular 40 x 40 x 27 mm (non-standard) specimens, and that lean mortar really is weaker than usual historic masonry lime mortars [15]. Of course, our “weak” mortar was very well compacted and integrated, and was not intended to model the typical disintegration of degraded mortars. In spite of the fairly good cohesion of the tested mortar, we observed a quite high loss of surface material during treatment, even though the treatment was very delicate.

Tension strength was tested on a small sample of the specimens, and the value given in Fig.5 should be considered only as informative.


Test results. The results from the test series presented here (each treatment variant included 5 test specimens) are plotted in Figure 5 (for compression) and summarized in Table 1 (for tension). It is apparent that a significant consolidation effect on lean lime mortars can be achieved by a short application, lasting only one day, of an alcohol suspension of calcium hydroxide. The strengthening effect is higher than many repeated applications of lime water or barium water, i.e. of saturated water solutions of calcium hydroxide or barium hydroxide.

Figure 5. Effect of various agents on the compressive strength of a lean lime mortar. However, the treatment of lean mortars, well compacted and cohesive, with a high number of

repetitions of lime water wetting, can also cause significant compression strengthening. After 161 cycles of lime water applications, a remarkable increase in compression strength is observed, (though still much less than the increase when alcohol suspensions of calcium hydroxide are repeatedly applied). From this limited range of experiments it can be concluded that some effect on similar lime mortars can be achieved with more than 50 cycles, and a substantial effect is likely after 160 cycles. The experiments further confirmed that barium water treatment causes a more noticeable increase in strength than lime water.

Table 1. Effect of various agents on the tension strength of a lean lime mortar.

Consolidation agent

Untreated reference mortar

Lime water – 161 cycles of application

CaLoSiL 15 40 cycles

CaLoSiL 25 40 cycles

Tension strength [kPa] 96 137 316 806


Figure 5 shows that the most efficient agents are suspensions of calcium hydroxide in alcohols, which are marketed under the trade name CaLoSiL and produced by the third author (IBZ Freiberg). Their major advantage is quite rapid application in comparison with tens to hundreds of lime water cycles. The reported research did not aim to optimize the application of various agents, but to make a comparison under specific conditions. This should be understood, in order to avoid misinterpreting the results. The efficiency of suspensions in alcohol is signalled by the one-day application of CaLoSiL 15 (7 cycles), where there is a significant strengthening effect.

Structural observations

Suspension agents deliver a much greater amount of calcium oxide into the mortar and help to create a stronger binding structure. Figure 6 illustrates the basic differences in the treated mortars from the macroscopic point of view. The amount of new calcite after lime water treatment is sufficient to improve the shear cohesion characteristics, and thus also the surface cohesion characteristics. However, the tension strength is little changed, (Table 1), therefore the overall consolidation effect is quite low, as reported in numerous publications, see e.g. [11]. On the other hand, the suspensions have a binding effect which substantially improves the tension strength, too, and the consolidation is remarkably high.

Figure 6. Macroscopic appearance of a lean (1:9) lime mortar before treatment (upper left quarter), after 40 cycles of CaLoSiL 15 (lower left), after 58 cycles of Ba(OH)2 (upper right) and after 161 cycles of Ca(OH)2 (lower right). The length of the base of the individual images is equal to 2 mm. (Photo J. Frankl)

The milky matrix connecting larger grains of sand in Fig. 6 (lower left quarter) has a significantly

different character from the larger crystals precipitated in the pores from the lime or barium water. The fine structure of CaCO3 generated from the suspensions is well visible in Fig. 7. Here the large pores are not completely filled with the new matrix but the grains are covered with a relatively thick


and smooth layer of the consolidating substance. In this case, the size of the grown crystals is of the order of about 150 nm and they have the tabular shape that is typical for calcite. Of course, this example shows the result of a quite high number of repetitions of treatment cycles, and such dense filling is not necessary for consolidating degraded mortars in practice. Even a much thinner and more porous matrix has sufficiently high strengthening effects.

Figure 7. SEM images of a lean mortar repeatedly treated with CaLoSiL 25 (40 cycles of application) under different levels of magnification. This rather compact and homogeneous consolidation matrix with nanosize pores resulted in a substantial increase in tension strength compared to other CaLoSiL applications. (Photo L. Nosál)

Lower concentrations of CaLoSiL and also a reduced number of treatment repetitions resulted in

a thinner matrix layer covering and connecting the composite grains. Fig. 8 shows a comparison on an identical scale of SEM images of typical CaCO3 formation after various treatments. It seems that in the untreated mortar calcium carbonate has grown in the columnar form together with tabular crystals. The matrix is quite thin, with weak bridges.


Figure 8. CaCO3 and BaCO3 formation after various treatments: lime mortar without treatment (upper left), after 161 cycles of periodically dried lime water treatment (upper right), after CaLoSiL 15 treatment – 7 cycles in one day (centre left) and 40 cycles (centre right), after 40 cycles of CaLoSiL 25 (lower left), and after 58 cycles of BaCO3 (lower right). (Photo L. Nosál)


After 161 cycles of lime water treatment in the mode of drying between subsequent applications, the matrix is filled with layers of newly formed calcium carbonate in a platelet form. Nevertheless, the new material creates discontinuous clusters without regular and dense bridging. The application of alcohol suspensions results in regular and evenly distributed new matrix with a density apparently dependent on the calcium hydroxide concentration as well as on the number of applications.

It seems that the thinner suspension of CaLoSiL supports the formation of needle or column-like crystallite of a quite fine nanosize, whilst the denser suspension results in more tabular or round shaped matrix elements. In both cases, the matrix creates very well and evenly distributed bridging layers. Figure 8 clearly illustrates the differences between the consolidating matrices of lime and barium water, where the latter obviously builds a denser and better-connected substance, which reflects in the reported higher efficiency of the barium hydroxide consolidation treatment. In fact, in both cases a nanosized precipitated phase of carbonates is apparent.

Conclusion

The comparison of the different agents shows that, as far as strengthening is concerned, the best results can be achieved with suspensions of calcium hydroxide in alcohol. Such an agent is marketed under the trade name CaLoSiL. It is applied sufficiently rapidly, meaning that there is no need for long term scaffolding, and both the compression strength and the tension strength are substantially increased. The positive effects on mortar described above indicate a possibility that degraded limestone can also be safeguarded with the use of calcium hydroxide suspensions. However, due to its typical pore size, the nanoparticles have to be accordingly small.

Nanomaterials offer many advantages for stone conservation such as: • Possibility to penetrate deep into damaged zones, no limitations due to particle size, • High reactivity and fast reactions (such as carbonation) in the treated zones, • High purity and defined composition. Further development and testing of nanomaterials for consolidating natural and artificial stone

forms the main part of the European STONECORE project1. Sols based on colloidal calcium hydroxide, calcium-magnesium hydroxide, barium hydroxide, barium carbonate, calcium carbonate and calcium sulphate will be developed and produced in amounts sufficient for tests on a laboratory scale and for use on demonstration objects.

Acknowledgement

The authors gratefully acknowledge support from Institutional Research Plan grant AV0Z 20710524, grant GAČR 103/06/1609, and the help of M. Benešová, J. Frankl, I. Frolíková, J. Hodrment, L. Nosál and P. Zíma in preparing test specimens and in carrying out and evaluating the tests.

1 Six SMEs, four universities, one public research organisation and one governmental organisation from seven

countries have joined together to find a new approach for conserving natural and artificial stone. The following applications are in the centre of interest of the STONECORE project:

Consolidation of limestone, marble and related materials due to the formation of calcium carbonate from calcium hydroxide sols. Use of colloidal calcium carbonate suspensions as an injection agent for filling voids and cracks.

Consolidation of mortar, plaster and sandstone by combining calcium/barium sols with conventional stone strengtheners such as silicic acid esters.

Destruction of mildew and algal growth by colloidal, alcohol-based hydroxide suspensions combined with consolidation of the treated area due to the formation of calcium carbonate or barium carbonate.

Consolidation and conservation of stucco by converting anhydrite from colloidal calcium sulphate suspensions into gypsum.

Stabilisation of mortar, plaster and wall paintings by forming carbonates from barium or calcium hydroxide sols. Neutralisation of acidic, sulphate-containing zones by converting barium carbonate sols into insoluble barium

sulphate. The project has duration of three years, and started in October 2008.


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http://dx.doi.org/10.1002/aoc.1462

Α Nano Approach to Consolidation of Degraded Historic Lime Mortars

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