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Biomineralization for Limestone

Consolidation

Science and Engineering of Natural Stones and Glass

HS 2019

Letizia Caderas

ETH Zürich, December 2019

Contents

1. INTRODUCTION ...... 3

2. BACKGROUND ...... 4

3. ONSITE TESTS OF THE GRANADA METHOD ...... 5 The gardens of Queluz Palace ...... 5 Loulé church main portal ...... 6 4. EVALUATION OF THE BIOMINERALIZATION TECHNIQUE ...... 7

5. CONCLUSION ...... 8

REFERENCES ...... 9

1. Introduction

Carbonate stones have been used for many millennia as a building material, which has led to a heritage of buildings and monuments of cultural significance that is considered worth preserving today. However, physical, chemical and mechanical processes lead to a degradation of the carbonate stone structures [1]. The strength of carbonate stones is given trough cohesion, which is provided by either bridges or a mechanical interlocking effect [2]. Therefore, a major problem is the dissolution of the substrate, which decreases the cohesion of the building material and increases the porosity of the stone [3]. Consolidation treatments aim to restore the cohesion between grains of the deteriorated layer of the stone [4]. For the choice of a suitable consolidation treatment, some criteria should necessarily be taken into account. The factors “effectiveness – harmfulness – durability” of the treatment need to be regarded as a magic rule. The effectiveness is a measure of the strengthening effect of the consolidation, whereas the harmfulness considers whether an incompatibility between the treatment and the stone can induce additional damage. Unfortunately, effectiveness or compatibility can often only be increased at the expense of the other [5].

One of the most widely used and commercial available consolidation products nowadays are based on alkoxysilanes. In general, these consolidation products achieve to re-establish the cohesion of the loosened grains through Si─O─Si network by either filling the interspace and thus locking the grains, or by glueing the grains together via adhesive bridges [6][7]. The Si ─ O ─ Si network is formed after a sol-gel process. Apart from the effectiveness of alkoxysilanes consolidation treatments, an increased penetration depth due to low viscosity, reduced visual alterations, low material costs and a simple application are among the advantages of this consolidation treatment [1]. However, two issues limit the successful application of alkoxysilanes based treatments on porous carbonate stone. First, it had been observed that the polymerisation process is altered in the environment of the carbonate substrate. Second, on carbonate substrate, a lack of strong bonds to the Si─O─Si network attenuates the consolidation effect. The silica network and the loosened grain cannot establish a strong chemical bond since anchor points on the carbonate substrate are missing, that could react with the hydrolysed species [8][9].The choice of the right coupling agents can improve the adhesion between the silica network and carbonate substrate. This way, improvements in strength are reported, although the role of the coupling agents on the consolidation is not fully investigated yet [1].

An alternative option to consolidate carbonate stone is given by biomineralization. Biomineralization is the formation of minerals by living organisms. The phenomena are widespread and occur in almost all groups of organisms [10]. It was shown that under suitable environmental conditions, all bacteria can form precipitates [11]. In 1990 Adolphe et al. were among the first to test biomineralization as a consolidation technique for ornamental stone and applied for the patent of the so-called Bioconcept technique. From this point on, much research has been done to find suitable bacteria, and different consolidation approaches were developed in research based on the biomineralization. This report deals with the principles of biomineralization and presents two different consolidation methodologies: Calcite Bioconcept technique and the Granada method. Two application examples of the Granada method are examined. Based on this, biomineralization as a viable consolidation technique is evaluated. 2. Background biomineralization

In biomineralization, two general mechanisms are considered: biologically controlled mineralization (BCM) and biologically induced mineralization (BIM). In the controlled precipitation mechanism, the organism is responsible for the nucleation and growth of the minerals. The minerals are deposited on or within the cells of the organism. Mineral structures like or shells are formed by BCM. Different to that, BIM describes a process where the precipitation of minerals is a consequence of the microbial metabolic activity. The minerals are deposited in the environment, whereby the precipitation of the minerals is highly dependent on environmental conditions. In the case of calcium carbonate precipitation, the governing factors are the pH, the amount of dissolved inorganic carbon, the calcium concentration and the availability of nucleation sites [12]. It was shown that under suitable environmental conditions, all bacteria can form calcium carbonate precipitates [11]. Many metabolic pathways induce the precipitation of calcium carbonate. One example is the hydrolysis of urea, which leads to fast precipitation of calcium carbonate. In this pathway, urea is degraded to dissolved inorganic carbon and ammonium. A simplification of this process is illustrated in Figure 1. It is their negatively charged bacterial , that attracts the positively charged calcium ions. Along with the dissolved inorganic carbon in the close environment, this leads to an oversaturation and calcium carbonate is precipitated. This is one out of many possible metabolic pathways. Further pathways are not discussed in this report. Through the various studies on biomineralization as a consolidation technique, two basic approaches have evolved. One is covered by the Calcite Bioconcept patent and the other by the Granada method. In the approach of Calcite Bioconcept bacterial strains are inoculated to the stones substrate. Contrary to this method, in the Granada method, only a nutritional medium is inoculated, such that the resident bacterial strains are activated. This method relies on the findings that under suitable conditions, most bacteria induce the precipitation of calcium carbonate [5].

Figure 1: Simplified illustration of the hydrolysis of urea that induces the precipitation of calcium carbonate. (A) Through the metabolic pathway of the bacteria, dissolved inorganic carbon and ammonium (DIC) and ammonium (AMM.) accumulate in the surrounding of the bacteria. Due to the negatively charged cell wall of the bacteria, calcium ions gather around the bacteria. Due to a local supersaturation, calcium carbonate precipitates. (B) Precipitated calcium carbonate amasses around the cell. (C) The cell becomes encapsulated, limiting the nutrient transfer, which leads to the death of the cell. [4]

3. Onsite tests of the Granada method

In a study by J. Delgado Rodrigues and A.P. Ferreira Pinto, the Granada method was tested onsite. The commercial product MYXOSTONE M3P of a nutritional medium was used. The nutritional medium was applied by spraying several times for one week to keep the damaged surface wet. Moreover, the treated area was sheltered from sunlight and in some cases, additionally from extreme temperature or relative humidity conditions. The shelters were installed for one month [5]. This way, two already greatly deteriorated structures were treated. Trial tests were conducted on Ançã stone, a very soft and porous limestone, in the garden of Queluz Palace in Portugal. Further tests were then conducted on some parts of the main portal of the Loulé church, made of limestone of moderate porosity [5]. The assessment of the consolidation performance was obtained with non-destructive characterization methods. Water absorption tests by the pipe and sponge and tape peeling tests were performed. Colour variations were investigated as well. Concerning the stones in the garden of Queluz palace, drilling resistance measurements were accepted additionally.

The gardens of Queluz Palace The trial tests in the gardens of Queluz Palace showed that the biomineralization processes lead to a hardening of the treated surfaces. In Figure 2 the transition between an untreated and treated surface can be seen. It can be recognised, that the treatment led to a yellowish colouring. Additionally, some grey spots were visible, possibly the formation of fungi was promoted. The results of drilling resistant measurement in Figure 3 demonstrated that the treatment led to a surface of increased drilling resistance with a 2-4 mm depth. In areas where the stone was already greatly deteriorated, a strengthening effect could still be observed, although the drilling tests revealed a more diffuse profile.

Figure 2: Image of the transition zone between an untreated (left) and treated (right) area. The treated area appeared after the treatment with a yellowish colouring. The greyish spots indicate that the formation of fungi was possibly enhanced [5].

Figure 3: Drilling resistance test of treated versus untreated stone. A consolidation depth of 2 to 4 mm can be recognized (upper graphs). In the case where the stone already revealed intense powdering and scaling, the drilling resistance profile is more diffuse, still, an increase in drilling resistance could be achieved [5].

Loulé church main portal In the case of the Loulé church main portal, which showed intense scaling, reparation actions with weak mortar were intended. However, the surface was considered so fragile, that first a consolidation treatment with MYXOSTONE M3P was conducted. The consolidation by biomineralization led to a generalized strengthening. The peeling tests revealed that the mass losses were lower than before the treatment in all cases. The graphs obtained by the peeling tests are presented in figure 5. Further, the water absorption was investigated before and after the treatment. It turned out that the water absorption slightly decreased after the treatment, but no substantial changes were observed. Trough the treatment the deteriorated areas became more stable, which enabled an application of soft lime mortars to stabilize the unstable fragments further.

Figure 4: Results of the peeling test before and after the consolidation treatment. 5 consecutive tests were conducted in the same area [5].

4. Evaluation of the biomineralization technique

To evaluate biomineralization as a viable consolidation technique, various aspects need to be considered. There are some practical aspects as the application and the cost of the consolidation treatment, but also the key points in stone conservation as “effectiveness – harmfulness – durability”.

Concerning the practical application, biomineralization is associated with great efforts. A shelter is required to establish controlled environmental conditions. Additionally, the bacterial strain or the nutritional medium needs to be applied several times for weeks. Therefore, many working hours are required. The labour intensive application is responsible for the high costs of the consolidation treatment. Additionally, the price of the constituents is higher compared to the ones of the traditional surface treatments. Thus the biomineralization treatment can compete with the commercial treatments on an ecological basis [4]. The multiple application of nutrients is responsible for another disadvantage of the biomineralization technique. Due to the repeated application, the surface of the stone is kept constantly wet. The treated area needs to remain wet since require a minimal amount of water to induce calcium carbonate precipitation. The longer the substrate has to remain wet, the higher the risk for formation and hence plugging. During an extended wet period, the outgrowth of fungi or other gets likely. Although in the example described above a strengthened layer of 2-4 mm was reported, the ineffectiveness for in-depth consolidation is discussed in the literature, reaching only a few microns [13][14]. The penetration may depend on the method and the specific composition of the treatment used. In any case, promising penetration depth results obtained from immersed stone samples are not comparable to a sprayed application. Depending on the metabolic pathway, further challenges arise. In addition to the precipitation of calcium carbonate, biomineralization also involves other processes that cannot be easily controlled. For instance, in the hydrolysis of urea, the urea is degraded to dissolved inorganic carbon and ammonium. On the one hand, ammonium can form ammonium acetate or ammonium chloride and induce salt damage depending on the concentration of the salt. On the other hand, ammonium can be converted to nitric acid by the activity of nitrifying bacteria in the long term and damage the stone [4]. Up to now, no significant problems were reported concerning that topic. Nevertheless, as one tries to increase the efficiency of the biomineralization technique, that might become an issue.

Despite the listed disadvantages, biomineralization is a promising consolidation technique for carbonate stones. It was shown in sonification tests, that the precipitated calcium carbonate crystals are strongly attached to the substrate, having the potential to re-establish an efficient adhesion between loosened grains [15]. Biomineralization is known to be a weak consolidation treatment. Nevertheless, the Granada method applied in situ demonstrated that a mild strengthening can be obtained. Thus, the biomineralization technique can strengthen the surface in such a way that further treatments can be applied. It is probably the most significant advantage that the consolidation by calcium carbonate precipitation is compatible with a carbonate substrate in mineralogical and chemical terms. Moreover, the contrast in physical properties of the treatment compared to the carbonate stone is low, what minimized the risk of incompatibility [5]. This treatment has thus a lot of potential for application where harmfulness is a major concern.

5. Conclusion

The consolidation of carbonate stones is a challenge in the field of stone conservation. Commercial products like alkoxysilane based consolidation treatments are often inadequate for carbonate stones, since a lack in bond between carbonate substrate and silica network, leads to an inefficient strengthening. Moreover, the risk of damaging the stone due to incompatibility exists. Even though coupling agents can improve the consolidation performance of alkoxysilane based treatments on carbonate stones, the need for an alternative consolidation treatment is given [1]. Over the last 20 years, the biomineralization technique has gained a lot in interest in research [12]. This technique is based on the calcium carbonate precipitation induced by microorganisms. On the one hand, one is still trying to find the most suitable carbonatogenic bacterial strain. On the other hand, in situ trials have already been carried out, and the first commercial products are available. Studies such as those in the gardens of Queluz Palace or on the main portal of the Loulé church show that biomineralization can be used as a weak consolidation technique [5]. However, the success of this consolidation technique is highly dependent on the time required for carbonate precipitation. Multiple application of the nutritional medium and hence, an extended duration during which the treatment area is kept wet, lead to high treatment costs and the risk of biofilm formation [4]. Considering the assessment rule “effectiveness – harmfulness – durability”, the advantage of the treatment is given by the minimized harmfulness. Therefore, it is worth it to further investigate and develop biomineralization as a consolidation technique, such that biomineralization can be understood as a viable option, if all other common options would imply to risk damage due to incompatibility. For sure, further research is needed to gain more knowledge concerning the “in situ” application and to translate this into practical applications.

References

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