MECHANISM OF EFFICIENCY OF SELECTED NANOSYSTEMS INTENDED FOR CONSOLIDATION OF POROUS MATERIALS

Over the past 20 years, the possible usage of disperse and colloid systems of organic consolidation agents as well as mineral systems with particle size in nanodimensions in the context of cultural heritage care has been examined. Nanomaterials, with particle size of up to 1μm whose properties are at a certain intersection between properties at the level of molecules and mass are the subject-matter of extensive scientific research for their positive physical and mechanical properties such as high chemical efficiency, excellent plastic, consolidation and diffusion properties, low sintering temperature, cleaning capabilities and others. Nanomaterials whose consolidation efficiency has been tested by experimental research, include, in particular, hydroxide nanodispersions. The article summarises basic conditions for application and principles of efficiency of selected nanosystems for consolidation of historic lime-based porous materials, i.e. in particular lime-based plasters that form integral part of historic buildings


INTRODUCTION
Consolidation -improving physical and mechanical properties and stabilising or, alternatively, slowing down or completely stopping degradation -is one of the fundamental objectives of restoration of historic plasters. The plaster consolidation principle is based on returning the binding material into their structure, filling up any cavities that arise and joining up cracks or increasing the adhesion of the plaster to the base and reducing salt content in the plaster porous system. Plaster layers can be strengthened and stabilised by a sequence of individual interventions and the specific procedure and scope of care cannot be universal and generally applicable. It is always an ad hoc process, affected by a number of factors, different for different buildings, conditions and properties of preserved historic materials. The general requirement is that renovation of historic buildings and cultural heritage should not damage the original preserved surface finishes and the original method and that the selected material for renovation should respect the preserved condition and follow the original composition of the plaster. After a conservation intervention is completed, the appearance, colour as well as structure of the new supplements and renovated parts should be as close to the original material as possible.
In recent decades, nanotechnologies have started to appear in restoration of surface finishes of historic buildings; these technologies introduce new ways of protection of building structures using specific, functionalised properties of materials caused by increasing the proportion of the area of the surface of the particles to the volume of the particles in nanomaterials. Nanomaterials, when compared to macro-materials, excel in extremely high "performance" in

METHODS
In the NAKI research project, extensive research focusing on possible usage of nanomaterials in restoration of historic materials (in particular, plaster and stone) has been conducted. The research is carried out in co-operation with the Faculty of Civil Engineering of the Czech Technical University in Prague and the Centre of Polymer Systems of the Tomáš Baťa University in Zlín.

Consolidation materials and their properties
After the basic steps in restoration of lime-based surfaces of historic buildings are carried out (i.e. mainly cleaning, removing the causes of damping and reduction of salt content), the surface can be consolidated. The strengthening product should penetrate through the entire degraded layer, recover its structure and adhesion to the "healthy" base and not to create surface crust. The important properties of the consolidation product include: stability (against moisture, alkaloids and acids, light, air and microbiological attack); -UV radiation resistance; -display high penetration capability; -it should not affect the appearance of the treated material (lustre, colour, surface structure, etc.); -the coefficient of thermal expansion close to the coefficient of expansion of the original material; -health-related fitness, easy application and affordability.
At the same time, the consolidation agent must be compatible with the original material, i.e. it must not change or affect materially the physical properties of original materials, e.g. porosity, vapour-permeability, thermal or moisture expansion, elasticity (modulus of elasticity). High durability and resistance against aging of the strengthening product and the consolidated material is also very important. The requirement of removability (reversibility) is among basically unrealizable requirements for products and procedures used in treatment of historic materials; it is tempered by the requirement of possible retreatability so as not to prevent subsequent care in later years. From this perspective, materials on inorganic basis, compatible with the treated material and, at the same time, stable with high efficiency following application, seem as most suitable consolidation products.
Great focus is placed primarily on penetration capabilities of the consolidation agent; the material is strengthened only when it penetrates into sufficient depth from the surface and spreads evenly in the mass of the substance to be strengthened (a sharp transition between nonstrengthened and heavily strengthened material leads to development of tension between layers and subsequent mutual separation). The rate of penetration of the consolidation agent depends mainly on the properties of the strengthened material (in particular, its porosity, moisture and pH value), properties of the solution (size of solved molecules, viscosity) and the consolidation method. The higher surface tension, the lower viscosity, density and contact angle at the interface between a solid and a liquid the consolidation liquid displays, the better its penetration. When disperse consolidation products are used, the size of particles and possibility of their penetration into the porous system is important.

Possibilities of production of nanosuspensions
Until recently, nanoparticles of calcium hydroxide have been obtained either through hydrolysis of calcium hydride (CaH2) under specific experimental conditions or through reaction of calcium oxide or calcium hydride CaH2 with water in organic environment in the presence of surfractants [15].
A more modern possibility of synthesising crystallised nanoparticles of calcium hydroxide is the reaction of sodium hydroxide (NaOH) and calcium chloride (CaCl2) that is added (drop by drop) into water solution at 90 °C and the reaction produces calcium hydroxide Ca(OH)2 and sodium chloride (NaCl) that is removed from the suspension subsequently. In the last stage of production, the lime particles are broken in a ball mill to particles of several μm to nm in size; the resulting clusters of particles are subsequently separated using ultrasound [16].
Another known production method involves the reaction of metallic calcium with water in alcohol medium [17] to [18]. The reaction occurs pursuant to this equation: Ca + 2H2O → Ca(OH)2 + H2. Yet another alternative is a method based on ion exchange between resin (anion) and water solution of calcium chloride at 20 to 25°C. Clean suspension containing nanoparticles of Ca(OH)2 can be obtained quickly after separation of the resin from the suspension without any need for cleaning. Exhausted resins can be regenerated and re-used for repeated production of nanolime [19].
When manufacturing nanosuspension, it is relatively difficult to create accurately the required concentration; different concentrations are achieved by evaporating the solvent during manufacture. Stability of suspension is proportionate to the particle size -the smaller the particle, the lower stability. Kinetically stable dispersions can be obtained with short-chain aliphatic alcohols. The benefit of these alcohols is that they evaporate fast (the cycles can be repeated quickly) and, when compared to other solvents, they have low toxicity.

Verifying the efficiency of selected nanosuspensions
In the NAKI research project, lime nanosuspensions were designed and manufactured by the most frequent procedure of synthesis of Ca(OH)2; these nanosuspensions were subsequently modified and enriched by other components that had been assumed to display positive potential for use in cultural heritage care. The nanosuspensions have been manufactured in close cooperation with the Centre of Polymer Systems of the Tomáš Baťa University in Zlín. Based on the results of preceding research, 3 various lime nanosuspensions have been prepared and tested [20] - [21]: Ca4: Ca(OMe)2 dispersed in isopropyl alcohol;

Ca4O: calcium oxide dispersed in ethanol;
CaMg1: Ca and Mg pre-cursors have been dissolved in distilled water; subsequently, NaOH solution was added.
(Each nanosuspension was prepared in concentration of 5 g / l.) In order to determine the crystalline structure of products, XRD powder analysis on the Rigaku MiniFlex 600 diffractometer was used. Each diffractogram was compared to the JCPDS Ca(OH)2 (JCPDS 01-076-0571) (Figure 1) card.
The basic parameters in assessment of potential success of strengthening include the capability of the plaster to absorb the consolidation agent and the depth of its penetration associated therewith. The average values of the depth of penetration of consolidation products range between several millimetres and several centimetres, depending on the size and character of the porous system and the size of particles of the strengthening agent. The laboratory research verified the depth of penetration of the tested nanosuspensions on plaster beams using phenolphthalein (Figure 2). The research demonstrated sufficient depth of penetration and equal distribution of the consolidation product. Another important characteristic is provided by assessment of sub-surface cohesion of the plaster (cohesion, ultimate tensile strength) that represents the condition of the historic material before and after consolidation, in terms of efficiency and quality of consolidation intervention to the degraded plaster surface. The efficiency of consolidation products can be verified in the case of plaster-coated surfaces using, for example, the "Scotch tape test" that can be accompanied by scanning electron microscopic examination. This will provide information about the deposit of the strengthening agents (i.e. lime) in the porous system of the degraded plaster. The scratch "Scotch Tape" test provides a reliable estimate of surface and shallow sub-surface strengthening after application of consolidation products and it provides a clear presentation of the differences between individual types of treatment or agents. However, the measuring of the cohesion is without any link to correlations to solidity characteristics; the result of the test is a "strengthenednot strengthened" statement ( Figure 3).

RESULTS
Nanosuspensions of calcium hydroxide in alcohols represent an alternative of almost waterfree restoration of lime-based historic surface finishes. Clear advantages of lime nanosuspensions prepared in alcohol include their material compatibility with the original structural binding agent (i.e. lime), small size of lime particles (this should have positive impact upon the depth of penetration), low number of application cycles which does not support mobilisation of water-soluble salts in the material.
The success of the strengthening process is influenced by a number of factors. The basic properties and parameters that need to be taken into consideration when designing the strengthening technology include: chemical and mineralogical composition of the material to be strengthened, hardness of the material before strengthening, size and shape of open pores, porosity, size of ions, molecules or particles of the active strengthening agent in the strengthening product, chemical composition and concentration of the active agent, speed of drying and hardening of the active agent in the strengthened substrate at specific temperature and moisture conditions of material and ambient air. The knowledge of the above parameters is a good basis when designing the strengthening technology -the type of the strengthening product, concentration of the active agent, method of application of the consolidation agents, number of repeated applications within one consolidation cycle, method of verification of substrate after impregnation.

CONCLUSION
At the moment, nanomaterials are applied in historic and cultural heritage structures in the Czech Republic occasionally and sufficient know-how and experience for their reliable and permanent application are not available. However, if these new materials should be applied, e.g., to surfaces or to below-surface layers of materials, verified know-how about long-term behaviour of these materials (i.e. verifying the stability in light of attack of outside environment, influence of radiation, interaction with the ambient environment, transparency, diffuse transmittance, resilience against chemical and biochemical degradation, etc.) would be required so as to avoid gradual and irrecoverable damage of valuable historic structures such as paintings, plaster, wooden, stone elements, etc. The tested nanosuspensions comply with the assumptions of their future use to consolidate porous lime-based materials.