Consolidation methods for calcareous materials

Abstract

The disclosure relates to consolidation methods that result in deep penetration of calcium hydroxide into a calcareous substrate and subsequent formation of calcium carbonate in decayed areas. In particular, methods comprise O2 plasma pretreatment of, and / or the application of electrical potential across, calcareous substrates to reinforce the structural integrity of a deteriorated substrate by the application of consolidant materials such as nanolime.

Description

[0001] Consolidation Methods for Calcareous Materials Field of the Invention The disclosure relates to consolidation methods that result in deep penetration of calcium hydroxide into a calcareous substrate and subsequent formation of calcium carbonate in decayed areas. Background of the Invention The consolidation of porous calcareous substrates with calcium hydroxide based consolidate materials such as nanolime is a conservation treatment which shows a growing importance since 2000 [1,2]. Nanolime (a term of art used to describe a composition comprising calcium hydroxide nanoparticles dispersed in an alcoholic medium) proved to be efficient in the consolidation of wall paintings, frescoes [3-7], plasters, renders [8-10] and lime-mortars [11,12]. Nanolime is chemically compatible with calcareous stones, in particular limestone, and restores the cohesion loss by regenerating mineral bridges with new calcite [13-15]. Methods for the production of Ca(OH)2 nanoparticles or nano hydraulic lime are for example disclosed in WO2014 / 02515 and WO2022125027, respectively. Nevertheless, the penetration of nanolime into fine-grained substrates characterised by low porosity ( ^^ < 20 %) in particular can sometimes be limited to a few millimetres at best, and so yield ineffective consolidation. Quick evaporation of the solvent used for the nanolime dispersion [16,17] and the low substrate porosity [18,19] are the main factors responsible for the observed insufficient penetration and consolidation efficiency. During the absorption of the alcoholic nanolime by low-porosity substrates, the calcium hydroxide nanoparticles tend to migrate back towards the surface as the alcohol evaporates. This phenomenon is known as “back-migration” and, in addition to limiting the penetration of nanolime, it also contributes to the formation of a white haze over the treated surface [16,17,20]. To tackle this issue and improve the penetration of the nanolime, previous work investigated the use of a number of solvents characterised by different physical properties [20-22]. It was proven that mixing the nanolime with solvents with a higher boiling point and surface tension (such as water, butanol, and propanol) helps to slow down the evaporation rate of the alcoholic dispersion and gives the nanolime sufficient time to penetrate the stone matrix [23,24]. In addition to the solvent properties, the penetration of nanolime can be affected by different environmental factors such as the relative humidity (RH) [25-28], the CO2 level and the air temperature [29-32], but also by the type and porosity of the limestone [33,34], the application methodology [2,25, 35-40] and the properties of the nanolime dispersion [16,21,23,41,42]. Other methods to reinforce the surface of weathered limestone are disclosed in US2021179504A using a low-temperature plasma source loaded with carbon dioxide to improve the carbonation yield of nanolime to strengthen weathered stone cultural relics. However, despite these improvements, consolidation of calcareous stones such as limestone, in particular fine-grained compact calcareous stones with low porosities, is typically limited to the surface and deep penetration into the substrate has not been achieved yet. The inventors developed consolidation methods using energetically-induced processes by means of electric currents and / or plasma surface activation, to enhance the penetration of consolidant product and restore the mineral cohesion of the decayed porous mineral matrix, with the view of improving the compressive strength and / or consolidation efficiency of deteriorated calcareous stones including, but not limited to limestone, marble, dolomite, biocalcarenite, travertine etc). Statements of Invention The present invention, in its various aspects, is as set out in the accompanying claims. According to a first aspect of the invention, there is provided a method for reinforcing the structural integrity of a deteriorated substrate or article, said method comprising: (i) applying a liquid consolidant composition comprising calcium hydroxide (Ca(OH)2) onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface; and (ii) curing said coated substrate, wherein, after step (i) and prior to step (ii), an electric current is applied across said deteriorated calcareous substrate. As used herein, the term calcareous substrate refers to a solid material that is formed from or contains a high proportion of calcium carbonate (CaCO3). Additionally, the calcareous substrate is substantially free of calcium sulfate containing materials such as plasters and / or clay-based siliceous ceramics. Preferably, the calcareous substrate comprises a stone material, and more preferably comprises one or more of the following substrates: limestone, biocalcarenite, marble, dolomite, and travertine. Most preferably, the calcareous substrate comprises limestone. Consolidation of calcareous materials is well-known in the art and describes the reinforcement of the structural integrity of a deteriorated or weathered substrate with a consolidant substance (i.e. nanolime) that restores mechanical properties, such as improved surface hardness and (compressive) strength, by regeneration of mineral bridges within said substrate with newly-formed calcite by the reaction of Ca(OH)2and gas / atmospheric CO2and air moisture. It has been found that the application of an electric current to the coated calcareous substrate not only enhances the penetration of calcium hydroxide-based consolidant beyond the coated surface and into the underlying subsurface, but also increases consolidation efficiency, particularly for the treatment of fine-grained compact calcareous substrates characterised by a low porosity. Therefore, in preferred embodiments, said calcareous substrate has a porosity ( ^^), which can be determined by UNE-EN 1936-2007 standard or by Mercury Intrusion Porosimetry (MIP), of about 20 % or less, and more preferably from about 11 % to about 18 %. In preferred embodiments, the consolidant composition comprises a dispersion of calcium hydroxide particles in a short-chain aliphatic alcohol, preferably a C1-C4alcohol and most preferably ethanol. Alternatively, or additionally, the calcium hydroxide is preferably provided in the form of nanoparticles, i.e. approximately plate-like / planar, and preferably approximately hexagonal, particles having a size of from about 50nm to about 500 nm, and more preferably from about 100 nm to about 300 nm. Further, in exemplary embodiments, the consolidant composition is characterized by one or more, and preferably all, of the following: a density of from about 0.6 g / cm3to about 1.0 g / cm3; a dynamic viscosity of from about 1.1 mPa.s to about 1.5 mPa.s; and a zeta potential (i.e. the potential difference existing between the surface of the nanolime particle immersed in the alcohol carrier and the bulk alcohol per se) from about 55 mV to about 60 mV. As will be appreciated, such consolidant compositions are readily available from numerous commercial sources and are typically marketed as nanolime dispersions. For example, the nanolime dispersion CaLoSiL®E5 (5 g / L Ca(OH)2 particles (50- 150nm) in ethanol), which was supplied by IBZ Salzchemie GmbH (Germany), was used in the proof-of-concept consolidation experimental work disclosed herein. Alternative nanolime dispersions suitable for use in the present invention include Nanorestore Plus®E5 and E10 (5g / L and 10 g / L of Ca(OH)2 particles (100-300 nm) in ethanol, respectively). Preferably, the consolidant material is combined with one or more poultice material. As will be readily appreciated, a poultice can be created from a variety of substances including sponges, fibre mats, powdered cellulose fibres, cellulose ethers, and natural or synthetic hectorite clays such as Laponite. However, the poultice material is preferably selected from water soluble cellulose ethers (e.g. Methocel®), powdered cellulose fibres (e.g. Arbocel BC 1000®), and synthetic hectorite clays (e.g. Laponite RD®). In exemplary embodiments, the poultice material is an aqueous dispersion comprising from about 2 to about 15 w / v%, and preferably about 6 w / v% of a synthetic hectorite clay such as laponite RD®. Such synthetic clay materials are advantageously combined with the consolidant material for two reasons: firstly, being a poultice material, it acts as a reservoir gradually delivering the consolidant material through the treated surface; and secondly, it will provide a constant humidity level that is required to make the calcareous substrate surface conductive enough to facilitate the desired penetration of the consolidant material by the application of an electric potential to the calcareous material. In preferred embodiments, the method further comprises, prior to step i), a preconditioning step wherein the surface of said calcareous substrate is cleaned and / or wetted with water or other aqueous solution having a high electrical conductivity (i.e. ^^ ≥ about 50, and preferably from about 60 to about 100 µS / cm) and / or a high permittivity (i.e. ^^ ^^ ≥ about 60, and preferably from about 75 to about 90). In particularly preferred embodiments, preconditioning is carried out using tap water. Alternatively, or additionally, said preconditioning introduces from about 5 wt.% to about 15%, and more preferably no more than about 11 wt.% of said water or other aqueous solution into the porous matrix of the calcareous substrate. Tap water is preferably used in this preconditioning step because its high permittivity (i.e. the ability to hold electrical energy and to create an induced electric field), which is about 80.37 at 25 °C, is more than three times higher than that of alcohol (i.e. ethanol)

[0049] , which is typically contained in the nanolime consolidant compositions used to reinforce the integrity of the calcareous substrate. Further, water is a polar solvent, and its electrolytic conductivity depends on the types and concentration of the impurities it contains (i.e., ions such as calcium, chlorides, nitrates, and sulphates) which are in constant motion. Therefore, when exposed to an electric field, under direct current (DC) configuration in the method of the invention, the cations contained in the water migrate towards the negative electrode (cathode) and the anions towards the positive electrode (anode), thereby producing an electric current flow

[0050] , resulting in deeper and more homogenous penetration as well as a higher consolidation efficiency. Preferably, the pre-conditioning step comprises introducing water into the calcareous substrate subsurface by means of capillary absorption; localised application by brushing, pipetting, spraying, or via ultrasonic air humidification; steam cleaning the substrate with water, or a combination of the methods aforementioned being particularly suitable for this purpose. In particularly preferred embodiments, the calcareous substrate is steam cleaned as this has been shown to wet and so increase the conductivity of the deteriorated substrate more rapidly when compared to other pre-conditioning methods. In alternative preferred embodiments, the pre-conditioning step may comprise a combination of steam cleaning and ultrasonic air humidification (UAH). Where UAH is employed, a spray volume capacity of 25 – 35 mL / h is preferably used). Where steam cleaning is employed in said preconditioning step, the substrate is preferably steam cleaned from a period of from about 1 to about 10 minutes, and preferably from about 5 to about 10 mins, to maximise efficiency of the conditioning step whilst avoiding damaging the surface of the substrate. Similarly, steam cleaning is preferably carried out whilst maintaining a gap between the steam source and calcareous substrate surface of from about 20 cm to about 30 cm. Further, a thin layer of tap water is preferably applied to the surface of the substrate, preferably by brushing, immediately after completion of the steam cleaning process to further increase electrical conductivity. Alternatively, or additionally, the pre-conditioning step may comprise covering the surface of the calcareous substrate with a thin flexible layer of a porous material. Suitable porous materials include, but are not limited to, Japanese tissue having a thickness of from about 2 to about 7 gr / m2,, fine woven medical gauze swabs and / or thin layers of hydrogel in absorbent pad. Covering the surface of the calcareous substrate in this way serves to maintain a residual humidity in the substrate during the consolidation method. In the above method, the consolidant composition (and, optionally, the poultice material) may be applied to at least onto a portion of the calcareous substrate by any suitable application means (e.g. brushing, nebulisation with a spray or an airbrush, or injection). An electric current, preferably a direct electric current, is then applied across the coated substrate to enhance the consolidant penetration into the substrate. In order to apply such an electric current, anodic connection is made to the first (deteriorated) surface of the calcareous substrate and cathodic connection is made to the second surface, opposite the first. Such connections can be made by wrapping the respective surfaces of the substrate with any conductive material such as aluminium foil or fine mesh. However, to avoid any short circuit between the anode and cathode a gap should be present, and preferably about a 5 mm to about 8 mm wide gap, between the cathodic and anodic conducting material. Preferably, the electric current is applied to the calcareous substrate using a DC power supply, which in some embodiments is set to a voltage of from about 12 V to about 36 V (e.g. about 24 V) and a current of from about 10 mA to about 40 mA (e.g. about 20 mA). Alternatively, or additionally, the electric current is preferably applied to the calcareous substrate for a period of from about 1 to about 10 hours, more preferably for a period of from about 2 to about 6 hours and most preferably for about 4 hours. In preferred embodiments, the calcareous substrate is at least partially, and more preferably entirely, covered with a non-breathable material such as a plastic (e.g. Cling Film) layer whilst the electric current is applied in order to reduce the evaporation of water and / or alcohol during this treatment step. Following application of the electric current, the coated calcareous substrate is cured in step ii) of the above method. Preferably, this step comprises storing the treated substrate in a curing environment for a period of at least about one month, and more preferably for at least about two months. Preferably, any residual consolidant composition and / or poultice material, together with any layers of porous, conductive and / or non-breathable materials are removed from the treated substrate before starting step ii). In preferred embodiments, the coated calcareous substrate is stored / submitted in step ii) in / to an environment having at least one and preferably all of the following: a relative humidity (RH) of 70 ± 5 %; a temperature of 20 °C ± 1°C; and a CO2 level of about 400 ppm to about 600 ppm. These environmental conditions are considered optimal to ensure efficient carbonation of alcoholic nanolime dispersions [26-28,55]. However, it should be noted that an increase in RH (≥ 80 %) and / or CO2levels (at least about 1 %, and preferably from about 5 % to about 20 %), would help increase the carbonation rate and so yield of the alcoholic nanolime dispersion(s) used. Therefore, in alternative embodiments, the coated calcareous substrate is stored / submitted in step ii) in / to an environment having a relative humidity (RH) of 80% and / or a CO2level of at least about 1% and preferably from about 5% to about 20 %. According to a second aspect of the invention, there is provided a method for reinforcing the structural integrity of a deteriorated substrate, said method comprising: (i) applying a liquid consolidant composition comprising Ca(OH)2onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface; (ii) curing said coated substrate, wherein, prior to step (i), said first surface is chemically functionalised by an O2plasma treatment process. It has been found that the wettability of the substrate surface can be modified by plasma surface activation by atmospheric pressure to increase the surface energy of the calcareous substrate and reduce the contact angle ( ^^ ^^), as shown in Figure 1, thereby increasing the adhesion and penetration of Ca(OH)2-based consolidant materials, particularly in fine grained compact calcareous substrates characterised by a low porosity. Therefore, in preferred embodiments, said calcareous substrate has a porosity ^^, which can be determined by UNE- EN 1936-2007 standard or by Mercury Intrusion Porosimetry (MIP), of less than about 20 %, and more preferably from about 11 % to about 18 %. Indeed, the deposit of polar functional groups (i.e. radicals) delivered by the plasma over a rough and uneven surface such as fine grained, compact and deteriorated calcareous substrate favours the imbibition and the spreading processes of the liquid applied over the surface. It will be appreciated that, in addition to the plasma action, wettability can also be affected by the surface roughness which is generally expressed as following

[0060] : ^^ ^^ ^^ ^^W= ^^ ^^ ^^ ^^ ^^0(1) where ^^W is the (Wenzel) contact angle characterising the rough surface, ^^0 the contact angle on (ideal) smooth surface as defined by Young’s equation, and ^^, the Wenzel roughness factor ( ^^ > 1). According to the Wenzel model, a liquid applied on a rough and porous calcareous surface follows a specific dual wetting behaviour. In this case, some of the liquid stays over the wetting front and the rest of it follows the surface topography and spreads into the porous mineral matrix by capillarity action

[0061] . Therefore, as shown in Figure 2, it can be assumed that the combination of plasma treatment and surface roughness will greatly reduce the contact angle, as the fraction of the liquid present over the wetting front should migrate through the activated surface. The principle of O2 plasma functionalisation or activation is straightforward and requires the use of an oxygen rich gas such as atmospheric gas (compressed air in this case) which is sent between the central and the insulated ring electrodes present in the plasma gun. Then, high voltage creates an arc discharge between the electrodes and ionised species are generated in the corona discharge zone. The plasma is subsequently blown out through the nozzle and highly reactive, oxygen containing, functional groups (also named as “radicals”) are deposited across the surface of the sample. The efficiency of the proposed technique is dual and relies on the combination of chemical and thermo-mechanical actions generated by the plasma, as shown in Figure 3. First, the plasma creates polar binding sites over the surface in coating this later with oxygen active species such as superoxide (O2-) and peroxide (O22-), hydroxyl (OH-), nitric oxide (NO-), or hydrogen peroxide (H2O2) and carbon free radicals, which will affect the hydrophilicity of the treated surface [62,63]. Then, the energy and the heat generated by the plasma affects the surface morphology, leading to an increase of its roughness

[0064] and the creation of microchannels inside the mineral matrix. As per the first aspect, in preferred embodiments of the second aspect the consolidant composition comprises a dispersion of calcium hydroxide particles in a short-chain aliphatic alcohol, preferably a C1-C4alcohol and most preferably ethanol. Alternatively or additionally, the calcium hydroxide is preferably provided in the form of nanoparticles, i.e. approximately plate-like / planar, and preferably approximately hexagonal, particles having a size of from about 50nm to about 500nm, and preferably from about 100 nm to about 300 nm. Further, in exemplary embodiments, the consolidant composition is characterized by one or more, and preferably all, of the following: a density of from about 0.6 g / cm3to about 1.0 g / cm3; a dynamic viscosity of from about 1.1 mPa.s to about 1.5 mPa.s; and a zeta potential (i.e. the potential difference existing between the surface of the nanolime particle immersed in the alcohol carrier and the bulk alcohol per se) from about 55 mV to about 60 mV. Such consolidant compositions are readily available from numerous commercial sources and are typically marketed as nanolime dispersions. For example, the nanolime dispersion CaLoSiL®E5 (5g / L in ethanol), which was supplied by IBZ Salzchemie GmbH (Germany), was used in the proof-of-concept consolidation experimental work disclosed herein. The consolidant material may or may not be combined with one or more poultice material. When present, the poultice material is preferably selected from water soluble cellulose ethers (e.g. Methocel®), powdered cellulose fibres (e.g. Arbocel BC 1000®), and synthetic hectorite clays (e.g. Laponite RD®). In exemplary embodiments, the poultice material, if present, is an aqueous dispersion comprising from about 2 to about 15 w / v%, and preferably about 6 w / v% of a synthetic hectorite clay such as Laponite RD®. In the O2 plasma treatment process, plasma is preferably applied to the calcareous substrate surface in the form of atmospheric plasma. In exemplary embodiments, plasma is applied at a frequency of from about 150 kHz to about 200 kHz (preferably about 170 kHz), a plasma flow of from about 20 l / min to about 60l / min (preferably 40 l / min (± 2 l / min)), and / or an air pressure of from about 2 bar to about 10 bar (preferably about 4.5 bar to about 6 bar). As will be readily appreciated, the required air pressure can be provided by various means, such as by an air compressor. It will be readily appreciated by the skilled person that several parameters influence the effect of plasma surface activation, in particular: the type of gas used for plasma activation (i.e. Ar, O2, He, CO2, N2, NH3, H2, CF4, SF4, and C4F8); working distance ( ^^ ^^);), contact angle ( ^^ ^^) between the plasma nozzle and the surface of the sample; plasma exposure time ( ^^ ^^ ^^); plasma flow ( ^^ ^^); and cooling period ( ^^ ^^) after the plasma application. As a result of experimental analysis, optimum settings have been derived: In preferred embodiments, plasma is applied at a ^^ ^^ of from about 1 to about 12 cm, more preferably about 12 cm. In preferred embodiments, plasma is applied at a ^^ ^^ of from about 45 ^ to about 90 ^, more preferably about 90 ^. In preferred embodiments, plasma is applied at a ^^ ^^ ^^ of from about 1 to about 15 mins, more preferably about 15 min. In preferred embodiments, plasma is applied at a ^^ ^^ of from about 3 to about 5 cm / s. In alternative, embodiments, plasma is applied at a ^^ ^^ of from about 20 to about 30 cm / s. In preferred embodiments, plasma is applied with a ^^ ^^ of between 1 to 24 h, more preferably 2 h. During this cooling period, the activated calcareous substrate is preferably covered with a non-breathable material to protect the activated substrate from the transfer of air pollutants and / or moisture and / or the surface temperature of the substrate is reduced to below about 20 ºC. Once the plasma treatment process is complete, the liquid consolidant composition is applied onto at least a portion of a deteriorated first surface of the plasma treated porous calcareous substrate. Preferably, the liquid consolidant composition is applied to the substrate surface in an enclosed environment by brushing in order to reduce the rate of evaporation of the alcohol component of the consolidant composition. In particularly preferred embodiments, after application of the consolidant composition, water is sprayed over the substrate and / or the treated substrate is covered in a non-breathable material such Cling Film®to prevent back migration of the consolidant material towards the treated surface. The treated substrate is then left for a period of at least about 12 hours, and more preferably at least about 24 hours to allow the consolidant material to penetrate through the calcareous material and react. The coated substrate is then cured in step ii) of the above method. As per the first aspect, this step preferably comprises storing the treated substrate in a curing environment for a period of at least about one month, and more preferably for at least about two months. Preferably, any residual consolidant composition and / or poultice material, together with any layers of porous, conductive and / or non-breathable materials are removed from the calcareous substrate before starting step ii). In preferred embodiments, the coated substrate is stored in step ii) in an environment having at least one and preferably all of the following: a relative humidity (RH) of 70 ±5 %; a temperature of 20 °C ±1 °C; and a CO2level of about 400 to about 600 ppm. However, it is again noted that an increase in RH (≥ 80 %) and / or CO2 levels (at least about 1 %, and preferably from about 5 % to about 20 %), would help increase the carbonation rate and so yield of the alcoholic nanolime dispersion(s) used. Therefore, in alternative embodiments, the coated calcareous substrate is stored / submitted in step ii) in / to an environment having a relative humidity (RH) of 80% and / or a CO2level of at least about 1% and preferably from about 5% to about 20 %. It will be appreciated that the methods of the first and second aspects can be combined into a single method combining an O2plasma pre-treatment step with the application of electrical potential across the coated substrate. Therefore, in a third aspect of the invention, there is provided a method for reinforcing the structural integrity of a deteriorated substrate, said method comprising: (i) applying a liquid consolidant composition comprising calcium hydroxide (Ca(OH)2) onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface); and (ii) curing said coated substrate, wherein prior to step (i), said first surface is chemically functionalised by an O2 plasma treatment process, and wherein after step (i) and prior to step (ii), an electric current is applied across said calcareous substrate. Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to” and do not exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Throughout the description and claims of this specification, the term “about” includes a variation of ± 5%, preferably ± 4%, more preferably ± 3%, still more preferably ± 2% and most preferably ± 1% unless the context otherwise requires. All references, including any patent or patent application, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. Further, no admission is made that any of the prior art constitutes part of the common general knowledge in the art. Other features of the present invention will become apparent from the following examples. Generally speaking, the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including the accompanying claims and drawings). Thus, features, integers, characteristics, compounds or chemical moieties described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith. Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose. The Invention will now be described by way of example only with reference to the Examples below and to the following Figures wherein: Figure 1. Theoretical model describing the impact of the plasma surface activation on the wettability of a porous limestone substrate; Figure 2. Theoretical model describing the impact of the plasma surface activation on the imbibition and the spreading of a liquid applied on a rough and porous surface (characterised here by successive spikes). It will be noted that in this case the plasma treatment applied on a rough surface reduces at the maximum contact angle ( ^^ ^^ < 1°); Figure 3. Theoretical model describing the chemical (deposit of radicals over the treated surface) and thermo-mechanical (formation of microchannels into the porous matrix) impacts induced by the treatment with plasma surface activation and the resulting improvement of the nanolime penetration; Figure 4. ^^ ^^ ^^ − ^^2test samples: (a & b) sound and (c & d) weathered Caen limestone samples; Figure 5. MIP results showing the average pore size distribution (PSD) for the sound and weathered Caen limestone ( ^^ ^^ ^^ − ^^2); Figure 6. View of limestone samples treated by the plasma for a same ^^ ^^ ^^ (15 mins) and ^^ ^^ = 12 cm and 2 cm; Figure 7. Examples of ADSAP pictures of the surface of sound and weathered Caen samples: (a & b) not pre-treated with plasma; (c & d) pre-treated with plasma (the red dotted circle outlines the area where the droplet touched the surface); Figure 8. Compressive strength ( ^^) profiles obtained after DRMS testing on (a) Sound and (b) Weathered Caen samples treated by the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique, the Control treatment, plus the As-received; Figure 9. Bar chart showing the compressive strength data for the Sound and Weathered Caen samples treated by the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique, the control treatment, plus the as- received (before treatment); Figure 10. Average Pore size distribution (MIP) characterising the sound and weathered Caen limestones ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^); Figure 11. Experimental phases describing the conditioning of the samples and the application of the ^^ −nanolime technique on a Caen limestone sample: (a) Sample before treatment; (b) Steam cleaning for 10 mins; (c) Sample after steam cleaning; (d & e) Brushing wet Japanese tissue layers over the sample surfaces; (f & g) Application of the aluminium kitchen foil at the bottom / sides of the sample; (h & i) mix of the laponite (6% W / V in H2O) gel with 6 ml of nanolime (CaLoSiL®E5 undiluted); (j) Application of the nanolime-laponite gel over the surface to be treated; Figure 12. Experimental phases describing the final steps of the application of the ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique on a Caen sample: (a) Injection through the aluminium foil of more nanolime into the gel; (b & c) Top and sides of the sample wrapped with an aluminium foil leaving a gap to avoid any short circuit between the conductive aluminium foils; (d) Monitoring the conductivity between the electrodes; and (e) Covering the sample with Cling®Film; Figure 13. (a) Corrosion products formed at the lower surface of a sample at the end of the ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ treatment, and (b) corrosion products brushed off; Figure 14. Compressive strength ( ^^) profiles obtained after DRMS testing on (a) sound and (b) weathered Caen samples treated by the ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique and the Control (Ctrl) treatment plus the as-received (before treatment); Figure 15. Bar chart showing the compressive strength data for the sound and weathered Caen samples treated by the ^^-nanolime technique and the Control treatment plus the as- received samples (before treatment); Table 1. Summary of parameters tested to find the ^^ ^^ ^^ − ^^2settings; Table 2. Summary of Caen samples used for ^^ ^^ ^^ − ^^2efficiency experiments; Table 3. DRMS results of the samples ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 treated with the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique and the samples ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 & #2 treated with the nanolime only. "DR" stands for “Drilling Resistance”; Table 4. Examples of determination of the water contents introduced into the samples ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1–3; Table 5. Phenolphthalein results for the samples ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1–3treated with the optimal ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique, and the control samples ( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 & #2) treated with the nanolime only; Table 6. DRMS results for the samples ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#3 & #4treated with the optimal electro- nanolime technique, and the control sample ( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#3 & #4) treated with the nanolime only. " ^^ ^^" and " ^^" stand for “drilling force” and “compressive strength”, respectively. Example 1 – Plasma Surface Activation (‘PSA-O2’) Method Materials and Methods Nanolime For the consolidation experiments, the commercially available nanolime dispersion CaLoSiL®E5 (5 g / L in ethanol) provided by IBZ Salzchemie GmbH (Germany) was used, the physical properties of CaLoSiL®can be found in the literature

[0051] . Limestone Caen limestone was used for the experiments which were carried out on sound and weathered prismatic blocks with dimensions varying from approximately 3 x 3 x 3 cm to 5 x 5 x 5 cm. The sound samples were provided by the Cathedral of Canterbury, UK and originally cut from sound quarried slabs imported from France. The weathered samples were chiselled from a historical architectural block which was removed from an unstable masonry section of the cathedral (Figure 4). Polarised Light Microscopy (PLM) was used to provide the petrographic characterisation of the Caen limestone. For the observations of the thin sections (made by Petrolab®Ltd. UK), a Nikon®Eclipse E400-Polarised Light Microscope working with Koehler-type diascopic illumination optics connected to a Nikon®Digital Sight Monitor was used. The results confirmed the clastic sedimentary origin of the Caen samples composed mainly of intraclasts, bioclasts and peloids, bound together by a micritic calcite matrix. According to Folk’s classification of sedimentary rocks

[0052] , the Caen limestone used for the experiments can be classified as intrabiopelmicrite. The mineralogical composition of the samples was determined by means of X-Ray Diffraction (XRD). Samples in the forms of powder were examined using a Philips X'Pert Pro MPD diffractometer with Cu detector Kα in the angular range 15 - 75° 2θ. The XRD patterns were recorded with a step size of 0.026˚ 2θ in the angular range 5 - 70° 2θ, Rietveld refinement was used to provide quantitative analyses, and X-ray data was fitted using the pseudo-Voigt profile function. Specimen displacement, polynomial coefficients for the background function, lattice parameters, profile parameters and Gaussian and Lorentzian profile coefficients were refined. Profile Fit Software (High Score Plus, PANalytical) was used to elaborate each experimental diffraction pattern and the ICSD and ICDD reference databases were consulted to identify each crystalline phase. X-Ray Fluorescence (XRF) was combined to XRD analyses to determine the elemental composition of the samples. For the analyses, a PANalytical MagiX Pro X-ray Fluorescence working with a Rhodium anode (as X-ray source) was used. The equipment allows analyses of around 77 chemical elements ranging from Boron (B) to Americium (Am) with concentration capabilities from a few ppm to 100 %. To prepare the XRF samples analysed as pressed aluminium pellets, 0.1 g of sample was mixed with approximately 0.1 g of cellulose binder which was then sprinkled on top of 2 g of cellulose. The pressed pellets were then compacted at 20 tons in a Retsch PP40 hydraulic press and loaded into the PANalytical MagiX Pro X-ray Fluorescence spectrometer to collect XRF spectra. PANalytical standardless "IQ+" software was used to quantify by weight % the concentration of the chemical elements present as oxides content. The results revealed that the Caen samples are essentially composed of calcite (CaCO3, ICSD #01-078-4614, # 01- 072-4582) with traces (1% < Wt.%) of Mg, Fe, Al and Si. The pore-size distribution (PSD) and the average porosity of the samples were determined by means of Mercury Intrusion Porosimetry (MIP) using a Pascal 140 / 240 equipment. The porosity was calculated as the average of three tests carried out on sound and artificially weathered samples measuring approximately 8 x 8 x 10 mm. The samples were oven dried at 70 °C for 24 h prior to the analysis and the mercury contact angle was taken to be 140°. The results showed that the sound and weathered samples are characterised by unimodal pore size distribution and average porosities of 16% and 17%, respectively (Figure 5). The strength of the Caen samples used in this project was determined before and after the application of the treatment by means of a Drilling Resistance Measurement System (DRMS) from SINT-Technology. The DRMS measures the drilling force ^^ ^^ ( ^^) required to drill a hole at constant rotational speed (rpm) and a lateral feed rate (mm / min). It has been acknowledged [67-71] that from ^^ ^^, it is possible to determine the specific compressive strength of a material ( ^^) using the penetration law below

[0068] (1): where ^^ is the radius of the drill bit and ^^ the cutting depth per revolution ( ^^ ^^), which is determined by Equation (2)

[0058] Where ^^ is the penetration rate (mm / min) and the revolution speed (rpm). DRMS tests were performed on cubes using a drill bit of 5 mm Ø, ^^ of 600 rpm, ^^ of 3 mm / min, and a drilling depth of 20 mm. Drilling resistance values were calculated as a mean of six tests carried out on each sample, giving a total of twenty-four readings to assess the consolidation efficiency of the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique. Upon completion of the DRMS tests, the compressive strength was determined using the aforementioned formulas (1) and (2). To finish, the Axisymmetric Drop Shape Analysis Profile (ADSAP) technique was used to determine the contact angle ( ^^), thus the wettability of the Caen samples. An OCA 15 Plus Dataphysics Instrument was used carry out the tests before and after treatments following a method described in the literature

[0033] . The water droplet contact angle which forms on a material's surface defines its hydrophobicity. The higher the contact angle, the more hydrophobic is the material's surface. For this test, a water droplet (5μl, deionised water) was placed over the surface of the substrate by means of a Hamilton 50 μl DS 500 / GT syringe. Pictures of the water droplet on the surface were taken after approximately 1 second after applying the water drop and the contact angle was calculated by the software. The static contact angle (θ) was calculated as the average of four measurements, taken at room temperature (21 ± 1 °C). A note of caution should be given concerning the measurement of contact angle on porous stone though. Indeed, according to the literature

[0072] , because of the hydrostatic and capillary pressures and the roughness of the surface, the precision and accuracy of the measurement can be questioned. Therefore, in this work, the (ADSAP) technique was used to obtain a qualitative assessment of the surface wettability and not to measure the actual contact angle. Plasma Module A ULS-1 Nano plasma module used in this work was provided by Active Surface Technology UK Ltd, and all experiments were carried out using the default plasma settings, i.e. a frequency of 170 kHz, a plasma flow of 40 l / min (± 2 l / min) and an air pressure between 4.5 bars and 6 bars, provided by an air compressor. During plasma application, the plasma module and the air membrane dryer were placed outside a fume hood and the plasma gun was fitted inside the fume hood for the extraction of exhaust gases. The limestone samples were placed at the centre of an aluminium tray which was also fitted inside the fume hood. To help contain the active chemical species delivered by the plasma, an experimental setup using an enclosed pipe made of stainless-steel fine gauze covered with layers of aluminium kitchen foil was constructed. To prevent any damages which could be caused to the plasma gun, the top of the pipe was left open to avoid generating too much heat and trapping exhausted gases during the application of the plasma. Plasma and nanolime treatment Plasma Surface Activation by Atmospheric Pressure ( ^^ ^^ ^^ − ^^2) Different parameters known to affect the treatment efficiency were tested, these are summarised in Table 1 Table 2. Parameters tested Settings ^^ ^^ 1 cm to 12 cm ^^ ^^ 45 º and 90 º ^^ ^^ ^^ 1 min to 15 mins ^^ ^^Fixed Flow, Slow Flow (3 – 5 cm / s), Rapid Flow (20 – 30cm / s) ^^ ^^ 1 h to 24 h In determining the optimum settings, it was noted that the shorter the working distance between the plasma nozzle and the surface of the sample, the more heat is generated, with damage to the stone surface potentially occurring at shorter working distances (see samples with ^^ ^^ = 12 cm and 2 cm in Figure 6). Although, the ULS-1 Nano plasma module technical guide advises to set the ^^ ^^ between 1 cm and 2 cm to minimise the loss of energy delivered by the plasma, it was decided to fix it at 12 cm as no heat damage was evident at this distance. However, it is believed that the combination of a long period of application ( ^^ ^^ ^^ = 15 mins) and the use of an enclosing pipe could offset the adverse effect of the long ^^ ^^, by containing the radicals and metastable and giving them enough energy and time to reach the surface of the sample. Therefore, to summarise, upon completion of the laboratory tests, the settings for an optimal plasma treatment were found to be ^^ ^^ = 12 cm; ^^ ^^ ^^ = 15 mins; Fixed ^^ ^^; ^^ ^^ = 90º; and ^^ ^^ = 2h. Nanolime application Once treated by the plasma, the samples were wrapped in aluminium kitchen foil to protect the surface from the transfer of air pollutants and moisture and left to cool down for 2 h to reduce the temperature at the surface (< 20 ºC). Then, 2 ml of CaLoSiL®E5 (undiluted) was applied by brushing in an enclosed environment to limit the impact of draught which could accelerate the evaporation of the ethanol. Finally, water was sprayed over the treated surface and the samples were wrapped in Cling Film®for 24 h. Assessment of the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique efficiency To assess the efficiency of the ^^ ^^ ^^. ^^2technique, ten samples were used in total, as described in Table 2. Two of them, named ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 were treated using the ^^ ^^ ^^. ^^2 technique, and two control samples named ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 were treated using the nanolime only (no plasma treatment on the surface). In addition to these four samples, four limestone cubes named ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 and ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 were used to assess the impact of the plasma application on the wettability performance against control (no plasma) samples by means of ADSAP. Finally, two specimens named ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 & #2 were used to determine the mechanical properties of the samples before treatment with plasma and nanolime.

[0002] Table 2. Sample ID PSA-O2 Nanolime Surface wetting Test type ^^ ^^ ^^ − ^^^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ ^ ^ ^ sound Caen ^^ ^^ ^^ − ^^^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ ^ ^ ^ weathered Caen DRMS ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ x ^ ^ sound Caen ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ x ^ ^ weathered Caen ^^ ^^ ^^ − ^^^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ ^ x ^ sound Caen ^^ ^^ ^^ − ^^^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ ^ x ^ weathered Caen CAM ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ x x ^ sound Caen ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)# ^^ x x ^ weathered Caen ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ # ^^ x x x DRMS sound Caen ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ # ^^ x x x DRMS weathered CaenTwo months after being cured inside an environmental chamber with RH at 70 % (± 5 %), 20°C (± 1 °C), and a CO2 concentration at ≈ 400 – 600 ppm, the samples ^^ ^^ ^^ −^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 and the ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1& #2 were tested by means of DRMS to assess the consolidation efficiency. Finally, samples ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 and ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 were all assessed using the ADSAP technique. Results Change of surface property The contact angles measured with the ADSAP technique showed that plasma treatment improves the wettability of the stone surface by increasing its surface energy. Indeed, the average contact angles measured at the surface of sound (Figure 7, a) and weathered (Figure 7, b) Caen limestone samples not pre-treated by the plasma ( ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & 2) is about 34.07° (± 6.61°) and 19.02° (± 4.68°), respectively. Conversely, when the surface of the sound (Figure 7, c) and weathered (Figure 7, d) samples is activated with the plasma ( ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & 2), the average contact angle is below the limit of the detection in both cases. Consolidation efficiency DRMS testing shows that pre-treating the sample by the plasma before applying the nanolime greatly improves the consolidation performance of the treatment. The data presented in Table 3 show that the samples treated with the ^^ ^^ ^^. ^^2 technique are characterised by the highest drilling resistance which in turn leads to the highest compressive strength as calculated using Equations 1 and 2 above. For example, concentrating on the compressive strengths of the sound Caen, the ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 samples yielded a strength of 17.38 N / mm2. The ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 which was treated with nanolime only increased its strength to 27.63 N / mm2but with the plasma intervention beforehand, the strength increased to 76.38 N / mm2for ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1. With regards to the weathered Caen, the strength of the ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #2 sample was 16.75 N / mm2. Following the application of nanolime, the compressive strength increased to 21.75 N / mm2for ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#2 but following the plasma treatment, the strength increased to 27.50 N / mm2for ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #2. The compressive strength (σ) profiles are shown in Figure 8. This illustrates the high efficiency of the ^^ ^^ ^^. ^^2 technique in increasing σ to a great extent after only one application of the nanolime. Indeed, in comparison with the profiles of the ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 & #2 and ^^ ^^ ^^ ^^ ^^ ^^ ^^( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 samples, the profiles for the ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 & #2 samples showed a constant increase in σ from the surface to the tested depth (i.e.2cm). Table 3. Sample ID Test N° Fd (N) Average Fd (N) σ (N.mm²) Hole #1 6.01 1 N.mm² = 1 MPa Hole #2 6.22 Hole #36.516.11 76.38 PSA-O2 - Nanolime #1 Hole #46.11± 0.31 ± 3.88 Hole #5 5.58 Hole #6 6.23 Hole #1 2.34 Hole #22.26Hole #32.532.20 27.50 PSA-O2 - Nanolime #2 Hole #4 2.02 ± 0.22 ± 2.75 Hole #5 2.07 Hole #6 1.98 Hole #11.89Hole #21.58Hole #3 2.22 2.21 27.63 Control #1 Hole #4 2.37 ± 0.41 ± 5.13 Hole #5 2.68 Hole #6 2.51 Hole #1 1.72 Hole #2 1.71 Hole #3 1.67 1.74 21.75 Control #2 Hole #4 1.77 ± 0.09 ± 1.13 Hole #51.91Hole #61.67Hole #1 1.42 Hole #2 1.32 Hole #31.271.39 17.38 As-received #1 Hole #41.38± 0.09 ± 1.75 Hole #51.52Hole #6 1.43 Hole #11.38Hole #21.36Hole #31.141.34 16.75 As-received#2 Hole #4 1.44 ± 0.10 ± 2.25 Hole #5 1.36 Hole #6 1.38 The bar chart presented in Figure 9 confirms that ^^ ^^ ^^. ^^2 improves the mineral cohesion of the porous mineral matrix. Indeed, when applied on sound Caen limestone, the technique increases the compressive strength of the sample ( ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#1 ) to 76.38 N / mm2, which is more than four times the compressive strength characterising the ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1 sample (i.e. 17.38 N / mm2). The data obtained for weathered Caen show that when the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique is applied on this limestone, the compressive strength is increased but to a lesser extent though. Indeed, the compressive strength of the sample ^^ ^^ ^^ − ^^2( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^)#2 is 27.50 N / mm2, compared to 16.75 N / mm2obtained before treatment on the ^^ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #2 sample. Finally, it will be noted that in comparison with the control samples, those pre-treated with the plasma are characterised by higher compressive strengths, thus validating the consolidation efficiency of the ^^ ^^ ^^. ^^2− ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique on sound and weathered Caen limestones. The results also indicate the benefits of using ^^ ^^ ^^ − ^^2as a preventative measure against deterioration on sound limestone which increases the strength and therefore durability against weathering. Example 2 – Electro-nanolime (‘ ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^’) Treatment Method Nanolime dispersion The commercially available nanolime dispersion CaLoSiL®E5 (5 g / L in ethanol) provided by IBZ Salzchemie GmbH (Germany) was used throughout this work for the consolidation experiments. CaLoSiL®is composed of plate-like hexagonal nanoparticles ranging in size between 50 nm and 250 nm

[0051] . CaLoSiL®E5 is characterised by a density of 0.79 g / cm3, a dynamic viscosity of 1.3 mPa.s and a zeta potential (i.e. the potential difference existing between the surface of the nanolime particle immersed in ethanol and the bulk of ethanol) of 57.8 mV

[0051] . Four types of nanolime dispersion were considered for use as electrolytes for the experiments, these are: CaLoSiL®E5 (referred to as CAL-E5), CaLoSiL®E5 diluted at 5 % V / V in tap water (referred to as ^^ ^^ ^^ − ^^5. ^^5), CaLoSiL®E5 diluted at 10 % V / V in tap water (referred to as ^^ ^^ ^^ − ^^5. ^^10), and CaLoSiL®E5 diluted at 20 % V / V in tap water (referred to as ^^ ^^ ^^ − ^^5. ^^20). Limestone samples The experiments were carried out on sound and artificially weathered Caen limestone blocks of prismatic shapes, with dimensions varying from approximately 3x3x3 cm to 5x5x5 cm. The samples were cut from quarried slabs imported from France and provided by the Cathedral of Canterbury (UK). A total of ten samples were employed in the research and sized to maximise output from the low volume of limestone available. The petrographic characterisation of the Caen limestone was carried out by means of Polarised Light Microscopy (PLM). Thin sections (prepared by Petrolab®Ltd. UK) were examined using a Nikon®Eclipse E400-Polarised Light Microscope working with Koehler-type diascopic illumination optics connected to a Nikon®Digital Sight Monitor. The results showed that the studied rock is of clastic sedimentary origin and composed mainly of intraclasts, bioclasts and peloids, bound together by a fine-grained calcite matrix (micrite). Based on these results, the rock can be classified as an intrabiopelmicrite according to Folk’s classification of sedimentary rocks

[0052] . The mineralogical composition of the Caen limestone was determined using X-Ray Diffraction (XRD). Powder samples were examined by means of a Philips X'Pert Pro MPD diffractometer with Cu detector Kα in the angular range 15 - 75° 2θ. The XRD patterns were recorded with a step size of 0.026˚2θ in the angular range 5 - 70° 2θ. Quantitative analyses were carried out by means of Rietveld refinement. The X-ray data was fitted using the pseudo-Voigt profile function. Specimen displacement, polynomial coefficients for the background function, lattice parameters, profile parameters and Gaussian and Lorentzian profile coefficients were refined. Each experimental diffraction pattern was elaborated by means of a Profile Fit Software (High Score Plus, PANalytical) and each crystalline phase was identified using the ICSD and ICDD reference databases. In addition to XRD, X-Ray Fluorescence (XRF) was used to determine the elemental composition of Caen limestone. A PANalytical MagiX Pro X-ray Fluorescence working with a Rhodium anode for the X-ray source was used, which analyses a 77- wide range of chemical elements from Boron (B) to Americium (Am) with concentration capabilities from few ppm to 100 %. XRF samples were prepared as pressed powder samples by mixing approximately 0.1 g of sample with approximately 0.1 g of cellulose binder and sprinkling the resulting mix on top of 2 g of cellulose. The aluminium boat was then compacted at 20 tons in a Retsch PP40 hydraulic press to form a pressed pellet. The pellet was loaded into the PANalytical MagiX Pro X-ray Fluorescence spectrometer to collect XRF spectra. The XRD and XRF data for weight % concentration was analysed using the PANalytical standardless "IQ+" software. The results show that the Caen limestone used is mainly composed of calcite (CaCO3, ICSD #01-078-4614, # 01-072-4582) with traces (1 % < Wt.%) of Mg, Fe, Al and Si. The porosity and pore-size distribution (PSD) of the samples were determined by Mercury Intrusion Porosimetry (MIP) using a Pascal 140 / 240 instrument. Porosity was determined as the average of three tests carried out on sound and weathered fragments measuring approximately 8x8x10 mm. Prior to the analysis, the samples were oven dried at 70 °C during 24 h. The mercury contact angle was taken to be 140 °. The results showed that the tested sound and weathered Caen samples are characterised by a unimodal pore-size distribution (Figure 10), and average porosities of 16 % and 17 %, respectively. Weathering of limestone samples Caen samples were artificially weathered by means of a Binder®climatic chamber (model MKFT 240 – 228 L. The water saturated limestone samples were exposed to freeze-thaw cycles running for twelve hours at -12 °C and twelve hours at +12 °C. The cubes were saturated with tap water, wrapped with Cling film®and then put into sealed freezing bags to prevent water evaporation during the freeze-thaw cycles. Electro-nanolime treatment The impacts of the following parameters were tested: power settings, period of application of power, conditioning techniques, water content of the stone, type and position of the electrodes, type of the nanolime dispersions and the method of application. Upon completion of the laboratory tests, an optimal ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique has been proposed. To validate its efficiency, six confirmation samples were treated with the selected ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique, of which three samples were dedicated for the nanolime penetration assessment ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1–3) and the remaining three ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#4-6) were used to assess the consolidation effectiveness. In addition to these six samples, four control samples ( ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1–4) were treated with the nanolime only using existing application techniques. The optimal ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique consists of three main steps : Conditioning of samples For the first step, the surface to be treated by the nanolime was steam cleaned (Figure 11, a– c). To avoid damaging the surface by the action of the steam cleaning, the distance between the steam cleaner nozzle and the surface of the substrate was maintained between 20 cm and 30 cm and the period of steam cleaning limited to 10 mins maximum. Based on the laboratory tests, the longer the steam cleaning period, the better is the efficiency of the conditioning. Immediately after the steam cleaning, tap water (4-6 ml) was brushed over the sample surfaces to increase the electrical conductivity. Unconditioned samples stored in a laboratory environment (20 °C ± 1 °C and 35 % RH ± 5 %) were used as controls. The results showed that the conditioning technique introduces less than 11 % by weight of water into the porous matrix. Application of the nanolime After the conditioning, wet Japanese tissue layers (2-7 gr / m2) were brushed over the surfaces of the sample to maintain a residual humidity (Figure 11, d & e). Then, the bottom surface and part of the sides of the sample were wrapped up with a sheet of aluminum kitchen foil (Figure 11, f & g) and the anode plugged onto it using a crocodile clip and cable. Next, the nanolime dispersion (undiluted) was mixed with a Laponite RD®(6 % W / V in H2O) and applied over the surface to be consolidated (Figure 11, h-j). The exact quantity of nanolime requried was dependent on the size and condition of the limestone treated. The larger and more deteriorated the surface to be treated, the higher the quantity of the nanolime required. If a high quantity is needed, injecting at regular intervals (i.e. every 15-30 mins) the nanolime through the aluminium foil during the treatment period can also be considered (Figure 12, a). After the application of the nanolime-laponite mix, the top surface, and the rest of the sides of the sample were wrapped down with another sheet of aluminum kitchen foil, and the cathode plugged onto the aluminum foil using a crocodile clip and cable (Figure 12, b). It will be noted that the closer the distance between the anode and the cathode (i.e. between both aluminium sheets), the higher the conductivity. However to avoid any short circuit between the anode and the cathode a 5-8 mm gap was ensured to avoid direct contact between the aluminum sheets (Figure 12, c). Application of power During the treatment, the electric current and voltage were delivered by a Thurlby Thandar Instrument PL320QMD DC power supply (32V – 2A, 240W). Once the electrodes were plugged onto the aluminiun kitchen foils, the power settings were adjusted to ensure the optimal electrical conductivity ( ^^) which was checked with a multimeter (Figure 12, d), it should be stressed that the higher the ^^, the better the treatment performance. Upon verification of the electrical conductivity, the voltage was set at 24 V and the current at 20 mA for all the experiments. Then, the power was left on until ^^ was reduced to the threshold conductivity limit (fixed in this project to below 2 mA). For the whole experiments, the power was left on during 4 h and then, the treated samples were covered with Cling Film®to limit the evaporation of moisture and ethanol during the treatment (Figure 12, e). Direct Current was preferred to Alternating Current (AC) because the former generates a unidirectional flow of electrons which should favour the migration of nanolime from the treated surface to the core of the stone sample. However, it should be noted that because of the reduction-oxidation reactions happening at the electrodes

[0053] , the top surface of the sample (treated with the nanolime) will become more alkaline during the treatment, while the bottom surface of the sample will be more acidic. Therefore, (electrolytic) corrosion will occur at the anode as the pH becomes more acidic, thus promoting the formation of aluminium corrosion products which will be deposited over the sample surface during the treatment (Figure 13, a). Nevertheless, in this case, it is a question of superficial corrosion happening at the (bottom) surface only, as the aluminium corrosion products can easily be brushed off straight after the removal of the aluminium sheet (Figure 13, b). However, it will be noted that in the case of a fragile substrate, brushing off the corrosion could lead to a removal of materials. Curing of limestone samples At the end of the treatment, the power was switched off, all the materials used in the process were removed (i.e. the aluminium foils, Japanese tissue layers, and the laponite-nanolime gel), and the treated samples were stored in a curing chamber for at least two months. This consisted of a plastic box containing a saturated solution of sodium chloride, which was used to maintain RH at 70 % (± 5 %). Air temperature and CO2levels in the room where the box was placed remained steady at 20 °C (±1 °C) and around 400 ppm, respectively. The selected environmental conditions are considered optimal to ensure an efficient carbonation of the nanolime [26-28,55]. RH, CO2 level, and temperature were constantly monitored by means of an Eltek®thermohygrometer working with an Eltek®Squirrel 1000 series system. Assessment of the ^^ −nanolime technique efficiency At the end of the established treatment period (i.e.4 h), the samples ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1–3were split with a chisel and tested by means of phenolphthalein GPRTM(1 % W / V diluted in 60 % of ethanol and 40 % of H2O) which was pipetted over the cross-sections to assess the penetration of the nanolime. The ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1 & #2, which were also wrapped with Cling Film®during 4 h after the application of the nanolime, were tested by means of phenolphthalein at the same time. The mechanical properties of the Caen limestone used in this project were determined before and after the application of the treatment. Two months after being stored in the curing chamber, the samples ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ # 4-6 and the ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #3 & #4 were tested by means of a Drilling Resistance Measurement System (DRMS) from SINT-Technology, to assess the drilling resistance profile of the treated samples. The DRMS measures the drilling force ^^ ^^ ( ^^) required to drill a hole at constant rotational speed (rpm) and a lateral feed rate (mm / min). It has been acknowledged [56-60] that from ^^ ^^, it is possible to determine the specific compressive strength of a material ( ^^) using the penetration law below

[0057] (3): where ^^ is the radius of the drill bit and ^^ the cutting depth per revolution ( ^^ ^^), which is determined by Equation (4)

[0058] Where ^^ is the penetration rate (mm / min) and the revolution speed (rpm). DRMS tests were performed on cubes using a drill bit of 5 mm Ø, ^^ of 600 rpm, ^^ of 3 mm / min, and a drilling depth of 20 mm. Drilling resistance values were calculated as a mean of six tests carried out on each sample, giving a total of twenty-four readings to assess the consolidation efficiency of the ^^ −nanolime technique. Upon completion of the DRMS tests, the compressive strength was determined using the formulas (3) and (4) aforementioned. Results Determination of the water content into the limestone matrix Details on the techniques applied to condition the samples are provided above. This involved increasing the conductivity of the stone by steam cleaning followed by brushing tap water over the cleaned surface for enhanced surface conductivity. Steam cleaning was used as a method for rapidly increasing the conductivity of the stone as opposed to adopting a water immersion procedure which takes longer and may not be practical in a real situation. Their respective effectiveness in enhancing conductivity is given in Table 4. Referring to Table 4, the sample IDs ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^# 1-4) and weights in the dry state are given. Following immersion for nine days in water, new sample weights are given which are converted to percentages of water (range: 9.05 % to 11.51 %). Dry samples were then subjected to steam cleaning for 10 minutes followed by re-weighing to establish the percent of moisture retained in the samples. This was calculated between 8.93 % and 10.46 %. Therefore, comparing the moisture retention between the two samples, the immersion method averaged 10.67 % whereas the steam cleaning method averaged 9.82 % giving a small difference of only 0.85%. Therefore, in terms of introducing moisture to the stone to increase conductivity, it has been shown that the much quicker and more practical method of steam cleaning is only marginally worse than immersion. However, it will be noted that the experiments by immersion were carried out with a tap water at ambient temperature. It is believed that an immersion with warm tap water of which the temperature could be gradually increased, could open more the pores and increase the ratio of water introduced into the porous matrix. As a comparison, it was disclosed above that the porosity of Caen limestone used in this study averaged about 17 %. Therefore, if full saturation was to be achieved through completely filling all the pores in the samples with water, this would lead to an increase in weight of about 15 g (taking an average of all three weights of the samples in Table 4). Therefore, since the percentage of water introduced is about 10 %, this demonstrates that the samples are not completely saturated, but moisture has penetrated the surfaces only making them more conductive. Since it is the surface of the limestone that suffers deterioration, partial penetration is beneficial since it limits the solubilisation of potential salts in the pores. Table 4. Sample Sample Sample weight Percentage of Sample Percentage of ID weight at at a saturation tap water weight after tap water a dry state (i.e. after content conditioning content state immersion in introduced with steam introduced (g) H2O for 9 days) after cleaning after (g) conditioning (10mins) conditioning by by immersion (g) steam cleaning (% Wt / Vp) (% Wt / Vp) ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ 88.35 98.52 11.51 97.26 10.07 #1^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ 88.31 98.47 11.51 97.55 10.46 #2^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ 88.05 96.01 9.05 95.92 8.93 #3 ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ 85.17 94.77 11.27 93.44 9.70 #4 Penetration of the nanolime Phenolphthalein tests The phenolphthalein tests carried out 4 h after the treatments show that the use of an applied voltage during the application of the nanolime greatly improves its penetration into the limestone (Table 5). The samples were split open using a hammer and chisel and immediately the phenolphthalein solution was applied over the cross-sections. The results of the sound ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#1 & #2) and weathered ( ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#3) samples treated by the technique developed in the laboratory show that the nanolime penetrated about 2 cm to 4 cm in-depth, in comparison with the sound ( ^^ ^^ ^^ ^^ ^^ ^^ ^^ #1) and weathered ( ^^ ^^ ^^ ^^ ^^ ^^ ^^ #2) control samples where almost no penetration of the nanolime was observed. Two points should be made regarding the measurement of the applied current before and after the treatment. First, although the current setting was fixed at 20 mA for all the experiments, data showed in Table 2 revealed that the actual current delivered by the power supply before the treatment was less than 20 mA for all the samples. This is due to the fact that the current was applied on limestone, which is generally characterised by a high electrical resistivity, thus reducing the actual current delivered by the power supply. Second, it will be noted that the current measured at the end of the treatment is lower than the one measured at the beginning. It will be recalled here that the ^^ −nanolime technique relies on the surface conductivity of the limestone sample treated. Indeed, as the water introduced into the limestone matrix during the conditioning migrates through the pores by capillarity effect, this reduces the moisture content present at the near- to-surface layers. Therefore, the migration of water affects the flow of electrical charges between the electrodes, thus leading to a decrease of the electrical conductivity of the sample. Table 5. Samples Water Nanolime DC Current Phenolphthalein results content treatment settings measured carried out on the cross- in Details before the sections at the end of stone application of the treatment period (i.e. matrix the nanolime 4 h after application of after (bef) and at the nanolime and the conditi the end of the applied voltage oning treatment (4 h) (Wt% / Vp) CaL E5 ^^ ^^ ^^ ^^ ^^ ^^ ^^#1No (8ml) NO mixed in Condit VOLTAGE laponite Sound Caen ioning gel (6 % W / V in H2O) CaL E5 ^^ ^^ ^^ ^^ ^^ ^^ ^^ #2 No (8ml) NO mixed in Condit VOLTAGE laponite Weathered ioning gel (6 % Caen W / V in H2O) CaL E5 (2 ml) mixed in ^^ − 24 V (DC) laponite 16.95 mA (bef) ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#110.07 gel (6 % - 11.72 mA (4h) % W / V in 20 mA Sound Caen H2O) + 6 ml of CaL E5 injected (1ml / 30 mins) ^^ − CaL E5 ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#2(8ml) 24 V (DC) 16.17 mA (bef) mixed in - 9.45 mA (4h) laponite Sound Caen 10.46 gel (6 % 20 mA % W / V in H2O) ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #3 8.93 CaL E5 24 V (DC) 19.80 mA (bef) % (8ml) - 11.87 mA (4h) Weathered mixed in 20 mA Caen laponite gel (6 % W / V in H2O) ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#48.93 CaL E5 24 V (DC) 20.04 mA (bef) % (8ml) - 11.87 mA (4h) Weathered mixed in 20 mA Caen laponite gel (6 % W / V in H2O) Consolidation efficiency DRMS tests The DRMS results presented in Table 6 show that in addition of increasing the nanolime penetration on a fine-grained Caen limestone characterised by a reduced porosity ( ^^ ~ 16-18 %), the ^^ −nanolime technique improves the consolidation performance too. Indeed, the comparison of data obtained before and after treatment reveals that after one application of the ^^ −nanolime technique, ^^ ^^ is increased in average to 2.14 N (± 0.25 N) and 1.52 N (± 0.40 N) for sound and weathered samples, respectively, thus translating a respective increase of the drilling resistance to ~ 54 % and ~ 55 %.

[0003] Table 6. Sample ID Test N° Fd (N) Average Fd (N) σ (N.mm²) Average Fd and σ Hole #1 2.11 1 N.mm² = 1 MPa before treatment Hole #22.31Sound Caen Hole #3 2.22 2.14 26.75 E-nanolime #3 Hole #4 2.13 ± 0.25 ± 1.38 Hole #52.12Hole #6 1.98 1.39 N (± 0.14 N) Hole #11.23 17.38 N.mm² (± 1.75 N.mm²) Hole #2 1.28 Sound Caen Hole #3 1.53 1.44 18.00 Control #3 Hole #41.62± 0.18 ±2.00 Hole #5 1.46 Hole #6 1.53 Hole #11.93Hole #2 1.44 Weathered CaenHole #31.441.52 19.00 E-nanolime #4 Hole #41.60± 0.40 ± 2.75 Hole #5 1.43 Hole #6 1.29 0.98 N (± 0.19 N) Hole #1 1.02 12.25 N.mm² (± 2.38 N.mm²) Hole #2 1.38 Weathered CaenHole #31.411.37 17.13 Control #4 Hole #4 1.36 ± 0.20 ± 2.38 Hole #5 1.58 Hole #61.45Conversely, it can be seen that when a sound sample is treated by the nanolime only, the increase of the drilling resistance is reduced to a great extent, thus confirming the limited penetration of the nanolime observed with the phenolphthalein results. For instance, the average force of the sound ^^ ^^ ^^ ^^ ^^ ^^ ^^ #3 sample is about 1.44 N (± 0.18 N), which gives an increase in the drilling resistance of about 3.60 %. The compressive strength ( ^^) profiles obtained for sound samples (Figure 14, a) confirmed the phenolphthalein results which show that the ^^ −nanolime technique improved the nanolime penetration to a depth of about 3 cm. Indeed, in comparison with the sound ^^ ^^ ^^ ^^ ^^ ^^ ^^ #3 and before treatment (as-received) samples the graph of the sample sound ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#3is characterised by a higher and constant ^^ over the tested depth. Even though, according to Table 6, the ^^ −nanolime technique applied on weathered samples appears to be as efficient as when applied on sound samples, the profiles shown in Figure 14, b reveal that the increase in ^^ for the sample weathered ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#4is essentially due to the presence of three major peaks located between 8.4 – 9.7 mm, 12 – 13.8 mm, and 17.9 – 18.8 mm. It can also be stated that in comparison with the control treatment, the ^^ −nanolime technique appears to be efficient only after ~ 3 mm deep. This could be due to the action of the current which affects the motion of the nanolime by forcing the nanoparticles to migrate deeper into the porous matrix, whereas in the case of the control treatment, a high ratio of the nanolime might have remained on the surface thus increasing ^^ at the near-to-surface layers. Therefore, it can be assumed that the three broad peaks seen on the profiles of the weathered ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #4 could be related to a high ratio of newly formed calcite resulting from the presence of nanolime clusters located at a preferential depth. The bar chart presented in Figure 15 shows that after only one application of the ^^ −nanolime technique, sound and weathered samples are characterised by higher compressive strengths. Indeed, ^^ for the samples ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ – ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^#3and ^^ ^^ ^^ ^^ℎ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ – ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ #4 are about 26.75 N / mm2(± 1.38 N / mm2) and 19.00 N / mm2(± 2.75 N / mm2), thus translating an increase of about 54 % and 56 %, respectively, when comparing with the data characterising the as-received samples. When comparing the data of samples treated with the Control treatment (i.e. nanolime only) with those obtained before treatment, it can be seen that the Control treatment leads to a reduced consolidation efficiency, especially for the sound samples. Indeed, ^^ for the sample ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ – ^^ ^^ ^^ ^^ ^^ ^^ ^^ #3 is about 18.00 N / mm2(± 2.00 N / mm2), against 17.38 N / mm2(± 1.75 N / mm2) for the as-received sample, hence resulting in an increase of only ~ 4 %. As expected, ^^ increase for the ^^ ^^ ^^ ^^ℎ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ – ^^ ^^ ^^ ^^ ^^ ^^ ^^ #4 is higher (~ 40 %), this is due to the more deteriorated porous matrix of the former sample, which allowed a deeper penetration of the nanolime. Summary ^^ ^^ ^^ − ^^2: A new conservation technique involving the use of O2plasma showed the benefits that plasma surface activation by atmospheric pressure could have on the increase of the consolidant performance. This technique proved to be efficient in improving the consolidation performance of nanolime consolidant when applied on low porosity (i.e. ^^ < 20%) Caen limestone. Indeed, experiments carried out on sound and weathered Caen limestones showed that the samples treated with the ^^ ^^ ^^. ^^2 technique are characterised by an average compressive strength which is higher than the one characterising the as-received samples and those treated with the nanolime only. ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^: An innovative way to improve the performance of nanolime by the action of an applied voltage has been discovered. The technique proved to be effective in increasing both the penetration of the nanolime and its consolidation efficiency, when applied on a fine- grained compact Caen limestone characterised by a low porosity. It is believed that the effectiveness of this could be due to a synergetic action involving the interaction of the following factors: (1) The action of an applied electric field on a wet limestone matrix generates electrophoresis / osmosis phenomena which together affect the wetting properties of the surface of the treated sample, thus the migration process of the nanolime through the limestone; (2) The redox reactions happening at the electrodes during the treatment application increase the exchange of surface charges. Therefore, changes are also evident on the conductivity of the limestone sample favoured by the deprotonation / protonation of basic surface group (from the nanolime), the preferential dissolution of ions (i.e. Ca2+, CO32-and HCO3-) from the carbonated crystal lattice, the differential adsorption of ionic species (H+and OH-) from the electrolytes (i.e. nanolime and water present at the surface and in the porous matrix of the sample); (3) The induced Faradaic current generated nearby the electrodes during the electrowetting process favours the penetration of electrically charged nanolime particles through the wet stone matrix. (4) The variation of pH at the interface solid-liquid during the electrolysis cause a preferential ionic absorption of the nanolime, thus its migration through the electrically charged sample matrix. The ^^ − ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ technique is easy to implement and requires only the use of basic materials such as: a DC power supply, kitchen aluminium foil, and conservation resources (i.e., nanolime and laponite gel). The effectiveness of the proposed technique relies on the electrical conductivity of the limestone sample treated. A conditioning technique using steam cleaning was found to be very effective in increasing the conductivity of the sample without introducing excessive water into the porous stone matrix (less than 10 % Wt / Vp of H2O). Upon completion of the treatment, the nanolime shows an impressive penetration of about 2 to 3 cm in-depth. According to the literature, it is believed that to date, no other known stone conservation technique can increase the penetration of the nanolime to such extent on a fine-grained compact limestone characterised by a porosity which is lower than 20%.

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Claims

Claims 1. A method for reinforcing the structural integrity of a deteriorated substrate, said method comprising: (i) applying a liquid consolidant composition comprising calcium hydroxide (Ca(OH)2) onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface; and (ii) curing said coated substrate, wherein, after step (i) and prior to step (ii), an electric current is applied across said deteriorated calcareous substrate.

2. The method according to claim 1, wherein said method further comprises, prior to step i), a preconditioning step wherein the surface of said calcareous substrate is cleaned and / or wetted with water or other aqueous solution having an electrical conductivity ^^ ≥ about 50 and / or a permittivity ^^ ^^ ≥ about 60.

3. The method according to claim 2, wherein said preconditioning is carried out using tap water and / or wherein said preconditioning step introduces from about 5 wt.% to about 15 wt.% of said water or other aqueous solution into the porous matrix of the calcareous substrate.

4. The method according to claim 2 or claim 3, wherein said preconditioning step comprises steam cleaning, optionally for a period of from about 1 to about 10 minutes.

5. The method according to any of claims 2 to 4, wherein said preconditioning step comprises covering the surface of the calcareous substrate with a thin flexible layer of a porous material6. The method according to any of the preceding claims, wherein said electric current is a direct current, optionally having a voltage of from about 12 V to about 36 V and a current of from about 10 mA to about 40 mA.

7. The method according to any of the preceding claims, wherein the calcareous substrate is at least partially covered with a non-breathable material whilst said electric current is applied.

8. A method for reinforcing the structural integrity of a deteriorated substrate, said method comprising: (i) applying a liquid consolidant composition comprising Ca(OH)2 onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface; (ii) curing said coated substrate, wherein, prior to step (i), said first surface is chemically functionalised by an O2 plasma treatment process.

9. The method according to claim 8, wherein plasma is applied to the calcareous substrate surface in the form of atmospheric plasma.

10. The method according to claim 9, wherein said plasma is applied at a frequency of from about 150 kHz to about 200 kHz, a plasma flow of from about 20 l / min to about 60l / min, and / or an air pressure of from about 2 bar to about 10 bar.

11. The method according to any of claims 8 to 10, wherein said plasma is applied: a. at a working distance ( ^^ ^^) of from about 1 cm to about 12 cm; b. at a contact angle ( ^^ ^^) between the plasma nozzle and the surface of the sample of from about 45 ^ to about 90 ^; c. at a plasma exposure time ( ^^ ^^ ^^) of from about 1 to about 15 mins; d. at a plasma flow ( ^^ ^^) of from about 3 to about 5 cm / s; and / or e. with a cooling period ( ^^ ^^) of between about 1 hour and about 24 hours.

12. The method according to any of claims 8 to 11, wherein the liquid consolidant composition is applied to the substrate surface in an enclosed environment by brushing.

13. The method according to any of claims 8 to 12 wherein, after application of the consolidant composition, water is sprayed over the substrate and / or the treated substrate is covered in a non-breathable material before being left for a period of at least about 12 hours to allow the consolidant material to penetrate through the calcareous material and react.

14. A method for reinforcing the structural integrity of a deteriorated substrate, said method comprising: (i) applying a liquid consolidant composition comprising calcium hydroxide (Ca(OH)2) onto at least a portion of a deteriorated first surface of a porous calcareous substrate to provide a coated substrate (sub)surface; and (ii) curing said coated substrate, wherein prior to step (i), said first surface is chemically functionalised by an O2 plasma treatment process, and wherein after step (i) and prior to step (ii), an electric current is applied across said calcareous substrate.

15. The method according to any of the preceding claims, wherein the calcareous substrate comprises a stone material, and optionally comprises one or more of the following substrates: limestone, biocalcarenite, marble, dolomite, and travertine.

16. The method according to claim 15, wherein the calcareous substrate comprises limestone.

17. The method according to any of the preceding claims, wherein the calcareous substrate has a porosity ^^ of about 20 % or less.

18. The method according to any of the preceding claims, wherein the consolidant composition comprises a dispersion of calcium hydroxide particles in a short-chain aliphatic alcohol, preferably a C1-C4alcohol.

19. The method according to claim 18, wherein said calcium hydroxide is provided in the form of nanoparticles, and wherein said nanoparticles are optionally plate-like / planar particles having a size of from about 50nm to about 500 nm.

20. The method according to any of the preceding claims, wherein the consolidant composition is combined with one or more poultice material, and wherein said poultice material is optionally selected from water soluble cellulose ethers, powdered cellulose fibres, and synthetic hectorite clays.

21. The method according to claim 20, wherein the poultice material is an aqueous dispersion comprising from about 2 to about 15 w / v% of a synthetic hectorite clay.

22. The method according to any of the preceding claims, wherein the curing of step (ii) comprises storing the coated substrate in a curing environment for a period of at least about one month, and wherein said curing environment optionally has at least one and preferably all of the following: a relative humidity (RH) of 70 ± 5 %; a temperature of 20 °C ± 1°C; and a CO2level of about 400 ppm to about 600 ppm.

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