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International audience; The durability of building materials with respect to salt crystallization is commonly determined by accelerated weathering tests, carried out in the laboratory. An effective laboratory weathering test should assess the durability and, in the case of conservation of historic buildings, the compatibility of repair materials with those existing. Besides, the test should provide reliable results within a reasonable period of time, accelerating the deterioration process without however altering its mechanism. Despite several national and international standards, recommendations and guidelines, a commonly accepted testing protocol does not yet exist. Researchers often develop and apply their own procedure, a fact that complicates comparison between different studies. The RILEM Technical Committee 271 ASC has been set up with the scope of developing improved test procedures for the assessment of the behaviour of materials under the influence of salt crystallization, which should overcome the limitations of existing standards and recommendations. This paper constitutes one of the first results of the work of the Technical Committee. It critically reviews the literature on salt crystallization tests, identifies advantages and limitations of the several test protocols and provides new ideas for the development of improved salt crystallization procedures.

The use of ammonium phosphate solutions has proven to be very promising for protection and conservation of marble. However, all the studies carried out so far have been performed on uncontaminated marble. Unfortunately, this is rarely the case in the field, because marble artifacts exposed outdoors are often affected by sulfation, i. e. formation of a gypsum crust on the surface. Because gypsum is much more soluble than calcite, the outcome of the ammonium phosphate treatment is expected to be sensibly altered by the presence of gypsum. Therefore, in this study the nature and morphology of the new calcium phosphate phases formed by reacting gypsum with aqueous solutions of diammonium hydrogen phosphate (DAP) were investigated. In particular, the effect of DAP concentration, ethanol addition (aimed at reducing gypsum solubility), and pH were explored. The result is that phase formation can be controlled by suitably tuning the above mentioned parameters. Phases with low solubility (such as tricalcium phosphate and hydroxyapatite) can be obtained by increasing the ethanol concentration, the DAP concentration or the pH. However, their formation is associated with diffused cracking, likely because of excessive growth of the new phases. Among the investigated formulations, treatment with a 0.1 M DAP solution with 30 vol % ethanol at pH=8 seems to be the most suitable one, as it leads to formation of brushite (about 30 times less soluble than gypsum), without cracking, so that a reduction in gypsum solubility in rain is expected.

Consolidation of sugaring marble (i.e., marble affected by granular disaggregation) still lacks fully effective solutions. Consequently, the use of an innovative phosphate-based treatment, aimed at bonding calcite grains by formation of hydroxyapatite at grain boundaries, has recently been proposed. In this paper, firstly a novel method for producing artificially decayed marble samples, by contact with a heating plate, is proposed. Then, some results are presented about the effectiveness and the compatibility of two different formulations of the phosphate treatment, differing in terms of concentration of the phosphate precursor (3.0 M or 0.1 M aqueous solutions of diammonium hydrogen phosphate, DAP), possible ethanol addition to the DAP solution and number of DAP solution applications (1 or 2). The results of the study point out that the new weathering method allows to obtain specimens with a gradient in microstructural and mechanical properties with thickness, just like naturally weathered samples. Both phosphate treatments were able to significantly improve marble cohesion, without causing significant changes in thermal behaviour and aesthetic appearance after treatment. The addition of small quantities of ethanol to the DAP solution seems to be a very promising method for favouring HAP formation and improving the treatment performance.

In this study, a novel method for consolidation and improvement of resistance to water of gypsum-stuccoes was preliminarily investigated. The idea is treating gypsum with an aqueous solution of diammonium hydrogen phosphate (DAP, (NH4)2HPO4) to form hydroxyapatite (HAP, Ca10(PO4)6(OH)2), which has much lower solubility than gypsum. Tests carried out on gypsum paste samples, manufactured to resemble historic stuccoes, showed that, after treatment with the DAP solution, a significant improvement in mechanical properties was achieved and brushite (CaHPO4∙2H2O) was formed (alongside some other by-products, that can be removed by an additional poultice treatment). Even if brushite is more soluble than HAP, still its formation is expected to be beneficial for stuccoes conservation, as brushite is significantly less soluble than gypsum.

Resin injection is a widespread technique to fight rising damp in historic brick masonry buildings, but the evaluation of its drying effectiveness is often hard, also due to the lack of standard procedures to test the different damp-proofing materials. Some testing procedures have been proposed in national guidelines and scientific literature to assess the effectiveness of chemical injection in laboratory, but they reflect very different approaches and are hard to compare. This paper firstly provides a critical overview of the presently available methods to assess the effectiveness of chemical injections in brick masonry at the laboratory scale, highlighting their main advantages and drawbacks. Then, two different experimental test walls are proposed, having large and small size, respectively. The test walls were manufactured and subjected to a continuous capillary rise of water, monitoring the moisture amount by a micro-destructive method based on “permanent sampling holes”. Both test walls allowed to effectively reproduce rising damp occurring in real masonry walls and to monitor the moisture amount during time by an easy and reliable method. Large walls involve a long curing time (more than 1 year in laboratory conditions) and are quite space consuming, but they are less affected by the microclimatic variations. Conversely, small walls are more easy to handle, but also more affected by RH changes. The proposed test walls aim at contributing to the future development of a new testing procedure to evaluate chemical damp-proof courses in laboratory.

Dissolution in pure and acid rain is among the main environmental degradation processes of marble. Coatings based on hydroxyapatite (HAP) have been proposed to prevent marble corrosion, providing encouraging results. In this paper, the resistance to dissolution was investigated on marble treated with different HAP-treatment formulations and compared to commercial ammonium oxalate. The ability of the coating to prevent corrosion was evaluated by performing wetting/drying cycles, by a custom-designed apparatus simulating rain. When ethanol is added to the solution used to form HAP, a significant reduction in calcium ions leached from marble is found, which is indicative of reduced corrosion.

The response to thermal variations is the primary cause of marble deterioration in ancient and modern buildings. In this study, the thermal behavior of Carrara marble after consolidation by an innovative hydroxyapatite-based treatment was investigated in comparison with ammonium oxalate and ethyl silicate. Samples were subjected to heating-cooling cycles up to 80 °C. All the consolidating treatments were found to be fairly compatible, as in no case the residual strain after the thermal cycles was found to increase compared to the unweathered untreated marble. Anyway, the hydroxyapatite-treatment has the advantage of causing the highest increase in marble cohesion and the lowest residual strain, besides being more chromatically compatible and durable than the alternative commercial consolidants. Keywords: Hydroxyapatite, Calcium oxalate, Thermal expansion coefficient, Residual strain, Dilatometry

Salt crystallization can produce severe damage in porous stones, with a dramatic impact on cultural heritage conservation. Such damage is related to the fact that repulsive forces arise between the salt crystals and the pore wall, generating a disjoining pressure that frequently exceeds the tensile strength of stone. In this paper, new treatments are proposed, aimed at preventing salt damage by depositing a thin layer of polymeric coatings over the stone’s pore surfaces. These coating are expected to change the surface chemistry, eliminating the repulsion between the growing crystals and the pore wall and hence the development of the disjoining pressure. Several biopolymers were tested on these substrates: silica glass, calcite, and calcite subjected to a pre-treatment with diammonium hydrogen phosphate (DAP), aimed at preventing calcite dissolution and acting as an anchoring substrate for the polymer coating. Selected polymer treatments were applied to porous Globigerina limestone samples, which were subjected to crystallization tests with sodium sulfate, obtaining promising results (i.e., significant reduction in stone damage), especially when the polymers were applied after the DAP treatment., This research has also been funded by the Spanish Government (Grants CGL2015-70642-R, CGL2015-73103-EXP), the University of Granada (‘‘Unidad Científica de Excelencia’’ UCE-PP2016-05), the Junta de Andalucía (P11-RNM-7550), and the research group RNM-179 of the Junta de Andalucía.

The durability of a new phosphate consolidant for limestone, based on formation of hydroxyapatite (HAP), was investigated in comparison with ethyl silicate (ES). Untreated, HAP- and ES-treated samples were subjected to repeated wetting–drying, freezing–thawing and salt crystallization cycles. The weathering effects were monitored in terms of alterations in visual appearance, weight, dynamic elastic modulus, tensile strength and pore size distribution. HAP samples performed better than untreated samples and underwent less deterioration in original properties than ES samples. ES samples, subjected to salt crystallization cycles when the treated layer was still hydrophobic, experienced detachment of the consolidated layer.

For consolidation of weathered carbonate materials (such as marble, limestone and lime-based mortars) no fully satisfactory treatment currently exists. In this paper, an innovative phosphate treatment was investigated as a possible consolidant for limestone and compared with a commercial ethyl silicate (ES). The two treatments were evaluated in terms of effectiveness (i.e., ability to restore cohesion and mechanical properties, by measuring penetration depth, dynamic elastic modulus, tensile strength, resistance to abrasion) and compatibility (i.e., lack of any negative consequence on the original substrate, by assessing mechanical match, colour change, new phases composition, pore size distribution, water and water vapour transport properties, drying rate and thermal behaviour). The phosphate treatment proved to be very promising, being able to overcome some ES limitations.

Bowing of thin marble slabs is a phenomenon affecting both historic monuments and modern buildings. In spite of the ubiquity and destructiveness of this phenomenon, no fully satisfactory treatment is currently available to arrest and/or prevent bowing. In this study, a treatment based on formation of hydroxyapatite (HAP) was investigated as a possible route to arrest and possibly prevent bowing of Carrara marble slabs. Four different formulations of the HAP treatment were tested and compared to ammonium oxalate and ethyl silicate (widely used in the practice of marble conservation). The treatments were applied onto pre-weathered and unweathered specimens to investigate their ability to arrest and prevent bowing, respectively. Marble behavior was studied in terms of residual strain and bowing after thermal cycles up to 90 °C in dry and wet conditions. Marble cohesion was assessed before and after the thermal cycles by ultrasound. The HAP treatments exhibited promising results, as the residual strain and the bowing after the cycles were always lower or equal to the untreated references, while marble cohesion was always higher. Surprisingly, ammonium oxalate caused marked worsening of marble thermal behavior. In the case of ethyl silicate, most of the initial benefit after consolidation was lost after the thermal cycles. In general, the results of the study point out the importance of evaluating marble thermal behavior to assess the suitability of any conservation treatment and suggest that treatments able to strengthen marble without causing excessive pore occlusion and stiffening are preferable to enhance durability to thermal cycles.

Research is needed on novel consolidants to restore weathered marble, because the available products exhibit limitations. To test new consolidants, samples with features similar to naturally weathered marble (i.e., near-surface damage) are recommended. We developed a novel method that produces decayed samples with a gradient in properties that can be tuned to any desired level, in terms of depth from the surface and decrease in dynamic elastic modulus. The idea is to produce near-surface damage by putting a marble sample in contact with a plate at a certain temperature for a certain time, calculated by a mathematical model. The model was used to predict the heating conditions to produce a certain decrease in dynamic elastic modulus in the first 1 cm from the surface, comparable to naturally weathered marble. Good confirmation of the model predictions was found experimentally. The suitability of using thermally “weathered” samples for testing consolidants was demonstrated in the case of a hydroxyapatite-based treatment. The mathematical model can be tailored to other lithotypes or ceramics, using the relevant property data.

Reversibility is a mandatory requirement for materials used in heritage conservation, including hydrophobic protectives. Nevertheless, current protectives for stone are not actually reversible as they remain on the surfaces for a long time after their hydrophobicity is lost and can hardly be removed. Ineffective and aged coatings may jeopardise the stone re-treatability and further conservation interventions. This paper aims at investigating the performance of PHAs-based coatings for stone protection, their main potential being the ‘reversibility by biodegradation’ once water repellency ended. The biopolymer coatings were applied to three different kinds of stone, representative of lithotypes used in historic architecture: sandstone, limestone and marble. Spray, poultice and dip-coating were tested as coating techniques. The effectiveness and compatibility of the protectives were evaluated in terms of capillary water absorption, static and dynamic contact angles, water vapour diffusion, colour alteration and surface morphology. The stones’ wettability after application of two commercial protectives was investigated too, for comparison. Finally, samples were subjected to artificial ageing to investigate their solar light stability. Promising results in terms of efficacy and compatibility were obtained, although the PHAs-based formulations developed here still need improvement for increased durability and on-site applicability.

The effect of adding ethanol and isopropanol to aqueous solutions of diammonium hydrogen phosphate, used to create a passivating layer of calcium phosphates over marble, was investigated. Thanks to its weakening effect on the hydration sphere of ions in solution, ethanol allowed complete coverage of marble surface by a crack-free and pore-free layer of octacalcium phosphate. Even better results were obtained using isopropanol, because it has lower adsorption affinity to calcite than ethanol. Treatments involving alcohols provided good acid protection and also restored cohesion among calcite grains in weathered marble.

In this study, we investigated by FIB-SEM the microstructure of protective coatings formed by two inorganic treatments (namely, ammonium oxalate and ammonium phosphate solutions), with the aim of identifying the factors limiting the protective efficacy of the coatings and outlining possible improvement strategies. In the case of the oxalate treatment, the resulting layer of whewellite was found to contain vertical channels, which can allow water to reach the marble surface and trigger dissolution. Possible prevention of the formation of these channels by addition of a calcium source to the oxalate solution was attempted, but even micromolar additions led to rapid precipitation in the solution. In the case of the phosphate treatment, the resulting hydroxyapatite and octacalcium phosphate coating was found to be cracked and porous. To prevent cracks and pores, diminishing the ammonium phosphate concentration and adding ethanol to the solution were found to be effective strategies, as cracks were prevented and pores were reduced almost to zero. The resulting protective efficacy of the coating was found to be significantly improved, although still not perfect.

The present paper aims at investigating the influence of the mortar joints on the FRP-masonry debonding process and providing some insights on the reliability of experimental tests carried out on single bricks and theoretical formulas available to predict the debonding force. Masonry panels were constructed using two types of bricks and two types of mortars with significantly different properties. The masonry specimens were strengthened using glass fiber reinforced polymers and the debonding force was evaluated by a single-lap push-pull set-up. The obtained debonding forces were compared to those obtained by applying the same reinforcements to single bricks and calculated by applying theoretical formulas. The results of the study point out that neglecting the presence of the mortar joints and taking into account only brick mechanical properties, thus neglecting their surface and microstructural properties, may lead to significant inaccuracies in the estimation of the debonding force.

Soiling of external surfaces is an increasing issue for conservation of architectural heritage. Providing architectural surfaces with self-cleaning ability is one of the most promising routes of preventive conservation. To this aim, several methods have been proposed in the literature, based on the use of photocatalytic TiO2 nanoparticles, either directly applied onto the surfaces or incorporated in protective coatings. However, when nano-TiO2 is directly applied onto architectural surfaces, the particles are easily removed by rain. When TiO2 is incorporated in polymeric coatings, durability issues arise as well, because the photoactivity of TiO2 nanoparticles can promote degradation of the polymer. Here, we present an innovative alternative method, based on combination of TiO2 nanoparticles and hydroxyapatite (HAP). The incorporation of nano-TiO2 into an HAP coating protects the nanoparticles from leaching by rain, thanks to the chemical bonding between TiO2 and HAP, without diminishing their photoactivity. As a result, marble treated with HAP-TiO2 composites exhibits high self-cleaning ability and high durability, with results superior to those achieve by direct application of nano-TiO2 onto the surface, as frequently performed on site.

To prevent soiling of marble exposed outdoors, the use of TiO₂ nano-particles has been proposed in the literature by two main routes, both raising durability issues: (i) direct application to marble surface, with the risk of particle leaching by rainfall; (ii) particle incorporation into inorganic or organic coatings, with the risk of organic coating degradation catalyzed by TiO₂ photoactivity. Here, we investigated the combination of nano-TiO₂ and hydroxyapatite (HAP), previously developed for marble protection against dissolution in rain and mechanical consolidation. HAP-TiO₂ combination was investigated by two routes: (i) sequential application of HAP followed by nano-TiO₂ ("H+T"); (ii) simultaneous application by introducing nano-TiO₂ into the phosphate solution used to form HAP ("HT"). The self-cleaning ability was evaluated before and after prolonged exposure to simulated rain. "H+T" and "HT" coatings exhibited much better resistance to nano-TiO₂ leaching by rain, compared to TiO₂ alone. In "H+T" samples, TiO₂ nano-particles adhere better to HAP (having flower-like morphology and high specific surface area) than to marble. In "HT" samples, thanks to chemical bonds between nano-TiO₂ and HAP, the particles are firmly incorporated in the HAP coating, which protects them from leaching by rain, without diminishing their photoactivity and without being degraded by them.

Fiber-reinforced composites with inorganic matrix (FRCMs) have been recently proposed as a more compatible and durable route to strengthen masonry structures with respect to FRPs (fiber-reinforced polymers), but their weathering mechanisms in aggressive environments are still scarcely known. In this paper, brick masonry specimens were reinforced with FRCM strips, made of galvanized steel fibers embedded within a hydraulic lime-based mortar, and were subjected to an artificial weathering procedure designed by the authors, involving capillary water absorption and salt crystallization cycles. Finally, direct shear tests were performed on the FRCM-masonry joints. To reproduce conditions that may be found in real buildings, shear tests were also carried out after applying the FRCM composite on salt laden masonry blocks. Interfacial debonding behavior was interpreted based on the salt distribution within the masonry joints and the porosity of materials. The permeable matrix did not hinder the migration of the saline solution used in some cycles, which in turn resulted in no significant accumulation of salts beneath the composite strip, and no micro-crack opening in the composite after accelerated weathering. Some corrosion of the galvanized steel cords in presence of high amount of chlorides was observed in the composite.

In the first half of the twentieth century, a variety of dry-mix renders with different trade marks spread across Europe. Their stated performance made them attractive for buildings where the overhanging eave was absent, hence they found broad application in rationalist buildings and districts. However, their composition and properties are barely known. In this paper, the Terranova render of the Engineering Faculty in Bologna was characterised, to investigate the reasons for its outstanding durability. Unexpectedly, the render was found to be constituted by dolomitic lime and only a small amount of white cement and exhibited a remarkable strength. The presence of air voids seems due the use of air entraining agents and may have contributed to the frost resistance of this render during 80 years. The results are expected to contribute not only to the knowledge of Terranova render, but also to a better understanding of durability issues for new renders.

Given the lack of satisfying treatments for consolidating marble affected by thermally induced grain detachment (the so-called "sugaring"), the use of aqueous solutions of diammonium hydrogen phosphate (DAP) has recently been proposed. The idea is to form a new binding mineral (hydroxyapatite, HAP) as the reaction product between the DAP solution and the calcitic substrate. In this study, we investigated the effects of adding small quantities of ethanol (EtOH) to the DAP solution, with the aim of favoring HAP formation. The results of the study indicate that, when a 0.1 M DAP and 0.1 mM CaCl2 solution in 10 vol% EtOH is used, complete coverage of marble surface with a crack-free coating with reduced porosity is achieved (whereas no coating is formed without EtOH addition). This is thought to be a consequence of the weakening of hydration shells of phosphate ions in the DAP solution, thanks to the presence of ethanol molecules. When used to restore mechanical properties of weathered marble, the treatment with 10 vol% EtOH was found to significantly improve the dynamic elastic modulus after a single application and to completely restore it after a second application.

Fiber-reinforced cementitious matrix (FRCM) composites have been recently employed as a strengthening system to retrofit historic masonry structures, since they are more compatible with masonry, less subject to ageing, and compliant with conservation criteria for cultural heritage than fiber-reinforced polymer composites. However, the durability of FRCMs, such as the resistance to salt attack, has not been deeply investigated yet. In this paper, the mechanical behavior of steel FRCM strips bonded to fired-clay brick masonry blocks is studied experimentally. Single-lap shear tests are conducted in laboratory on control FRCM-masonry joints and on additional joints that underwent artificial weathering cycles in a solution of sodium sulfate decahydrate. Peak loads, slips at failure at the loaded end as well as failure modes are investigated. Pore size distribution of the constituent materials and the amount and distribution of salt within the specimens are reported and discussed in order to evaluate the transport and crystallization mechanisms of sulfate in masonry elements strengthened with externally bonded FRCM strips.

This paper aims at evaluating how the FRP/brick debonding force is influenced by brick mechanical properties (compressive, tensile and flexural strength) and physical-microstructural properties (surface roughness, porosity, pore size distribution, rate of water capillary suction). The depth of epoxy resin penetration into bricks and the thickness of brick layer detached at the end of bond test were also determined, in order to investigate the role of resin penetration into the brick on the FRP performance. All the tests were carried out on different brick surfaces (namely “bed” and “face”), to account for possible brick anisotropy. The results of the study point out that brick mechanical properties alone are not sufficient to accurately predict the maximum debonding force, but a better estimation can be achieved taking into account also brick surface roughness and resin penetration depth. If brick anisotropy is not considered, in some cases a remarkable overestimation of the maximum debonding force may be found.

When consolidants are applied to porous stones, redistribution may occur inside the pore system after the treatment application, which influences their final penetration depth and performance. In this paper, the kinetics of redistribution were investigated for a 2-step hydroxyapatite-based consolidant, applied by brushing and immersion. The results of the study indicate that both the application method and the drying phase are crucial in determining the final penetration depth, the amount of hydroxyapatite that forms and the redistribution kinetics. In all cases, the limewater poultice applied during the second step of the treatment proved to be effective in removing soluble compounds.

Fired clay elements are widely diffused in architectural heritage, for both structural and ornamental purposes, and are susceptible to severe decay forms, hence their conservation is a very urgent task, requiring appropriate consolidants to restore their cohesion and mechanical strength. As fired clay bricks and terracotta artefacts contain silicate fractions, allowing ethyl silicate (ES) to chemically bond to the substrate, ES is supposed to be suitable for their consolidation and indeed it is used in some conservation works on this kind of material. However, its behaviour and performance on fired clay elements has not been systematically evaluated in the scientific literature, mostly when durability is concerned. This aspect is particularly relevant as diverging data can be found in the literature about the impact of ES on substrate durability, to such an extent that it was found to either increase or hamper materials resistance to decay, depending on the specific case in exam. In this study, the behaviour towards salts crystallisation of fired clay bricks treated by ethyl silicate was investigated, specifically focussing on the role of the application procedure, an aspect which is usually left to the conservatorists’ skilfulness but which is however of paramount importance for the treatment outcome. In particular, ES was applied by five and ten brush strokes, to assess how the different amount of product absorbed and its different distribution inside the porous substrate influence the final resistance of bricks to salt weathering. As a matter of fact, salt crystallization is commonly considered one of the most frequent and severe decay mechanism affecting bricks on site. Despite both treatments providing significant increases in mechanical properties and a limited modification in samples microstructure, durability was found to be dependent on the treatment conditions. Particular attention should be devoted to the amount of product applied, as higher product absorption, despite generally leading to better efficacy, could cause higher microstructural modifications, possibly hampering materials durability.

Alkali-activation of brick waste has recently been proposed as a sustainable route to develop pastes/mortars with tailored mechanical properties and pore system. In this study, the suitability of using pastes from brick waste alkali-activation for repointing existing masonries (i.e., filling the most external part of mortar joints, lost due to deterioration processes) was investigated. Five different formulations (having SiO 2 /Al 2 O 3 molar ratio ranging from 1.4 to 0.4) and two different curing temperatures (room temperature and 50 °C) were investigated. Open porosity and efflorescence formation were found to decrease for decreasing SiO 2 /Al 2 O 3 ratio. Curing at high temperature generally favored geopolymerization and reduced efflorescence formation. Pastes with SiO 2 /Al 2 O 3 =0.8 and 0.9 exhibited open porosity and water vapor permeability fairly similar to those of historic lime-based mortars, thus proving to be potentially compatible with them. Further optimization of the mix design seems however necessary to reduce the formation of efflorescence.