1,721,054 research outputs found
Phosphate‐based treatments for conservation of stone
Full text linkTo overcome the limitations of currently available protectives and consolidants for carbonate stones (such as marble and limestone), in 2011 the use of calcium phosphate was proposed. The idea is forming calcium phosphates (ideally hydroxyapatite) as the reaction product between the substrate and an aqueous solution of a phosphate salt, that the stone is treated with. In this paper, the studies aimed at identifying the best treatment conditions (in terms of nature and concentration of the phosphate precursor, solution pH, reaction time, ionic and organic additions) are first briefly summarized. Then, the efficacy of the phosphate treatment in protecting marble from dissolution in rain and restoring cohesion of weathered marble and limestone is discussed. Some recent studies on the use of the phosphate treatment on alternative substrates and some future steps for research on the topic are finally outlined
Hydroxyapatite and Other Calcium Phosphates for the Conservation of Cultural Heritage: A Review
Full text linkThe present paper reviews the methods and the performance of in situ formation of calcium phosphates (CaP) for the conservation of materials belonging to cultural heritage. The core idea is to form CaP (ideally hydroxyapatite, HAP, the most stable CaP at pH > 4) by reaction between the substrate and an aqueous solution of a phosphate salt. Initially proposed for the conservation of marble and limestone, the treatment has been explored for a variety of different substrates, including sandstones, sulphated stones, gypsum stuccoes, concrete, wall paintings, archaeological bones and paper. First, the studies aimed at identifying the best treatment conditions (e.g., nature and concentration of the phosphate precursor, solution pH, treatment duration, ionic and organic additions to the phosphate solution, mineralogical composition of the new CaP phases) are summarized. Then, the treatment performance on marble and limestone is reviewed, in terms of protective and consolidating effectiveness, compatibility (aesthetic, microstructural and physical) and durability. Some pilot applications in real case studies are also reported. Recent research aimed at extending the phosphate treatment to other substrates is then illustrated. Finally, the strengths of the phosphate treatment are summarized, in comparison with alternative products, and some aspects needing future research are outlined
Experimental study on the salt weathering resistance of fired clay bricks consolidated by ethyl silicate
No full textFired 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 5 and 10 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
An innovative phosphate-based consolidant for limestone. Part 2: Durability in comparison with ethyl silicate
No full textThe 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
An innovative phosphate-based consolidant for limestone. Part 1: Effectiveness and compatibility in comparison with ethyl silicate
No full textFor 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
Repair of sugaring marble by ammonium phosphate: Comparison with ethyl silicate and ammonium oxalate and pilot application to historic artifact
No full textNo fully satisfactory consolidant for sugaring marble currently exists, hence the use of ammonium phosphate to form hydroxyapatite (HAP) inside marble micro-cracks has recently been proposed. This study was aimed at: (i) investigating different treatment formulations (application of a 3 M aqueous solution of diammonium hydrogen phosphate (DAP), with/without addition of 1 mM or 3 mM CaCl2, with/without subsequent limewater poultice application) and characterizing the new Ca-P phases; (ii) systematically testing mechanical effectiveness and compatibility with the substrate of the most promising formulations, in comparison with ethyl silicate and ammonium oxalate; (iii) performing a pilot application of the most promising formulation to a real marble artwork affected by sugaring. The results of the study indicate that application of a 3 M DAP solution, followed by limewater poultice application, produces remarkable consolidation of weathered marble, with only slight alterations in pore size distribution and color change, thus providing much better results than both ethyl silicate and ammonium oxalate. Notably, while the above-described treatment conditions caused formation of tricalcium phosphate and octacalcium phosphate in artificially weathered samples, the same conditions caused formation of HAP in the historic artwork, presumably because some gypsum residues were present and calcite grains had high surface roughness
TEOS-based treatments for stone consolidation: acceleration of hydrolysis–condensation reactions by poulticing
No full textEthyl silicate (TEOS) is nowadays the most widely used consolidant for stone, due to its efficacy on silicate stones and also on limestones containing small amounts of quartz. However, because of the very long time required for TEOS curing reactions to be completed, the final mechanical improvement caused by the treatment is reached only after several months and, during this period, the treated stone is temporarily hydrophobic. This latter aspect is one of the most relevant drawbacks of TEOS, as it makes it impossible to perform water-based interventions after consolidation. Therefore, in this study the possible acceleration of TEOS curing reactions by prolonged contact with water, some time after consolidant application, was investigated. A commercial TEOS-based product was applied on Globigerina limestone, a porous stone from Malta containing also small amounts of quartz. After that preliminary tests indicated that prolonged contact with water is actually effective in both removing hydrophobicity and improving mechanical strength, a new consolidation cycle, based on TEOS application followed by application of a water-impregnated poultice was tested. The results of the study indicate that water application by poultice, after curing for 7 days, allows to remove the hydrophobicity and achieve higher mechanical properties than those obtained after curing for 4 weeks in laboratory conditions. Moreover, such mechanical properties are around 90% of those achieved after curing for as much as 7 months, which points out that the proposed method is a very promising technique for accelerating TEOS curing reactions
Experimental study on the physical-mechanical durability of innovative hemp-based composites for the building industry
Full text linkFor reducing the environmental impact of the building sector, novel sustainable composites have recently been developed, by bonding hemp hurds with a new hybrid organic-inorganic binder. These composites, designed as substitutes for traditional insulating materials or as substitutes for formaldehyde-bonded wood particle boards, exhibit very promising thermal, physical and mechanical properties. To ensure that the panel performance is maintained during the building operation phase, durability needs to be specifically evaluated as well. Therefore, in this study three composite types with low, medium and high density (LD, MD and HD, respectively) were subjected to accelerated ageing and the alterations in their physical-mechanical properties were evaluated. Composite resistance to accelerated ageing is strongly correlated with bulk density. HD composites, the only ones actually designed to be directly exposed to rainfall, exhibited almost negligible decreases in mechanical properties and hence a substantially satisfactory behavior. MD and LD composites, designed to provide thermal insulation and hence to be sheltered by HD panels, were affected to a larger extent by accelerated ageing, which however was definitely more severe than the real exposure conditions of the composites during their service life. Further studies are currently in progress to optimize the composites formulation and physical-mechanical durability
Valorization of brick waste by alkali-activation: A study on the possible use for masonry repointing
No full textAlkali-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 SiO2/Al2O3 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 SiO2/Al2O3 ratio. Curing at high temperature generally favored geopolymerization and reduced efflorescence formation. Pastes with SiO2/Al2O3=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
Some Recent Findings On Marble Conservation By Aqueous Solutions Of Diammonium Hydrogen Phosphate
Full text linkGiven 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
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