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Conservation of sugaring marble by hydroxyapatite: some recent developments on producing and treating decayed samples Enrico Sassoni 1,2, Gabriela Graziani 1 , Elisa Franzoni 1 , George W. Scherer 2 1 Dept. Civil, Chemical, Environmental & Materials Engineering (DICAM), University of Bologna, Italy 2 Dept. Civil & Environmental Engineering (CEE), Princeton University, USA Dept. Civil & Environmental Engineering (CEE), Princeton University, USA [email protected] , Website: https://events.unibo.it/hap4marble/ BACKGROUND HELP! Architectural decorations and sculptures made of marble, when they are exposed outdoor, deteriorate because: In 2011, we proposed the use of hydroxyapatite (HAP) to preserve carbonate stones [1] HELP! • Temperature variations cause the opening of cracks between calcite grains, so HAP can be formed directly inside marble and over marble surface, from the reaction between calcite and an aqueous solution of diammonium • Rain causes the dissolution between calcite grains, so that grains detach and fall calcite and an aqueous solution of diammonium hydrogen phosphate (DAP) in mild conditions • Rain causes the dissolution of calcite grains, so that the marble carved surface is lost The products currently available are not effective and/or durable in preserving marble effective and/or durable in preserving marble against these deterioration causes RESEARCH AIMS 1 Accelerated ageing 2 Treatment parameters 4 Protection 3 Consolidation 1 2 4 3 To study the consolidating efficacy of the HAP treatment, artificially aged samples with characteristics similar to naturally The treatment parameters need to be optimized to reduce the porosity of the HAP layer, increase the surface The ability of the HAP treatment to prevent the dissolution of marble surface in rain, by formation of a dense The ability of the HAP treatment to re- establish cohesion between calcite grains and restore the strength of weathered sugaring marble (i.e. micro-cracks more diffused near the surface) are needed coverage and prevent the formation of soluble calcium phosphate phases coating with low solubility, needs to be investigated marble, without causing over- consolidation, needs to be investigated MATERIALS AND METHODS RESULTS AND DISCUSSION MATERIALS AND METHODS RESULTS AND DISCUSSION 1 Ageing was produced by heating samples over a hot Heating at 200 °C for 20 sec was predicted to cause an ∂ T ∂ = ∂ ∂ k( x , t ) ∂ T ∂ T = temperature t = time x = distance from plate UPPER 1 cm Experiments Theoretical model d heating samples over a hot plate, the temperature and time of heating being calculated by a theoretical was predicted to cause an average ΔE d = -35% in the first 1 cm from the surface, leaving the rest undamaged. 200 °C for 20 sec before ageing after ageing ∂t = ∂ x k( x , t ) ∂ x x = distance from plate k = thermal diffusivity of initial E d R 1 cm R 1 cm calculated by a theoretical model. Damage was assessed by measuring the modulus E d leaving the rest undamaged. Experimental results confirmed the prediction travelling time t 1 → E d1 time t 2 > t 1 → E d2 < E d1 LOWER 1 cm % o Distance from the plate (cm) UPPER LOWER 2 Ethanol addition favors marble surface coverage and The influence of several treatment parameters 1 d1 2 1 d2 d1 Organic additions pH control SEM marble surface coverage and helps reduce the porosity of the HAP layer, as it weakens the hydration shell of the treatment parameters (ethanol addition, pH control, double treatments [2]) was investigated and the effects additions Ca 2+ ions additions e - 3 the hydration shell of the phosphate ions in solution investigated and the effects evaluated by SEM-FIB The increase in marble 3 Double application of the HAP treatment with 10 vol% ethanol was able to fully The increase in marble cohesion after consolidation was assessed by measuring after consolidation consolidation before consolidation HAP inside cracks 10 vol% EtOH restore the E d , thanks to HAP formation inside cracks formed by artificial ageing the increase in modulus E d and observing the new phosphate phases by ESEM travelling time t 1 travelling time t 2 < t 1 from t 1 → E d1 from t 2 → E d2 > E d1 cracks 4 formed by artificial ageing The HAP coating was able to reduce marble dissolution, phosphate phases by ESEM The resistance to dissolution was assessed by measuring pH measurement from t 1 → E d1 from t 2 → E d2 > E d1 10 vol% EtOH reduce marble dissolution, even if cracks appeared after the test. The orientation of the underlying calcite grains was assessed by measuring the increase in pH vs time of an aqueous solution of HNO initially at pH=5 HNO 3 solution initially at pH=5 the underlying calcite grains influences the film durability HNO 3 initially at pH=5 (simulating slightly acid rain) REFERENCES ACKNOWLEDGEMENTS CONCLUSIONS AND FUTURE WORK [1] Sassoni E., Naidu S., Scherer G.W., The use of hydroxyapatite as a new inorganic consolidant for damaged carbonate stones, J Cult Herit 12 (2011) 346-355 [2] Graziani This project has received funding from the European Union’s Horizon 2020 programme through the Marie Sklodowska-Curie HAP4MARBLE The developed model allows to produce samples with a desired level of damage. The addition of ethanol is effective in Ethanol has a competitive effect, as it weakens the hydration shell of the phosphate ions in the solution but it is adsorbed on calcite surface. Herit 12 (2011) 346-355 [2] Graziani G., Sassoni E., Franzoni E., Scherer G.W., Hydroxyapatite coatings for marble protection: Optimization of calcite covering and acid resistance, Sklodowska-Curie HAP4MARBLE Project (Multi-functionalization of hydroxyapatite for the restoration and preventive conservation of marble The addition of ethanol is effective in promoting HAP formation and reducing the film porosity, thus allowing to achieve a good consolidating and protecting ability but it is adsorbed on calcite surface. Future research will be aimed at identifying by NMR possible alternative solvents that may weaken the hydration shell without being adsorbed on calcite calcite covering and acid resistance, Appl Surf Sci, 368 (2016) 241-257 artworks, grant agreement No 655239) consolidating and protecting ability hydration shell without being adsorbed on calcite
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