Ammonium Phosphate on Gypsum

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by AMS Acta - Alm@DL - Università di Bologna HMC 2016 4 th Historic Mortars Conference Preliminary study on the use of ammonium phosphate for the conservation of marble-imitating gypsum-stuccoes Enrico Sassoni 1, Gabriela Graziani 2, George W. Scherer 3 and Elisa -ranzoni 4 0Tmh dqrhsxneAnknfm+)skx+ [email protected] 1Tmh dqrhsxneAnknfm+)skx+ faqhdk-fqyhmh1.tmhan-hs 2 OqhmbdsnmTmh dqrhsx+MI+TR@+ rbgdqdq.oqhmbdsnm-dct 0 Tmh dqrhsxneAnknfm+)skx+ dkhr-eqmynmh.tmhan-hs Abstract6 In this study, a novel method for consolidation and im rovement of resistance to water of gy sum-stuccoes was reliminarily investigated. The idea is treating gy sum with an aqueous solution of diammonium hydrogen hos hate (DAP, (.H 4)2HPO 4) to form hydroxya atite (HAP, Ca 10 (PO 4)6(OH) 2), which has much lower solubility than gy sum. Tests carried out on gy sum aste sam les, manufactured to resemble historic stuccoes, showed that, after treatment with the DAP solution, a significant im rovement in mechanical ro erties was achieved and brushite (CaHPO 4ì2H 2O) was formed (alongside some other by- roducts, that can be removed by an additional oultice treatment). Even if brushite is more soluble than HAP, still its formation is ex ected to be beneficial for stuccoes conservation, as brushite is significantly less soluble than gy sum. Introduction Since antiquity, gy sum-based stuccoes have been frequently used to imitate recious white or colored marbles, when the use of real marbles was not ossible, because trans ort from faraway quarries was too costly a1,2b. 5hile gy sum stuccoes generally exhibit good durability in indoor environments, they may exhibit severe durability issues when ex osed outdoors (intentionally, such as in Islamic art a3b, or after events such as a building's roof colla se a1b). In articular, stuccoes can suffer dissolution and ulveri7ation, because of gy sum<s high solubility in water, with consequent surface recession and loss of decorations when stuccoes are ex osed to rain, condensation or rising dam a4b. However, also very low relative humidity and high tem erature may lead to decay of stucco aAb. To enhance durability of stucco, in ancient times it was common ractice to add organic com ounds (such as sugars, glycerin and citric acid) to reduce gy sum solubility a6b and to use fats and roteins (such as animal blood, egg whites and casein) to achieve water- re ellent ro erties a1-0b. .onetheless, gy sum-stuccoes ex osed outdoors are often in need of consolidation and rotection, as illustrated for instance in Figure 1. Since 110 HMC 2016 4 th Historic Mortars Conference traditional consolidants and rotectives (e.g. TEOS and olymeric roducts) have roved to have serious limitations when a lied to gy sum-stuccoes, there is a need to develo new consolidating and rotecting treatments s ecifically tailored for gy sum a4b. In this a er, a ossible new strategy for the conservation of gy sum stuccoes was investigated. The idea is to use an aqueous solution of diammonium hydrogen hos hate (DAP, (.H 4)2HPO 4) to react with gy sum and form hydroxya atite (HAP, Ca 10 (PO 4)6(OH) 2), according to the following reaction a10b/ 10CaSO 4ì2H 2O + 6(.H 4)2HPO 4 û (1) Ca 10 (PO 4)6(OH) 2 + (.H 4)2SO 4 + H 2SO 4 + 12H 2O. Since HAP has solubility in water about 4 orders of magnitude lower and dissolution rate about 1 orders of magnitude slower than gy sum a11,12b, the formation of HAP on the surface and inside ores of gy sum stuccoes is ex ected to im rove resistance to dissolution and ulveri7ation. The ins iration for the resent study came from research carried out by the authors in the last few years on the use of ammonium hos hate solutions for the conservation of carbonate stones, such as marble and limestone a11,13-1Ab. In the case of stone, the hos hate treatment is intended to form HAP starting from calcite, with the aim of roviding rotection against dissolution in rain (HAP being less soluble and having slower dissolution rate than calcite) and mechanical consolidation (HAP being able to bind loose calcite grains). Starting from this background, the interaction of the ammonium hos hate solution with gy sum resent in calcareous stones (as the result of stone sulfation) has recently been taken into consideration by the authors a1Ab and also other grou s investigated the ossible conversion of gy sum layers to stable hases a16b. Similarly, the use of ammonium hos hate solutions to transform gy sum into less soluble calcium hos hates was ro osed in the ast for the desulfation of lime aint layers a11b/ encouraging results were obtained, even if hase control remained quite challenging a11b. -ig. 1 Stuccoes ex osed outdoor in the church of San Francesco in Fano (Italy, XIX cent.) 302 HMC 2016 4 th Historic Mortars Conference Materials and Methods Samples Sam les of gy sum stuccoes were manufactured as in historic reci es. Pure gy sum binder (so-called " scagliola ", CaSO 4ì1/2H 2O) and water (water/gy sum weight ratio 0.A) were used to re are 4416 cm 3 risms, without any aggregate addition. After hardening, 12 cubic sam les (4 cm side) and 12 cylindrical sam les (A cm height, 2 cm diameter) were obtained from risms by sawing and core-drilling res ectively. Half of the sam les were treated with the ammonium hos hate solution, while the other half were left untreated and used as reference. The same treatment conditions reviously ro osed for carbonate stones a11,13b were ado ted also for gy sum stuccoes, in order to have a reliminary indication of the reactivity of gy sum to ammonium hos hate solution and to evaluate its ability to bind gy sum crystals. Sam les were treated with a 1 M aqueous solution of DAP (Sigma Aldrich, reagent grade), a lied by brushing until a arent refusal. After wra ing in a lastic film for 42 hours (to avoid DAP solution eva oration), sam les were characteri7ed in terms of changes in mechanical ro erties and mor hology and com osition of the new hases. Characterization techni)ues The consolidating ability of the hos hate treatment was evaluated by determining the increases in dynamic elastic modulus ( Ed), com ressive strength (m c) and tensile strength 2 (m t) after consolidation. Ed was calculated as Ed = • v where • is sam le density and v is the ultrasound velocity, measured across cubic sam les using a Matest instrument with AA kH7 transducers. m c was measured on cubic sam les, using an Amsler-5ol ert loading machine (maximum load 100 k.). m t was measured by Bra7ilian test on cylindrical sam les, using an Amsler-5ol ert loading machine (maximum load 10 k.). The mineralogical com osition of the newly formed hases was analy7ed using a Bruker D2 Discover X-Ray Diffractometer. Gra7ing incidence diffraction (GID, incidence radiation •=0.A°, detector scan range 2•=A-AA°) was erformed on untreated and treated sam les, both on the treated surface and at 1 mm de th. Thanks to the reduced enetration of the incoming radiation with •=0.A° (amounting to a few hundreds of nanometers from sam le surface), it was ossible to obtain information on new hases formed on the sam le<s surface. The mor hology and the elemental com osition of untreated and treated sam les, at different de ths from the treated surface, were analy7ed by an environmental scanning electron microsco e (FEI Quanta 200 FEG ESEM) equi ed with an energy dis ersive x- ray s ectrosco y device (Oxford Instruments EDS robe). 303 HMC 2016 4 th Historic Mortars Conference Results and Discussion After treatment, a significant mechanical im rovement was found (Table 1), which suggests that gy sum crystals were more effectively bonded after consolidation. Three different hases were detected by GID after treatment (Figure 2). Accordingly, three different mor hologies with different elemental com osition were assessed by ESEM/EDS on the treated surface (Figure 3). 2+ Brushite (CaHPO 4ì2H 2O) was found as the reaction roduct between Ca ions coming 3- from gy sum dissolution and PO 4 ions from DAP dissolution (Figure 2). Crystals containing Ca and P but not S, identified as brushite, are visible in the u er left art of Figure 3. A further magnification is re orted in Figure 4. Formation of brushite starting from gy sum was re orted also by Snethlage et al. a11b, who however detected brushite when gy sum was reacted with ammonium di-hydrogen hos hate (ADP) at H=6.A, while HAP was formed when gy sum was reacted with DAP at H=0.0. .onetheless, in the resent study no HAP eaks were detected after reacting gy sum with DAP. A ossible reason for this may be the evolution of H during reaction (not measured in this study). .able 1 Mechanical im rovement after consolidation ( Ed = elastic modulus, m c = com ressive strength, m t = tensile strength. Values are averages for 6 sam les, standard deviation in brackets). ,ntreated Treated Increase ahb Ed 0.1 (±0.1) 0.A (±0.1) +Ah mc 1A.0 (±1.0) 22.2 (±0.0) +42h mt 2.2 (±0.3) 3.3 (±0.4) +16h -ig. 2 XRD s ectra of untreated and treated sam les at different de ths According to Ishikawa et al.

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