Multi-scale laboratory routine in the efficacy assessment of conservative products for natural stones

Method

article

Multi-scale

laboratory

routine

in

the

ef

ficacy

assessment

of

conservative

products

for

natural

stones

S.

Raneri

a,b

,

G.

Barone

a

,

P.

Mazzoleni

a,

*

,

I.

Al

fieri

c

,

L.

Bergamonti

d

,

T.

De

Kock

e

,

V.

Cnudde

e

,

P.P.

Lottici

f

,

A.

Lorenzi

c

,

G.

Predieri

c

,

E.

Rabot

g

,

J.

Teixeira

g

a

UniversityofCatania,DepartmentofBiological,GeologicalandEnvironmentalSciences,C.soItalia57, 95129,Catania,Italy

b

UniversityofPisa,DepartmentofEarthScience,ViaSantaMaria53,53126,Pisa,Italy

c

UniversityofParma,DepartmentofChemistry,LifeSciencesandEnvironmentalSustainability,ParcoArea delleScienze17/A,43124,Parma,Italy

d_{UniversityofParma,DepartmentofEngineeringandArchitecture,ParcoAreadelleScienze187/A,43124,}

Parma,Italy

e

GhentUniversity,DepartmentofGeology,Krijgslaan281/S8,B-9000,Ghent,Belgium

f

UniversityofParma,DepartmentofMathematical,PhysicalandComputerSciences,ParcoAreadelleScienze 7/A,43124,Parma,Italy

g

LaboratoireLéonBrillouin(CNRS/CEA),CEASaclay,F-91191,Gif-sur-Yvette,France ABSTRACT

Theevaluationofconservativetreatments’efficacyonnaturalbuildingstonesareusuallybasedonstandard

recommendation routinesfinalized to evaluate compatibility and harmfulness of products in turn of the

substrate.However,thevisualizationandthequantificationofproductsinsideporestructureofnaturalstonesis

notimmediatethrough standardtests,so thatimagingand advancedtechniques arerecentlyproposed in

materialconservationfieldtoimproveknowledgeonpenetrationdepth,modificationofpore-airinterfaceat

differentscaleandmonitordynamicabsorptionprocesses.Moreover,naturalstonesareusuallycharacterizedby

complexstructure,whichchangesduetoconservativetreatmentshavetobeinspectedatdifferentscale(from

micrometertonanometer).

Inthisprospective,theassessmentoflaboratorypracticesabletointegratemultiscalemethodsandgivebacka

completeoverviewoninteractionbetweennewconservativeformulatesandnaturalstonesisofhighinterest.

In this paper, we propose a methodological routine for efficacy assessment of conservative products,

incorporatingclassicalandinnovativenondestructivetechniques.Validationoftheworkflowhasbeenverifiedon

ahighporousnaturalstonetreatedwithnewhybridformulatesappropriatelycustomizedforconservation

issues.

*Correspondingauthor.

E-mailaddresses:[email protected](S. Raneri),[email protected](G. Barone),[email protected](P. Mazzoleni), ilaria.alfi[email protected](I. Alfieri),[email protected](L. Bergamonti),[email protected](T. DeKock), [email protected](V.Cnudde),[email protected](P.P. Lottici),[email protected](A. Lorenzi), [email protected](G. Predieri),[email protected](E. Rabot),[email protected](J. Teixeira).

https://doi.org/10.1016/j.mex.2018.08.013

2215-0161/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

ContentslistsavailableatScienceDirect

MethodsX

Thestudyintendstoaddnewinsightsonproblemsrelatedtoconsolidationofhighporouscarbonatestone,

applicationmethodsinconsolidatingnaturalstonesandmethodstoevaluateefficacyofnewproducts.

A multi-scale laboratory investigation procedure is proposed by integrating standard and innovative

nondestructivemethods.Meritsandlimitsofeachappliedmethodarediscussedduringvalidation.

Thepossibilitytoincorporatestandardroutinesand/orsubstitutedestructivetestingwithnon-destructive

onesseem to beavalid alternativetoevaluate efficiencyand monitorbehaviorof stonestreatedwith

consolidatingproducts.

©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

ARTICLE INFO

Methodname:Advancedandmulti-scalelaboratoryinvestigation Keywords:Naturalstones,Conservativetreatments,Consolidants,Efficacy

Articlehistory:Received11June2018;Accepted29August2018;Availableonline5September2018

SpecificationsTable

Subjectarea
EarthandPlanetarySciences
Chemistry
Physic
Engineering
MaterialsScience Morespecific subjectarea
Masonry
Polymers
Conservation

Methodname Advancedandmulti-scalelaboratoryinvestigation

Methoddetails

A multi-scale laboratory investigation procedure finalized to inspect consolidants’ efficacy is proposed;itisbasedontheintegrationofinnovativenondestructivemethodstostandardwellknown laboratorytests.

Theroutineincludestwooperativesteps,namelytheapplicationof(a)standardrecommendation proceduresand(b)thenoninvasiveadvancedtestingbyX-rayandneutronsources(Fig.1).Alltests havetobeperformedonlaboratorysampleswhichdimensionandshapeisdeterminedinaccordance withstandardguidelines,whenapplicable.

Conditioninglaboratorysamples

ArtificiallyagedvsUnweatheredfreshlaboratorysamples

Asafunctionofgeneralproprietiesofthenaturalstonetotest,preliminaryagingprocedureaimed tomimethenaturalweatheringstatecanbeperformedbeforethetreatment[1].Theselectionof agingmethodcanbecriticalandhastobeselectedinfunctionofsubstratepropertiesandusualdecay phenomena. Generally, salt crystallization is the most used one to mime the real weathering conditions;however,limitationandpossibleby-productsduetothetesthavetobeevaluated,aswell thekindofsinglesaltand/orsaltmixestouse[2].

Treatmentmethod

Inordertopossibleevaluatemeritsoftechniquesandefficacyofproductsinfunctionoftreatment method(i.e.:immersion,brushing,poultice)[3],laboratorysamplescanbetreatedbyfollowingone onmoreapplicationprocedures.

Curing

Aftertreatmentwiththenewproducttotest,laboratorysampleshavetobedriedat60_Cuntil

constantmass,accordingtoUNI10921[4].Appropriatecuringoflaboratorysampleshastobeassured, infunctionofemployedformulate(usually,almostonemonthatroomtemperatureandhumidity). Quantityofproduct

Calculatethequantityofproductabsorbedbylaboratorysamplesasweightpercentgain(WPG%): WPG%=100[(Mt–M0)/M0),withM0,themassofdrysamplebeforetheproductapplication,andMt, themassofdrysampleafterproductapplication.

Standardrecommendation

Performphysicalandmechanicalstandardtests(UNIEN15886[5],UNIEN15801[6],NORMAL7/ 81[7],NORMAL29/88[8],artificialweatheringtests[9,10]DRMS[11])toevaluatewaterabsorption anddesorptionatenvironmentalpressure,colordifference,resistancetoarti_ficialweathering,and microdrillingresistanceonreferenceuntreatedandtreatedlaboratory samples.Perform mercury intrusionporosimetry(MIP)measurementsonvolumesof1cm3_{sampledatthesurfaceoftreated} samples where coated is created. Collect intrusion curves and compare results with untreated referencesamples.Performatleastthreemeasurementsforeachcase,toverifyrepeatabilityofdata. Comparemodal porevalues and average cumulativevolumes beforeand afterthe treatmentto evaluatewhereproductislocated.Limitsininspectedrangeisrelatedtoinstrumentalcharacteristics; usually,MIPallowstoinspectporesintherange10nm0.1/1mm[12].

Noninvasiveadvancedtesting X-rayimaging

Beforeapplyingthis methodverifythat productshavea differentattenuationcoefficientwith respecttostone[13];otherwise,useatracertodopetheproductandassureitsvisualization.Insome studies,3-bromopropyltrimethoxysilanehasbeensuccessfullyemployedtobettervisualizeproducts intotheporousstructureofthestone[14_–16],inthecaseofbothwaterrepellentsandsilane-based consolidants;however,somemetals(suchasAg)canbealsousedastracer,exhibitingnointeraction withthepropertiesofthetestedproducts[17].

Use X-ray

m

-CT[18] toinvestigate the internalstructure of untreated andtreated laboratory samplesandcharacterizetheir3Dporestructure.Tosetthebettermeasurementconditionsevaluate thespatialresolutionrequiredtoquantifytheprocessestoobserve.Rememberthatcriticalpointsare theobjectsourcedistanceandthesamplesize[19];forthelatterone,keepinmindthatvoxelsize(and thusyourresolutionintermofsmallestfeaturesdetectableandquantifiable)isafunctionofsample size.

Imagesofproductsinsidetheporestructureofinvestigatedmaterialcanbeobtainedbycollecting scansbeforeandafterthetreatmentsoncylinder(whichdiameterhasbeendeterminedinfunctionof texturalfeaturesofstudiedstone).

Thequantificationofporestructurebeforeandafterthetreatmentcanbethereforeobtainedby subtractingimagesbeforetreatmentfromimagesaftertreatment,afterregistrationofbothvolumes; inthiswaychangesintheporestructureduetoconsolidantpenetrationcanbeinspected.Inorderto workwithdifferentialimagesitiscrucialtoacquirethedatabyusingthesameparametersbeforeand aftertheapplication.

Thereconstructionofporestructurecanbeeasilyachievedbyusingdifferentdedicatedsoftware [19]allowingtovisualizeandquantifyscannedobjects.

Neutronimaging

Useneutronradiography[20]tovisualize thedistributionoforganicproductsinside thepore structure and monitormovement of water under dynamic conditions in untreated and treated laboratorysamples.

Asitiswellknown,byusingneutronsthegoodcontrastbetweenwater(andwatercontaining products,suchasorganicprotectivesandconsolidants)andstoneallowsthevisualizationofproducts containinghydrogenaswellaswaterintothesamples.Inthecaseofdynamicmeasurements,consider thepossibilitytouseheavywater[21].Theuseofneutronsasinvestigationprobeallowstoavoidthe dopingoftheproducts.

Collectneutronradiographsbyusingcoldneutronsource[21].

SDD(sample-detectordistance)andSSD(sample-sourcedistance)canbeselectedinfunctionof experimentalsetupandresolutionrequired[20].

Collectimagesindryconditiontovisualizethedistributionofproductinsidethenaturalstone; rememberthatneutronsaresensitivetopolymer/organic–basedproductscontaininghydrogen.

Forthefurthercorrectionsandquantitativeevaluation,collectdarkfieldandopenbeamimages. Acquirescansatregulartimeintervalduringcapillaryabsorptiontomonitorwaterbehaviorof consolidated stones; manual or automatic _filling of water container can beused in function of experimentalsetup.Itisadvisabletocollectpreliminaryscansontestsamples,toevaluatethespeed oftheobservedprocessandassesthebesttimeintervalstoapply.

Ideally, capillary tests might be performed until saturation; however, carefully consider the possibleevaporationprocessesoccurringduringtheexperiments,especiallyiftheyarecarriedoutin environmentalconditions(TC.RH%).Afterthesepreliminaryevaluations,selectfixedtimeintervals andamaximummonitoringtimeforallthestonesubstratesandallthestudiedproductsallowingthe successivecomparisonevaluation.

Aquantificationofwatercontentcanbeobtainedbyperformingcalibrationmeasurementsatthe beginningofyourexperiments[22].

Processimagesfirstlybycorrectingandnormalizingbydarkfieldandopenbeamimages.Quantify watercontentdistributionaccordingtoKimetal.[23]anddeterminesorptivityparameter(B)[24] fromtheradiographs.

Smallangleneutronscattering

Usesectionsofuntreatedandtreatedsamples(alsobyusingdifferenttreatmentmethods)which thickness avoids multiple scatteringeffects (usually, < 1mm). Select Q ranges as a function of experimentalsetupandresolutionrequiredintermofscalerangetoinvestigate.

Themethodallowstoobtaininformationaboutthesize,thenumberdensityandthecorrelation betweencomponentsofasample,especiallylookingatpore-airinterfaces.Inthisprospective,the existence of different arrangements inside the porous structure of the solid matrix can be demonstrated,evaluatinghowtheoccurrenceofproductsmodifythepore-airinterfaceofthepore system inthestudiedsubstrate.The measuredparameter(i.e.Q(I))hasinfact relationwiththe formalismassociatingroughnessandfractaldimensionofasurface.Inthisprospective,byusingthe followingcorrelationIðQÞ/Qð6DsÞ _{theroughnessoftheporessurfaceintermsofsurfacefractal}

dimension Ds [25] canbe determined and obtained values ondifferent treated samplescan be comparedtoevaluateefficacyinpenetrationofproductsaswellastheassessmentofhomogenous productlayersontoporesurfaces.Toobtainnumericalparameters,correcttwo-dimensionalintensity distributions for the background and normalize by measuring the incident beam intensity, transmission,andsamplethicknessfromtheI(Q)distributions.Then,determinefractaldimension foreachanalysedspecimen

Incorporationoftheresults

Changesinporestructure:

m

-CTvsMIPvswaterabsorptionvsSANS

Evaluatedifferencesin termof porosityand poreradiusdetermined byMIP,

m

-CTand water absorption.Compareresultstakinginconsiderationdiscrepanciesamongthemethods[12].

IntegrateMIP,

m

-CTandSANSdatatoevaluateporestructurecharacteristics(beforeandafter conservativetreatments)intherangefromnanometertomillimeter.

Waterbehavior:Absorptiontestsvsneutronimaging

Quantifywaterabsorptionchangesbygravimetrictests;evaluate sorpivityandquantifywater contentatsubsequenttimestepfromneutronimages.Compareresultsanddescribewaterbehavior, and possiblepreferentialpathwayinwaterabsorptiondue todistributionof consolidantinpore network.

Penetrationdepth:DRMSvsneutronimagingvs

m

-CTvsSANS

Compareobtainedresultsobtainedbydrillingresistancemeasurementsystem withimagescollected indryconditionbyneutronsource;takeadvantagefromvisualizationofproductdistributiontoexplain possiblediscrepanciesinindirectDRMSvalues.Use

m

-CT3Dreconstructionstovisualizein3Dthe productdistribution,improvinginformationobtainedbyDRMSandneutronimaging.

Finally, evaluatethesmallscalestructuralchangesandpenetrationinnano-sizedpores onthe basisoffractaldimensiondeterminedbyQ(I)scatteringdata.Rememberthatideallythepresenceof productsatpore-airinterfaceshoulddetermineasmoothingofporesurface,withaconsequential changeinfractaldimensioncalculatedrespecttountreatedsamples.

Methodvalidation

Inordertovalidatetheproposedroutine,wepresentanexampleappliedonhighporouslimestone. Thecompletecharacterizationofthestone,thedescriptionoftestedproductsaswelltheevaluationof theirsuitabilityandcompatibilitywithsubstrateisoutofthescopeofthepresentpaper;detailsabout stone,consolidantsandefficacyofproductsarereportedin[17].Briefly,thestoneusedforconsolidating testexhibitsaporosityofabout27%,withatotalporevolumeof0.14cm3_g1_{andamodalporeradiusof} about 8.5

m

m. Compositionally, it is constitutes mainly by calcite, with low amount of dolomite; the stone suffersmainlyofgranulardisintegration,forwhichitrequiresconsolidationactions.Theusedproducts consistin newhybrids formulationscustomizedforculturalheritageconservation;theyincludea patented consolidant modified by amine to promote interaction with limestone and inorganic commercialproductswhichshrinkageprocesswasimprovedbytheadditionoforganicchains.

In thecasestudy,waterabsorption,mercuryintrusionporosimetry,and

m

CTmethods offered togetheranoverviewonthechangesinporestructureofthestudiedstoneintherange0.007–200

m

m after the application of consolidating products. They provided consistent results, even if some discrepanciesduetotheintrinsicdifferencesamongtheappliedmethodswerehighlighted.

Neutronimagingwasdemonstratedasapowerfultechniqueforinvestigatingthepresenceandthe distributionofproductsintothestone,evenifpossiblelimitsduetothelowinteractionbetween neutron beam and polymeric consolidants were evidenced; the obtained data were indirectly confirmedbymicrodrilling,indicatingaverysharpincreaseoverthefirstmillimetersunderneaththe surface. As regards water behavior, neutron imaging provided also measurements of thewater absorptiondynamic,quiteinaccordancewithgravimetrictests;advantageswererepresentedbythe possibilityto visualizethe waterdistribution inside thestone, highlighting possiblepreferential uptake pathsdue to presence/absence of products.SANS data provided information onproduct penetrationinnano-pores.Thetechniquewas alsousefultoevidencesurfaceroughnesschanges relatedtotreatmentmethod;inthecasestudy,relevantchangeswereobservedforproductsapplied byimmersion,suggestingthatthismethodispreferablebecauseitensureshigherbondingofthe producttotheporesurfaceandthecompletepenetrationofconsolidantsintonano-sizedpores.

Overall,stone-productinteraction wasinvestigatedfrommicrometrictonanometricscale(byintegrating

m

-CT-MIP-waterabsorption-SANS),bondingabilitywasverifiedandevaluated(byintegratingDRMSvs neutronimagingvs

m

-CTvsSANS),waterbehaviorwasquantifiedandvisualized(byintegratingabsorption testsvsneutronimaging),andpreferableapplicationmethodswasassessed(bySANS).

Conclusions

Inthiswork,alaboratorymultiscalemethodologyforassessingconsolidant’sefficacyonnatural stoneshasbeenpresentedanddescribed.Theroutineishighlycustomizable;infact,setupofstandard andnon-destructivetestingcanbeestablishedcasebycasetakinginconsiderationsubstratefeatures, productscharacteristicsandpotentialofdifferentmethodstoinspectthewholerangeoftexturaland porosimetric properties. The methodology has been demonstrated to be suitable in better understandinginteractionsbetweenconsolidatingproductsandnaturalstones,supplying quantita-tivedataaboutporerangesinwhichconsolidantsinteractwithstone,andadequatelysupportingthe interpretationofmaterialbehaviorespeciallyagainstwater.

Themethodologicalroutinecanbeeasilyincorporatedintostudiesaboutconservativetreatments, claimingtheadvantagesinintegratingorevensubstitutingnondestructivetestingtoclassicalmethods. Acknowledgments

Thisresearchhasbeensupportedby:UniversityfundsbestowedbyDept.ofBiological,Geological and EnvironmentalSciences of theUniversity ofCatania, Universityfunds bestowedby Dept. of ChemistryoftheUniversityofParma,andFinancingFundofbasicresearchactivities(FFABR)ofMIUR. References

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