Applications Of Nano Calcium Hydroxide And Its Hybrids In The Protection Of Wall Paintings

As an important kind of cultural heritage,tomb wall painting is being increasingly damaged due to the aging.In order to extend the life span of the wall pintings,the conservators develop organic polymers such as polyacrylate resin to consolidate the wall paintings and achieve good effects.However,because of the aging of the organic materials and color-changing after its aging,and the poor compatibility of organic materials and the wall painting body,the wall painting may be further damaged after consolidated with organic materials.In order to tackle the issuses of organic materials,the conservators try to explore inorganic materials to protect the wall paintings,which shows long durability and has good compatibility with the wall paintings.To date,nano calcium hydroxide has been demonstrated to be one of the most promising wall painting consolidation materials.However,many urgent problems on the preparation of nano calcium hydroxide still need to be solved,such as,large size（～150 nm）,complicated,time-consuming,energy-consuming or costly.On the other hand,the calcium hydroxide shows slow carbonization rate,low carbonation degree and low consolidation strength when consolidated wall paintings.In this study,a low cost,low energy consumption,high yield and time-saving method to synthesize nano calcium hydroxide is developed through molecular inhibition of crystal growth.In addition,the synthesis mechanism by molecular inhibition of crystal growth is elucidated.Since the successful exfoliation of single-atmoic-layer graphene in 2004,it has been widely investigated for its excellent mechanical,carbon dioxide adsorption,optical and magnetic properties.Graphene quantum dots（GQDs）are the graphene that is composed of hundreds of carbon atoms.In addition to the properties of graphene,it also possesses the ability to absorb ultraviolet ray.Besides,hexagonal boron nitride has been also widely studied as a kind of two-dimensional materials,as well.In addition to excellent mechanical and carbon dioxide adsorption capabilities,hexagonal boron nitride also shows excellent thermal stability.In this paper,graphene and hexagonal boron nitride materials are firstly introduced into cultural heritage preservation.Calcium hydroxide/graphene quantum dots（Ca（OH）₂/GQD）and calcium hydroxide-hexagon boron nitride nanosheet（Ca（OH）₂-BNNS）nanohybrids materials are paprepaed by simple aqueous solution method.The synthesis mechanisms of them are elucidated.More important,the nanohybrids show great performances in wall painting consolidation.The main contents and results of the above study are as follows:（1）In view of the large size of nano calcium hydroxide particles and the high cost,complex operation,time-consuming and energy-consuming of the synthesis method,we propose a crystal growth inhibition method by surfactant molecules to realize the low-cost,low-energy,simple and rapid synthesis of nano-sized calcium hydroxide.Octyl phenyl polyoxyethylene ether（OPPE）and dimethyl formamide（DMF）are chose as surfactants.Nanosized calcium hydroxide is synthesized by adding optimal the volume percentage of OPPE and DMF.Density functional theory calculations find that OPPE preferentially adsorbs on the（0001）surface of calcium hydroxide,while DMF preferentially adsorbs on the（?1010）surface of calcium hydroxide.By the action of the two surfactant molecules,we can synthesize nano calcium hydroxide on a large scale.Characterizations show that the stability,carbonation rate and carbonation degree of the nano calcium hydroxide are better than commical Ca（OH）₂（C-Ca（OH）₂）.This method will greatly promote the application of nano calcium hydroxide in the protection of cultural relics.（2）Given that nano calcium hydroxide shows slow carbonization rate,low carbonation degree and low consolidation strength for wall painting,we propose to design and synthesize Ca（OH）₂/GQD nanocomposite with small and uniform particle size（about 80 nm）by taking advantages of GQDs.The effects of GQDs on the morphology and size of Ca（OH）₂ are studied.In addition,the possible combination way of Ca（OH）₂ and GQDs in Ca（OH）₂/GQD and the synthesis mechanism of Ca（OH）₂/GQD are investigated.Properties characterizations show that Ca（OH）₂/GQD possesses anti-ultraviolet（UV）ability,high carbonation rate and high carnation degree compared to C-Ca（OH）₂.More importantly,the Ca（OH）₂ in Ca（OH）₂/GQD is fully carbonated into a stable Ca CO₃ crystal phase―calcite‖because of GQD.The results of consolidation of simulated wall painting show that Ca（OH）₂/GQD greatly improve the consolidation strength and anti-UV abitily for wall painting.This chapter research opens the direction of wall painting conservation using two-dimensional materials.（3）In order to develop multifunctional materials to protect wall paintings,we propose to introduce hexagonal boron nitride into wall painting protection.By utilizing BNNS,we synthesize Ca（OH）₂-BNNS nanohybrid,which contains small Ca（OH）₂ particles and large size BNNS（i.e.,Ca（OH）₂-BNNS）.Additionally,the synthesis mechanism of Ca（OH）₂-BNNS is explored.The results show that because of the addtion of large size BNNS,the small calcium hydroxide particles are parted from the big size Ca（OH）₂ particle through supersaturation induced etching-striping mechanism.Properties characterizations show that Ca（OH）₂-BNNS has faster carbonation rate and high carnation degree.In addition,the results of consolidation of simulated wall painting show that Ca（OH）₂-BNNS has limited effects on wall painting color.More,surprisingly,Ca（OH）₂-BNNS endows good fire-proof abitily for wall painting.This study,for the first time,proposes the concept of wall painting fire-proof,which is of great significance to reduce the loss of cultural relics caused by fire disaster.（4）The consolidation effects of nano calcium hydroxide on real tomb wall painting are studied and evaluated.The results show that nano calcium hydroxide not only strengthens the wall painting,but also has no significant effects on its color,porosity and pore size distribution.The consoliation techologies of nano calcium hydroxide for real tomb wall paintings are summarized.We find that the technologis of wall painting consolidation mainly include dropping,spraying,and burshing.The consolidation technology is different when the wall painting is storaged in a different way.However,in order to achieve consolidation strength as high as possible,the applied time of nano calcium hydroxide should be as many as possible.Moreover,the relative humidity of the environment to stroage wall painting after consolidation should be improved.In addition,the consolidation strength and fire-proof effects of Ca（OH）₂-BNNS nanohybrid for the real tomb wall paintings are studied.The results show that Ca（OH）₂-BNNS endows high consolidation strength and good fire-proof abitily for wall painting.Therefore,nano calcium hydroxide and Ca（OH）₂-BNNS can be widely used in wall painting protection.