Biomimetic Conservation Of Historic Stones Inspired By Biomineralization

A good many of historic stones exposed out-of-doors are badly damaged due to natural and human factors. Their weathering state is badly serious. If people should not take effective measures to minimize damage from circumstances and slow down the ageing process of stones, a lot of historical value of historic stones will be fading away. However, hardly any protective materials do meet with all requirements now. Consequently, it is a very urgent task to explore new protective materials with a good performance which are fit for historic stones.In this paper, a review of the historic stone weathering and protective materials is summarized. Inspired by a crude biomimetic protective film of calcium oxalate, a novel idea on the application of biomimetic method to stone conservation is brought forward on the principle of biomineralization. Moreover, a series of inorganic biomimetic protective materials has been synthesized by biomimetic methods.The following results have been obtained by investigating:(1) Organic silicon, which was widely used in stone conservation now, was selected as surface protectant. Some common relic-used stone were used as protected objects, such as sandstone, tuff, dolomite, marble and granite. A series of artificial weathering experiments, which included single or cooperative effect of major ruinous factors—dry-wet cycle, salt crystallization, freezing and heating were simulated and researched. It is advanced that the surface hydrophobic protection can damage historic stones rather than protect in opposition to expectation. The reasons that decaying of stone is aggravated are a great contrast between hydrophilic stone and hydrophobic organic materials and aggravation of stress damage. An effective method to resolve this problem is to develop a new harmonious protective material which is non-absolutely hydrophobic, acid-resistant and soil- resistant, such as biomimetic materials.(2) In this work, the stone surface treatment, including surface cleaning and functionalization, was studied. The optimal processing parameters, by which stone surface was functionalized well, were made certain by tests of specimens' surface energy.Selecting chondroitin sulfate, a category of principal acidic mucopolysaccharide, as an organic macromolecule template, the supersaturated solution of calcium oxalate as precursor, a protective material was synthesized on the marble surface by a biomimetic method. The effect degree of various factors and optimal processing parameters of biomimetic synthesis were investigated by an orthogonal experiment. The effect degree order is: first, organic template; second, mineralization ion precursor; third, surface treatment; fourth, lay amount of thin film. In the process of biomimetic synthesis, first of all, the stone surface was pretreated with ethanol and functionalized with organic template solution, and then filmed repetitiously by using filtered CaC₂O₄ supersaturation solution as precursor.The composition and microstructure of the material were characterized by using XRD, SEM and AFM. The analysis results show that the stone was covered by a lay of calcium oxalate monohydrate thin film which is approximately 100 nm thick. The synthesis mechanism of the protective material is explained. The tests of hydrophobicity, acid resistance, soil resistance, UV-lighting aging and breathing proved that the protective film functions well.On the basis of biomimetic synthesis exploration of calcium oxalate protection material on the stone surface, a series of protective materials, including calcium oxalate, apatite and calcium carbonate, were further investigated in biomimetic synthesis and surface protection performance. The microtopography of materials was characterized by SEM images. The protective performance was characterized by testing acid resistance, soil resistance and UV-lighting aging. The results show that all of them have various degrees of capabilities in surface protection. Calcium oxalate is the best of them. For example, the stone sample coated calcium oxalate can resist a pH 0.8 acid solution and achieve 2nd level of soil resistance. Moreover, they can breathe and serve for long.(3) With a view to the loose structure of historic stones, the further investigation of biomimetic protection is developed in consolidation. Referring to the architectonics theory, the replicas of rotten stones were prepared by using the powders and particles of calcium carbonate. Biomimetic surface protection materials were applied to the replicas of historic stones. It was investigated how about the capabilities in consolidating of biomimetic surface protection materials. The results from testing compressive strength and water soaking resistance show that these materials have great capabilities of consolidation. The compressive strength and water soaking resistance of treated replicas have been greatly improved. Apatite is the best of them. The stone replica consolidated by apatite has an increase of about four times in compressive strength, and can resist soaking for more than six months.(4) On the basis of summarization of experiments, some protective materials, including calcium oxalate and other common organic protective materials, were selected to do a fieldwork experiment at the Longmen Grottoes in Luoyang. The initial results show that the biomimetic method function well, which open up a good prospect for the conservation of historic stones by biomimetic methods.In conclusion, the side effects of organic hydrophobic protective materials are proved. A series of biomimetic protective materials is investigated. The test results show that they can function well in surface protection and consolidation. The process is simple. Currently, the literature on it has not been found yet. The production is of great significance to the development of new materials for historic stones conservation. Meanwhile, this work can develop the research field of biomimetic materials.