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Stone conservation for the refurbishment of buildings

Final Report Summary - STONECORE (Stone conservation for the refurbishment of buildings)

Six SME’s, four universities, one public research organisation and one public body from seven countries joined together in project STONECORE in order to find a new approach for the refurbishment of stones, mortars and plasters used in the construction of historic monuments. The idea was to develop and test nano-materials compatible to those used during construction, together with new nondestructive methods for the assessment of stone. The development and testing of calcium hydroxide nano-sols was at the centre of interest. In detail, the following was required:

- Fundamental investigations into the synthesis of calcium hydroxide nano-sols and characterisation of their basic properties.
- Determination of the applicability of different nano-materials to natural and artificial stones, mortars and plasters.
- Documentation, sampling and characterisation of the objects foreseen for trial testing and demonstration.
- Demonstration of the efficacy of the developed materials by application on several objects within Europe
- Characterisation of fungal and algal growth on different stones, mortars and plasters as well as development of methodologies to remove biological growth using calcium hydroxide nano-sols.
- Methodology and hardware development for the non-destructive assessment of stone.

STONECORE has led to manifold new approaches for the conservation of stone, mortar and plaster as well as for the overall refurbishment of buildings. Many new consolidants based on calcium hydroxide nano-particles were developed. The nano-particles they contain are characterised by sizes in the range of between 50 and 250 nm. They are stable when dispersed in different alcohols. The small size and electrostatic repulsion forces guarantee that the particles do not sediment. Stable sols are formed. These are able to penetrate deep into deteriorated stones, mortars or plaster. After evaporation of the alcohol, calcium hydroxide particles are present in the treated materials. The alcohols evaporate without leaving any other residues. Conversion into calcium carbonate takes place in a similar way as for conventional lime by reaction with atmospheric carbon dioxide. The achievable strengthening, the distribution of the nano-materials in different stone and mortar samples as well as the development of favourable application techniques were explored through a large number of comprehensive investigations. Within the STONECORE project it could be shown that sols containing stably dispersed nano-lime particles are able to strengthen and disinfect areas affected by algae and fungi and to remove microbiological growth in an eco-friendly way, without chlorine or quaternary ammonium compounds. Safe removal of biological growth is achieved by the dehydrating action of ethanol in combination with the creation of alkaline conditions by the lime particles itself.

The development of new materials for structural consolidation was accompanied by comprehensive research in the field of non-damaging assessment methods. An enhanced peeling test to characterise the surface properties of natural and artificial stone was developed as well as a sophisticated, semi automated device to characterise the water-uptake of porous materials using the for Karsten tube technique. Advanced SEM investigations in combination with digital image analysis have have been used to detect calcium hydroxide nano-particle in treated materials. Comprehensive investigations have shown that Ground Penetrating Radar is a useful tool to detect fractures and voids in stones especially when the computer-controlled positioning device developed during the project is employed. A new borehole ultrasonic measurement device and a drilling resistance device were also developed and successfully tested.

The successful use of the developed calcium hydroxide nano-sols was demonstrated on 17 objects. Wall paintings, deteriorated natural stone as well as mortar and plaster was strengthened successfully. STONECORE has achieved fully its objectives and technical goals.