Preparation of water suspensions of nano-calcite for cultural heritage applications

The consolidation of carbonatic stone used in ancient monuments, such as middle age cathedrals, degraded because of natural ageing, is an important topic for European cultural heritage conservation. In conservation, usually tetraethyl-orthosilicate (TEOS) based treatments are used as consolidants because they lead to formation of stable amorphous silica inside stones pores1. While generally effective, this treatment is more compatible with silicate stones and it shows a tendency to cracking upon drying. It shows also the tendency to induce damages because of the differential dilatation behavior with respect to host stone. To consolidate carbonatic stones, a carbonate based consolidant should be reasonably developed. As an example, the use of nano-calcite, may result in an improved compatibility with the host stone. Moreover, its nano-dimension facilitates penetration into the stone micro-porosities, with generation of a very high surface area resulting in a high reactivity inside pores. For these reasons nano-calcite may provide a good alternative to TEOS. Nano-calcite can be prepared by the carbonation of calcium hydroxide in water in the presence of interfacial agents2. This reaction is very cheap and well known, but the control of reaction parameters to achieve nano-scale dimensions is quite complex. The present work concerns the preparation of a water suspension containing nano-calcite through the controlled carbonation of calcium hydroxide in water in a pilot plant. The reaction was optimized by keeping into account a statistical approach based on a simplified Design of Experiment technique. Thanks to this approach it was possible to understand the role played by several parameters in determining the final morphology of the nano-calcite. The morphology of nano-calcite and the mechanism of its agglomeration, advantageous for its effectiveness as consolidant but undesired in the formulated aqueous suspension, was studied by electron microscopy and light scattering techniques.