Procedure for reforming calcium carbonate in the restoration of stone monuments or the like

Pouchain, Ing. Maurizio and Melolago Albani, Prof. Ludovico

The invention concerns a procedure for restoring stone monuments by means of reconstituting the calcium carbonate, transformed by the atmospheric pollutants.

It is known that almost all ancient objects show clear signs of degradation arising from physical, biological and chemical causes.

Urban monuments suffer greatly from the action of sulfur bacteria which release sulfate ion S0₄ starting from the sulfur dioxide in polluted atmosphere due to burhing of fossil fuels (coal and its derivatives).

However, the chemical causes are undoubtedly the most important and above all-.the most difficult to handle. These are closely related to the characteristics of the material used, and interact to enhance their damage-with the physical and biological causes.

The most commonly used stone building materials are marble, limestone and sandstone. All three of these materials include calcite among their components, which is correctly held to be mainly responsable for their vulnerability.

From a chemical point of view, calcite is calcium carbonate CaCO₃, which is present in various crystalline forms in marble, where it is the major component. Limestone also contains a high precentage, although in less pure form, at times of biological origin, mixed with traces of iron and other metal salts, as in marls, etc.

Finally, it is also present in sandstone, where it is the substance which cements the quarz crystals to the other silicates.

Under neutral or alkaline conditions, calcium carbonate CaCo₃ is insoluble in water, but if the pH of the solution drops into acidity, the situation is created in which the compound undergoes chemical reactions which make it soluble.

The aqueous solutions of the dioxides of carbon, nitrogen and sulfur are among the most important of these solutions, and said dioxides are present in high concentrations in ubban atmospheres.

Although the mechanisms of nitrogen dioxide attack is poorly understood, those of the carbon and sulfur oxides are known perfectly.

Carbon dioxide CO₂ is the principal product of the- combustion of organic compounds, together with small quantities of carbon monoxide CO which is rapidly corverted to C0₂. This carbon dioxide is present in the gas state in ever increasing amounts in the atmosphere, and is extremely soluble in water, forming carbonic acid.

This solution effects calcium carbonate by transforming it into the highly soluble carbonic acid bicarbonate according to the following reaction:

An analogous process occurs for sulfur oxide derivatives. In fact, in aqueous solution S0₃ forms sulfuric acid H₂SO₄, which reacts with calcium carbonate to form soluble..:calcium sulfate hydrate, commonly known as chalk.

These transformations profoundly affect the appearance and consistency of the stone, for thicknesses of various millimeters and even centimeters. Not only surface alterations are involved, but also deep ones which begin an unstoppable process.

The pounding of the surfaces exposes the stone ever more deeply to new attack by these corrosive solutions.

The aim of this invention is to realize a procedure to retransform the calcium sulfate hydrate in the surface and in depth alterations of stone, marble, etc. monuments, back into calcium carbonate, restoring the original structures.

This aim is achieved with a procedure for treating the stone elements which selectively eliminates the S04 ion and replaces it with the CO₃ ion, in place.

According to the invention, the object is immersed in deionized water so as the exploit the solubility property of the calcium sulfate, then realizing a forced circulation through ion exchange resins which can selectively retain the sulfate while leaving the calcium hydroxide in solution, according to the following reaction:

The advantage of this method lies in the fact that it only removes the sulfate ion while releasing calcium ion to the original stone. Then in a second phase the object is exposed to the action of carbon dioxide under controlled conditions, in order to reprecipitate the calcium ion as the carbonate.

The difficulties lie above all in controlling the conditions for reforming the carbonate so that it precipitates in the calcite cuptalline form rather than argonite.

The thermodynamic equilibrium is in favor of calcite, but many factors may shift it to favor argonite: such as temperature, the presence of other cations, etc..

A preferred variant of the procedure may be obtained using the system schematized in the attached drawing.

The scheme shows in detail the treatment tank 1, in which the stone materials to be treated may be immersed or which may be placed around objects of larger size, which is filled through opening 2 with carbon dioxide free water, and whose temperature is controlled with thermometer 3, the liquid is then circulated by means of pump 4 through the shunts 5 - 8 in the anion exchange resin 6, and in column 7 with Ca(OH)₂, to return to the vat 1, according to the arrows, while the pH is measured in device 9 and the conductivity in conductometer 10.

The scheme for treating the stone materials may be described as follows:

• - characterization of the objects stone material according to-the NORMAL 6/81 A and B recommendations;
• - gathering of representative samples according to NORMAL 3/80 recommendations, p. 6;
• - determination of the distribution of the pore volume as a function of their diameter, in accord with NORMAL A/80;
• - determination of water absorption by total immersion and imbibing capacity, according to NORMAL 7/81;
• - elimination of the sulfate ions and all soluble anions according to the following specific procedure: immersion of the samples in C0₂-free deionized water in vat 1; forced circulation of the water with pump 4; through the ion exchange resin column 6; determination conductometically with 10 and by pH with 9 of the anions held on the column; determination on the basis of the Ca(OH)₂ concentration in solution of the quantity of Ca(OH)2 to add to restore the stone material to its original state.

After exclusion of the ion exchange column 6, Ca(OH)₂ is added to the solution and forced circulation is continued until equilibrium is reached, that is until the resin absorbs no more sulfur; the sample is then removed from the water, its surface is dried and it is placed in a C0₂ atmosphere at controlled temperature to facilitate formation of calcium carbonate in the form of calcite rather than argonite which, being crystallized, is extremely delicate.

The sample so treated than has its morphological characteristics checked with an incident light optic microscope.

If necessary, the imbibing capacity and water absorption on total immersion are compared with those obtained before treatment.

With regard to the Ca(OH)₂ concentration determination indicated in the procedure, the Vs = volume of the system, that is that of the water contained in the vat 1 and in the conductor, the Vc = volume of the stone piece being treated, and, at least approximately, the Va = volume presumably involved in the sulfation must be known so that, since Vs - Vc = volume of H₂0, (Vs - Vc)/Va, will be tha value by which the Ca(OH)₂ concentration must be multiplied.

Of course, while the principle of the finding remains the same, the froms of realization and the details of the procedure may be varied widely from that described and illustrated here, without going beyond the bounds of the present invention.