ELEMENTS FOR STONE DEGRADATION

The stones used for ornamental and covering ends suffer natural and artificial requests, that become causes of consuming, loss of mechanic resistance, fissuration, spotting, fading, formation of crusts (efflorescence of salts) and changes in coloration. The natural requests are related to the geologic exposure, deformation (tectonic and non tectonic) and erosion. The artificial requests are due to mining, processing, handling, applications and use.

As for coverings, the processes of degradation of the applied materials result from the action of physical, chemical and biological agents, as illustrated by IPT on Table 6.1:: see table

As alterações mais importantes ocorrem pelo ataque físico-químico dos minerais constituintes das rochas, podendo-se destacar alguns parâmetros conhecidos:

The most important alterations occur by physical and chemical attack of the rock constituent minerals, some of the well known parameters as follows:

-   The alkalies, for example, under the caustic soda form, attack silicate minerals present within granitoid and granitic rocks in general;

-   Calcite and dolomite, that are carbonates and main marble constituents, suffer the attack from all the acidulous solutions;

-  The oligoclase, silicated mineral of the calcium-alkaline feldspars family, and the nephelinite, also a silicated mineral of the feldspars type, is sensible to the chloride acid;

- The mafic minerals (dark) are more alterable by oxidation than the felsite minerals (light), and it is noticeable that the hypersthenes, mafic mineral of the pyroxenes family and constituent of the charnockits (green granites, Ubatuba type), are degraded by insolation and modify the stone chromatic standard;

- The sulphides, metallic minerals that occur as accessories in marbles and granites, serpentinites and quartzites, oxidize very fast when exposed to the atmospheric conditions, then constituting one of the main problems for their use of covering stones.

Simulation tests of these materials alteration, for a previous analysis of their performance in the normal conditions of use, should be executed. The basis for these tests are estimated on Table 6.2:: see table

ARestoration of stones already applied, must be done after specific analysis of the problem there observed. The most common procedures for removal of spots and other alterations, include new polishing, application of oxalic acid (solution of 10% in volume), application of oxygenated water (20 volumes), sand blasting (for non reflecting surfaces) and application of hot water and/or water value under pressure. For chips and cracks, the fulfilling with plastic mass, white cement or plaster, mixed to the powder of the affected rock, has been used. Specifically for travertines (for example Beige Bahia), there is a proper mixture (stucco) very used in holes.

The best products for regular stone cleaning, mainly on floors, are the pure soaps and detergents of neutral pH. The most common methods for this systematic cleaning, involve the washing itself, sweeping, vacuuming (non polishing surfaces) and use of humid mop. Also for the floors, it is important for the cleaning work to be done the most regularly possible, for besides its abrasiveness, dirt finishes by impregnating in the surfaces, following the pressure of pedestrian traffic.

The prevention of problems related to the absorption of liquids and grease, can be made possible through procedures to achieve hydro and oil repellent subsurface impermeability. The use of these sealers can only be done after specific tests, once the available products in the market do not present their composition adequately written and recommendations for use are too generic.

The best prevention against these and other observed problems are, even now, the correct specification of the stones for the intended uses. From the arrival of the new codes for the consumer’s defense and the ISO quality certifications, the knowledge and interpretation of the technological characteristics, about the commercialized stones, will become more and more demanded.