María José Varas

Determination of anisotropy to enhance the durability of natural stone

Anisotropy is a petrophysical property of natural stone and other construction materials that determines their quality and resistance to decay due to a variety of agents, such as water. A study was conducted on nine types of stone widely used in Spains built heritage, using six previously defined anisotropy indices. These indices can be used to determine the degree of anisotropy, which helps explain the differential decay observed in stone materials quarried in the same bed and used to build the same structure. The conclusion reached is that anisotropy should be determined in the natural stone used both to restore the architectural heritage and in new construction, since the appropriate choice of material quality ensures greater resistance to decay and, therefore, increased durability. Materials with the lowest possible anisotropy should be selected, as this property governs their hydraulic behaviour: the lower the anisotropy in a material, the better its behaviour in relation to water and the longer its durability.

Assessment of Different Methods for Cleaning the Limestone Façades of the Former Workers Hospital of Madrid, Spain

Abstract Five different methods for cleaning the limestone façades on what was originally the Workers Hospital in Madrid are analyzed in this paper. Due to the pollution in the surrounding air, just 20 years after a prior cleaning operation, sulfate crusts had developed on the entire stone surface of the building. The gypsum mortar used in the original masonry constituted an additional source of sulfur. Limestone is a traditional building material in Madrid and its surroundings. The petrography, mineralogy and petrophysical properties of the biomicrite, pelmicrite and biopelmicrite varieties of limestone identified in the hospital walls were determined. Analysis of a black layer on the stone surface showed that it consisted primarily of sulfate crusts. The cleaning methods tested were alkaline gels (sodium hydroxide and potassium hydroxide), pressurized hot water, glass bead blasting and latex peeling. The criteria for assessing the effectiveness and potential risks of the various cleaning systems included changes in the chromatic parameters of the stone, the formation of alteration products (i.e. salts) and modification of the stone surface. The stones cleaned with the three most effective methods, together with a rain-washed stone as a reference, were washed with water to generate an artificial runoff. The collected drain water was analyzed to determine the presence of any by-products from the cleaning process. The method found to be most effective and which caused the least alteration to the stone surface was glass bead blasting, particularly after adjustment of the bead size and pressure conditions used for the test.

The Influence of Past Protective Treatments on the Deterioration of Historic Stone Façades A Case Study

Abstract During a 2002 cleaning operation on the Royal Palace (Madrid, Spain), some stains actually became much more visible after the limestone and granite stones were cleaned. These stains were analysed by several instrumental techniques (both in situ and in the laboratory) and were found to result from past protective treatments. Some of these treatments had formed thin, impermeable films that did not allow the water inside the ashlar to evaporate. Sub- and crypto-efflorescences (gypsum, thenardite and nitratite) broke the protective film, generating efflorescences on the surface. The treatments had not penetrated the stone, and the stains were merely superficial. The method used for the cleaning of the stone façades (pressure water jet) was ineffective for the removal of these protective treatments, which at present are deteriorating the stone on which they were applied.

Limestone on the ‘Don Pedro I’ facade in the Real Alcázar compound, Seville, Spain

Abstract This paper discusses the research conducted prior to restoring the ‘Don Pedro I’ facade on the Real Alcázar or royal palace at Seville, Spain. The different types of stone on the facade were located and characterized, and their state of decay mapped. Although other materials (brick, rendering, ceramics, marble) are present on the facade, its main elements are made from two types of limestone: palomera and tosca, each in a different state of conservation and exhibiting distinct behaviour. Colour parameters, real and bulk densities, compactness, open porosity, water saturation coefficient and total porosity were determined to characterize the two varieties. In addition, ultrasonic techniques were used to map the various levels of decay on the facade, stone by stone, for future interventions. The findings show that owing to its petrographical and petrophysical properties, palomera stone is of a lower quality than tosca stone, and has undergone more intense deterioration.