A SYSTEM TO SIMULATE POLLUTED ATMOSPHERES AND ITS APPLICATION TO THE STUDY OF STONES USED IN ANDALUSIAN CATHEDRALS AND OF TREATMENT PRODUCTS

Abstract

One of the main problems affecting monuments today is the effect of atmospheric pollution on their materials. From this arises the need to develop test methods to evaluate these effects, on the original materials (to define weathering mechanisms), on new materials for restoration and on treatment products to be applied. An installation has been designed and developed that allows the simulation of polluted atmospheres (with several pollutants) and other climatic conditions. The response of the system is studied when simulating specific conditions, by submitting samples of stone used in important Andalusian monuments, treated with various water-repellent products, to an accelerated weathering test with a sulphur dioxide atmosphere and variable humidity conditions. The installation is of a modular type and can be divided into 10 sections. In sections 1 and 2 the conditioning of air used to generate the simulated atmosphere is carried out: the air is first dried with 96% sulphuric acid and then cleansed of solid particles, sulphuric acid vapour and organic compounds. In sections 3-8, pollutants are added to the airstream and their flow is controlled. Hydrogen fluoride is added to the air by evaporation from a solution; sulphur dioxide, ammonia and propene are added from bottles of liquified gas; nitrogen dioxide is added from a heated bottle of liquified gas. The degree of humidification is achieved by evaporation. The test chamber is made of polyester reinforced with glass fibre and has a volume of one cubic metre; it is cylindrical, with a thermostatic bath to control the temperature. The gas streams go into a mixing chamber with a sampling device and then into the test chamber. Samples are placed on a rack; there are spray nozzles to simulate rain over the samples or to generate salt vapour. With the normal air flow of 25 litres per minute, the residence time is 40 minutes. When the conditions change, 99% of new conditions are reached in 147 minutes. The radiation source has to be located outside the chamber, due to the corrosive nature of the atmospheres generated. As radiation transmitter, films of Teflon FEP are used, as it has good resistance to chemical attack and excellent transmission of ultraviolet radiation of every wavelength. The sampling device has five independent lines that allow sampling simultaneously at five points in the test chamber. To obtain samples of pollutant gases, sampling ports with the appropriate absorbing solutions are used. Five types of limestone used in cathedrals of Andalucia were studied, as well as samples from quarries in the region. Their most important characteristics are as follows: