Advanced Masters in Structural Analysis of Monuments and Historical Constructions

Rheinheimer, Vanessa. Sulphate Attack and the Role of Thaumasite in Historical Constructions. 2008. Dissertation (Master of Science). Advanced Masters in Structural Analysis of Monuments and Historical Constructions. Czech Technical University in Prague and Technical University of Catalonia. Barcelona. Spain. Some of important historic constructions around the world are located in centers of big cities, many times near to industrial areas and regarding of that it can be submitted to sulphate atmosphere, important degradation factor for the durability and serviceability of cementitious systems. On the other hand, in the architectural and monumental heritage, it is possible to find masonry with different types of mortars, or also with ancient concrete, cementitious materials subject to suffer sulphate attack. The attack on the cementitious system may occur in different ways: delayed ettringite formation, gypsum formation or, when there is a source of carbonate, thaumasite formation, object of this study. The thaumasite was found in 1965 in decayed concrete and in repairing mortars used for the conservation of the architectural heritage, and its formation is generally associated with relatively cold climate conditions, but nowadays the formation process of thaumasite is still not been explained. Thaumasite can be formed through a combination of sulphate attack and carbonation, and occur in concrete structures and in masonry walls of historic buildings. It provokes expansion, cracking, spalling, loss of strength and adhesion of cementitious materials, transforming hardened concretes or mortars in a pulpy mass because of the loss of strength. Regarding to the identification of the composite, the crystallographic structure of ettringite and thaumasite is very similar, in spite of the difference between their chemical composition, and consequently, the X-Ray diffraction patterns for this two composites are very similar. For this reason, different techniques are used together to identify and differentiate this composites. To develop this research, pastes were prepared and carbonated in laboratory under carbonation and sulphate conditions. The evolution of the mineralogy of the pastes was monitored through time with the aid of X-Ray Diffraction and Fourier Transform Infra-Red. In parallel, compressive and tensile strength were measured. The data sets were analysed to find interrelationships regarding to understand the effect of CO2, temperature and time in the formation of as ettringite and thaumasite. The results shows the sulphate attack products formation with the increasing of ettringite, but not the thaumasite formation after this exposition period.