Numerical simulation of gypsum transport and crystallization

Over the last few decades, brick masonry buildings in the UK, the Netherlands and Belgium have started suffering from a new type of salt efflorescence. Contrary to the common efflorescence of highly soluble salts, which easily wash off during rain events, this new efflorescence consists of lowly soluble gypsum, and is therefore strongly persistent. There is a progressively growing body of dissatisfied building owners, who primarily regret the aesthetic damage to their facades. Although gypsum efflorescence has already been known for several decades, there are still many open questions regarding the mechanisms behind this phenomenon. One highly puzzling issue is the reason why the phenomenon started appearing only recently, among others. Numerical simulation of the transport and crystallization of gypsum in porous materials can contribute to a better understanding of the underlying processes, hence being helpful in the search for causes and solutions. Additionally, such simulations can be used for the formulation of experimental procedures for accelerated efflorescence testing. It is well known that calcium sulphate is a salt commonly present in building materials. However, due to its low solubility and insensitivity to changes in temperature or relative humidity, it is usually considered as harmless. And that explains why calcium sulphate is generally not included in simulation software for salt transport and crystallization in porous materials. Therefore, this paper focuses on the numerical implementation of gypsum transport and crystallization in the Delphin simulation environment. The main objective is the study of the processes of transport and crystallization of gypsum in porous media, with relation to gypsum efflorescence on brick masonry. As initial applications, the simulation of uptake and drying tests is performed. The uptake test comprises an initially dry material sample in contact with gypsum solution at the bottom surface, while evaporation is allowed only at the top surface. The drying test involves an initially moist material sample, containing gypsum, exposed to drying conditions. In both cases, the sample’s moisture gain respectively loss and the gypsum concentration profiles are of main interest. These simulations make up the first step towards a better insight in how gypsum is transported, where gypsum crystallizes, and which conditions may lead to the development of gypsum efflorescence.status: publishe