Drying processes of gypsum products: Cause and prevention of efflorescence appearance

Gypsum plaster and blocks are widespread building materials which are usually applied in the interior of buildings. Since gypsum is a readily soluble salt, it dissolves when it comes into contact with water. When the hardened product is exposed to moisture penetration, the water will move towards the surface of the solid body, where it evaporates. Because the solid fraction is partially dissolved during this process, sulfate salts can form as efflorescence on the gypsum surface. This is a widespread phenomenon and causes severe damage in the building industry. This thesis investigates the occurrence of different efflorescence salts on gypsum products as well as countermeasures against them by adding small amounts of calcium formate (CF) or Ordinary Portland Cement (OPC). It aims to define exactly how efflorescence occurs on gypsum products and how the presented countermeasures work against it. Furthermore, the influence of CF on the hydration reaction of the raw powder is examined as well. To investigate the drying process, gypsum prisms were put into beakers filled with water and then stored in a defined environment. Water was absorbed into the prisms and then transported towards the surface, where it evaporated. Investigations were emphasised on the development of the porewater composition, and thermodynamic interactions between water and solid body. Water evaporation was recorded by repeated weighing of the samples. XRD experiments were conducted to analyze the phase composition of the efflorescence and the prism surfaces. To retrace changes in the porewater composition during transport through the prisms, they were cut into pieces and the porewater was then pressed out. The composition was then analyzed by ICP-MS. Thus, ion motion could be comprehended, and thermodynamic calculations showed the interactions between liquid water and the solid gypsum body at varying prism heights. With these findings, thermodynamic models were created, that can explain the formation of the efflorescence salts, and why the chosen additives prevent them. With increasing complexity of modern binder formulations, knowledge of the influence of applied additives on the hydration of gypsum based building materials becomes increasingly important. Experimental methods such as heat flow calorimetry, X-ray diffraction and porewater analysis were combined to record the raw powder hydration reaction of calcium sulfate hemihydrate with and without calcium formate. The formate salt retards the hydration reaction. Dissolution experiments of the raw powder, further porewater analysis and XRD measurements were combined, which revealed that CF initially hinders the dissolution of the raw powder and later adsorbs on growing gypsum surfaces which retards their growth. Calcium formate thus acts similar to usually applied retardation agents in gypsum plasters.