Properties of concrete containing desulphurised waste.

In the past, the emission of SO[2], NO[x] and CO[2] gasses, as a result of the combustion of fossil fuels within the power industry has been a major contributor to the pollution of the environment. Controls over the last few years have been implemented to combat the effects of air pollution on the environment. One such control is the reduction of sulphur dioxide (SO[2]) from power stations, which is achieved by fitting desulphurisation systems to new and existing installations. At present there are many types of desulphurisation processes available, which produce a variety of wastes that vary in quality and quantity. The majority of processes fit into three main categories, wet, semi dry and dry desulphurisation processes. Desulphurisation systems work by introducing alkaline sorbents, such as limestone, to the SO[2] gasses, either during or after combustion. The limestone reacts with the SO[2] gasses to form new insoluble materials such as gypsum (CaSO[4].2H[2]O). However, semi-dry and dry processes produce wastes that are usually a blend of fly ash and some form of calcium sulphate. The wide variation in chemical, physical and mineralogical properties of the wastes produced hinders their utilisation, because general experimental investigations are only relevant to specific types of wastes. In addition, the wastes can contain large quantities of sulphates, which are normally limited to a few percent in plain cements to regulate setting, and therefore, their initial potential appears limited.The current investigation aims to evaluate the performance of paste, mortar and concrete that contain large quantities of actual and simulated desulphurised wastes. Members of the Copernicus project (1999) provided various desulphurised wastes from Eastern European installations for investigation. Fly ash and gypsum from UK sources were blended in different proportions to simulate desulphurised wastes based on a range of SO[3] contents typically associated with actual desulphurised wastes produced throughout Eastern Europe. Several tests were conducted on paste, mortar and concrete containing actual and simulated desulphurised wastes to investigate basic physical, mechanical and durability properties. These include chemical shrinkage, porosity and pore size distribution, water absorption, length change, compressive strength, and sulphate resistance. The outcomes of investigation were positive in that cement based materials containing high levels of desulphurised wastes were produced, which exhibit physical, mechanical, and durability properties equal or superior to reference cements, and cements containing more common replacement materials such as fly ash and slag.Attempts were made to correlate specific properties such as chemical composition, porosity and pore size distribution, and chemical shrinkage with strength in order to determine the key factors influencing strength development of cement-based materials containing desulphurised wastes. The relationship between SO[3] content and compressive strength of pastes containing simulated desulphurised wastes (FA-G blends) was used to predict the theoretical strength of cement-based materials containing actual desulphurised wastes with reasonable accuracy. Thus reinforcing the decision to evaluate simulated desulphurised wastes due to the diverse nature of desulphurised wastes currently available across Eastern Europe.