Experimental design of an alkali-activated slag concrete railway sleeper

Abstract

This research aims to improve the sustainability of Dutch prestressed concrete railway sleepers. Approximately 100.000 to 200.000 sleepers are used every year, corresponding to 12.000 to 24.000 tonnes of concrete. This will be done by replacing cement with an alkali-activated binder. The cement industry is responsible for approximately 5 to 7% of the global CO2 emission, whereas the alkali-activated binder uses no cement. Instead, by-products will take over the role of cement in concrete, activated by high pH alkalies. This research can be subdivided into four parts: Deciding the requirements, designing for the mechanical properties, determining the durability properties and conducting a Life Cycle Analysis (LCA). The main requirements follow the product specification of the railway infrastructure manager ProRail. Some requirements on manufacturability follow from a market consult. Ground granulated blast furnace slag is used as a precursor due to its availability in the Netherlands, unlike other potential candidates. An iterative mix design procedure is conducted to come to three mixes, meeting the requirements for the fresh concrete and the mechanical properties. The three mixes differ based on the binder content and activator dosage. Nineteen different mixes are designed, of which the final three meet all the mechanical and manufacturability requirements. These three mixes are subjected to tests to determine non-required mechanical properties, such as the tensile strength or the material’s elastic modulus. Finally, the durability of the newly designed alkali-activated slag concrete is assessed. Results show the designed concretemeets the requirements for fresh and hardened concrete. The results of the other mechanical properties are similar or better to the concrete mixes used nowadays to produce sleepers. However, not all requirements on durability are met. Due to the unavailability of certain test conditions, a direct result cannot be given. Therefore, the three durability properties are assessed by comparing the results of this research with results from the literature. The comparison for electrical resistivity shows an excellent behaviour, better compared to similar OPC concretes and on par with CEM III/B concrete types. The alkali-activated binder demonstrates less damage under a freeze-thaw attack with de-icing salts than other alkali-activated concretes but still fails to meet the requirement set by ProRail. The accelerated carbonation test indicates a similar carbonation resistance compared to other alkali-activated concrete types but a lower resistance compared to similar concrete. More research is necessary to investigate these properties further. The environmental impact of the concrete used to produce a sleeper is reduced by 34 to 68%compared to a CEM III/B or OPC binder, respectively. Taking into account the global warming potential only, these numbers increase to 75 to 90% respectively. The total environmental impact of a complete sleeper, including steel and plastic components, is lowered by 12 to 35%. This is explained by the fact that the steel and plastic components contribute 75% to the environmental cost, and improving the cost of these components is beyond this research’s scope. This research shows a significant potential to improve the sustainability of concrete structural elements by replacing the cement binder with an alkali-activated binder. This binder is capable of meeting the mechanical requirements, shows good workability. The compressive strength values are comparable to high strength concrete. Further research has to be conducted in the fields of behaviour under carbonation and freeze-thaw attacks. With these results and conclusions, this research has attributes to realizing more sustainable railway sleepers using an alkali-activated binder.