High-tech analysis leads to a breakthrough concrete binder
Cement is primarily used as a binder in concrete, the most widely used material in the world next to water. It literally supports our everyday lives. However, its production consumes tremendous energy and natural resources – and generates up to 8 % of total global emissions. With the support of the Marie Skłodowska-Curie Actions programme, the GeoDust project developed a way to produce alternative concretes using a waste by-product of cement production, simultaneously lowering cement demand and production-associated emissions.
Cement kiln bypass dust as the alkali activator of a novel concrete binder
Portland cement, the most used cement worldwide, is made by heating raw materials to temperatures above 1 400 °C. A portion of the kiln gasses is removed via a bypass to prolong the kiln’s service life and increase product quality. After cooling, cement kiln bypass dust (CBPD) remains. CBPD contains alkali metals, particularly potassium chloride, which makes it a potential ‘activator’ of alkali-activated materials. These materials, also known as geopolymers, are not new. They are typically created from industrial by-products (precursors), and their use to produce a cement-like material has been the subject of intensive research over the last few decades. However, given the many precursors, activators, doses and curing conditions that can be used, the resulting wide range of product properties has not been well characterised.
Characterising the CBPD-to-concrete chain: from microstructure to macroscopic performance
The team used high-tech approaches to characterise CBPD complexity. For example, scanning electron microscopy allowed them to see particle size, shape and chemical composition (via colour coding according to chemical element). The variability of CBPD composition in the same cement plant from day to day was higher than expected, resulting in significant variability in binder properties. One of the reasons the team chose to investigate CBPD was its suspected potential to minimise the high shrinkage of alkali-activated materials. Scientists were surprised to discover how sensitive the volume changes are to the CBPD dose and curing conditions. In some cases, expansion was so great it destroyed the concrete. “The GeoDust project allowed us to understand CBPD’s role in alkali-activated slag binders. We gained insight into phenomena from chemical reactions and microstructure to the macroscopic performance of the final concrete,” explains Vlastimil Bílek of project coordinator Brno University of Technology. This led to successful development of a robust binder. GeoDust has published its results on the beneficial role of CBPD in the alkali-activation process and on the shrinkage-mitigating ability of alkali-activated binders. In addition, the project published results about alkali-activated binder preparation using 100 % recycled materials and the excellent durability of alkali-activated concretes based on CBPD.
Sustainable concrete binders and a human legacy
Bílek summarises: “Inorganic binders may not attract public attention like nanotechnology or biomedicine, but they are ubiquitous, impact our everyday lives and the environment, and are a challenge to characterise. Our project has contributed a piece to this puzzle, enhancing concrete’s sustainability.” A doctoral student when the project started, Bílek is now a PhD holder completing his first research project of which he is the principal investigator. In addition to the technical and environmental benefits, GeoDust has contributed to the global competitiveness of European academia and industry through training of early researchers, including Bílek.
Keywords
GeoDust, concrete, binder, cement, CBPD, alkali-activated, cement kiln bypass dust, scanning electron microscopy, slag