Self-compacting alkali-activated slag concrete

Abstract

Currently, Ordinary Portland cement (OPC) concrete accounts for the world's 8-10% anthropogenic carbon/ greenhouse (GHG) emissions, where 90-95% of the emissions are due to the production of OPC. A 200% increase in OPC demand is projected by 2050 from 2010 levels. Therefore, it is urgent to reduce these emissions arising from its production. Alkali-activated concrete (AAC) has gained significant attention from researchers worldwide for being a sustainable alternative to OPC concrete. AAC completely substitutes Portland cement with industrial by-products, such as BFS (blast furnace slag) and FA (fly ash). An alkaline activator is used to activate the BFS or FA to form a hardened binder. Despite comparable or even better technical performance than OPC concrete, worldwide usage of AAC is not yet observed. The challenges for the commercial scalability of AAC have rather increased with the onset of coal phase-out law in many parts of the world. This limits the availability of FA and thus forcing researchers to find alternative binders to make AAC, such as using BFS as the sole binder. Therefore, while other technologies develop and blast furnaces are phased-out, it is essential to effectively use the available BFS from the steel industries to produce alkali-activated slag concrete (AASC). Although various studies proved that AASC has significantly higher compressive strength than OPC concrete, numerous researchers have reported poor workability and rapid-setting. AASC also has higher drying & autogenous shrinkage. However, despite having a higher autogenous shrinkage, AASC shows moderate-low cracking potential compared to OPC concrete. A self-compacting AASC is expected to follow the same behaviour. However, very few studies have focused on the self-compacting behaviour of AASC, which, therefore, limits AASC for various structural applications where an SCC is required. In order to upscale the commercialization, a comprehensive study is needed that provides detailed insights on the development of a SCAASC (without FA) with an appropriate mixture-design and optimization procedure, desirable fresh properties, hardened properties, sustainability analysis and commercial viability. Hence, the main research question is: Is it possible to develop a self-compacting alkali-activated concrete made from 100% BFS as a binder and currently available raw materials without using any FA? If so, What is the optimal mixture design that results in desirable fresh and hardened properties suitable for real structural applications? Is the developed concrete sustainable and ready for commercialization? And if not, what are the commercial barriers? Attempts are made to answer these questions through various laboratory experiments and stakeholder interviews. The final mixture design is developed in a 5 step optimization process. It involves studying the effect of a particle packing based mixture design method, curing temperature, silica modulus (Ms) content and different admixtures on the workability and compressive strength (1, 7 and 28 days) development of the SCAASC. In this process, an optimal mixture design with desired workability and strength properties is developed and an optimal curing and mixing regime is also suggested. This developed SCAASC is further tested for all the SCC like fresh properties. Various hardened properties such as compressive and splitting tensile strength, elastic modulus, Poisson’s ratio, bond strength to reinforcement, free drying and autogenous shrinkage are also investigated at different ages for different curing regimes. The cracking potential is also investigated for the first time using a TSTM (Temperature stress testing machine). Furthermore, the environmental footprint and material costs are evaluated through an LCA (life-cycle analysis). Lastly, stakeholder interviews were conducted with an international and local construction company to validate the developed SCAASC and exploit its commercial viability. From the experiment results and conclusions drawn from the interviews, the developed SCAASC is commercially scalable for its desirable fresh and hardened properties. It can be used in various industrial applications where an SCC is used. The 80% less CO2 emissions from a reference OPC concrete further bolster its market acceptance. However, high market costs and lack of available standards are still the major challenges.