An Investigation on the Swelling Behaviour of Basic Oxygen Furnace Slag-Granulated Blast Furnace Slag Mixtures

Steel is essential to our contemporary way of life and economic development, and its inherent advantages offer it a versatile solution in an increasing variety of applications. About 70% of global steel is produced from iron ore, while steel scraps account for 30%. Around one tonne of steel produces about 400 kg of granulated blast furnace slag (GBFS) and basic oxygen furnace slag (BOFS), commonly known as steel furnace slag (SFS) or steel slag. While several re-purpose applications of GBFS is available, utilisation of BOFS is minimal. Recently, steel slag coarse aggregates (\u3e 6 mm) have been used in the asphalt mix for the surface course in pavement constructions. However, the applications of finer slag particles (\u3c 6 mm)) in construction applications are limited because of the unhydrated lime and magnesium oxide, causing a volumetric expansion of up to 10%. Therefore, this research investigates the potential of a composite mixture using GBFS in controlling the swelling of BOFS. A series of laboratory tests were conducted to evaluate the engineering properties, swelling and shear behaviour to identify the optimum mixture of the BOFS-GBFS. It is evident from the results that the introduction of GBFS has a significant influence on optimum moisture content and maximum dry density of BOFS. In addition, accelerated swell and long-term swell consolidation tests highlighted a reduction in the swell of BOFS with the addition of GBFS. For example, up to 77% of swelling reduction can be expected once 30% GBFS content is added to the composite. However, the peak deviatoric stress of BOFS was reduced linearly with an increase in GBFS content leading to a 15% reduction of internal friction with a 30% addition of GBFS content. The detailed microstructural studies using an optical microscope and micro-CT scan showed the bonding of BOFS-GBFS particles leading to the reduction in swelling of BOFS. The bond formed between BOFS-GBFS particles may be due to the chemical reaction between the free lime in BOFS with silica and alumina in GBFS in the presence of water to form C-S-H gel and tricalcium aluminate. A design chart was developed based on these laboratory findings and, the optimal mixture of GBFS to BOFS is about 25-30%, was identified where volumetric swelling is less than 1% and has a friction angle of about 47o and can be effectively used as the base course material in the pavements. These results highlight the potential of BOFS-GBFS mixtures as sustainable construction material for future infrastructure development