Modification of shrinkage and mechanical properties of concrete by the addition of ladle furnace slag (LFS)

Abstract

The decrease in humidity in a concrete element due to a difference in relative humidity between the surrounding environment and the element itself causes a volumetric deformation, which leads to a decrease in volume. This phenomenon, called shrinkage, can lead to the appearance of cracks in the concrete, which in turn can cause a decrease in the strength and durability of the structural element. One strategy to reduce shrinkage could be the addition of Ladle Furnace Slag (LFS) during concrete production, as this type of slag has expansive properties due to the presence of free lime and magnesia. LFS is a by-product of the steel industry, so its use in concrete production also has a beneficial effect on the environment by removing waste from the production chain. As the addition of LFS leads to a variation in the mechanical properties of concrete apart from a decrease in shrinkage, it is convenient to accurately evaluate the modification of all these properties when adding different proportions of LFS. The final aim is to define the optimum amount of LFS that can be added to reduce shrinkage while maintaining the mechanical properties of the concrete above the required limits. To do so, this research work evaluates the modification of shrinkage up to 90 days from concrete production, as well as the variation of two key mechanical properties, compressive strength and flexural strength, at two different curing ages, 28 and 90 days, when LFS in proportion of 5%, 10% and 20% is added to the concrete mix as cement addition. A reference mix was designed with a slump of about 70 mm and the other three mixes were defined by replacing the fine aggregate with LFS in the proportions referred and adjusting the water content to reach a slump equal to 80 ± 15 mm. The results showed a decrease in shrinkage when LFS was added to the concrete mix, although the water content of the concrete mix also played a key role. It was also found a modification of the compressive strength and flexural strength, which was related to the proportion of LFS added. Finally, it was also noted that the strength increased quicker when a lower proportion of slag was used.