Influence of the Steel Slag Particle Size on the Mechanical Properties and Microstructure of Concrete

In the field of construction materials science, optimization of concrete properties has always been an important issue [1,2]. With the increasing emphasis on environmental sustainability, the utilization of industrial by-products to improve concrete properties is not only a way to enhance resource recycling but also an important direction for the innovation of traditional construction materials [3,4,5].

Steel slag is the solid waste produced by smelting steel, which is the product of slag-making agents added for deoxidation, desulfurization, and dephosphorization in steelmaking, and the emissions account for about 12% to 20% of steel production. At present, the utilization rate of steel slag in China is less than 30%, and there is a big gap with the utilization rate of nearly 100% in developed countries such as the United States and Germany. The comprehensive utilization of steel slag resources can not only reduce the pollution to the environment but also produce benefits for iron and steel enterprises. The main chemical components of steel slag are CaO, SiO₂, Fe₂O₃, MgO, and Al₂O₃, of which the active components are CaO, SiO₂, and Al₂O₃, and the higher the content, the better the activity of steel slag [6,7]. Researchers have explored the use of steel slag in concrete, focusing on the effect of steel slag admixture on concrete properties such as density, strength, and durability [8,9,10]. Nan et al. [11] explored the mechanical properties, hydration products, and the process of hydration hardening of concrete prepared by combining steel slag with mineral slag. The study found that the concrete exhibited optimal mechanical properties when the steel slag content was 31.97%, with 28-day compressive and flexural strengths of 20.20 MPa and 7.25 MPa, respectively. Liu et al. [12] successfully developed alkali-activated slag/steel slag cementitious materials using silica fume and alkali slag. The optimal mix achieved 28-day compressive and flexural strengths of 36.2 MPa and 13.29 MPa, respectively, satisfying the requirements for ordinary slag Portland cement of grade 32.5. Masilamani et al. [13] carried out a study on the difference in performance between EOF steel slag and natural aggregate, and they compared the difference in the shape index, roundness, sphericity, form factor, and roundness of the two materials. The results showed that different grading size ranges of EOF steel slag, a hardened interface zone, and a rough surface texture were beneficial to the improvement of concrete strength.

Lai et al. [14] used BOF steel slag to replace coarse aggregate and fine aggregate, and their results showed that the optimal replacement rates of coarse aggregate and fine aggregate were 50% and 30%, respectively. Under these replacement rates, the late compressive strength and microstructure of concrete were significantly improved.

Current research shows that steel slag can enhance its cementitious activity through chemical and mechanical activation, and it performs well when incorporated into products like concrete [15,16,17,18,19]. However, most studies have focused on the overall amount of steel slag added, and relatively few studies have been carried out on the effect of the key factor of the steel slag particle size [20,21]. The properties of steel slag, such as the particle size, distribution curve, and specific surface area, determine the gelling activity of steel slag. Variations in the particle size of steel slag can significantly affect the microstructure of concrete, subsequently impacting its macroscopic mechanical properties. It is significant to study the influence of the change in the steel slag particle size on the mechanical properties and microstructure of concrete. Therefore, the present study is dedicated to investigating the influence law of the steel slag particle size on the mechanical properties and microstructure of concrete.