Preparation of C60 High-Fluidity Concrete-Filled Steel Tube Concrete Using Steel Slag Sand

Literature Overview

The paper by Ding Qingjun, Bao Jiacheng, Jin Hongcheng, and Zhou Peng, published in the Journal of Wuhan University of Technology in 2019, addresses a critical challenge in the construction of concrete-filled steel tubes (CFST): the significant shrinkage of the core concrete, which leads to void formation at the steel-concrete interface and compromises the structural integrity of the composite section. The authors propose the use of pre-treated steel slag sand as a partial replacement for natural river sand to prepare a C60 high-fluidity concrete with excellent volume stability. The research is supported by the National Basic Research Program of China (973 Program, Project No. 2015CB655101) and the National Key R&D Program (Project No. 2017YFB0310105), underscoring its strategic importance for sustainable construction materials.

Material Design and Mix Proportion Optimization

The steel slag sand is pre-treated to activate its latent hydraulic activity, which provides an alkali-activation effect and internal curing function. The alkali-activation effect promotes additional cementitious gel formation, densifying the interface transition zone (ITZ) between the concrete and the steel tube. The internal curing function is achieved through the pore water stored within the steel slag particles, which is released during the curing process to maintain adequate moisture for continued hydration.

The mix design incorporates a combination of superplasticizer, water-retaining agent, and shrinkage-reducing and toughening admixtures to achieve the required high fluidity and volume stability. Fly ash and silica fume are blended in appropriate proportions to exploit their pozzolanic effects, which contribute to the long-term strength development and microstructural densification. The optimized mix achieves a 28-day compressive strength of 76.9 MPa, exceeding the C60 specification, while maintaining excellent flowability for placement within the confined space of the steel tube.

Volume Stability and Interface Transition Zone

The key innovation of this study lies in the synergistic use of steel slag sand to address the shrinkage problem from multiple angles. The alkali-activation effect generates additional calcium silicate hydrate (C-S-H) gel, which fills the capillary pores and reduces the permeability of the concrete. The internal curing function ensures that the concrete maintains adequate moisture content during the critical early-age period, preventing premature drying and shrinkage. The combination of these effects results in a concrete with minimal shrinkage, which maintains intimate contact with the steel tube wall throughout the service life.

The interface transition zone (ITZ) is a critical region in CFST structures, as it governs the load transfer between the steel tube and the concrete core. The densification of the ITZ through the alkali-activation and internal curing effects of the steel slag sand significantly improves the bond strength and interfacial friction, enhancing the composite action of the CFST section. The reduced shrinkage also prevents the formation of voids at the ITZ, which would otherwise act as stress concentrators and initiation sites for cracking.

Engineering Practice and Sustainability Implications

The use of steel slag sand as a partial replacement for natural river sand offers significant sustainability benefits. Steel slag is a by-product of steel production, and its use as a construction material reduces the demand for virgin natural resources and diverts waste from landfills. The C60 high-fluidity concrete developed in this study is particularly suitable for the construction of high-rise buildings, bridge piers, and offshore platforms, where CFST columns and beams are commonly used for their high load-carrying capacity and compact cross-section.

From a construction quality perspective, the high fluidity of the concrete ensures complete filling of the steel tube without the need for vibration, which is critical for maintaining the integrity of the steel tube and preventing damage to the tube wall. The volume stability of the concrete eliminates the risk of void formation, which would compromise the structural performance and durability of the CFST component. Quality control measures should include slump flow testing, air content measurement, and non-destructive testing of the concrete fill to ensure proper placement and curing.

The research by Ding et al. demonstrates that the use of steel slag sand in CFST concrete is a technically sound and environmentally beneficial approach. The synergistic effects of alkali activation and internal curing provide a robust solution to the shrinkage problem, which has been a persistent challenge in CFST construction. Engineers and researchers should consider this approach as a viable option for sustainable CFST construction, particularly in regions with abundant steel slag resources and high demand for high-performance concrete. Future research should investigate the long-term durability, freeze-thaw resistance, and corrosion protection performance of the steel slag sand CFST concrete under various environmental conditions.