Creep Behavior of Steel Pipe Confined Red Mud-Slag Geopolymer Concrete

Literature Overview and Research Motivation

This study investigates the long-term creep performance of geopolymer concrete (GPC) confined within steel pipes, utilizing red mud and slag as primary binder precursors. Red mud, a byproduct of the Bayer process in alumina production, has accumulated to alarming quantities worldwide, creating both an environmental burden and a potential resource opportunity. By incorporating red mud and ground granulated blast furnace slag (GGBFS) into a geopolymer matrix, the research addresses waste valorization while exploring the mechanical behavior of the resulting composite under sustained loading. The steel pipe confinement introduces a hybrid system where the ductile steel shell interacts with the brittle geopolymer core, fundamentally altering the creep response compared to unconfined specimens.

Core Technical Content

Material Composition and Mix Design

The geopolymer binder system typically employs a sodium hydroxide and sodium silicate activator combination to dissolve the aluminosilicate phases present in red mud and slag. The water-to-binder ratio (W/B) is a critical parameter, generally maintained between 0.35 and 0.50 to ensure adequate workability while achieving target compressive strength. Red mud content is usually varied between 10% and 40% by mass of total binder, with slag providing the primary aluminosilicate source for geopolymerization. The steel pipe confinement ratio (C = f_y·A_s / f_c·A_c) ranges from approximately 0.1 to 0.3 in typical test configurations, where f_y is the yield strength of the steel pipe, A_s is the steel cross-sectional area, f_c is the unconfined compressive strength of the GPC, and A_c is the concrete cross-sectional area.

Creep Test Methodology

The creep tests follow a sustained axial load protocol, typically applying 30%–50% of the specimen's compressive capacity after a standard curing period of 28 days. Load is maintained for durations ranging from 90 to 180 days, with strain measurements recorded at regular intervals. The steel pipe, typically made of Q235 or Q345 grade steel with wall thickness of 4–8 mm, provides lateral restraint that suppresses microcrack propagation within the geopolymer matrix. Creep compliance is evaluated as the ratio of creep strain to applied stress, with both primary (transient) and secondary (steady-state) creep regions identified.

Key Findings on Creep Behavior

The confined geopolymer concrete exhibits markedly lower creep coefficients compared to conventional Portland cement concrete (PCC) under similar conditions. The primary creep phase is shorter due to the denser pore structure developed during geopolymerization, and the secondary creep rate is reduced by the confining pressure from the steel shell. Red mud content of approximately 20%–25% yields the lowest creep coefficient, while excessive red mud (>35%) introduces excessive porosity and increases creep susceptibility. The steel pipe confinement reduces total creep strain by 40%–65% relative to unconfined GPC specimens, demonstrating the effectiveness of the hybrid confinement mechanism.

Process and Standards Analysis

The testing methodology aligns with GB/T 50082 for compressive strength determination and references JGJ/T 70 for creep measurement procedures. The steel pipe fabrication follows GB/T 8163 standards for fluid transport pipes, with welding quality inspected per NB/T 47013. The geopolymer concrete mixing follows GB/T 2419 for slump measurement, ensuring consistency across specimens. From a metallurgical perspective, the steel pipe's microstructure—ferrite-pearlite for Q235 or fine-grained ferrite-pearlite for Q345—remains essentially unaffected during the geopolymer curing period, provided that the alkaline activator does not cause significant corrosion of the pipe inner surface.

Engineering Practice Implications

The research carries significant implications for the design of confined geopolymer concrete columns and pipe pile foundations. The reduced creep coefficient suggests that long-term deflections and prestress losses in structures utilizing this composite system will be lower than predicted by conventional PCC-based codes. However, the absence of extensive long-term data (beyond 180 days) and the limited number of load levels warrant cautious application in service-life-critical structures. Engineers should consider incorporating a safety factor of 1.2–1.5 on predicted creep strains when applying these findings to design calculations, until further long-term monitoring data becomes available.

Study Insights and Reflections

The integration of industrial waste (red mud) into a functional structural material represents a paradigm shift in sustainable construction. The creep data presented here provides a quantitative foundation for replacing traditional cement-based materials in pipe-confined applications, particularly in infrastructure projects where carbon footprint reduction is a design driver. Nevertheless, the long-term durability of the geopolymer-steel interface under cyclic or variable loading remains an open question that demands further investigation through accelerated aging tests and field monitoring programs.