With the rapid development of low-carbon buildings and structural-energy storage integration technologies, cement-based materials, as core components of building structures, are increasingly required to possess simultaneously load-bearing and energy storage functions. However, the inherently low ion transport efficiency of traditional cement-based materials, along with the difficulty in simultaneously improving mechanical and energy storage performance, limits their application in structural energy storage. Steel slag, a bulk solid waste with hydration activity, can be utilized as a supplementary cementitious material to regulate pore structure and optimize microstructural interfaces, thereby enhancing ionic transport and energy storage performance. Meanwhile, electrolyte type plays a crucial role in the electrochemical performance of cement-based energy storage devices. Therefore, a systematic investigation into the effects of steel slag on the mechanical and energy storage properties of cement-based materials in different electrolyte systems is of considerable theoretical value and practical significance.

Porous cement-based structural electrolytes were prepared using aluminum powder as a pore-forming agent and steel slag as a supplementary cementitious admixture, with a water-to-binder ratio of 0.4. Aluminum powder contents were 0.02%, 0.05%, and 0.10%, while steel slag contents were 0%, 15%, 30%, and 45% (by mass of cement). After standard curing for 28 d, specimens were immersed in 2 mol/L KOH or ZnSO₄solution for 7 d. Activated carbon (AC), acetylene black, and polyvinylidene fluoride were mixed, and N-methylpyrrolidone was added before grinding to form a uniform slurry, which was coated onto a stainless-steel foil current collector and dried to obtain AC electrodes. Subsequently, AC//AC symmetric supercapacitors and Zn//AC hybrid supercapacitors were assembled in the KOH system and ZnSO₄ system, respectively. The compressive strength of the samples was measured, and ionic conductivity was evaluated using an electrochemical workstation. The microstructure and phase composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray computed tomography (X-CT). The electrochemical performance of the supercapacitors was assessed using electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge measurements.

The synergistic effect of aluminum powder-induced pore formation and steel slag incorporation significantly promoted the development of an interconnected pore network, thereby increasing ionic conductivity. Ionic conductivity increased with increasing aluminum powder and steel slag contents, reaching 130.5 mS/cm (KOH) and 26.1 mS/cm (ZnSO₄). The type of electrolyte had a significant impact on the compressive strength of the samples. After immersion in KOH solution, the compressive strength of the samples remained essentially unchanged, indicating that KOH mainly contributed to pore-solution filling. In contrast, after immersion in ZnSO₄ solution, the compressive strength of the samples increased markedly. The compressive strength of the S1A5 specimen increased from 9.6 MPa to 17.5 MPa, corresponding to an improvement of 82.3%. Microstructural analysis revealed that ZnSO₄ reacted with cement hydration products to form compounds such as zinc hydroxide sulfate and gypsum, which effectively filled internal pores and refined the pore structure, thereby enabling the simultaneous enhancement of ionic conductivity and compressive strength.

Electrochemical measurements demonstrated that the devices based on the KOH system exhibited a typical electric-double-layer capacitive behavior, characterized by low internal resistance and excellent rate capability, but had a relatively low energy density of only 10.8 W·h/kg. In contrast, the ZnSO4-based devices displayed a hybrid energy-storage mechanism combining electric double-layer capacitance and pseudocapacitance, resulting in a significantly improved energy density of 72.2 W·h/kg. Both types of devices exhibited excellent cycling stability, with capacity retention exceeding 90% after 1000 charge-discharge cycles.

This study demonstrates that aluminum powder and steel slag synergistically regulate the pore structure and ionic transport properties of cement-based structural electrolytes. The ionic conductivity of the KOH system is markedly higher than that of the ZnSO₄ system. ZnSO₄ can react with cement hydration products to enhance compressive strength and ionic conductivity simultaneously, while also enabling high energy density in Zn//AC hybrid supercapacitors. Steel slag contributes to multi-performance optimization by promoting pore connectivity, facilitating ion transport, and improving interfacial electrochemical processes. These findings highlight the dual value of solid-waste reuse and functional structural regulation, and provide important guidance for the design and application of cement-based structural energy storage devices.