WEN Xiaoqing ^{1, 2}, DONG Weixia ¹, BAO Qifu ¹, GU Xingyong ¹, LI Yinming ³, LI Runfeng ³

(1. Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 2. Hebei Ruisuo Solid Waste Engineering

Technology Research Institute Co., Ltd., Chengde 067000, Hebei, China; 3. Beijing Building

Materials Academy of Sciences Research, Beijing 100041, China)

Extended abstract:[Background and purposes] Foamed ceramics, as a novel category of inorganic non-metallic materials, have found applications in building energy efficiency, due to their advantageous properties, including light weight, thermal insulation, sound-absorbing and fire resistance. However, the production of these silicate materials, to achieve porous structure through high-temperature foaming processes, relies heavily on clay-feldspar-quartz mineral systems. This dependence represents a significant consumption of high-quality natural mineral resources. Furthermore, challenges in pore structure control can detrimentally impact the overall performance profile. Consequently, strategies for performance enhancement through the synergistic combination of industrial solid waste substitution and optimized process control are of considerable importance. China, as a major consumer of industrial resources, generates vast quantities of industrial solid wastes, such as tailings and fly ash, alongside its economic development. The resource utilization of these wastes demands urgent breakthroughs. This study was aimed to address this need by utilizing representative iron tailings and fly ash from the Chengde region as the primary raw materials for fabricating lightweight high-strength foamed ceramics. The influence of fly ash content on phase composition, pore structure and key performance metrics (namely bulk density, compressive strength, and thermal conductivity) of the resulting ceramics was systematically examined, aiming to provide a robust technical pathway for the high-value utilization of these solid wastes.[Methods] Based on the fundamental composition-structure-property relationship, fly ash was incorporated as a mineral additive, which was specifically selected for its demonstrated ability to enhance mechanical performance. The approach leverages the beneficial high-temperature complementarity between the primary mineral constituents, iron ore tailings and fly ash. Single-magnetite iron ore tailings (Chengde source) and power plant fly ash served as the primary raw materials. The total solid waste content (iron tailings+fly ash) was fixed at 60 wt.%, while the foaming agent content was 1.5 wt.%. The fly ash to iron tailings ratio varied across the proportions (by weight, relative to the total solid waste) of 0:60, 10:50, 20:40, 30:30, 40:20, 50:10. The batched raw materials were blended with 0.1 wt.% anhydrous ethanol as a process control agent (PCA). The mixtures were then ball-milled using a powder grinder operating at 1500 r·min⁻¹ for 2 h (powder-to-ball weight ratio=1:2). The milled powders were sieved through a 325-mesh screen (aperture ~45 µm). The undersized fraction was collected and dried at 100 ℃ for 15 min. The dried powders were loaded into moulds, compacted using vibration, leading to green bodies with smooth surface. The green compacts were subject to sintering. The samples were heated from room temperature to 800 ℃ at a ramping rate of 5 ℃·min⁻¹. Subsequently, they were heated from 800 ℃ to the target sintering temperature of 1160 ℃ at a ramping rate of 3 ℃·min⁻¹. They were held isothermally at 1160 ℃ for 30 min.  The resultant foamed ceramic samples were characterized to evaluate the influence of the fly ash to iron tailings ratio on bulk density, mechanical properties (specifically compressive strength) and thermal conductivity. Mineral phase evolution, pore structure and microstructural morphology were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM).[Results] Porous multiphase ceramics, comprising quartz, mullite (3Al₂O₃·2SiO₂) and anorthite, were successfully fabricated through powder-based in-situ foaming, utilizing single-magnetite iron ore tailings (Chengde source) and coal fly ash as primary solid waste feedstocks. Fly ash content critically governs pore architecture. An optimal fly ash proportion (20–30 wt.% of total solid waste) promoted formation of uniform, closed-cell pores, thickened pore walls. This microstructural refinement enhanced mechanical integrity. Conversely, exceeding this optimal range led to pore coarsening, increased pore wall defects, which are detrimental to mechanical performance. Appropriate fly ash content optimized pore size homogeneity and closed porosity fraction, thereby improving thermal insulation by minimizing gas-phase convection. Excess fly ash induced large, interconnected pore channels, which significantly degrade both mechanical strength and thermal insulation efficacy. The addition of fly ash significantly promoted the phase formation of mullite (3Al₂O₃·2SiO₂). This crystalline phase served as the primary mechanical reinforcement agent, substantially enhancing compressive strength of the composites. Under the formulation of 60 wt.% total solid waste (fly ash + tailings) and 1.5 wt.% foaming agent, the ceramics achieved bulk density of 430–480 kg·m⁻³, compressive strength of 5.94–12.96 MPa and thermal conductivity of 0.18–0.42 W·m⁻¹·K⁻¹.[Conclusions] High-performance foamed ceramics with excellent properties were successfully prepared using iron tailings and fly ash from the Chengde area. The fly ash provides reactive sources of Si, Al and Ca, acting as a promoter for composite silicate-aluminate mineral phases. During high-temperature processing, it facilitates the formation of mullite-anorthite networks, which serve as the primary reinforcing phase within the ceramic matrix. In terms of pore structure regulation, the proportion of fly ash was the core factor in controlling pore morphology (uniformity, closed pore rate and pore size), directly affecting the balance between mechanical and thermal insulation properties of the materials. Under the formula framework of a total solid waste content of 60 wt.% and a foaming agent content of 1.5 wt.%, adjusting the proportion of fly ash (about 20–30%) could produce foamed ceramics with a bulk density of 430–580 kg·m⁻³, compressive strength of 5.94–12.96 MPa and thermal conductivity of 0.18–0.42 W·m⁻¹·K⁻¹. Our results can be used as a reference for the resource utilization of solid waste to prepare high-value-added building materials in Chengde and similar regions rich in solid waste, with significant environmental and economic benefits.

Key words: foamed ceramics; industrial solid waste; fly ash; single-magnetite iron ore tailings; mechanical properties; thermal insulation