Enabling the High-Volume Utilization of Iron-Rich Sewage Sludge Ash in Cement: The Role of Curing Temperature in Microstructure and Performance Enhancement

Abstract The massive generation of sewage sludge ash (SSA) poses severe environmental challenges worldwide. Ternary blended cement incorporating SSA and limestone represents a promising green cementitious material that enables high-volume SSA utilization, yet the retarding effect of SSA on cement hydration leads to unsatisfactory early compressive strength, which remains the primary bottleneck restricting its large-scale engineering application, particularly at high replacement levels. In this work, the compressive strength development, hydration heat evolution, phase assemblage, and pore structure of SSA-limestone-cement blends cured at 5 °C, 20 °C, and 40 °C were systematically investigated to reveal the temperature-dependent hydration mechanism. The results show that elevated curing temperature accelerates early hydration and improves the 3-day compressive strength, while curing at 20 °C yields the highest late-age strength. Based on hydration calorimetry, X-ray diffraction (XRD), thermogravimetric analysis, and mercury intrusion porosimetry (MIP), the underlying micro-mechanism is clarified: curing at 5 °C results in the lowest 3-day and 28-day strength due to sluggish hydration reactions and abundant harmful macropores. At 20 °C, the 3-day strength is slightly lower than that at 40 °C owing to the delayed hydration acceleration period, but the substantial formation of ettringite (AFt) and carboaluminate phases at later ages refines the pore structure and gives rise to the maximum 28-day compressive strength. Curing at 40 °C achieves the highest early strength via accelerated hydration, yet the transformation of AFt to AFm and the accumulation of portlandite (CH) generate an unfavorable pore structure and limit late-age strength growth. These findings elucidate the regulatory role of curing temperature in the hydration and microstructural evolution of high-volume SSA ternary blends, providing theoretical guidance for curing regime optimization in engineering practice. This work offers a feasible pathway to promote the high-value utilization of iron-rich SSA in cementitious materials and contributes to carbon reduction in the cement industry.