Overcoming phosphorus-induced hydration retardation in sewage sludge ash-cement systems: A comparative study of mechanical and chemical activation

The utilization of sewage sludge ash (SSA) as a supplementary cementitious material (SCM) offers a significant opportunity for resource recovery; however, its high phosphorus content poses a major challenge by inhibiting cement hydration. This study systematically investigates strategies to mitigate this inhibition in SSA-Ordinary Portland Cement (OPC) systems by comparing the effects of mechanical activation versus chemical activation (using Na₂SO₄, CaSO₄, and Triethanolamine (TEA)) on early-age hydration kinetics and mechanical properties. Results reveal that, contrary to the activation behavior of traditional SCMs, mechanical activation exacerbates phosphorus leaching from SSA, thereby retarding the early hydration process. In contrast, chemical activation can overcome this limitation via targeted regulation of the hydration of different clinker phases. Specifically, 1 % Na₂SO₄ accelerated early hydration by promoting C₃S dissolution, increasing the 3-day strength by 127 %, while simultaneously suppressing excessive U-phase formation to maintain late-age strength. 5 % CaSO₄ synergistically accelerated the hydration of both C₃S and C₃A, thereby enhancing the formation of C-S-H and AFt, which culminated in compressive strength enhancements of 190 %, 47 %, and 23 % at 3, 7, and 28 days, respectively, relative to the reference mixture. 0.01 % TEA selectively promotes C₃A hydration while minimizing the inhibitory effect on C₃S. This research provides effective activation strategies for incorporating phosphorus-rich SSA into cementitious materials, contributing to the development of low-carbon binders and supporting the synergistic goals of waste valorization in zero-waste city initiatives and decarbonization of the construction sector.