Multi-Scale Investigation on the Hydration and Microstructure of Portland Cement Blended with Inorganic Softening Sludge

Inorganic Softened sludge is a solid by-product generated during water treatment. Owing to its fine particle size and high calcium content, ISS has potential for use as a mineral admixture in cement-based materials. However, its specific role and underlying mechanisms in cementitious materials remain unclear. Therefore, this study systematically investigates the effects of different ISS replacement levels on the workability, mechanical properties, and hydration evolution of cement paste. The results showed that increasing the ISS replacement level progressively reduced the flowability of the paste and prolonged the setting time. Compared with the neat cement control, low ISS replacement levels (5–10%) increased the 3 days compressive strength, while causing only a slight reduction in the 28 days compressive strength; in contrast, high ISS replacement levels (15–30%) resulted in a pronounced decrease in compressive strength. Particle-size analysis showed that ISS had a smaller median particle size than OPC, suggesting a potential physical filling effect. Microstructural analyses indicated that ISS incorporation did not appreciably alter the types of major hydration products, while the detection of hemicarboaluminate (Hc) suggested that carbonate introduced by ISS may have participated in reactions involving aluminate phases. BSE/EDS observations revealed residual ISS-associated particles within the hydrated matrix, with Ca- and Si-enriched regions adjacent to some particles, consistent with the elemental composition of C-S-H-like hydration products. At low replacement levels, particle surface effects and possible physical filling effects may jointly contribute to the observed strength changes, potentially enhancing the early-age strength of the cementitious matrix.