The use of blast-furnace cements (CEM III) enables substantial CO2 reduction but is commonly associated with limited early-age strength development. In this study, the early strength of CEM III systems was enhanced by incorporating calcium sulfoaluminate (CSA) cement. For time-critical applications the rapid hydration of CSA cement must be deliberately retarded to ensure sufficient processability. However, retardation adversely affects later-age strength development, particularly in ternary systems with high ground granulated blast-furnace slag (GGBFS) contents. Experimental results showed that Portland cement recovered their strength development after delayed hydration, whereas retarded CEM III exhibited no comparable recovery within 28 d. Isothermal calorimetry indicated strongly suppressed heat release and temperature evolution in retarded CEM III systems. SEM revealed unhydrated particles, while TGA showed significantly reduced calcium hydroxide (CH) contents; additionally, X-ray diffraction identified unhydrated alite in retarded CEM III systems, indicating incomplete clinker hydration. The external addition of CH in the form of hydrated lime significantly improved strength development in retarded CSA–CEM III/B systems, confirming CH deficiency as the primary factor limiting the reactivity of GGBFS under retarded conditions. These findings further indicate that competitive interactions between CSA cement hydration, GGBFS activation, and retarder action govern GGBFS hydration and the associated strength development. In contrast, CH addition did not enhance the strength development of retarded CEM III systems without CSA cement, indicating that in this case clinker retardation was the primary cause of strength reduction in these mixtures.
Electro-osmotic (EO) dewatering is a potential technique for removing water from low-permeability fine-grained construction materials in various construction-related applications, such as soft-ground improvement and dredged fill treatment. However, conventional metallic electrodes can suffer from corrosion and interfacial voltage losses during sustained direct-current operation. This study evaluates practical pre-passivation surface treatments for improving the durability and service performance of such electrodes. Oil-quenched films and a brush-applied chemical conversion coating (Super Blue) were applied to mild steel (MS) and 304 stainless steel (SS) electrodes. Performance was assessed by quantitative surface characterization (passive overpotential, interfacial resistance, contact angle, and coating detachment) and by long-term service tests (energy-normalized drainage, energy-normalized mass loss, and cyclic reuse). The oil-quenched films were hydrophobic and tape tests showed that the coating detachment differed between the substrates and the coatings. On MS, oil quenching produced the strongest passivated response, improving energy-normalized drainage by approximately 10–20% and reducing energy-normalized mass loss by 23–30% in single tests. The four-layer oil-quenched film gave the best balance between drainage efficiency and durability, and maintained more stable drainage and lower mass loss during cyclic operation. On SS, both treatments improved drainage and reduced mass loss, but film peeling indicated weaker coating retention on the SS surface. These results indicate that electrode pre-passivation provides a practical material-engineering strategy for improving the durability and service performance of metallic electrodes in EO dewatering systems.
Phosphogypsum (PG), a byproduct of phosphate fertilizer production, can substitute natural gypsum (NG) in cement as a calcium sulfate source. However, PG introduces soluble phosphorus (P₂O₅) and fluorine (F⁻), which may retard hydration and affect early strength development. This study evaluated the hydration kinetics and phase assemblage of tricalcium silicate (C₃S) in the presence of soluble P₂O₅ and F⁻, simulating PG contributions. Concentrations ranged from 0.25 to 1.00 wt% P₂O₅ or F⁻, and also included a combination of 1.00 wt% P₂O₅ with 1.00 wt% F⁻. Analytical techniques included isothermal calorimetry, X-ray diffraction, and thermogravimetric analysis. Both P₂O₅ and F⁻ retarded C₃S hydration, extending the induction period and reducing early heat release, the retardation increasing monotonically over the dosage range investigated. At equal mass dosage, F⁻ produced the longer induction periods. The combined mixture, containing 1.00 wt% P₂O₅ and 1.00 wt% F⁻, extended the induction period from 1.78 h to 13.06 h and reduced the heat released at 9 h by approximately 96%, with portlandite undetectable at that age. No crystalline calcium phosphate or calcium fluoride phases were detected within the detection limits of the techniques employed. Within the 48 h window measured, cumulative heat release in all doped systems reached and then exceeded that of the reference between 24 and 48 h. These results indicate that P- and F-driven retardation in C₃S, while severe, is transient rather than permanent, opening the way for future studies extending this approach to more realistic PG-containing cement systems.
