Experimental Study of Workability and Mechanical Performance of Cellulose Nanofiber-Modified Underwater Non-Dispersible Concrete

This study investigates cellulose nanofiber (CNF) suspension as a nano-reinforcement for underwater non-dispersible concrete, aiming to improve the balance among workability, washout resistance, and compressive-strength performance. An L9 orthogonal array was employed to evaluate the dosage-dependent responses of CNF suspension, hydroxypropyl methylcellulose (HPMC), and a polycarboxylate ether-based water-reducing admixture (PCE). Within the investigated range, increasing the as-received CNF-suspension dosage from 0.15% to 0.60% was associated with an increase in the factor-level mean 28-day underwater-to-air compressive-strength ratio from 0.75 to 0.79 and a decrease in suspension pH from 11.92 to 11.73, suggesting higher underwater strength retention and lower alkaline dispersion. CNF suspension showed favorable dosage-dependent responses in the washout-related and compressive-strength indicators, while HPMC and PCE contributed to the overall regulation of measured workability and compressive-strength performance. Among the nine mixtures, Group 5 was identified as a balanced engineering-oriented candidate, combining a slump of 245 mm, a slump flow of 465 mm, and 7- and 28-day underwater-to-air strength ratios of 0.94 and 0.82, respectively. SEM observations revealed pores, interfacial gaps, and locally distributed acicular, plate-like, and filament-like features, providing qualitative microstructural context for the measured responses. Overall, the results support the potential of CNF suspension as a promising nano-reinforcement for underwater non-dispersible concrete with a favorable balance of workability, washout-related performance, and compressive-strength retention.