Hydration Characteristics and Mechanical Property of Microfine Cement–Silica Fume Composites Modified with Cellulose Nanocrystals

Abstract Cementitious composites for fractured rock reinforcement often suffer from insufficient mechanical performance, and widely used inorganic nanomodifiers are nonrenewable, which limits their application in rock stabilization engineering. To address these limitations, this study prepares a novel microfine cement-based composite (MCB) in which microfine cement serves as the primary cementitious binder, silica fume serves as the supplementary cementitious material, and cellulose nanocrystals (CNCs) serve as the modifying additive. The combined properties of the MCB were evaluated through macroscopic and microscopic testing, supplemented by gray correlation analysis. Experimental results indicated that the specimen with 1% CNC dosage level (MCB-1%) exhibited the optimal comprehensive performance among all dosage levels. After 28 days of curing, its compressive strength increased by 53.97% compared to the unmodified specimen. The dominant failure mode was characterized by microcracking, and a dense dendritic calcium silicate hydrate (C–S–H) gel was formed within the matrix. Gray correlation analysis indicated that the correlation degree of CNC dosage levels with composite material performance was 16.44% higher than that of curing ages, demonstrating that CNC dosage levels served as the dominant factor governing the properties of the MCB. CNCs bond with the C–S–H gel through Ca–O ionic bonds and hydrogen bonds. With their catalytic activity, CNCs accelerate the pozzolanic reaction between the silica fume and Ca(OH)2 hydroxide, thereby effectively promoting additional C–S–H gel formation. Concurrently, CNCs cooperate with hydration products to construct a fiber-bridging network, which restrains crack propagation and significantly enhances the mechanical integrity of the matrix.