Regulation of early hydration and microstructure of cement slurry by cellulose nanofibers

In this study, cellulose nanofibres (CNFs) with different structures were prepared using ball-milling and 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated (TEMPO) oxidation. Then, microcalorimeter, scanning electron microscope, electrical conductivity, and micro-CT were used to investigate the interaction mechanisms between the CNFs and cement slurry. The results showed that the combination of ball-milling and TEMPO oxidation produced CNFs with carboxyl functional groups. When the NaClO concentration was 2 mmol/g and oxidation time was 2 h, the carboxyl content on the surface of the nanofibres was 0.0856 mmol/g. Based on the hydration rate and electrical conductivity results, the carboxylated CNFs reduced the hydration rate of the cement slurry and delayed the exothermic peak by 0.5–2.3 h. The decrease rate of the electrical conductivity of the cement slurry during transition stage increased from 2.805 to 3.543–5.142 mS/(cm·h). The electrical conductivity of the solid–liquid two-phase cement slurry was mainly provided by the pore solution. The decrease in the electrical conductivity of the cement slurry during the transition stage could mainly be attributed to the formation of hydration products that changed the microstructure of the cement slurry from ‘dispersed particles’ to a ‘gel grid’ during the hydration acceleration stage, the porosity and pore connectivity of cement slurry are reduced. The zeta potential results indicate that CNF in the cement slurry provides a location for the nucleation and growth of early hydration products through adsorption and bridging effects, changes the microstructure of the cement slurry, and accelerates the transition from liquid to solid state.