Chloride diffusion and chloride binding capacity of cellulose nanocrystal modified cement pastes

Penetration of chloride ions is one of the main factors that lead to the rusting of steel reinforcement within concrete. This study investigates the effects of cellulose nanocrystal (CNC) on the chloride ion erosion resistance of cement pastes by examining the chloride diffusion behavior, the chloride binding capacity, and the bound chloride desorption behavior under different pH environments. It is found that for cement pastes with a water-to-cement ratio of 0.3, the addition of 0.1 % CNC can reduce the chloride ion penetration depth and the free chloride ion content, with the chloride ion diffusion coefficient reduced by approximately 35 %; for cement pastes with a water-to-cement ratio of 0.5, the impact of CNC on chloride ion penetration depth is not significant, but it decreases the free chloride ion content. When the CNC dosage increases to 0.2 %, the chloride ion diffusion coefficient can be reduced by approximately 40 %. By analyzing the chloride ion diffusion coefficient and the surface chloride ion concentration, a chloride diffusion model for CNC-modified cement pastes is established. Additionally, the use of CNC can increase the chloride binding capacity and inhibit the bound chloride desorption of cement pastes. Specifically, the addition of 0.1 % CNC increases the 28-day physically and chemically bound chloride ion contents by approximately 58.71 % and 29.54 %, respectively. The desorption of bound chloride ions from cement pastes varies under different pH conditions. At pH= 12 and pH= 9, the addition of 0.1 % CNC reduces the 28-day desorption rate from 33.13 % to 21.49 % and from 80.21 % to 43.00 %, respectively.