Using viscosity modifying admixture to reduce diffusion in cement-based materials: Effect of molecular mass

•VMAs is a promising alternative to reduce the permeability of cementitious materials.•The molecular mass range of VMAs is optimized to efficiently reduce ions diffusion.•It is microviscosity rather than macroviscosity of a solution affects ion diffusion.•The steric hindrance of the organic solute determines the solution microviscosity. The viscosity modifying admixtures (VMAs) can increase the pore solution viscosity of cement-based materials and thus slow down the diffusion of corrosive chemicals. As a key parameter, the increase of the molecular mass of VMAs can improve the anti-corrosion performance. However, if it becomes too large, the VMAs will significantly increase the bulk viscosity of the solution but do not improve the resistance to corrosive chemicals. Aiming at determining the range of molecular mass in which the resistance of cement-based materials can be optimized, the diffusion behavior of KCl in VMAs solutions with various molecular mass was explored. Then the influence of molecular mass of VMAs on chloride diffusion, mechanical properties, hydration and pore structure development of mortar were systematically investigated. The results indicated that within molecular mass range of 600–1500, VMAs can efficiently reduce the ion diffusion in cement-based materials. After 28 days of exposure, the samples containing 5% of polyethylene glycol (PEG) with the molecular mass of 1000 exhibits the minimum diffusion coefficient, which is 30.00% lower than that of the control group. The steric hindrance, which is mainly dependent on the molecular mass of the organic solute rather than its structure, directly determines the microviscosity of a solution. Although the addition of VMAs unfortunately coarsens the pore structure of mortar, and thus leads to the decrease of mortar strength, this study provides a novel approach to improve the durability of cementitious materials by optimizing the molecular mass of VMAs.