What is the primary mechanism by which silica fume improves the durability of concrete against chloride ingress?

Silica fume, a very fine powder composed primarily of amorphous silicon dioxide, improves concrete's resistance to chloride ingress primarily by two key mechanisms: pore refinement and pozzolanic reaction. Pore refinement refers to the reduction in the average size and connectivity of pores within the concrete matrix. Silica fume particles, being much smaller than cement particles, fill the spaces between cement grains, creating a denser and less permeable microstructure. This finer pore structure makes it more difficult for chloride ions to penetrate the concrete. The pozzolanic reaction is a chemical reaction between silica fume and calcium hydroxide, a byproduct of cement hydration. Calcium hydroxide (CH) is relatively soluble and can be leached out of the concrete over time, weakening the structure and increasing its permeability. Silica fume reacts with the CH to form calcium silicate hydrate (C-S-H), the main binding component in concrete. This secondary C-S-H gel produced by the pozzolanic reaction further densifies the microstructure, reduces the amount of CH available for leaching, and consumes CH that might otherwise react with aggressive substances. The combination of pore refinement and the pozzolanic reaction significantly reduces the rate at which chloride ions can penetrate the concrete, thus protecting the reinforcing steel from corrosion, which is initiated by chloride exposure. For example, in marine environments where chloride exposure is high, silica fume concrete exhibits substantially lower chloride diffusion coefficients compared to ordinary Portland cement concrete, indicating superior durability.