Colloidal silica combats corrosion of reinforcing steel

Chlorides are a major cause of reinforcing steel corrosion, the primary reason for the failure of reinforced concrete structures. They penetrate the passive layer around the rebar inside the concrete. This passive layer is a film created by the high alkalinity of concrete through which oxygen cannot penetrate. Chloride ions increase the solubility of the passive layer, causing it to give way at a threshold concentration. The corrosion action then no longer needs chlorides to progress. This action is typically self-sustaining, requiring only the rebar, water and oxygen to continue. After this occurs, the pore structure of the concrete and the electrical resistivity that they provide become the primary focus.

Carl White, Managing Director of Spraylock Africa, says that colloidal silica post-placement pozzolan technology is a one-time application solution for combatting corrosion of concrete reinforcing steel. “It achieves this in several ways. Firstly, colloidal silica post-placement pozzolan technology can significantly deaccelerate the transmission of chlorides through the concrete. This potentially extends the corrosion initiation period by years compared to untreated concrete. Secondly, it waterproofs concrete. This reduces access of the corrosion reaction to water and significantly restricts oxygen availability. Moreover, colloidal silica post-placement pozzolan technology can increase the electrical resistivity of the concrete to levels where corrosion is unlikely to occur,” White says. Spraylock Africa is the official African representative for Spraylock Concrete Protection’s cutting-edge SCP products, which are based on colloidal silica post-placement pozzolan technology.

SCP products can, therefore, provide an advantage over conventional corrosion inhibitors. This is by affecting the initiation period and removing some of the water and oxygen that is needed for corrosion to continue. Typically consisting of calcium nitrite, corrosion inhibitors are promoted to chemically alter the corrosion process. Laboratory studies have shown that corrosion inhibitors only extend the corrosion initiation period. Just a few of these products provide a short delay in corrosion initiation. Moreover, once corrosion has started, the effectiveness of corrosion inhibitors is less significant than previously reported.

Considering colloidal silica post-placement pozzolan technology’s extremely small particle size, it has a tremendous amount of pozzolanic potential. The pozzolanic reaction of colloidal silica post-placement pozzolan technology is greater than even that of undensified silica fume. This pozzolanic reaction takes place in the capillary voids and pore space in concrete. In this way, colloidal silica post-placement pozzolan technology fills the void space with calcium-silicate hydrate (C-S-H). C-S-H is the same reaction product that provides concrete with its strength and durability traits. This action also restricts movement of water through concrete even under hydrostatic pressure. SCP, therefore, considerably reduces water-borne contaminant ingress, including chloride transport.

The performance of colloidal silica post-placement pozzolan technology has been tested in chloride bulk diffusion situations in conventional concrete by three United States-based laboratories.

White explains that chloride bulk diffusion involves ponding salt water on top of concrete to test chloride concentrations at various depths according to AASHTO T-259 or ASTM C1543. “This information is used to derive an average diffusion coefficient (D) that can be used to calculate the amount of time that it will take for chlorides to reach reinforcing steel and penetrate the passive layer around it. These predictions are modelled with software, such as Life 365. If the diffusion coefficient can be reduced by years, time can be added to the period it takes for chlorides to reach the rebar. This is the way in which colloidal silica post-placement pozzolan significantly extends a structure’s life expectancy,” White concludes.