“If these ions are floating around, if they’re moving through honey instead of water, they’ll be significantly slowed down,” says Bentz. “The trick is to find the right chemical that will change the viscosity of the solution.”
The researchers took a cue from the food industry, which uses additives as thickeners in everything from salad dressings to carbonated drinks. Bentz searched for similar additives that would both increase the viscosity of the water solution found in concrete and slow ion diffusion; he even tried using food thickeners, including xanthum gum, which is used in sauces and ice cream.
After screening multiple additives in water solution in order to model the behavior of ions in concrete, the team found that those with a smaller molecular size were more successful at slowing the rate of ion diffusion. Additives that occur in small molecular chains, with branches of hydrogen and oxygen, were particularly good at increasing a solution’s viscosity. Bentz says this might be due to the fact that such hydrogen and oxygen branches can interact with water molecules to form a barrier against infiltrating ions, making it harder for them to penetrate.
The team also tested various additives within small cylinders of cement mortars–essentially, concrete without the aggregates. Bentz mixed the additives with cement, let the mortars dry, and placed each mortar into a chloride solution for up to one year. After removing the mortars from the solution, he and his team broke apart each mortar and analyzed how far chloride ions were able to penetrate. Compared with mortars without any additives, those with additives showed significant reduction in chloride diffusion.
However, the technique may not be quite ready for industry-scale application, mainly due to potential costs. Bentz says to get such results he had to make the additive as much as 10 percent of the cement solution. “The industry is comfortable with one percent, so there’s a cost factor, in that it’ll cost 10 percent more,” says Bentz. “We’ve demonstrated proof of concept, and now we would like to find an additive that works at 3 to 5 percent concentration.”
Jason Weiss, professor of civil engineering at Purdue University, works on improving concrete mixtures and increasing the material’s long-term performance. He says that such a technique may one day make bridges and roads less susceptible to corrosion. “This has an enormous potential,” says Weiss. “This would imply that a bridge that could last 30 years would now last 40 to 45 years under the same type of chemical attack.”