ARCHIVED - New concrete's little secrets

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July 12, 2008— Ottawa, Ontario

It may look dull and grey, but cement concrete is still the most widely used material in construction. And it has been attracting considerable scientific attention now that we face the costs of rebuilding aging bridges and other structures. A more durable concrete would save millions of dollars.

Dr. Laila Raki and her team at the NRC Institute for Research in Construction have discovered ways to make ordinary concrete made from Portland cement more flexible and workable. Through nanotechnology, they've found a key to preventing cracks in concrete as cement ages, a much-needed advance in Canada where extreme heat, cold and salt take their toll.

Since 2005, Dr. Raki has led a multidisciplinary team that aims to provide nanotechnology-based materials for the construction industry. The team has focused primarily on developing new cements, admixtures and concretes.

Canada's harsh climate can take a toll on our roads and buildings.
Canada's harsh climate can take a toll on our roads and buildings.

"Over the years, we've come to understand what happens when we change the mix design, add other materials, and slow down or speed up the chemical processes that create concrete," says Dr. Raki. "But before we could experiment with different admixtures and the timing of reactions, we had to understand what was happening at the molecular level."

Dr. Raki has focused on the hydration phase of cement because it essentially controls the micro and macro properties of concrete. The setting and hardening of cement is caused by water-containing compounds — or hydrates — that form through reactions between cement components and water. "The reaction produces calcium silicate hydrate," says Dr. Raki. "We wanted to examine how that hydrate interacts with various organic additives at the nanoscale to see how we might address the durability problems of concrete."

Her team has experimented with adding industrial byproducts — including fly ash and silica fume — a practice that mitigates carbon emissions by consuming less energy than the mining of new materials would require. These additives create a denser, more durable concrete, but some of them also slow down the hardening. So the researchers have looked at nanoparticle additives that offset the slower setting time while retaining the hardness.

"By adding nanoparticles to cement mixes that include fly ash, we've been able to speed up the hydration and hardening process, while filling in pores," says Dr. Raki. "This is important because cement pores are a route for salt and other chemicals to enter concrete and break it down. We've also greatly enhanced the durability of concrete by adding carbon nanotubes, which hinder crack propagation."

One of the construction industry's challenges is to control the timing of cement mixing and delivery to the worksite. If a cement mixer gets stuck in traffic, the cement will harden in the truck. To help manage this problem, Dr. Raki and her team have developed and patented a controlled-release "super-plasticizer" that can speed up or slow down the hydration (setting phase) of cement. Cement manufacturers will be able to time the hydration process more accurately so concrete will be used at the right time.

"A world-leading chemical company will soon be testing our formula with its own admixtures," says Dr. Raki. "If our formula works, we will just need a business partner to commercialize it."

Enquiries: Media relations
National Research Council of Canada

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