June 3, 2005

Pump up your concrete strength with slag

Looking for a way to improve the quality of concrete, increase strengths, enhance workability, upgrade pumpability, and make a sustainable product even greener? Think slag.

Slag cement, a post-industrial by-product of the iron-manufacturing process, is a carbonless cementitious product that has no effect on entrained air.

Growing numbers of concrete producers are building or dedicating silos to add slag cement to their product lines. A dedicated cement silo allows them to offer a variety of Portland slag and even ternary blends — a capability that lets them win more contracts, work more efficiently, improve customer satisfaction, and set themselves apart from traditional competitors.

Some customers prefer slag cement blends because of their enhanced strength and durability. Others like their placing and finishing characteristics. Some choose slag for aesthetic reasons: The light color of certain blends allows decorative finishers to achieve a more brilliant color.

Strong sales

Slag cement sales are at an all-time high and growing. A record 3.5 million metric tons of slag cement was shipped for use in concrete and construction applications in 2004, a 15.5 percent increase over 2002, according to the Slag Cement Association. Shipments of Portland cement grew at 5.5 percent in the same period. Since 1996, slag cement production has more than tripled, growing faster than any other cementitious material.

"Slag cement is the fastest-growing part of our cement business," says Greg Daderko of Lafarge North America, a supplier based in Herndon, Va. "We have made significant investments to increase our capacity, and we think its popularity will continue to grow."

In 2004, Lafarge installed a slag dryer, a key component to making slag cement, at its Seattle cement plant. Using the same mills the company grinds cement in, it is able to produce Grade 120 slag cement.

This and other facilities are keeping supplies plentiful and costs competitive, even as shipping shortages and overseas demand are putting a crimp in Portland cement supplies in North America.

Sustainability issues are also spurring more interest in slag cement. Concrete, arguably one of the most sustainable building materials available, enables one to design a building that can last hundreds of years, reduce energy costs, decrease heat island effects, use locally manufactured and harvested materials, create virtually zero waste, be recycled and use post-industrial recycled materials.

One benefit of using slag cement is that, as a post-industrial by-product, it reduces the embodied energy and emissions in concrete and can help achieve LEED certification credits from the U.S. Green Building Council.

Local producers

Cowden Ready-Mix, a family-owned company based in Bellingham, has been using slag cement since August 2004 from two plants. The mix is used for residential, commercial and paving projects.

"Slag cement enables us to differentiate from our competition and give our customers more choices and a superior product," says Steve Cowden, owner of Cowden Ready Mix.

Cowden was recently awarded the concrete on a LEED-certified project that requested 30 percent replacement on all of its concrete.

"Due to the improved strengths achieved in mixes using slag, we use 30 percent in non-LEED projects. Slag cement eliminates a lot of the risk in high replacement mixes typically seen with fly ash," says Cowden.

Stanwood Redi-Mix also began using slag cement in its mix designs in August 2004.

"Initially we tried slag cement because it offered a solution that alleviated the low strengths and fluctuating air percentages the industry was seeing in fly ash mixes," says Ken Christoferson, owner of Stanwood Redi-Mix.

"The more familiar we became with using slag cement the more we came to realize the terrific strength benefits it provides to concrete," says Christoferson.

Miles Sand & Gravel, a second-generation producer serving heavy and light commercial, residential and paving clientele, earlier this year began using slag cement. Miles has used slag cement to improve its high-performance concrete offerings.

"We try to stay on top of new innovative ways of producing a higher quality and superior finishing concrete," says Jerry Trudeau, vice president and general manager of Miles Sand & Gravel. "The ready-mix industry has used slag cement out East for over 20 years and when Lafarge introduced it in Seattle we decided to take a look at it."

Miles did extensive testing on the slag cement and was pleased by its 28-day strength improvements.

"Slag cement isn't like fly ash, the strength gains are remarkable," Trudeau says. "We have done the necessary testing and can recommend optimal replacement levels based on the application."

Slag cement replacement levels can vary from 10 to 85 percent (mass concrete pours), based on the concrete application. It's always recommended to consult with your ready-mix producer first.

Miles recently supplied upwards to 60,000 cubic yards of concrete for an Interstate 5 project, replacing 25 percent of the Portland cement with slag. "We have enjoyed significant gains in flexural strength in slag cement concrete over typical straight Portland cement mixes," says Trudeau.

Miles Sand & Gravel is involved in the green building movement. "Please tell me, what building material is more durable and more sustainable than concrete," says Trudeau. "Sometimes these attributes are forgotten because concrete has been around for thousands of years. What products like slag enable us to do is make a green product even greener."

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