Slag flowing from kettles

No longer a waste material, slag has found an important place in the building industry. (Copyright: graf_montekristo / 123RF Stock Photo)

Many people probably don’t know the One World Trade Center shares a characteristic with Roman Empire roads, railroads, and 18th century masonry work in Europe.  That characteristic goes by the unassuming name of “slag.”

Slag, the leftovers from the metal smelting process was a key component used as a base material in Roman roads. And while Germans made cannon balls from the stuff in the 1500s, it wasn’t until the 1800s that Europeans and Americans got serious about slag for building.  The Europeans used cast iron slag stones in masonry work, and about the same time, both Europeans and Americans also started using it for road building.  But for half a century slag in the U.S. was chiefly used as ballast for railroad tracks.

There was a lot of the stuff available. U.S. steel mills were turning out 20 million tons of it a year in 1918. One account placed the slag production in Mahoning Valley in Pennsylvania at 6 million tons just in 1920. Some of it went to commercial use, but the bulk of it was waste.

Eventually, people started chipping it up and grinding it into finer and finer forms until it was usable as an additive to cement. Today it is used to replace between 25% and 80% of the Portland cement in a  batch of concrete. Since preparing the slag for this secondary use requires less energy than making Portland cement, it reduces the embodied energy in the concrete.

And, that’s how slag comes to be in the new financial center in New York.

The One World Trade Center includes a massive cast-in-place, reinforced concrete inner core that runs the full height of the tower—an extra-strong backbone that provides support for gravitational loads as well as resistance to wind and seismic forces. The concrete core walls measure three feet thick or more above ground, and up to twice that below grade. Higher up, the concrete core walls slim down to two feet thick.

There were 200,000 cubic yards of concrete used in the tower’s superstructure. The supporting columns on the first 40 floors were made from 12,000 to 14,000-psi self-consolidating concrete. The upper floors used 8,600 to 10,000-psi mix designs. To meet the compressive strength requirements, the design and engineering team specified a highly specialized concrete mix that included Lafarge Portland Type I/II and NewCem® slag cements, along with other supplementary cementitious materials. The NewCem slag cement helps achieve greater strength potential and helps control shrinkage, creep and cracking in mass concrete structures.

The concrete also made a strong contribution to the sustainable design of the building, which is expected to receive LEED® Gold certification when it opens later this year.



Press Release supplied by Lafarge –



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