Slag in Bulk

By S. Towns

Edited By C. Cherenfant

Spring 2025

Slag isn’t something most people picture when they think about the Port of Baltimore, but a surprising amount of it comes through the port every year. According to the ImportInfo trade database, the port received seven bulk shipments of slag in 2024. Maryland Port Administration records reflect that the Port of Baltimore received about 354,000 U.S. tons of slag that year, closely matching the delivery weight of the seven bulk shipments. A bulk shipment means the product wasn’t moved by container ships but by bulk carriers. A bulk carrier is a type of merchant ship specifically designed to transport unpackaged bulk cargo, such as coal, grain, and iron ore. They are known for their efficiency in moving large quantities of raw materials over long distances.

The image above reflects worldwide iron and steel production from 1940-2018.

(USGS, by Jgarabit. Wikimedia Commons, CC BY-SA 4.0)

Slag is a leftover material created during the steel and iron-making process. When impurities in metal ores like silica and alumina mix with materials such as limestone, they form melted slag that separates from the purified metal. When it cools, it hardens into a rocky material with useful qualities. There are different types of slag, including blast furnace slag, steel slag, and non-ferrous slags, and each type works best for certain industries.

The Port of Baltimore is an important entry point for slag imported from steel-producing countries like Germany, China, and South Korea. The United States makes some slag on its own, but imports help fill the growing need for strong and sustainable building materials. Baltimore’s deepwater docks and bulk terminals make it an attractive port for handling bulk cargoes. (Photo to left: Slag piles. Credit: USGS)

Slag has many practical uses.

In construction, it’s used as a tough, long-lasting material for building roads. Granulated blast furnace slag is also used in cement and concrete, helping them last longer and stand up to moisture, chemicals, and heat. In farming, slag can reduce soil acidity and add nutrients like calcium and phosphorus to help crops grow. Environmental engineers are even studying whether slag can help filter pollution or capture carbon dioxide. Granulated slag (Photo by Blast Furnace Chip Worker, Wikimedia Commons, CC BY-SA 3.0)

Each use of slag takes advantage of its strength, chemistry, and flexibility. In concrete, it reacts with water to form strong bonds that help prevent cracking. In roadbeds, its rough texture helps roads stay firm under heavy traffic. In farm fields, it helps fix worn-out soil by restoring its pH balance.

Photo to left: Molten slag being dumped as part of a manufacturing process. (Photo by Javier Rubilar, Wikimedia Commons, CC BY-SA 2.0)

By handling this often-overlooked material, the Port of Baltimore helps connect global steelmaking with America’s infrastructure and agriculture. Every ton of slag that moves through the port quietly supports the roads, bridges, and farms that people across the Mid-Atlantic rely on every day.