What Is Slag In Construction?

What Is Slag In Construction?

Slag is a by-product of smelting ores and used metals, which can be classified as ferrous (by-products of iron and steel processing), ferroalloy (a by-product of ferroalloy production), or non-ferrous/base metals (by-products of recovering non-ferrous materials like copper, nickel, zinc, and phosphorus).

These slags can be further categorized by their precursor and processing conditions, such as blast furnace slag, air-cooled blast furnace slag, basic oxygen furnace slag, and electric arc furnace slag.

Despite recycling and upcycling efforts, slag production has significantly increased due to high demand, with the World Steel Association estimating that 600 kg of by-products, mostly slags, are generated per tonne of steel produced.

Slags are typically a mixture of metal oxides and silicon dioxide, but can also contain metal sulfides and elemental metals.

The major components of slags include oxides of calcium, magnesium, silicon, iron, and aluminum, with smaller amounts of manganese, phosphorus, and others depending on the raw materials used. Slags can also be classified based on the abundance of iron among other major components.

Types Of Slags

Slag is a byproduct of various industrial processes, and there are three main types: ferrous, ferroalloy, and non-ferrous.

Ferrous slag is created during the iron and steelmaking processes, and its properties can vary based on the stage of production and the rate of cooling.

For example, air-cooled slags tend to be more crystalline and denser, making them useful as an aggregate, while water-quenched slags have greater amorphous phases and latent hydraulic properties, similar to Portland cement.

The major phases of ferrous slag include calcium-rich olivine-group silicates and melilite-group silicates. However, trace elements such as arsenic, iron, and manganese can accumulate in groundwater and surface water to levels that exceed environmental guidelines.

On the other hand, non-ferrous slag is produced from non-ferrous metals of natural ores, and it can be characterized as copper, lead, and zinc slags due to the ores’ compositions.

Non-ferrous slag has more potential to impact the environment negatively than ferrous slag, as it can contain higher concentrations of heavy metals such as cadmium and lead.

For example, copper slag, the waste product of smelting copper ores, was studied in an abandoned mine in California, USA, and samples collected from the reservoir showed higher concentrations of cadmium and lead that exceeded regulatory guidelines.

In conclusion, slag is an industrial byproduct that can have varying properties based on the type and the production process. Ferrous slag is mainly composed of calcium and silicon, whereas non-ferrous slag can be broken down into copper, lead, and zinc slag.

Ferrous slag can have environmental impacts such as the accumulation of trace elements in groundwater and surface water, while non-ferrous slag can have more severe environmental impacts due to the presence of heavy metals.

Applications Of Slag

Slags are a byproduct of the smelting process, which involves heating a metal ore to separate the metal from its impurities. Slags can serve various purposes, such as controlling the temperature of the smelting process, preventing re-oxidation of the metal, and even being a valuable product in some smelting processes.

In the past, slags were used to make glassware, jewelry, and ceramics. Today, slags have a wide range of modern uses in construction, wastewater treatment, agriculture, and emerging applications such as CO2 capture and storage.

In the construction industry, slags have been used for centuries in the form of ground granulated blast furnace slags (GGBFS) which are used in combination with Portland cement (PC) to create “slag cement.” GGBFS reacts with portlandite (Ca(OH)2), which is formed during cement hydration, to produce cementitious properties that contribute to the later strength gain of concrete, leading to concrete with reduced permeability and better durability.

Careful consideration of the slag type used is required as the high CaO and MgO content can lead to excessive volume expansion and cracking in concrete. These hydraulic properties have also been used for soil stabilization in road and railroad constructions. Slag is also used as aggregate in asphalt concrete for paving roads.

In wastewater treatment, slags can be used to precipitate out metals, sulfates, and excess nutrients (nitrogen and phosphorus). Similarly, ferrous slags have been used as soil conditioners to rebalance soil pH and fertilizers as sources of calcium and magnesium.

Because of the slowly released phosphate content in phosphorus-containing slag, and because of its liming effect, it is valued as fertilizer in gardens and farms in steel-making areas.

In recent times, slags have been studied for their potential use in CO2 capture and storage (CCS) methods, such as direct aqueous sequestration, and dry gas-solid carbonation among others.

Slags have one of the highest carbonation potentials among industrial alkaline waste due to their high CaO and MgO content. However, slags’ high physical and chemical variability can result in performance and yield inconsistencies.

Hence, some scientists have proposed performing a series of experiments to test the reactivity of a specific slag material or utilizing the topological constraint theory (TCT) to account for its complex chemical network.

Effects Of Slag On Environment

Slags, which are byproducts of metal smelting and refining processes, can have negative impacts on the environment if not properly disposed of.

They are often transported to “slag dumps,” where they are exposed to weathering and can potentially leach toxic elements and hyper alkaline runoffs into the soil and water, endangering the local ecological communities. Leaching concerns are typically around non-ferrous or base metal slags, which tend to have higher concentrations of toxic elements. However, ferrous and ferroalloy slags may also have these elements, which raises concerns about highly weathered slag dumps and upcycled materials.

The dissolution of slags can produce highly alkaline groundwater with pH values above 12. The calcium silicates in slags react with water to produce calcium hydroxide ions that lead to a higher concentration of hydroxide in groundwater.

This alkalinity promotes the mineralization of dissolved CO2 to produce calcite, which can accumulate to as thick as 20 cm. This can also lead to the dissolution of other metals in slag, such as iron, manganese, nickel, and molybdenum, which become insoluble in water and mobile as particulate matter.

Fine slags and slag dust generated from milling slags to be recycled into the smelting process or upcycled in a different industry can be carried by the wind, affecting a larger ecosystem. They can be ingested, posing a direct health risk to the communities near the plant, mines, and disposal sites.

According to the 2019 International Energy Agency (IEA) report, the iron and steel industry directly contributed 2.6 Gt to global CO2 emissions and accounted for 7% of global energy demand.

The IEA report and the WSA project a continual increase in demand in the coming years, prompting greater efforts to move toward a circular economy.