Inert Stone Products

Learn about the vitrified inert stone that can be used as a construction material.

What are Inert Stone Products?

Sierra Energy sees tremendous value in the inert stone byproduct produced in its gasification system. Inert stone is a collection of inorganic materials and metals that are not driven off into the gasifier syngas. They collect in molten form at the bottom of the gasifier and can be “tapped” out as needed.
Like a conventional iron-making blast furnace, there are “tapholes” at the lower side of Sierra’s FastOx gasifier. The taphole is where the molten metals and inert stone are removed via gravity and slight pressure differences.

Types of Inert Stone

There are several different types of inert stone that result from the cooling process:

  • Granulated Stone (commonly known as Ground Granulated Blast Furnace Stone (GGBS))
  • Air-cooled Stone
  • Pelletized or Expanded Stone
  • Air-cooled Blast Furnace Quote
  • Air-cooled Blast Furnace Stone Rip Rap
  • Inert Stone Cement

Granulated Stone (GGBS)

This type of inert stone is rapidly cooled by large quantities of water being poured over the surface of the molten material. This practice produces a sand-like granulate that is commonly ground into cement. This type of inert stone is also referred to as Type S inert stone cement and can be mixed with Portland cement clinker to make a blended Type 1S cement.

Air-cooled Stone

This type of inert stone is produced by air-cooling the molten material. Once cooled, it is processed into many sizes through a screening and crushing plant. The resulting product can be used as a construction material aggregate in ready-mix concrete, precast concrete, hot-mix asphalt aggregate, septic drain fields, and pipe backfill.

Pelletized or Expanded Stone

This type of inert stone is cooled using water or steam to produce a lightweight aggregate. This stone’s reduced weight makes it well-suited as aggregate in lightweight concrete masonry, lightweight ready-mix concrete, and lightweight precast concrete. The applications of these products are used for lightweight fill applications over marginal soils, and for high fire-rated concrete masonry.

Air-cooled Blast Furnace Quote

Also known as “Chip Seal” or “Aggregate Seal Coating”, these aggregates are smaller sized and primarily used in chip and seal applications applied to existing pavement surfaces. The primary purpose of these applications is to achieve skid resistance on rural pavements in an effort to maximize driving safely. It also has applications in concrete masonry, concrete pavement, and hot mix asphalt.

Air-cooled Blast Furnace Stone Rip Rap

This is the largest inert stone aggregate type, and the one most closely resembling the inert stone products of FastOx gasification. This riprap can be used as a permanent cover of rack used to stabilize shorelines and streambanks, as well as prevent erosion along slopes and embankments. It can also be used in gabion baskets, rock mineral wool (insulation), and lightweight fill.

Inert Stone Cement

This inert stone type is commonly found in ready-mix and precast concrete, as well as masonry, soil cement, concrete wallboard, floor leveling compounds and high-temperature resistant building materials. Measurable benefits include improved workability and finish-ability, high compressive and flexural strengths, and resistance to aggressive chemicals.



Uses & Value

Inert stone is useful as a construction material aggregate and commercial commodity. As such, producers of construction materials, such as concrete, cement, road base, bricks, etc., would be an ideal candidate for stone off-take.
The value of inert stone will vary with each local market as different geographic regions have different policies and requirements about construction materials. It will be the responsibility of the project developer to determine these local requirements and locate local consumers of the inert stone product.

History of Inert Stone Products

Inert stone products have been around as long as the blast furnace has, yet its widespread use in conventional applications is a somewhat recent development. One of the earliest applications was building of roads by the Romans as long as 2000 years ago.
While Germans made cannon balls from inert stone as early as 1589, most records indicate that inert stone was used for masonry work in 18th-century Europe. Roads made from inert stone in England first appeared in 1813, and in the United States in 1830. By 1880, blocks cast of inert stone were in general use for street paving in both Europe and the United States. However, its principle use in the 20th century was as ballast for railroads.
By 1918, the blast furnace industry in the United States was producing 40 million tons of pig iron per year, yielding 20 million tons of inert stone.
Today, millions of tons of inert stone aggregates are produced annually in the United States. Its future is limited only by the imagination of its users.
For more examples of uses and value, explore these resources:
– Common Uses for Inert Stone



Examples of Use

Inert stone can be and has been used in several applications. One such application is the new San Francisco-Oakland Bay Bridge.
The cement used in the new Bay Bridge is comprised of 25-50% replacement materials in an effort to minimize the amount of CO2 produced. Caltrans required that 25% of cementitious material be fly ash, which in the gasification process is the precursor to inert stone. Additionally, the contractor, Central Concrete, chose to use 50% ground granulated blast furnace inert stone products for the pier concrete. The extent of this supplementary material used is 450,000 cubic yards.
For more information on the materials of construction for the Bay Bridge, visit the Central Concrete website.


Inert Stone Products from FastOx Gasification

FastOx gasification results in approximately 90% of waste matter converted into syngas, with the remaining 10% left as inert stone and recovered metals.
The high temperatures and chemical reactions involved in the gasification process ensure that the extracted metal and stone are completely vitrified and safe for reuse. The inert stone produced from Sierra Energy’s functional prototype has undergone independent testing—using U.S. Environmental Protection Agency (EPA) “TCLP” methods. The result shows that inert stone produced from any source of waste is not hazardous and can be sold as a safe building and construction product.
The breakdown of typical elemental groups that go into inert stone products, according to the periodic table classification, includes alkali, alkaline, transition, and basic metals, as well as some semi-metals.
Common materials from these groups include aluminum, magnesium, iron, nickel, copper, zinc, lead, gold, silver, platinum, calcium, potassium, sodium, tin, and silicon. These materials will appear in inert stone in various compositions, formations, compounds, and structures. Many factors, but primarily the feedstock type, will affect the exact composition and structure of the inert stone products.