Portland Cement: Its History, Challenges, and Brimstone’s On-Spec Solution

Brimstone’s Senior Manager of Products, Will Nguyen, PhD, PE, details how Brimstone is transforming the portland cement manufacturing process by using carbon-free, globally abundant rocks.

Portland cement is a powder that can be mixed with water and aggregates to make the ubiquitous material known as concrete. Civilization depends on concrete for its ease of production, formation into any shape, structural load-bearing capacity, and longevity.

Many properties of concrete can affect the construction and serviceability of a structure, including the time the concrete is workable before it begins to set and stiffen, its ultimate strength, and how durable it is in different environments.

Considering the vast amounts of concrete produced every day— concrete is the second-most consumed material in the world after water—it’s crucial that builders work with concrete that behaves predictably and consistently. Because cement highly impacts the properties of concrete, it’s not surprising that nearly all concrete is produced with the same kind of cement: portland cement.

A brief history of portland cement

Portland cement was invented 200 years ago when Joseph Aspdin, an English bricklayer, heated limestone and clay together to make a material that could make a hardened product after mixing with water. Because the hardened product resembled portland stone found on the Isle of Portland off the southern coast of England, the material was named portland cement.

Today, roughly 4.4 billion tons of cement are produced globally each year, with 88 million produced in the U.S. Although manufacturing technology has improved since the invention of portland cement, several key aspects remain unchanged. Raw materials–called raw meal–are heated in a rotary kiln, forming a pyro-processed material called clinker. Clinker is the main component of portland cement, making up about 85% to 95% of cement by mass. Clinker consists largely of calcium, followed by silicon, aluminum, and iron.

Heating raw meal in a kiln is necessary to produce high-quality clinker. During heating, calcium, silicon, aluminum and iron elements will combine and form different crystalline compounds in the clinker. Alite is one of these desirable compounds and typically makes up 50% or more of clinker by mass but requires pyro-processing at temperatures up to 1450 °C to properly form.

Limestone and process emissions in cement production

Another aspect of modern cement production that has remained unchanged from its invention is the use of limestone as a raw feedstock. Limestone is a rock mostly composed of calcium carbonate (CaCO3) and serves as a primary source of calcium for clinker.

When placed into a kiln and heated to temperatures of roughly 800 °C, the calcined limestone will decompose into lime (CaO) and carbon dioxide (CO2):

CaCO3 → CaO + CO2

Although producing lime from limestone through calcination is well-understood and has been practiced in some form since even ancient Roman times, it comes with a major environmental drawback: the formation of carbon dioxide gases. These so-called process emissions account for 60% of total carbon emissions from portland cement production, which is responsible for about 8% of global anthropogenic carbon emissions.

Even if the remaining processes of cement production were decarbonized, including emissions related to kiln heating and operating other equipment, process emissions from limestone are unavoidable and present a major blocker to full cement decarbonization when using conventional raw materials.

Viable alternatives to limestone

Considering the raw calcium-rich materials available on the Earth’s surface, three kinds of rocks are present: carbonates, evaporites, and silicates. As previously mentioned, carbonates like limestone will emit significant CO2 when calcined to make portland cement. Evaporites like anhydrite, that is, calcium sulfate, were used in the early to mid-1900s to produce sulfuric acid as a primary product and portland cement as a secondary product. This anhydrite process became uneconomical in the 1970s with changing sulfur prices and is no longer practiced.

Silicates have not yet been used to produce clinker on an industrial scale. However, calcium-rich silicates, such as basalt, provide a compelling starting point for decarbonized clinker production because they: 1) don’t contain embedded carbon that is emitted during processing; 2) are globally abundant at the Earth’s surface; and 3) have the potential for co-production of materials that are economically important today.

Brimstone’s solution

At Brimstone, we’ve developed a process to extract the necessary calcium for clinker from calcium-rich silicates while eliminating all the process emissions related to limestone calcination. The extracted calcium is refined into lime, pyro-processed, and results in clinker with the same chemical compounds found in the same clinker used today. This clinker is then milled and blended with additives to produce a final cement powder. Brimstone not only makes portland cement, but also co-produces supplementary cementitious material (SCM), a key ingredient in concrete mixes, and alumina, which is a key ingredient in aluminum production. The Rock Refinery will break down common rocks into decarbonized value-added products.

In the United States and other countries, portland cement conforms to the requirements of a standard specification known as ASTM C150. Properties such as chemical composition, compressive strength, and setting time are determined and compared against limit values. In 2023, Brimstone proved that its portland cement made from calcium-rich silicates met the testing requirements of ASTM C150 by working with a third-party, accredited testing laboratory. This means that Brimstone’s portland cement has no barriers in concrete applications where portland cement is already specified and used. Brimstone’s portland cement is designed to produce concrete with fresh and hardened properties that are typical of concrete currently used today but with significantly reduced global warming potential.

Portland cement is a critical ingredient for making high-performance concrete but needs decarbonizing. As Brimstone has shown, by changing the source of calcium to calcium-rich silicates, carbon dioxide emissions related to limestone calcination can be avoided entirely while still producing a portland cement that meets ASTM C150 specifications and industry expectations.

Will Nguyen, PhD, PE is the Senior Manager of Products at Brimstone.