Most bricks are made primarily of clay and sand, fired at high temperatures to create a hard, durable building block. But “brick” is a broad term, and the exact ingredients depend on the type. Clay bricks, concrete bricks, mud bricks, and fly ash bricks each use different raw materials and different processes to hold together.
What’s Inside a Standard Clay Brick
The classic red brick you see on most buildings is a mix of naturally occurring minerals, dominated by two ingredients: silica (sand) at 50% to 60% by mass, and alumina (clay) at 20% to 30%. The remaining fraction includes 2% to 5% lime, up to 7% iron oxide, and less than 1% magnesia. These aren’t measured and blended like a recipe. They’re simply the minerals already present in the clay deposits that brick manufacturers mine from the earth.
Raw clay is shaped into bricks using one of three methods: soft mud molding, dry pressing, or extrusion (pushing clay through a die like a pasta machine, then cutting it into blocks). After shaping, the bricks are fired in a kiln at temperatures between 1,000°C and 1,200°C. That intense heat triggers chemical changes in the clay minerals, fusing the particles together into a solid ceramic that won’t dissolve in water or crumble under load.
Why Bricks Come in Different Colors
Iron oxide is the ingredient responsible for the familiar red color. During firing, iron compounds in the clay oxidize and produce that warm reddish hue. The more iron present, the deeper the red. When impurities mix with the iron, the color shifts toward orange.
Pure clay with no iron or other transition metals fires white. This type of clay is called kaolin, and it’s the same material used in fine porcelain. Manganese, which sometimes appears naturally in clay deposits, produces dark brown or black tones. Brick manufacturers can also add synthetic iron oxide pigments or industrial byproducts to shift the color intentionally, adjusting the amount to control the final shade. So the wide range of brick colors you see, from pale cream to deep charcoal, comes down to which metals are in the clay and how much oxygen reaches them during firing.
Concrete Bricks: A Different Formula
Concrete bricks look similar to clay bricks but are made from completely different materials. They’re a mixture of Portland cement (the binder), sand, aggregate (crushed stone or gravel), and water. In an efficient production facility, cement typically makes up less than 16% of the brick by mass. Aggregate takes up the majority of the volume.
The key difference is that concrete bricks don’t need a kiln. Instead of being fired at over 1,000°C, they harden through a chemical reaction between cement and water that happens at room temperature. This makes them cheaper to produce since manufacturers avoid the fuel costs and infrastructure of high-temperature kilns. The tradeoff is that Portland cement requires significant processing and energy to manufacture on its own, so the environmental cost shifts rather than disappears.
After mixing, the wet concrete is pressed into molds and cured, sometimes with steam to speed the process. The result is a dense, uniform block that performs well structurally but has a different texture and appearance than fired clay.
Mud Bricks: The Oldest Building Material
Long before kilns existed, people made bricks from mud and dried them in the sun. These adobe or mud bricks are simply a mixture of clay-rich soil and water, shaped by hand or in wooden molds and left to harden in the heat. On their own, sun-dried mud bricks are fragile and vulnerable to water damage, so builders have added stabilizers for thousands of years.
Straw is the most iconic additive. There’s evidence of straw-reinforced mud bricks dating back to biblical and Roman times. The fibers work like rebar in concrete, holding the brick together and reducing cracking as it dries. Animal hair, animal manure, and various plant fibers have also been used historically, depending on what was locally available. For stronger results, lime and cement are two of the most effective stabilizers, both with deep historical roots. More experimental options include gypsum, volcanic pumite, fly ash, and even crushed coconut shells.
Fly Ash Bricks: Made From Industrial Waste
Fly ash is the fine powder left over when coal is burned in power plants. Rather than sending it to landfills, manufacturers press it into bricks. A typical fly ash brick contains 60% to 80% fly ash, 10% to 20% lime, about 10% gypsum, and about 10% sand. These ingredients are fed into a mixer with water, pressed into shape, then dried and water-cured in the open air for about 21 days.
Like concrete bricks, fly ash bricks skip the kiln entirely. The lime and gypsum react with the fly ash over time to form a hard, stable bond. The result is a lightweight brick that repurposes an industrial byproduct, reducing both waste and the demand for mined clay. They’re especially common in regions with large coal power industries that generate abundant fly ash.
How the Type Affects Performance
Each brick type has practical differences that matter for construction. Fired clay bricks are extremely durable, naturally moisture-resistant, and hold up well over centuries. Many historic buildings still standing after hundreds of years are built with fired clay. Concrete bricks offer good strength at lower cost but can be more porous and may not age as gracefully. Mud bricks work best in hot, dry climates where rain exposure is minimal. Fly ash bricks are lighter than clay bricks, which makes them easier to work with, and their smooth, uniform surface requires less plastering.
Color consistency also varies. Clay bricks can shift in color from batch to batch depending on the mineral content of each clay deposit and conditions inside the kiln. Concrete and fly ash bricks tend to be more uniform since their ingredients are measured and controlled during manufacturing.