What is MgO Board Made Of?

If you’re involved in construction, renovation, or are simply a homeowner researching durable, sustainable building materials, you’ve likely come across the term “MgO board.” Touted as a versatile and superior alternative to traditional drywall, cement board, and even plywood, its popularity is on the rise. But what exactly is it that gives MgO board its impressive set of properties? The answer lies in its unique composition.

This article will deconstruct MgO board, examining each of its core ingredients, explaining how they work together, and demystifying why this material is becoming a go-to choice for builders seeking a fire-resistant, mold-resistant, and environmentally friendly building solution.

   

The Core Components: Breaking Down the Acronym

The name “MgO board” is derived from its primary binding agent: Magnesium Oxide, often abbreviated as MgO. However, it’s a composite material, meaning it’s made from a mixture of several components that, when combined, create a product greater than the sum of its parts.

A typical MgO board is composed of a precise blend of the following ingredients:

Component	Primary Function	Key Property it Imparts
Magnesium Oxide (MgO)	Binder / Matrix	Fire resistance, structural integrity, high pH
Magnesium Chloride (MgCl₂) or Magnesium Sulfate (MgSO₄)	Reactant / Solvent	Catalyzes the reaction, forms the binding cement
Reinforcing Fibers	Strength & Flexibility	Crack resistance, impact resistance, tensile strength
Fillers (e.g., Perlite, Sawdust)	Bulking Agent	Lightweight properties, thermal insulation, cost control
Water (H₂O)	Solvent & Reactant	Facilitates the chemical reaction during curing

   

Magnesium Oxide (MgO): The Fire-Resistant Heart

Magnesium Oxide is the star of the show. It’s a white, powdery mineral compound derived from naturally occurring magnesite ore (MgCO₃) or extracted from seawater or brine.

Sourcing: The magnesite is calcined (heated at high temperatures) to drive off carbon dioxide, leaving behind reactive magnesium oxide, often called caustic calcined magnesia. The quality and reactivity of the MgO are critical to the final board’s performance. High-purity sources are essential for creating a strong, durable board.

Key Role: When mixed with a magnesium salt solution, the MgO particles undergo a fascinating chemical reaction, forming a cementitious bond that is incredibly hard and stable. This magnesium oxychloride or oxysulfate cement is what gives the board its core structure.

Why it Matters: Magnesium oxide itself is completely non-combustible. It does not burn, melt, or release toxic fumes when exposed to fire, making it the fundamental source of the board’s exceptional fire-resistant qualities.

   

Magnesium Chloride (MgCl₂) or Sulfate (MgSO₄): The Catalyzing Partner

You can’t have the reaction without the reactant. MgO powder on its own is inert; it needs a liquid catalyst to activate the cement-forming process. This is where magnesium chloride (often derived from seawater or ancient seabed deposits like China’s Qinghai-Tibet Plateau) or magnesium sulfate (Epsom salts) comes in.

The Reaction: When mixed with water, these magnesium salts create a solution that reacts with the MgO powder. This reaction forms a gel-like paste that eventually hardens into a solid, rock-like matrix that binds all other components together.

The Delicate Balance: The ratio of MgO to the magnesium salt is absolutely critical. Historically, some early MgO boards suffered from issues like efflorescence (white, powdery deposits) and “curing sweat” due to an imbalance of leftover unreacted salts. Modern, high-quality manufacturing has largely solved this by using precisely controlled formulas and highly reactive MgO, ensuring a complete reaction and a stable final product.

Reinforcing Fibers: The Backbone of Strength

While the magnesium cement is hard, it can also be brittle. To add flexibility, tensile strength (resistance to pulling forces), and impact resistance, reinforcing fibers are added to the mix. This is what prevents the board from cracking easily during installation or from stress.

Types of Fibers: The most common reinforcing material is fiberglass mesh, embedded within the board, much like rebar in concrete. Some manufacturers also use organic fibers or specialized cellulose fibers. The type, length, and quantity of these fibers directly influence the board’s flexibility and screw-holding strength.

   

Fillers: Lightweight and Insulating Agents

Fillers are added to the slurry for several reasons: to reduce the overall density of the board, to improve its thermal insulation properties, and to optimize material costs.

Common Fillers: Perlite, a volcanic glass that expands when heated, is a very popular filler. It is lightweight, inert, and provides slight insulating value. Wood chips or sawdust from sustainable sources are also sometimes used as a low-cost, organic filler. These fillers are dispersed evenly throughout the magnesium matrix.

   

Water: The Essential Solvent

Water is the medium that brings everything together. It dissolves the magnesium chloride or sulfate salts to create the reacting solution and allows the MgO powder and other components to be mixed into a smooth, workable slurry. This slurry is then formed into sheets and pressed. Most of the water is consumed in the chemical reaction or evaporates during the extended curing process.

   

   

The Manufacturing Process: From Slurry to Sheet

Understanding the ingredients is half the story; how they are combined is the other.

Mixing: The precise amounts of MgO powder, fillers, and reinforcing fibers are dry-mixed. Separately, the magnesium salts are dissolved in water.

Slurry Formation: The liquid solution is combined with the dry ingredients in a high-shear mixer to form a homogeneous, thick slurry.

Forming: This slurry is poured onto a moving conveyor belt, typically onto a lower layer of fiberglass mesh. Another layer of mesh is laid on top, and the slurry is squeezed between rollers to form a sheet of consistent thickness.

Curing: This is the most critical phase. The formed sheets are left to cure for several days under controlled temperature and humidity. This allows the chemical reaction to complete fully, resulting in a hard, stable board. Proper curing is essential to prevent the aforementioned issues with efflorescence.

Drying, Trimming, and Finishing: After curing, the boards are dried in ovens to remove any remaining moisture, trimmed to precise sizes, and can be sanded or have their edges shaped.

   

Why This Composition Makes MgO Board a Superior Choice

The specific blend of minerals and fibers results in a material with a standout set of properties:

Unmatched Fire Resistance: The mineral-based composition means it is classified as a non-combustible building material (often achieving a Class A fire rating). It can typically withstand direct flames for over two hours without failing.

Exceptional Resistance to Mold and Mildew: The high pH (alkaline) environment of the magnesium cement is inherently hostile to biological growth. Unlike paper-faced drywall or wood, mold and mildew have nothing to feed on and cannot thrive, making it ideal for high-humidity areas.

Durability and Strength: The resulting board is hard, dense, and impact-resistant. It boasts excellent moisture resistance, holding up far better than drywall in damp conditions and not disintegrating like cement board can over time.

Environmental Friendliness: MgO board is often highlighted as a sustainable building material. Its production requires less energy than Portland cement or gypsum board. It is free from asbestos, fiberglass (aside from the optional mesh), and other harmful VOCs (Volatile Organic Compounds), contributing to better indoor air quality.

   

Applications and Considerations

Thanks to its properties, MgO board is used as:

Exterior sheathing underneath siding or stucco.

Interior wall and ceiling panels in place of drywall.

Substrate for tile in bathrooms, showers, and kitchens.

Underlayment for floors and roofs.

Fire-rated assemblies for walls, doors, and partitions.

When selecting an MgO board, it’s crucial to source it from a reputable manufacturer who uses a balanced formula and proper curing techniques to ensure long-term performance.

   

Conclusion

So, what is MgO board made of? It’s a sophisticated composite material engineered from a reactive magnesium oxide cement, reinforced with fibers, and modified with lightweight fillers. This specific mineral-based composition is the direct source of its remarkable fireproof, water-resistant, and rot-proof characteristics. Moving beyond traditional materials, MgO board offers a durable, versatile, and eco-friendly solution for modern construction challenges, proving that the future of building may indeed be founded on a bedrock of magnesium.