Understanding MgO Purity Grades
Magnesium oxide (MgO) is not a single, uniform material — it comes in a range of purity grades that directly determine where and how it can be used. Whether you're sourcing MgO for a construction board, a refractory lining, or an agricultural application, understanding the difference between a 55% grade and a 90%+ grade is essential to making the right choice.
The percentage figure refers to the magnesium oxide content by mass in the material. The remainder is made up of impurities including calcium oxide (CaO), silica (SiO₂), iron oxide (Fe₂O₃), and other trace compounds. Higher purity generally means better performance in demanding environments — but it also comes at a higher cost.
The Main MgO Grade Tiers
55% MgO Grade
This is one of the lowest commercial grades, sometimes called low-grade calcined magnesite. It is commonly used in:
- Agricultural soil treatment and pH correction
- Animal feed additives (where trace minerals are acceptable)
- Basic filler applications
- Certain cement and board manufacturing formulations where purity requirements are relaxed
At this grade, the high level of impurities means it's unsuitable for precision industrial or refractory uses, but its lower cost makes it attractive for bulk agricultural or low-spec construction applications.
66% MgO Grade
The 66% grade bridges the gap between agricultural and light industrial uses. It is frequently found in:
- MgO board manufacturing (standard building boards)
- Basic refractory products
- Environmental remediation projects
- Waste water treatment
Many commodity MgO construction boards use a 66–75% MgO content binder. This grade offers a reasonable balance of performance and cost for non-critical structural panels.
85% MgO Grade
At 85% purity, MgO enters the territory of mid-grade industrial and construction applications. Common uses include:
- Higher-performance construction boards and fire-resistant panels
- Refractory bricks and castables for moderate-temperature furnaces
- Rubber and plastics manufacturing additives
- Chemical processing applications
The reduced impurity load improves mechanical strength, temperature resistance, and dimensional stability — all critical for demanding build environments.
90%+ MgO Grade (High Purity)
Grades above 90% — and especially those reaching 95–98% — represent high-purity or fused magnesia. These grades are essential where performance cannot be compromised:
- Steel industry refractory linings (electric arc furnaces, converters)
- Premium fire-resistant and insulation boards
- Pharmaceutical and food-grade applications
- Electronics and specialty ceramics
- High-temperature kiln furniture
Grade Comparison at a Glance
| Grade | MgO Content | Typical Uses | Relative Cost |
|---|---|---|---|
| Low Grade | ~55% | Agriculture, basic fillers | Low |
| Standard Grade | ~66% | Standard boards, remediation | Moderate |
| Mid Grade | ~85% | Fire panels, industrial uses | Moderate–High |
| High Purity | 90%+ | Refractories, pharma, steel | High |
How to Choose the Right Grade
When selecting an MgO grade, ask yourself these key questions:
- What is the operating temperature? Higher temperatures demand higher purity.
- Are there regulatory or certification requirements? Food, pharma, and certain fire ratings require documented purity.
- What are the mechanical strength requirements? Higher purity generally improves density and compressive strength.
- What is the budget? Don't over-specify — if 66% meets your needs, paying for 90%+ adds unnecessary cost.
Always request a Certificate of Analysis (CoA) from your supplier to verify the actual MgO content and impurity profile before committing to a purchase.