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The water absorption rate of a high-quality Magnesium Oxide (MgO) board after 24 hours of total immersion typically ranges between 8% and 15% by weight.

However, this value can fluctuate based on the board’s density and its chemical formulation (Chloride-based vs. Sulfate-based).

1. Performance by Board Density

MgO boards are categorized by their density, which directly correlates with their porosity and water resistance:

• Standard Density (800–900kg/m3): Absorption is usually on the higher end, around 12%–18%. These are often used for internal wall linings.
• High Density (1,000–1,300kg/m3): Absorption is significantly lower, typically 8%–12%. These are the industry standard for external facades, SIP panels, and sub-flooring.

2. The “Swelling” Factor (Thickness Change)

In technical submittals, the percentage of weight gain is often less critical than the Thickness Swelling Rate. This measures the dimensional stability of the board—how much it expands when wet.

• High-quality MgO board: Thickness swelling after 24 hours of immersion is usually < 0.1% to 0.5%.
• In comparison, traditional wood-based products like OSB or plywood can swell by 10% to 20% under the same conditions.

3. Chloride-free (Sulfate) vs. Chloride-based

The type of Magnesium salt used affects how the board interacts with moisture over the long term:

• Magnesium Sulfate Boards: These are preferred for high-humidity environments. They maintain better structural integrity during prolonged immersion and do not exhibit the “sweating” (hygroscopic) issues common in lower-grade boards.
• Hydrophobic Treatments: Many manufacturers apply a factory-sealed silane or siloxane coating to the “smooth” side of the board. This can temporarily drop the initial absorption rate to < 5%, though the core remains breathable.

4. Impact on Strength

A key advantage of MgO over gypsum or wood is its wet-strength retention:

• While the board absorbs a small amount of water, it does not lose its structural “bond.”
• Once the board dries out, it returns to its original dimensions and recovers nearly 100% of its original flexural strength (Bending Strength), whereas gypsum boards would turn to “mush” and OSB would permanently lose structural capacity.