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The primary reason engineers and architects switch from standard MgO boards to MgO Sulfate Boards is specifically to eliminate the “sweating” (hygroscopic) and “leaching” (efflorescence) issues.

Here is the technical breakdown of why MgO Sulfate boards perform differently in high-humidity environments.

The Chemistry: Why it doesn’t “Sweat”

To understand why Sulfate boards don’t sweat, you have to look at what causes the problem in older versions:

• Standard MgO (Chloride-based): Uses Magnesium Chloride (MgCl2).This salt is hygroscopic, meaning it naturally attracts water molecules from the air.³ In high humidity, the salt pulls in so much water that it forms droplets on the surface—this is “sweating.”
• MgO Sulfate Board: Uses Magnesium Sulfate (MgSO4).This compound is not hygroscopic. It creates a stable crystalline structure that does not react to atmospheric moisture. Even at 90% humidity, the board remains dry to the touch.

The short answer is no. The primary reason engineers and architects switch from standard MgO boards to MgO Sulfate Boards is specifically to eliminate the “sweating” (hygroscopic) and “leaching” (efflorescence) issues.

Here is the technical breakdown of why MgO Sulfate boards perform differently in high-humidity environments.

1. The Chemistry: Why it doesn’t “Sweat”

To understand why Sulfate boards don’t sweat, you have to look at what causes the problem in older versions:

• Standard MgO (Chloride-based): Uses Magnesium Chloride (¹$MgCl_2$).² This salt is hygroscopic, meaning it naturally attracts water molecules from the air.³ In high humidity, the salt pulls in so much water that it forms droplets on the surface—this is “sweating.”
• MgO Sulfate Board: Uses Magnesium Sulfate (⁴$MgSO_4$).⁵ This compound is not hygroscopic. It creates a stable crystalline structure that does not react to atmospheric moisture. Even at 90% humidity, the board remains dry to the touch.

2. Leaching and Efflorescence

“Leaching” occurs when free-floating salts move to the surface of the board, leaving a white, powdery residue (efflorescence) or a sticky “tears” effect.

• Sulfate Stability: In a high-quality MgO Sulfate board, the chemical bond is more complete. Because there are no free chloride ions moving through the board’s capillaries, there is no “leaching” of corrosive liquids that could damage paint, wallpaper, or steel studs.
• Surface Integrity: Because it doesn’t leach, the adhesion of finishes (like tile adhesive, plaster, or paint) remains permanent. In chloride boards, leaching often causes paint to bubble and peel.

3. Protection of Metal Fasteners

A major side effect of “sweating” in chloride boards is the formation of a “brine” (salty water) that eats through galvanized screws and steel frames.

• Since MgO Sulfate boards do not sweat or leach, they are non-corrosive.
• You can safely use standard galvanized or stainless steel fasteners without the fear of them rusting away inside the wall.

• Comparison at 95% Relative Humidity