A practical guide to adding magnesium hydroxide to fire-resistant gypsum board for construction
——Analysis of key technologies to improve fire resistance and environmental protection
As the core material of building fire protection system, the performance of fire-resistant gypsum board is directly related to the escape time of personnel and fire loss control. Magnesium hydroxide (Mg(OH)₂) has become a key additive to improve the fire resistance limit of gypsum board due to its non-toxic smoke suppression and high-temperature decomposition and heat absorption.
1. Fire resistance mechanism and core advantages of magnesium hydroxide
Triple flame retardant mechanism
Heat absorption and cooling: decompose at 340–490℃ (heat absorption 1.3 kJ/g), delaying the temperature rise rate of gypsum board.
Gas phase dilution: release 18.6% mass fraction of water vapor, reduce oxygen concentration and dilute combustible gas.
Ceramic barrier: The generated MgO covers the surface of gypsum to form a dense insulation layer (thermal conductivity <0.5 W/m·K).
Environmental protection and safety characteristics
The decomposition products are only H₂O and MgO, without halogen, dioxin and other toxic substances, and meet the GB 8624-2012 A-level fire protection standard.
The combustion smoke density is reduced by more than 50% compared with traditional gypsum board (NBS smoke box test peak <400).
2. Key parameter design and selection guide
1. Index screening of magnesium hydroxide
Parameter Technical requirements Impact on gypsum board performance
Purity ≥93% Impurities reduce flame retardant efficiency, and the ignition loss needs to be controlled at 31-33%
Particle size D50=1-3μm (specific surface area>15 m²/g) Nano-scale particles fill pores and improve gypsum density
Morphology Hexagonal flakes or flower balls Flake structure enhances mechanical strength and reduces cracking risk
2. Addition amount and synergistic system
Basic formula: gypsum powder 70% + magnesium hydroxide 25-40% + reinforcing fiber 5-10%.
Compounding and enhancing efficiency:
Compounded with aluminum hydroxide at a ratio of 2:1, the total addition amount is reduced to 30%, and the peak heat release rate is reduced by 28%.
Adding 2% zinc borate promotes gypsum carbonization and increases the fire resistance limit to 120 minutes.
III. Process optimization and dispersion technology
Necessity of surface modification
Unmodified magnesium hydroxide is easy to agglomerate, resulting in a 15-20% decrease in the flexural strength of gypsum board. Recommended process:
Silane coupling agent treatment: KH-570 builds molecular bridges and increases interfacial bonding by 300%.
Stearic acid coating: Oil absorption value is reduced to 33.4%, and fluidity is improved.
Key points of mixing and molding
Dry mixing stage: Premix the modified magnesium hydroxide and gypsum powder in a high-speed mixer (speed ≥800 rpm, time ≥5 min).
Wet mixing control: water-paste ratio 0.6-0.7, temperature <40℃ to prevent Mg(OH)₂ from pre-decomposition.
Pressing molding: pressure 8–10 MPa, holding time 30 seconds, avoid delamination.
IV. Performance verification and industry application cases
Test standards and results
Fire resistance limit: GB/T 9978.1 standard test, gypsum board with 35% modified magnesium hydroxide added for 105 minutes (40% higher than the benchmark board).
Smoke and toxicity control: EN 13823 smoke density peak <375, CO generation <100 ppm.
Typical engineering applications
Fireproof partition walls of high-rise buildings: Shanghai Tower uses composite system gypsum board and passed LEED Platinum certification
.
Subway tunnel lining: Xiongan subway project verification, hydrogen cyanide concentration in fire is stable <5 ppm.
V. Common problems and solutions
Problem 1: Gypsum board strength decreases after addition
→ Solution: Use zirconium-modified spherical magnesium hydroxide (particle size D50=1.5μm), and the flexural strength increases by 8%.
Problem 2: Partial yellowing of the board surface
→ Solution: Control the heavy metal content (Pb < 0.1%, Cd < 0.01%), and perform XRF screening before feeding.
The application of magnesium hydroxide in fire-resistant gypsum board has been upgraded from a simple flame retardant to a synergistic optimizer of safety, environmental protection and mechanical properties