Technical guide · Pandaboard

Magnesium Oxide Board: Salt Blooming, Priming and Fixing

Direct answer

Magnesium oxide board is made from magnesium oxide, magnesium chloride and a glass fiber mesh, typically 1220×2440 mm and 3–20 mm thick, Class A1 non-combustible, strong and moisture resistant. Fix it to steel or timber framing: studs at 400–600 mm, furring at 400 mm, screws at 150–200 mm and 10–15 mm from edges, heads sunk 0.5–1 mm, joints 3–5 mm wide and taped. The main risk is salt blooming: unreacted magnesium chloride migrates to the surface as white crystals and lifts coatings and tile. Specify a low-chloride formula, seal faces, cut edges and screw heads with primer, and use galvanized or stainless screws.

Magnesium oxide board (MgO board) is a composite of magnesium oxide, magnesium chloride (or magnesium sulfate) and glass fiber mesh. It is Class A1 non-combustible, strong, moisture resistant, and can be sawn and nailed. Its greatest risk is not installation but salt blooming — after absorbing moisture the surface exudes white salt, weeps water and corrodes metal. This document makes salt-blooming prevention its main thread.

Salt blooming is a fatal defectOnce salt blooming occurs it is irreversible: the face whitens and powders, the coating peels off, and screws and framing corrode, so the ceiling or partition fails. There are only three controls: a low-chloride formulation, humidity control, and a sealed surface.

01Key specifications and suitability

Suitable

• Bathroom and kitchen partitions and ceilings

• Shaft and duct encasement

• Fire-rated partitions in malls and schools

• Exterior wall lining and facade backing

• Modular and prefabricated wall units

• Damp storage rooms

Not suitable

• Inside water tanks and pools

• Sealed spaces above 85% RH with no ventilation

• Exposed faces with no sealing system

• Concrete formwork used repeatedly

• 3–5 mm thin board that needs heavy on-site chasing

02Before Installation

2.1 Site conditions

• The framing system is complete and accepted;

• Relative humidity ≤75 %, temperature ≥5 ℃;

• Deliver boards 24 h in advance and store them flat indoors — never in the open;

• The ceiling void is well ventilated and free of persistent condensation.

2.2 Materials and tools

Table 2-1 — Materials and tools

Parts list and reference quantities：Per 100 m² of one face, studs at 400 mm; includes normal wastage.

2.3 Delivery and site conditioning

• Move boards indoors at least 24 h before installation and stack them flat on bearers at ≤600 mm centres so moisture content equalises with the room and shrinkage joints do not open later

• Use one batch and one shade per room or elevation; mixing batches shows as a visible colour difference

• Spot-check 5 % on arrival: flat face, intact edges, no blistering or scratches; reject boards with diagonal deviation over 2 mm

• Cut in a separate area, clear dust immediately and seal cut edges as described in this guide

03Installation Steps (Partitions and Ceilings)

Figure 1 — Framing and board build-up Studs @400/600, joints landing on studs; MgO board is hard and brittle, so screw spacing must be uniform.

1. Check the framing - Spacing 400 (8 mm board) or 600 (10 mm and above); galvanized coating intact, cut ends touched up.

2. Set out - Mark the joint lines, stagger by ≥300, and land joints on framing.

3. Pre-drill - Pre-drill 10–15 from the board edge to prevent breakout.

4. Fix - Self-drilling screws φ4×25 at 150–200 spacing, 10–15 from the board edge, heads sunk 0.5 mm — do not overdrive.

5. Leave gaps - Board to board 2–3 (MgO board expands less than gypsum board); board to structure 5–8.

6. Jointing - Fill with compound → apply alkali-resistant mesh tape → second coat to level → sand.

7. Seal - Apply 1–2 coats of alkali-resistant sealing primer to the surface, especially board edges and cuts — this is the key step against salt blooming.

8. Finish - Skim and paint, tile (with a proprietary bonding agent in wet areas) or apply fire-rated laminate.

   

Figure 2 — Joint treatment 2~3 joint, proprietary compound, alkali-resistant mesh tape; cut edges must be sealed with primer.

Never skip the sealing primerCut edges and board edges are the paths by which moisture enters and leaves. Unsealed cuts bloom first when humidity fluctuates: a ring of white salt appears around the cut and the coating then blisters. Seal every cut, drilled hole and board edge with primer.

04Salt Blooming Control Measures

Table 4-1 — Five measures

05Quality Acceptance

Table 5-1 — Permissible deviations

06Common Problems

Table 6-1 — Typical defects

07Storage and Handling

Figure 3 — Stacking methods Boards must be stored flat, dry and clear of the ground; once MgO board takes up moisture and blooms, the whole batch is scrap.

Table 7-1 — Storage conditions

Stock checks: inspect board edges and cuts for white salt once a month; if salt appears, quarantine the batch immediately and do not use it. Issue stock on a first-in-first-out basis.

08Transport and Safety

• Load flat with dunnage supports ≤600 apart, stack ≤1.2 m, protected from rain throughout;

• No open-air transfer and no rain exposure;

• Cutting dust is alkaline: wear a mask and goggles and avoid eye contact;

• Wash skin with clean water after contact with dust;

• Use a harness for work at height and never stand on the boards.

09Appendix: Checklist

9.1 Goods receiving

• □Size, thickness and density match the order

• □Faces flat, no white salt or signs of blooming

• □Edges and corners intact, no delamination

• □Packaging dry and undamaged

• □Class A1 and salt-blooming test reports supplied

9.2 Before installation

• □Ambient humidity ≤75 %

• □Framing galvanizing intact, cut ends touched up

• □Stainless screws available

• □Alkali-resistant sealing primer available

• □Boards stored flat and dry for 24 h

10Frequently asked questions

What is salt blooming and how do I spot it?

Unreacted magnesium chloride absorbs moisture, migrates to the surface and reacts with air into white crystals that feel powdery and redissolve in water. It lifts coatings and debonds tiles. Ventilate and dry, remove the crystals, then re-apply sealing primer.

How do I prevent salt blooming?

Do all three: specify a low-chloride formula that has been fully cured, keep site humidity at or below 80%, and apply two coats of alkali-resistant sealing primer to faces, cut edges and screw heads. One measure alone will still bloom in a humid season.

MgO board or calcium silicate — which should I choose?

MgO is stronger and lighter, works in thin sheets and shaped details, and suits duct boxing and modular units. Calcium silicate is more stable with far less blooming risk, so it suits large partitions and tile substrates. In sealed humid spaces, prefer calcium silicate.

Can tiles be fixed to MgO board?

Yes, with 8 mm board or thicker and framing closed to 400 mm, using a full-bed thin-set. The face must be dry and primed, and the adhesive should be low absorption so the bed does not stay wet and trigger blooming under the tile.

Do the screws rust?

Yes, ordinary carbon steel screws rust quickly in wet areas and the stain bleeds through. Use galvanized or stainless screws, sink the heads and touch each with anti-rust paint before jointing.

Can MgO board be used outdoors?

Yes, as a system: 8 mm or thicker board, protected framing, weatherproof sealant over a backer rod at joints, an exterior coating or fluorocarbon finish, and a drained, ventilated cavity behind. Avoid permanently wet or poorly ventilated locations.

About Pandaboard

Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA

We manufacture and export gypsum board, mineral fiber and fiberglass ceiling tiles, calcium silicate and magnesium oxide boards, fiber cement board, aluminum ceilings, WPC and SPC wall panels, exterior decking, fencing and handrails, together with the matching grid and framing systems. The methods on this page are compiled from product behaviour and standard trade practice; for any project, follow the approved drawings, local codes and the specification confirmed on your order.

Website: www.pandamanufacturer.com Contact sales: contactus

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Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA
This guide is technical information for specifiers and installers; it does not replace project drawings or local codes.