Magnesium Oxysulfate Board vs Oxychloride Board: Chemistry, Performance and Plant Choice

A magnesium oxysulfate board binds magnesium oxide with magnesium sulfate; an oxychloride board uses magnesium chloride. The oxychloride binder phases are strong, but in damp air the board can release chloride, absorb moisture, sweat brine and corrode steel. The oxysulfate board contains almost no chloride, but it needs reactive magnesium oxide and chemical modifiers to reach comparable strength.

Two Binders Behind “MgO Board”

“MgO board” is a family name, not a single product. Every MgO board is held together by a magnesia cement, formed when reactive magnesium oxide (MgO) reacts with a concentrated magnesium salt solution. Two salts are used in practice:

• Magnesium chloride (MgCl₂) gives magnesium oxychloride cement, often shortened to MOC. This is the traditional formulation. In China it is the binder of glass fiber and magnesium cement board, the product covered by building-materials industry standard JC 688.
• Magnesium sulfate (MgSO₄) gives magnesium oxysulfate cement, or MOS. Boards made with it are sold as sulfate MgO board or magnesium oxysulfate board.

The two boards can look identical. The difference sits in the crystals that form inside the binder, and those crystals decide how the board behaves in a humid wall. For a new plant, the binder fixes the raw materials, the modifiers and the markets the boards can be sold into.

Note that “3·1·8” means two different compounds depending on the salt. Ask any supplier which salt they mean.

Magnesium Oxychloride Board: Phase 5, Phase 3 and Free Chloride

In MOC, the two phases stable at ordinary temperatures are phase 5 and phase 3. By the reaction stoichiometry, phase 5 corresponds to MgO, MgCl₂ and water in a 5:1:13 ratio and phase 3 to 3:1:11 (Ahmad et al., 2024). The review notes that phase 5 starts to form about two hours after mixing and dominates when the paste cures at around 40 °C, while lower curing temperatures favor phase 3 and leave some MgO unreacted.

Phase 5 gives oxychloride board its hardness and bending strength. Its weaknesses come from the same chemistry:

1. Water attack. In water, magnesium chloride leaches out and phases 5 and 3 transform into loose brucite, Mg(OH)₂. The review cites a softening coefficient (wet strength divided by dry strength) of only 0.31 for plain MOC paste at 28 days (Ahmad et al., 2024).
2. Carbonation. In air, phase 5 slowly converts to phase 3 plus Mg(OH)₂, and further carbonation forms a chloride-bearing carbonate, Mg(OH)₂·MgCl₂·2MgCO₃·6H₂O. The board keeps changing after it leaves the factory.
3. Free chloride. MOC typically holds roughly 1.5–6% free chloride ions, and chloride breaks down the passive film on steel. The review concludes that MOC is unsuitable for steel reinforcement (Ahmad et al., 2024).

Free chloride is the root of brine sweating, or “tears.” Magnesium chloride is strongly hygroscopic; where it is not bound into phase 5, it draws water from humid air, and the brine runs onto studs, screws and timber.

Magnesium Oxysulfate Board: 3·1·8, 5·1·7 and Why Modifiers Matter

Plain magnesium oxysulfate cement made at room temperature is weaker than MOC. Without additives, the reaction gives the plate-like 3·1·8 phase plus brucite, and both the 3·1·8 and 1·1·5 (Mg(OH)₂·MgSO₄·5H₂O) phases are described as unstable at room temperature (Jin et al., 2026).

The change came from modifiers. Small amounts of weak acids slow the precipitation of Mg(OH)₂ and steer the reaction toward the needle-like 5·1·7 phase, which Jin et al. (2026) describe as the dominant strength-bearing component of modified MOS. Needles interlock and fill pores in a way plates do not. Cements built on this phase are often called basic magnesium sulfate cement.

Two consequences for a board producer:

• “Sulfate board” does not guarantee 5·1·7. When the Technical University of Denmark analyzed two commercial sulfate boards by X-ray diffraction, it found brucite, Mg₄(OH)₆SO₄·8H₂O (the 3·1·8 phase written another way) and, in one board, Mg₆(OH)₁₀SO₄·3H₂O (the 5·1·3 phase), not 5·1·7 (Wøhler Nielsen et al., 2019). Formula and process decide the phase, not the label.
• Heat limits. The 5·1·7 phase starts to lose its water at about 200 °C (Jin et al., 2026), one reason MgO boards are cured and dried gently.

Brine Sweating and Chloride Corrosion: What Independent Tests Show

Most of what buyers fear about MgO board comes from humid-climate failures of chloride boards. Several public test programs have compared the two binders.

Denmark (DTU, 2019). Researchers at the Technical University of Denmark tested two sulfate boards and one chloride board at 20 °C while stepping humidity from 85% to 95% RH over seven weeks. The chloride board, with about 21% chlorine by XRF, showed its first droplets at 89% RH after gaining 43% in mass; they held 44,123 mg/L of chloride, more than double the 20,000 mg/L the authors cite for Atlantic seawater, and mold followed at 93% RH. The sulfate boards held out until 95% RH, but by then had gained around 60% in mass. The conclusion is blunt: both types can absorb unacceptable amounts of water from humid air, and they did not consider MgSO₄ boards an alternative for Denmark and countries with a similar humid climate (Wøhler Nielsen et al., 2019).

New Zealand (BRANZ, 2023). BRANZ tested four boards. Two contained 0.04% and 0.073% chlorine and around 4–5% sulfur, consistent with an oxysulfate binder; two contained about 10% chlorine. After 90 days at 30 °C and 90% RH, stainless steel screws (grades 304 and 316) showed no corrosion in any board, and the two low-chloride boards caused no corrosion on any screw type. Non-stainless (plated and electroplated) screws corroded in the two chloride boards, mildly to severely depending on the screw type. In the same 90-day humidity exposure, all four boards met the PAS 670:2021 acceptance criterion for bending strength retention. Other tests were less favorable for every board, sulfate ones included: all four gained 27–39% in mass in a soak of just over 48 hours, and none met the warm-water soaking criterion (BRANZ, 2023).

Australia (Olofinjana et al., 2022). An Australian study found no chloride by EDS in new oxysulfate boards and water absorption comparable to fiber cement boards, which the authors attribute mainly to the absence of residual chloride.

Canada (CCMC, 2018). The Canadian Construction Materials Centre advised that non-evaluated MgO boards should not be used above 84% RH or in direct contact with metal other than stainless steel fasteners. The advisory addressed oxychloride boards; CCMC had not then evaluated oxysulfate boards (CCMC advisory, 2018).

The pattern is consistent: removing chloride removes the brine and the fastener corrosion, but it does not by itself guarantee that a board passes humidity or hot-water tests. Formula, MgO quality and curing decide that, so boards for humid markets should be tested to PAS 670, ICC 1125 or the buyer’s standard.

Strength and Water Resistance Compared

Oxychloride board’s advantage is mechanical. High-density chloride boards are somewhat tougher in bending than sulfate boards, which is why they are still preferred for heavy hanging loads. Our MgO board lines make sulfate boards, and we state that trade-off up front.

Water resistance is where well-made sulfate board pulls ahead. In one 2026 study, a paste modified with 0.5% citric acid and silica reached 78.8 MPa compressive and 18.9 MPa flexural strength at 28 days, and after 90 days in water it still measured 57.3 MPa, against 32.6 MPa for the study’s baseline mix (Jin et al., 2026). These are paste results, not board properties.

On our reference design, sulfate boards reach a softening coefficient of up to 0.9, meaning they keep about 90% of their strength after long immersion in water. Oxychloride can also be pushed into that range, but only with modifiers. The 2024 review lists 30% fly ash raising MOC’s softening coefficient to about 0.85, combinations of fly ash and silica fume reaching 0.95–1.0, and 0.5–1.0% soluble phosphates keeping retention above 0.8 after 60 days in water (Ahmad et al., 2024). The chloride binder becomes more water-resistant, but its free chloride remains.

Modifiers for Oxysulfate and Oxychloride Boards

Both binders need additives in real production. They do different jobs:

For magnesium oxysulfate board:
– Citric acid, boric acid and trisodium citrate. A study in the Journal of Materials in Civil Engineering tested each at 0.5%, 1.5% and 2.5% of MgO weight. All three improved strength and water resistance and cut drying shrinkage by promoting a needle-like basic magnesium sulfate phase. The best dosages were 0.5% citric acid, 2.5% trisodium citrate and 0.5% boric acid (JMCE, 2018).
– Tartaric acid, sodium malate and other weak-acid salts also favor 5·1·7 and suppress Mg(OH)₂ (Jin et al., 2026).
– Set control. Retarding acids lengthen working time; 1% boric acid delayed setting to 73 minutes in a study cited by Jin et al. (2026). The mixer, forming speed and curing schedule have to match the setting time the formula gives.

For magnesium oxychloride board:
– Phosphoric acid and soluble phosphates form insoluble phosphates that slow the breakdown of phase 5 in water; fly ash, silica fume, slag and metakaolin form gels and fill pores (Ahmad et al., 2024).

The modifier is part of the board formula. A supplier who cannot explain which modifier system a sulfate formula uses, and why, has not finished the process design.

Raw Materials: MgO Activity, Magnesium Sulfate and Magnesium Chloride

Magnesium oxide. Both boards use light-burned (caustic-calcined) magnesia. Calcination temperature controls reactivity: MgO burned at about 600–1000 °C is reactive, while hard-burned (1000–1500 °C) and dead-burned (1500–2000 °C) material reacts little or not at all. The 2024 review cites one-day MOC strengths of 5–7 MPa for MgO calcined at 600 °C against 25–32 MPa at 900 °C, and MgO calcined at 1500 °C did not set within 24 hours (Ahmad et al., 2024). On our reference design, the line buys light-burned MgO with 60–80% active MgO. Total MgO content alone is not enough to qualify a supplier: one magnesia in the 2026 study analyzed at 93.2% MgO but only 63.2% active MgO by the hydration method (Jin et al., 2026).

Magnesium sulfate. On our reference design the sulfate is made up as a 15–25% solution. Solution temperature needs watching: one US patent for a chloride-free board dissolves the sulfate in water warmed to about 40 °C, then cools the solution below about 35 °C and checks it for clarity before the MgO goes in (US11319249B2).

Magnesium chloride. Very soluble and very hygroscopic. That makes the solution easy to prepare, and it is also what lets any MgCl₂ left unbound in the board pull water from the air later.

Supply. Magnesite supply is concentrated. China mined 12.9 of the world’s 21.1 million tonnes in 2024, and Liaoning Province, its main producing region, cut its number of magnesite-mining enterprises from 114 in 2020 to 63 in 2024 (USGS, 2026). A plant should qualify more than one MgO source and test the activity of every lot.

Choosing Between Magnesium Oxysulfate and Oxychloride for a New Plant

For partitions, ceilings, linings and substrates, the case for magnesium oxysulfate board is strong. It removes the chloride behind brine sweating and fastener corrosion, and it is what our MgO board lines are built for. It does not by itself guarantee a pass in humidity tests, so for humid or coastal markets such as Southeast Asia, West Africa or the Middle East, plan the PAS 670 or ICC 1125 tests from the start. The decision does not end there:

1. Fix the target standard first. BSI PAS 670:2021 covers oxychloride and oxysulfate boards for partitions and internal and external linings, including performance in a humid environment; flooring, sarking and roofing are outside its scope. ICC 1125-2026 covers wall sheathing, tile backer, and floor and roof sheathing, with flexural strength, moisture movement, freeze–thaw and corrosion resistance requirements. These tests shape the formula.
2. Test your own magnesia and sulfate. Run trial boards from the MgO and MgSO₄ you can buy before the formula is fixed.
3. Test the board with the fixings. The BRANZ results show that the fastener grade matters. State the screws and studs in your installation guide.
4. Keep chloride boards for dry interiors. Uses that depend on bending toughness can justify a high-density chloride board, but not humid or external applications.

Field problems, from sweating to warping, are covered in MgO board problems; how the binder carries through mixing, forming and curing is set out in the MgO board manufacturing process.

FAQ

Is magnesium oxysulfate board chloride-free?

A properly made one contains almost no chloride. BRANZ measured 0.04% and 0.073% chlorine in two boards that appeared to use a sulfate binder, against about 10% in chloride boards, and an Australian study found no chloride by EDS in new oxysulfate boards. But the two sulfate boards DTU tested held about 2% chlorine by XRF, so check incoming MgO and magnesium sulfate for chloride.

Does magnesium oxysulfate board still absorb moisture?

Yes. In DTU’s 2019 humidity test, sulfate boards did not drip until 95% RH, against 89% RH for the chloride board, but they had gained around 60% in mass by then. In BRANZ tests, sulfate and chloride boards alike gained 27–39% in mass in a 48-hour soak. Formula, MgO quality and curing decide the final result.

What are the 5·1·7 and 5·1·8 phases?

They are the main strength-giving crystals in the two binders. The 5·1·7 phase, 5Mg(OH)₂·MgSO₄·7H₂O, is the needle-like phase in modified oxysulfate cement. The 5·1·8 phase, 5Mg(OH)₂·MgCl₂·8H₂O, also called phase 5, is the main phase in oxychloride cement. The numbers give the molar ratio of magnesium hydroxide, magnesium salt and water.

Which is stronger, oxysulfate or oxychloride MgO board?

High-density oxychloride board is somewhat tougher in bending, which keeps it in use for heavy hanging loads. Modified oxysulfate binders reach high strength in laboratory tests and keep far more of it in water: on our reference design, sulfate boards reach a softening coefficient of up to 0.9. Unmodified oxysulfate cement is weak, so the modifier system is essential.

Can an oxychloride board line be converted to sulfate board?

The mechanical equipment, from mixers to forming and curing, is broadly similar, so much of a line can be reused. What changes is the chemistry: salt dissolving and solution handling, the modifier dosing system, the MgO specification and the curing schedule, since sulfate boards harden more slowly. Trial boards from local raw materials should confirm the new formula before conversion.

Planning an MgO Board Plant?

Choosing the binder is the first decision; the raw materials, modifiers, curing area and test program follow from it. See how a sulfate-based MgO board production line is configured, and use our MgO board plant setup guide to plan the standard, raw material testing and site before you compare suppliers.

Sources

• Ahmad, F., Rawat, S., Zhang, Y. (2024). Magnesium Oxychloride Cement: Development, Opportunities and Challenges. Applied Sciences, 14(7), 3074 — https://www.mdpi.com/2076-3417/14/7/3074
• Jin, K., Wang, H., Xue, H., et al. (2026). Mechanistic insight of white carbon black in regulating hydration and improving water resistance of magnesium oxysulfate (MOS) cement. Journal of Building Engineering, 130, 116958 (author manuscript, Brunel University repository) — https://bura.brunel.ac.uk/bitstream/2438/33613/4/FullText.pdf
• Modification of magnesium oxysulfate cement by incorporating weak acids (2018). Journal of Materials in Civil Engineering, 30(9) (record, University of Technology Sydney) — https://opus.cloud1.lib.uts.edu.au/handle/10453/132237
• Wøhler Nielsen, S., Rode, C., Bunch-Nielsen, T., Kielsgaard Hansen, K., Kunther, W., Grelk, B. (2019). Properties of magnesium oxide boards used as sheathing in exterior walls. MATEC Web of Conferences, 282, 02091 — https://www.matec-conferences.org/articles/matecconf/pdf/2019/31/matecconf_cesbp2019_02091.pdf
• BRANZ (2023). Research Now: Materials #3, MgO boards — https://www.branz.co.nz/documents/4290/BRANZ_RESEARCH_now_Materials_3_MGO_final.pdf
• Olofinjana, A., Jays, N., Young, D., Mata, J., Haque, R. (2022). Hygroscopic Performance of Sulphate-Based MgO Construction Boards. Springer, LNNS 335 (record, University of the Sunshine Coast) — https://research.usc.edu.au/esploro/outputs/conferencePaper/Hygroscopic-Performance-of-Sulphate-Based-MgO-Construction/99629640502621
• CCMC MgO Board Advisory Update, March 2018 (published by the Saskatchewan Building Officials Association) — https://sboa.sk.ca/ccmc-mgo-board-advisory-update/
• BSI, PAS 670:2021, Magnesium oxide-based boards for use in buildings. Specification — https://knowledge.bsigroup.com/products/magnesium-oxide-based-boards-for-use-in-buildings-specification
• International Code Council, ICC 1125-2026, Standard for Specification of Magnesium Oxide Boards and Construction — https://shop.iccsafe.org/icc-1125-2026-standard-for-classification-of-magnesium-oxide-board-and-construction.html
• US11319249B2, Structural assembly board and method of manufacturing same, MgO Systems Ltd (Google Patents) — https://patents.google.com/patent/US11319249
• JC 688-2006, Glass fiber and magnesium cement board (standard record, chinesestandard.net) — https://www.chinesestandard.net/PDF/English.aspx/JC688-2006
• US Geological Survey, Mineral Commodity Summaries 2026: Magnesium Compounds — https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-magnesium-compounds.pdf