Magnesium Oxide vs Magnesium Hydroxide: Key Differences

2026-09-19 16:57:45

They share a metal, a white powder appearance, and a spot on the same supplier catalog page - yet Magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) behave so differently in process that substituting one for the other can stop a line. This guide gives buyers and formulators a clear, practical comparison: the chemistry, how each is made, a property table, and a decision map by application.

The Quick Answer

·         Magnesium oxide (MgO) is the *reactive alkaline* workhorse: acid acceptor, neutralization alkali, precipitation agent, sintering raw material.

·         Magnesium hydroxide (Mg(OH)2) is the *stable, mild and heat-responsive* workhorse: halogen-free flame retardant, gentle neutralizer, wastewater treatment alkali.

One is a sponge for acids; the other is a heat-triggered water releaser. Most buying mistakes come from forgetting which job you are hiring for.

The Chemistry and the Conversion

The two are related by a single reversible reaction:

MgO + H2O -> Mg(OH)2 (hydration), and Mg(OH)2 -> MgO + H2O (thermal decomposition above ~330 degrees C).

In water, MgO actively hydrates and consumes - it wants to react. Mg(OH)2 is already the reacted, low-solubility end state - it sits quietly until heated or until an acid arrives. This single difference explains nearly every application assignment below.

How Each Is Made

·         MgO (light/active grades): controlled calcination of magnesium carbonate or hydroxide at relatively low temperatures, producing a fine, high-surface-area, reactive powder. Higher calcination temperature = denser "dead-burned" magnesia with low activity for refractory duty.

·         Mg(OH)2: produced by hydrating MgO or by chemical precipitation from brines and salt-lake magnesium salts; synthetic routes deliver high purity and controlled hexagonal-plate morphology for flame retardant duty.

Wanfeng produces both families from mineral and salt-lake brine routes - see the high purity MgO and high purity Mg(OH)2 raw powder product pages.

Property Comparison Table

Application Mapping: Which One Does the Job

A Five-Question Decision Guide

1. Does the process need a chemical reaction from the additive? (neutralization, precipitation, thickening) -> MgO.

2. Does the additive need to survive processing and act only when heated? (flame retardancy) -> Mg(OH)2.

3. Is overdosing risk a concern in water treatment? -> Mg(OH)2, self-buffering.

4. Is the medium acidic and halogenated? (rubber with HCl evolution) -> light active MgO.

5. Is the operating temperature above 200 degrees C in a polymer? -> Mg(OH)2 stays stable past 330; MgO is stable but does not contribute fire protection.

Can You Substitute One for the Other?

Rarely one-for-one. MgO in a flame retardant formulation adds alkaline reactivity and moisture uptake but no endothermic water release; Mg(OH)2 in a neutralization duty reacts only slowly because of its low solubility. Where a conversion is feasible, it is a formulation change - not a drop-in swap - and should be piloted.

FAQ

Which is more reactive, MgO or Mg(OH)2?

MgO. It actively hydrates and neutralizes acids; Mg(OH)2 is the low-solubility product of that same reaction and behaves as a mild, buffered alkali.

Does MgO turn into Mg(OH)2 during storage?

Partially, if exposed to humidity - and that conversion consumes the activity you paid for. Keep active MgO sealed and dry.

Which is safer to handle?

Both are non-toxic powders, but active MgO is moisture-reactive and mildly irritating to skin and eyes, requiring sealed storage; Mg(OH)2 is gentler in handling.

We need a flame retardant - which one?

For polymers processed above ~200 degrees C, magnesium hydroxide. For epoxy, PU and low-temperature resins, Aluminum Hydroxide (ATH) is the usual economic choice. See our MDH and ATH-vs-MDH guides.

Talk to a Supplier Who Makes Both

Choosing between MgO and Mg(OH)2 is easier when one technical team produces both families. Wanfeng covers nine product series and 150+ grades, exporting to 30+ countries. Contact us for a recommendation matched to your process.