(See also Synthesizing amorphous magnesium silicate.)
Suppose you want reactive magnesia in order to make, for example, Sorel cement for X-ray 3-D printing (see Can you 3-D print Sorel cement by inhibiting setting with X-rays?), or exothermic refractory magnesium phosphate cement (see SHS of magnesium phosphate) , but it isn’t sold locally, but other soluble magnesium salts are available. What can you do?
Well, calcining the carbonate or the hydroxide produces magnesia at only 350°, and according to WP:
Magnesium carbonate can be prepared in laboratory by reaction between any soluble magnesium salt and sodium bicarbonate... If magnesium chloride (or sulfate) is treated with aqueous sodium carbonate, a precipitate of basic magnesium carbonate—a hydrated complex of magnesium carbonate and magnesium hydroxide—rather than magnesium carbonate itself is formed
So magnesium oxide can be made with a soluble carbonate or bicarbonate (such as baking soda) plus the citrate, the chloride, or the sulfate of magnesium, thus forming the carbonate of magnesium (possibly, or possibly the bicarbonate), and just a bit of heat. Though WP also says that if you don’t heat it to at least 700° it’s so reactive that it will recarbonate from the air.
An alternative alkali-free approach is taken in Kiwami Japan’s “sharpest Seawater kitchen knife in the world”. They begin by boiling down the filtered seawater twice to crystallize some salts (presumably sodium chloride?) which are filtered out, leaving a solution of mostly magnesium chloride (solubility 72.6 g/100mℓ at 100°, twice NaCl’s 36 g/100mℓ).
Then they calcine seashells in a microwave-oven kiln made of insulating firebrick, using charcoal as the susceptor rather than (or perhaps in addition to) the more conventional silicon carbide, for 15' at 1000 W, reaching a bright yellow heat, twice. This yields pure white quicklime seashells, which are ground to a granulometry around 100 μm using a mortar and pestle and a meat grinder. This heats from 25° to 65° upon hydration with water from a cow-shaped pitcher. A little limewater (1.5 g/ℓ slaked lime) is decanted and filtered off.
Adding some limewater to concentrated seawater (containing a significant amount of magnesium chloride) precipitates some magnesium hydroxide (solubility 0.00064 g/100mℓ) over the next 30', which is filtered off, along with some sodium chloride. This is washed with water from another cow-shaped pitcher to remove the salt, dried, and ground to get dry magnesium hydroxide, which is then moistened with magnesium chloride solution to produce a weak Sorel cement (11 Shore D). Calcining the hydroxide in the microwave-oven kiln (with charcoal as susceptor, 15', 1000W, heating to a yellow heat measured as 970° with a pyrometer) yields the oxide. Moistening the oxide with the chloride solution yields a much harder Sorel cement (102-109 Shore D, but I think that may be as high as his meter goes).
An open rectangular mold was filled with this cement and vibrated with a Handy Massager to remove bubbles; when set, the demolded rectangle was cut with an abrasive wire saw into the rough shape of a kitchen knife. While wearing rubber gloves equipped with reptilian claws, a gavel was used to drive a scrap of Sorel cement through the top of an Altoids tin, damaging the face of the gavel. The knife blank was then ground with a series of files and abrasives to form the knife edge, which then successfully cut a cucumber extruded from the reptilian glove.
David Reid’s experiments with “Microwave melting of metals” found that magnetite works well as a susceptor for microwave-oven kilns, more efficiently than graphite, up to 900° (though its Curie point is only 580°, and you would expect it to become transparent to microwaves above that temperature). He reports that the magnetite tends to flux the sand and melt it, though. His use of uninsulated crucibles explains the need for more-efficient susceptors; Kiwami Japan’s use of a few centimeters of insulating refractory ceramic avoids this problem. Previously Reid had used ceramic-fiber furnaces.
Some other recent work has used graphene paint as a susceptor.