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It's part of the CALCIUM CARBONATE chain. Sometimes in our differing countries. Things are listed as not exactly as what they are...... Limestone based calcium's have natural Calcium Oxide content. They also contain Mag...
In EU countries, it's common to list Ca as CaO and Mg as MgO. They are not exactly what you think they are...
I supply these to help you understand.
This is from the greenhouse growers site.....
Limestone Nutrient Content
There are four types of carbonate-based limestone that are available. Calcite is pure calcium carbonate (CaCO3, 40 percent Ca). Calcitic lime is composed of mostly CaCO3 (> 30 percent Ca) with some MgCO3 (<5 percent Mg). Dolomitic lime contains less CaCO3 (<30 percent Ca) and more MgCO3 (>5 percent) than calcitic lime. Finally, dolomite also contains CaCO3 and MgCO3, but at a specific ratio of 22 percent Ca to 13 percent Mg.
Calcium hydroxide (Ca(OH)2), also known as hydrated lime, or calcium magnesium hydroxide (CaMg(OH)4), also known as dolomitic hydrated lime, are sometimes used as a fast reacting liming source because they will react quickly and leave no residual. Calcium hydroxide contains approximately 54 percent Ca, while calcium magnesium hydroxide contains about 30 percent Ca and 18 percent Mg.
Sometimes the calcium and magnesium content of the limestone is listed as CaO or MgO on the label. To convert CaO to actual Ca, multiply the CaO value by 0.71. To convert MgO to actual Mg, multiply the MgO value by 0.6.
The below is from
https://www.britannica.com/science/calcium#ref89814
Calcium oxide, CaO, also known as lime or more specifically quicklime, is a white or grayish white solid produced in large quantities by roasting calcium carbonate so as to drive off carbon dioxide. At room temperature, CaO will spontaneously absorb carbon dioxide from the atmosphere, reversing the reaction. It will also absorb water, converting itself into calcium hydroxide and releasing heat in the process. The bubbling that accompanies the reaction is the source of its name as “quick,” or living, lime. The reaction of quicklime with water is sometimes used in portable heat sources. One of the oldest known products of a chemical reaction, quicklime is used extensively as a building material. It is sometimes used directly as a fertilizer, although calcium carbonate is usually preferred for that purpose. Large quantities of quicklime are used in various industrial neutralization reactions. Limelights, used in the 19th century in stage lighting, emit a very brilliant white light upon heating a block of calcium oxide to incandescence in an oxyhydrogen flame, hence the expression “to be in the limelight.”
A large amount of calcium oxide also is used as starting material in the production of calcium carbide, CaC2, also known simply as carbide, or calcium acetylide. Colourless when pure (though technical grades are typically grayish brown), this solid decomposes in water, forming flammable acetylene gas and calcium hydroxide, Ca(OH)2. The decomposition reaction is used for the production of acetylene, which serves as an important fuel for welding torches. The drip of water on calcium carbide produces a steady stream of acetylene that is ignited in carbide lamps. Such lamps were commonly used in lighthouse beacons and by miners in the early 20th century and still find some use in spelunking. Calcium carbide also is used to make calcium cyanamide, CaCN2, a fertilizercomponent and starting material for certain plastic resins.
The average Joe grower, couldn't even keep CaO around the house in pure, powdered form. It would end up Ca Hydroxide - CaOH2