Carbonates are a very useful addition to numerous potting mixes for two main reasons.

a. Carbonates are alkaline. When properly applied, they correct and stabilize the pH of the soil, just enough to reach a non-corrosive equilibrium. They do not overcorrect under normal conditions. AND

b. Dissolve slowly and supply the soil with nutrients such as Calcium, Magnesium, Iron and Manganese, plus numerous trace elements.

From the numerous "Carbonates" known to man, three are used in horticulture.

Calcite (CaCO3): Improves the pH and ads Calcium to the mix.

Dolomite ((Ca-Mg)CO3): Improves the pH and ads calcium, magnesium and iron to the mix.

Magnesite (MgCO3): Improves the pH and ads Magnesium to the mix.

All the above are crushed and screened to sizes to suit or even pulverized.

Lime (CaO) and Limestone (CaCO3) are two completely different materials.

Lime is a compound. It is the result of “burning” of  Limestone in special kilns at temperatures above 825oC. The reaction is simple: CaCO3 (Limestone) --> CaO (Lime) + CO2 (Carbon Dioxide).

Lime (CaO) is very dangerous to use simply because when it comes in contact with water it reacts violently and temperature increases very quickly to over 150 degrees Celsius. Lime is used as a disinfectant in stables etc. When water is added, it hydrates and changes to a "Yogurt" like product.  This "Yogurt" like product, diluted, is used in horticulture to paint after a day or two tree trunks. It is alleged that the "paint" stops ants and other insects going up and down the tree and transferring pests like "Scales", etc. Also used to paint houses in summer.

Make sure when you buy "Lime" you actually buy "Limestone"