High-calcium limestone, in its native rock state, has the chemical composition CaCO3, calcium carbonate. CO2 (carbon dioxide) is driven out of the limestone in the process of firing the rock creating CaO, calcium oxide (also called "quicklime" or "lump lime"). Dolomitic limestone, on the other hand, has the formula CaMgCO3, and when fired ends up as CaMgO, losing a CO2 molecule but retaining both the calcium and magnesium. When either of these fired limestones, or quicklime, is recombined with water (a process referred to as slaking), an exothermic reaction takes place that converts the calcium oxide into calcium hydroxide (Ca (OH)2), or hydrated lime. The main difference is that the high calcium variety reacts several times more quickly than the high magnesium sample. When this calcium hydroxide, commonly known as lime putty, is mixed with sand and applied onto a wall surface as a plaster, another chemical reaction takes place that essentially reintroduces carbon dioxide (CO2) back into the lime;over time it will harden and revert back to limestone. This process is known as carbonation and, just as the two types of limestone react differently when slaked, they also recarbonate at different rates, the high calcium variety being the fastest.
Unfortunately, quicklime is not readily available to the owner/builder without a lot of investigative searching. The hazards associated with the slaking of quicklime also act as a deterrent to the first time do-it-yourselfer. As a result, the most readily available building lime in the US is the pre-bagged variety, commonly referred to as Type S-Hydrated Lime.
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