Calcium (Hydroxide and Carbonate) and the Lime Cycle

When learning about Group 2 elements a few days ago, an odd thought struck me:

Why don't we see more products made with calcium metal? 

To this, I had one theory as to why - calcium is particularly reactive, which is why you're more likely to know of products or landforms containing calcium compounds, such as chalk. As a result, you wouldn't want calcium-lined pipes because who knows what could happen to them. React calcium with water and calcium hydroxide (Ca(OH)₂) forms - a highly alkaline and corrosive substance, with a pH not too dissimilar to that of bleach. So you wouldn't want to put those two items next to each other if you're trying to keep something in good condition. (Indeed, all calcium compounds are alkaline, so they're commonly used for treating acidic products, such as in food production and crops, as well as when treating water for drinking.)

Calcium is in fact near the top of the reactivity series - a simple way to compare just how reactive certain metals are compared to each other, as well as carbon and hydrogen for good measure. Indeed, only the alkali metals, barium and strontium are higher up than calcium - and it is thus hard to produce calcium metal. You'd need to electrolyse a calcium compound solution to do so, but this method's only been around for about 200 years - and should you do this, you'd then need to separate calcium (if electrolysis occurs in an aqueous solution) as it won't form on the electrodes (hydrogen would instead since it's less reactive). It's therefore not too surprising that native metals such as gold and platinum are often used - they're not very reactive, and you can easily extract them in large doses.

Where calcium metal seems to shine most is in steelmaking, where it is used to treat steel and rid it of sulphur and oxygen - as it is so reactive, CaS and CaO will form, which can eventually be separated from the steel. Calcium plays a similar role in treating lead (fifth paragraph) by reacting with bismuth, which can then be removed. Other than that, however, it's not commonly used as it's so reactive. In much the same way, potassium and sodium metals aren't commonly used in manufacturing, and whilst the more reactive lithium is often used in battery production, it's typically involving the more stable Li⁺ ions.

The following isn't intended to be comprehensive - merely some kind of overview.

So you'll more typically see calcium as part of a compound. Calcium carbonate (CaCO₃) is one such example, often identifiable as a white powder. It comes in various crystal forms, such as calcite, aragonite and vaterite (which vary by their lattice structure), and you may have seen some as part of a cliff or any similar coastal landform, such as Durdle Door (as in photo) in Devon. This coastal arch is made of limestone, which mostly consists of CaCO₃, specifically as calcite and aragonite. As CaCO₃ is alkaline and thus very soluble, it is more quickly weathered by the sea than harder rock (such as granite), which can cause these coastlines to become jagged, such as along headlands and bays. Other cliffs such as Beachy Head are composed of chalk, a form of limestone which can also be used to produce chalk for writing and drawing. 

How do you get this limestone? From quarries, obviously. After extracting the limestone from quarries, you could burn the limestone in a kiln to produce various forms of lime (a substance containing calcium oxides/hydroxides) in a process known as the lime cycle. (This is where Limehouse, a suburb in the Docklands, gets its name, as chalk would be burned in kilns to produce lime in the area.)

The lime cycle works like this: start with calcium carbonate and heat it to a very high temperature. It will thermally decompose into calcium oxide and carbon dioxide, as in this reaction:

CaCO₃ → CaO + CO₂

The calcium oxide produced is also known as quicklime, and could be used to produce cements for building. But if you wanted to go one step further, you could react it with water to form calcium hydroxide:

CaO + H₂O → Ca(OH)₂

If you wanted to go full circle, you could react the calcium hydroxide (also known as slaked lime) with carbon dioxide to get back to where you started:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

The aforementioned Ca(OH)₂ is useful due to its high alkalinity, even if it could prove deadly (in large doses). It can be used as a food additive (E526) to control the acidity of food, since it being alkaline means any harmful acids can be neutralised. It can also be used to reduce the spread of diseases amongst livestock, and was actively recommended as such during a foot-and-mouth outbreak in Japan. I could continue mentioning various calcium compounds until the end of time, but all that needs to be said is that calcium keeps cropping up in various places.

Should limestone be pressurised and heated under the ground for a very long time, marble might form - used to make elaborate statues and decorate elaborate houses. Gypsum is a form of hydrated calcium sulphate, and can be used for plastering. That's not even mentioning the prevalence of calcium minerals in bone, teeth, and blood tissue, as well as in food such as dairy products. It's the most abundant mineral in the body, apparently.

As with most elements with a relatively low number of protons (at calcium, you're only about 17% through the periodic table), calcium isn't particularly difficult to find (one form or another). We may not see many products made with calcium metal, but calcium itself is more prominent than I'd have expected.