Yes, Calcium is a Metal!

A photographic montage showing a human skeleton pulling on a giant container of calcium supplements.Building the World

Most of us are familiar with the idea that our bodies need calcium.  And calcium is indeed the key element in our bones.  Calcium is the most abundant metal in the human body – and those of animals too.  The fifth most abundant element on Earth and our World’s chosen architectural building block.  Yes, calcium is a metal.  Do we really appreciate its true value?

Organic Material

Many living organisms use calcium to build the structures that house and support them.

These structures are:

  • skeletons,

  • egg shells,

  • mollusc shells,

  • coral reefs and

  • exoskeletons of krill and other marine organisms.

When these organisms die, their shells and skeletons sink to the bottom of the sea and collect in great drifts.  Over millions of years, they compact to form limestone, chalk and marble.


Limestone, Chalk and Marble – Taj Mahal and Coccolithophores…

Limestone (CaCO3)

A photograph showing fossil fusulinids in limestone.
Fossil fusulinids seen in Limestone (8x), Polarized Light. Photograph: Ron Sturm, Construction Technology Laboratories, Inc., Illinois, USA. Source: Wired/Nikon Small World

Limestone is a sedimentary rock, composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

Limestone solubility in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years.  Most cave systems formed through limestone bedrock.

If you look very closely at a piece of limestone, you can see the tiny fossils of ancient marine creatures from which it is composed.  Around 10% of all sedimentary rock is limestone, which is pretty extraordinary when you consider that it represents the concentrated bodily remains of living marine creatures.


Chalk (CaCO3)

Chalk is a soft, white, porous sedimentary carbonate rock – a form of limestone composed of calcite or CaCO3.  Calcite forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells, shed from micro-organisms called coccolithophores.


Marble (CaCO3)

A photograph of the Taj Mahal in Agra, India.
Taj Mahal, Agra, India  Source: NationalGeographic

Marble is a non-foliated metamorphic rock composed of re-crystallized carbonate minerals, usually calcite or dolomite.  To a geologist, “marble” is essentially metamorphosed limestone.  To an art lover, it represents perfection…


Building Material

Limestone has many uses.  From building material and road aggregate, to white pigment or filler in everyday products, such as toothpaste or paints, and as a chemical feedstock for the production of lime, calcium is also the key ingredient in man’s most important structural material – cement.

A trick humankind learned very early on…

So how do you turn limestone into concrete?


Limestone, Lime and Concrete

A diagram showing the operation of an early lime kiln.
An early operating lime kiln. Source: British Geological Survey

The key is extracting the calcium from limestone – a pretty simple process, you just need to heat up the limestone.  This can be done on a large scale using a lime kiln – a specially designed heated chamber, for the purpose of de-carbonating limestone.

A diagram showing the basic operating principle of a lime shaft kiln.
A basic shaft-type lime kiln Source: Wikipedia

What you do is place your limestone – calcium carbonate, CaCO3 – in a fire where the temperatures are high enough to drive out the carbon atoms as carbon dioxide CO2 into the atmosphere.  That leaves you with calcium oxide CaO – more commonly known as lime.

In permanent draw kilns, the chalk was layered with coke and lit.  As it burnt through, lime was extracted from the bottom of the kiln and further layers of chalk and coke added to the top.

A chemical equation showing the reversible reaction that decarbonates limestone to create lime and carbon dioxide.
The reversible chemical reaction that turns limestone CaCO3 into lime CaO and carbon dioxide CO2.

A common feature of early kilns was an egg-cup shaped burning chamber, with an air inlet at the base (the “eye”), constructed of brick.  Limestone was crushed by hand to fairly uniform 20-60 mm (1 to 2.5 inch) lumps.

Successive dome-shaped layers of limestone and wood or coal were built up in the kiln on grate bars across the eye.  When loading was complete, the kiln was kindled at the bottom, and the fire gradually spread upwards through the charge.

When burnt through, the lime was cooled and raked out through the base.

A photograph showing a full-size replica of a Robins kiln.
Robins kiln, c. 1860 Such kilns rarely survived, because if not regularly warmed up they would quickly collapse. Source:

The theoretical heat (or standard enthalpy) of reaction required to make high-calcium lime is around 3.15 MJ per kilogram of lime, so the batch kilns were only around 20% efficient.

The key to development in efficiency was the invention of continuous kilns, thus avoiding the wasteful heat-up and cool-down cycles of the batch kilns.  The first were simple shaft kilns, similar in construction to blast furnaces.  These are counter-current shaft kilns. 

Modern variants include regenerative and annular kilns.  Output is usually in the range 100-500 tonnes per day.

Lime is the basis of most cements – the glue that hold rocks and particles of sand together to make concrete.  Recent archaeological discoveries show some prehistoric people created concrete, long before they had discovered the first metals.


Predating Pottery?

Over the last two decades, a German archaeologist working in Turkey has uncovered what he believes is the world’s first temple.  It is a complex of carved stones erected about 11,000 years ago – 6,000 years before Stonehenge.

A photograph showing the ancient site of Göbekli Tepe in Turkey, where the first attempt at cement is thought to have originated.
The Early Beginnings of Architecture at Göbekli Tepe, Turkey Source: Wikimedia

The site is called Göbekli Tepe (Pot-bellied Hill in Turkish).  It features floors made of very early cements.

Built on a hilltop in southeastern Turkey, Göbekli Tepe was referred to be one of the oldest temples existing in the World.  It is also said that the site of the temple was most likely to be crafted by animal hunters before the age of wheels, animal husbandry and agriculture came into existence, about the 10th millennium BC.

The houses or temples are round megalithic buildings while the walls are made of unworked dry stone and include numerous T-shaped monolithic pillars of limestone that are up to 3 metre (10 ft) high.

Göbekli Tepe not only predates pottery, and the invention of writing or the wheel, but it was also built before the beginning of agriculture and animal husbandry.

A photograph of the pont du Gard in southern France - a 2,000 year old Roman aqueduct which is still standing to this day.
The Pont du Gard is a 2,000 year old Roman aqueduct that crosses the Gardon River in the south of France. Located near the town of Vers-Pont-du-Gard, the bridge is part of the Nîmes aqueduct, a 50-kilometer system built in the first century AD to carry water from a spring at Uzès to the Roman colony of Nemausus (Nîmes). Source: Wikipedia

Over millennia, the technology was refined.

Two magnificent examples of early Roman buildings – the Pantheon and the Pont du Gard at Nimes – showed the amazing potential of concrete.  Here cement was used it to enclose space with an unsupported dome, and to bridge considerable spans without reinforcement.


The Romans – Masters of Sustainable Cement

Our entire architecture, all our great building and engineering projects start with calcium, because cement is the basis of the most widely used man-made substance on Earth – concrete.

A photograph of the coliseum in Rome - another masterpiece of Roman construction.
Building an Empire on calcium. The Romans knew the “secret” to a long-lasting mortar. Source:

Ancient Romans were master builders.  Many temples, roads and aqueducts constructed during Roman times have held up remarkably well, despite the wear-and-tear they had to endure – in the form of military invasions, tourist mobs and natural disasters, such as earthquakes.  Geologists and engineers have long been fascinated by Roman harbours, many of which stand almost intact after 2,000 years or more, despite constant pounding by seawater.

Pliny the Elder (c. AD 23 – August 25, AD 79) wrote that the best maritime concrete was made from volcanic ash found in regions around the Gulf of Naples, especially from near the modern-day town of Pozzuoli.  Its virtues became so well-known that ash with similar mineral characteristics – no matter where it was found in the world – was dubbed pozzolan.


Concrete Studies

A couple of maps showing the location of Pozzuoli Bay in Naples, Italy - the site of an ancient Roman seawater concrete-built harbour.
Ancient Roman seawater concrete harbour sites, central Italian coast, with Al-tobermorite in relict lime clasts, mid-first century BCE to first century CE.  (a) Drill sites (green circles): 1 Santa Liberata, 2 Portus Cosa, 3 Portus Claudius, 4 Portus Traianus, 5 Portus Neronis, and 6 Portus Baianus, Portus Iulius, and Baianus Sinus,  (b) Volcanic districts (red triangles) Flegrean Fields caldera and concrete drill cores, including Baianus Sinus BAI.2006.03 (after Orsi et al. 1996). Thick black lines: Campanian Ignimbrite caldera rim, thin black lines: Neapolitan Yellow tuff caldera rim;; red areas: volcanic craters.  Source: Monteiro et al. 2013

By analyzing the mineral components of the cement taken from the Pozzuoli Bay breakwater at the laboratory of U.C. Berkeley, as well as facilities in Saudi Arabia and Germany, an international team of researchers was able to discover the “secret” to Roman cement’s durability.  They found that the Romans made concrete by mixing lime and volcanic rock to form a mortar.

To build underwater structures, this mortar and volcanic tuff were packed into wooden forms.  The seawater then triggered a chemical reaction, through which water molecules hydrated the lime and reacted with the ash to cement everything together.  The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) bond is exceptionally strong.


Portland Cement

By comparison, Portland cement (the most common modern concrete blend) lacks the lime-volcanic ash combination, and does not bind well compared with Roman concrete.

Portland cement, in use for almost two centuries, tends to wear particularly quickly in seawater, with a service life of less than 50 years.

The production of Portland cement also produces a sizeable amount of carbon dioxide, one of the most damaging of the so-called greenhouse gases.

According to Paulo J.M. Monteiro, a professor of civil and environmental engineering at the University of California, Berkeley, and the lead researcher of the team analyzing the Roman concrete, manufacturing the 19 billion tons of Portland cement we use every year “accounts for 7% of the carbon dioxide that industry puts into the air.”

Therefore, in addition to being more durable than Portland cement, Roman concrete also appears to be more sustainable to produce.  Such early concretes remained brittle and weak, which is why most buildings continued to be made of stone and brick.

A photograph showing a wheelbarrow full of modern Portland cement.
A wheelbarrow full of Portland cement.  Source: itech-Dickinson

To manufacture Portland cement, carbon is emitted by the burning fuel used to heat a mix of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit), as well as by the heated limestone (calcium carbonate) itself.

To make their concrete, Romans used much less lime, and made it from limestone baked at 900 degrees Celsius (1,652 degrees Fahrenheit) or lower – a process that used up much less fuel.

The real breakthrough came in the 1840s.


Modern Portland Cement

“Modern” Portland cement was first developed by William Aspdin.  In 1843, Aspdin set up a cement manufacturing plant at Rotherhithe, near London, where he was soon making a cement that caused a sensation among users in London.  He had discovered that a significantly different product, with much wider applications, could be made by modifying his father’s cement formulation.

By increasing the limestone content in the mixture, and burning it much harder, a slow-setting, high-strength product suitable for use in concrete could be obtained.  Aspdin discovered that when this was ground to a fine powder, it produced an exceptionally powerful cement.  And very soon, he got the perfect opportunity to test out his new product.

However, the product was also substantially more expensive to make, in terms of cost of extra limestone, cost of extra fuel, and difficult grinding of the hard clinker.

Like his father, William Aspdin had little chemical training, and his innovations were likely the result of luck.  Although he was unaware of its chemical significance, Aspdin’s contribution to the modern world of building construction was to make the first cement containing alite, as an active ingredient.  As such, he is credited with launching the “modern” Portland cement industry.

Actually, it is testament to the strength of the cement – and the power of calcium – that, 150 years later, Londoners are still using the sewers that civil engineer Sir Joseph Bazalgette (1819-1891) built to flush away their waste in response to the “Great Stink” of 1858.

A photograph of the Dubai skyline in the United Arab Emirates.
The ever-expanding Dubai skyline requires millions of tonnes of concrete to build.

The incredibly strong concrete modern Portland cement can create has transformed the building industry across the World – as the skyline of every major city shows.

The World produces about 3.5 billion tonnes of cement every year.


Given that cement is usually between 10% and 15% of the mix in concrete, this represents enough cement to produce about four tonnes of concrete for every person on Earth each year.

Fortunately, there’s a lot of calcium about – the soft grey metal is the fifth most abundant element in the Earth’s crust.

Much of it is dissolved in the sea.

For millennia, marine organisms have been combining it with carbon dioxide they fix from the atmosphere to make shells of calcium carbonate.

The problem is that creating all the cement for all that concrete is doubly, or even triply polluting!


Environmental Impact of Portland Cement Production


The CO2 associated with Portland cement manufacture comes from 3 sources:

  1. CO2 derived from de-carbonation of limestone,

  2. CO2 from kiln fuel combustion,

  3. CO2 produced by vehicles in cement plants and distribution.

Source 1 is fairly constant: minimum around 0.47 kilos of CO2 per kg of cement, maximum 0.54, typical value around 0.50 worldwide.

Source 2 varies with plant efficiency: efficient pre-calciner plant 0.24 kilos of CO2 per kg cement, low-efficiency wet process as high as 0.65, typical modern practices (e.g. UK) averaging around 0.30.

Source 3 is almost insignificant at 0.002–0.005 kilos.

Typical total CO2 is around 0.80 kg CO2 per kg finished cement.  This omits the CO2 associated with electric power consumption because it can vary according to the local generation type and efficiency.  Typical electrical energy consumption is of the order of 90 – 150 kWh per tonne of cement, equivalent to 0.09–0.15 kg CO2 per kg finished cement, if the electricity is coal-generated.


You need vast amounts of energy to get your kiln hot enough to bake all that limestone, and that usually means burning fossil fuels.  And the limestone itself produces vast amounts of greenhouse gases, as all the carbon dioxide fixed by those ancient sea creatures is driven into the atmosphere.

Every ton of cement produces almost a ton of CO2.  That’s why the concrete industry is reckoned to be one of the most polluting on Earth, responsible for up to 5% of total CO2 emissions.


Pure and Silver

A photographic animation of the chemical reaction of calcium and water.
The fiery reaction of calcium and water. Source: Giphy

Anyway, pure calcium is a silvery metal, a little harder than lead.  Although it is the fifth-most-abundant element by mass in the Earth’s crust, free calcium metal is actually too reactive to occur in nature.

Calcium plays an important role in the human body as a cellular ionic messenger with many functions.  Calcium also serves as a structural element in bone.  It is the relatively high-atomic-number calcium in the skeleton that causes bone to be radio-opaque.  Of the human body’s solid components after drying and burning of organics in cremation, about a third of the total “mineral” mass remaining is the approximately one kilogram of calcium that composes the average human skeleton, with the remainder being mostly phosphorus and oxygen.  It makes you think…

From coccolithophores… to us…  It really does make you think…