“Our lives are full of artificial materials – plastic bottles, steel structures and glass facades. But while these may seem like the most common man-made materials, our world is built on concrete. As the most popular man-made material on earth, it is the second most consumed substance after water.”
So says the giant Korean-based construction and engineering company Samsung C&T, and writer Robert Courland agrees. He titled his 2011 book Concrete Planet: The Strange and Fascinating Story of the World’s Most Common Man-Made Material.
It’s important to understand that although the words cement and concrete are used interchangeably, cement is simply an ingredient of concrete. But it is the ingredient.
As the Portland Concrete Association of the US makes it clear, “concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden”.
And “portland”, in the lower case, is correct. “Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver,” explains an article published by the Concrete Contractors Association of Greater Chicago.
The story of cement – and concrete – is one with ancient roots.
The article “The History of Concrete and the Nabataeans”, which draws on work by Joseph Davidovits, on expert on ancient cements and concretes, states that “the Romans are generally credited as being the first concrete engineers, but archaeological evidence says otherwise”.
Archaeologists have found a type of concrete dating to 6500 BCE, “when stone-age Syrians used permanent fire pits for heating and cooking”, it says. “These fire pits, built from area limestone, showed a primitive form of calcining on the exterior faces of the limestone rocks that lined the fire pits and led to the accidental discovery of lime as a fundamental building material.”
In an article for the International Association of Certified Home Inspectors, Nick Gromicko and Kenton Shepard report that in 1300 BCE Middle Eastern builders found that thin coats of burnt limestone mixed with water, and applied to the outsides of pounded-clay walls, dried to form a hard, protective surface.
“This wasn’t concrete,” they say, “but it was the beginning of the development of cement.”
Today portland cement is made by combining a mixture of limestone and clay in a kiln at 1400-1600 degrees Celsius, says an article on the scientific principles of cement published by the University of Illinois.
“It begins with the quarrying of limestone, CaCO3, which is crushed and mixed with clay or shale, sand, and iron ore, and ground together to form a powder.”
In the kiln it passes through “four stages of transformation”. Free water in the powder is lost by evaporation, decomposition occurs from the loss of bound water and carbon dioxide – this is called calcination. The third stage is called clinkering, as calcium silicates are formed. The final stage is the cooling stage.
The marble-sized pieces produced by the kiln are referred to as clinker, which is cooled, ground and mixed with a small amount of gypsum “to produce the general-purpose portland cement”.
And why is it called portland cement? An article in the Journal of Light Construction tells the tale.
It explains how, in 1824, Joseph Aspdin, an English bricklayer from the city of Leeds, patented a material by that name – “the cement being similar in colour to Portland stone, popular in England at the time”.
However, the article says, “this claim is at best only partly true”, and although Joseph experimented with various formulas, his cement was no better than any other on the market.
The Journal hands the story over to Robert Courland, who, it says, claims “the secret of Portland cement as we now know it was stumbled upon by Aspdin’s son, William, who he calls “a self-promoting swindler and con man who also happens to be the true father of portland cement”.
Joseph, the Journal says, would grind up limestone, mix it with water and clay, dry the mixture, kiln it until it was hard, and grind it up again to make his cement powder. He discarded some of the product – the overbaked “clinkers” – because it was too hard to grind.
“It was William, the son, who seems to have had the bright idea of saving and grinding up the clinkers. And it was the clinkers that yielded the cement that actually rivaled Portland stone for hardness and durability.”
William’s cement became a huge success but, the Journal says, “his penchant for cheating his partners brought him to trouble” and he died “alone and friendless” in Germany on 11 April 1864.