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How Concrete Started—Part 4: Lime

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Limestone rocks were turned into mortar.
In Part Four of this six-part series, concrete historian Luke Snell looks at the many steps involved in lime production and speculates on how it may have been discovered.

Concrete historians have speculated that the first cement (one that is similar to modern portland cement) was lime. This conclusion is based on that the production of both materials is similar and both materials use limestone as a basic ingredient.

Archeologist have found evidence of lime being used as a cement in what is today Turkey and Israel from about 7000 BC. This ancient lime cement closely resembles modern day concrete. Thus, we need to look at the many steps involved in lime production and speculate how these steps may have been discovered.

To make lime from limestone (calcium carbonate, CaCO3) is a simple chemical formula but a relatively complex process that requires many steps. First, the limestone is heated to higher than 1650°F resulting in the following reaction:

CaCO3 CaO + CO2

The carbon dioxide (CO2) is released as a gas leaving calcium oxide (CaO also called quicklime), which is not stable. L on its own, quicklime will react with the CO2 in the air and revert back to the limestone (calcium carbonate). If it is protected from the air and has water available, it can become a lime mortar as shown below:

CaO + H2O Ca(OH)2+ heat

This process is called slaking and can take a considerable amount of time. In the United States, one of the early ways of slaking was to put the quicklime in a hole in the ground to protect it from CO2 and let the moisture from the soil react with it.

This slaked lime (Ca (OH)2 or calcium hydroxide) is very caustic and the reaction with water is violent, therefore making it in the ground eliminated many of the problems of handling the material during the slaking process. Once the slaking process is complete, what is left is lime putty that can be used as a mortar. One of the ways people knew this process was complete was to stick a trowel into the mixture. If nothing stuck to the trowel the process of slaking was considered complete. The mortar then could be placed between stones or bricks to join them together or if mixed with aggregates to make a simple concrete. The chemical reaction is:

Ca(OH)2 + CO2 CaCO3

When the mortar reacts with carbon dioxide in the air, the limestone cycle is complete, and limestone has been re-created. Now, however it is the cement that holds the bricks, stones, or aggregate together into a solid mass. Archeologist have found evidence of lime being used to make concrete floors in 7000 BC. We can conclude that they must have understood the process by this time and knew how to mass produce lime putty and mortar.

One of the main difficulties our ancestors faced was how to get enough heat to start the process. Again without written records, we can only speculate on how they developed a process of getting the needed high temperatures (above 1650°F) to create quicklime. In article three of this series, we noted that a campfire only gets to about 1000°F, a temperature well below what was needed to make quicklime. Article 3 also showed that they knew how to fire clay to make figurines and pottery at least 5 to 10 centuries before the first known use of lime. 

Our ancestors must have learned by experience while making and firing figurines and pottery that hotter fires gave them better and more durable figurines or pottery. Thus began their quest of discovering methods to get higher temperatures.

To better understand the steps in getting higher temperatures, we need to trace some of the major developments of man. Archaeologists tells us that the discovery of how to make and control fire occurred by at least 125,000 BC, but there is no agreement on the exact date. In this article, I am taking a conservative approach and use dates that have the most consensus.

These ancient fires were most likely wood fires except in areas where trees were scare, where dung was used as fuel. Dung is still used in some countries today. In rural Mongolia, dung is collected from sheep and goats each morning with a dung catcher. In other areas of the world, this is done by hand.

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Gathering and drying dung in Mongolia to use as fuel.

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The dung is then dried and stored for heating and cooking fires. In some developing countries this remains a major fuel source. In the American west in the 1800s, settlers did the same thing, collecting “buffalo chips” using them for fuel.

The smoke from dung fires can cause severe health problems and many health agencies have worked to help people using dung for cooking and heating to find alternative fuel sources. Dung is free and available so for those in poverty, the practice continues.

The first attempts at making clay figurines or pottery could be achieved at temperatures consistent with a wood or dung fire. The clay would be formed into the figurine or pottery and then covered with wood or dung. The wood or dung was set on fire and the clay was fired. For those interested in more details on this process, there are several YouTube videos on how to make pottery with open fires using this technique.

People living in areas where wood was scarce learned that dung fires were superior to wood fires, providing uniform heat that resulted in less breakage during the firing process. Modern researchers using infrared thermometers have found that sheep dung (thought to be the first domesticated animal) provides the highest temperature fires compared to other animal dung.

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Modern day experiments using dung fires to make pottery.

Those making pottery in ancient times could not measure the temperature but through trial and error they were able to evaluate the effectiveness of each fuel they tried. The pottery made using sheep dung had less breakage while in the fire and produced a much stronger and more durable pottery than those achieved using other fuels. They would realize that the hottest fires provided the best results, so they began searching for ways of getting even higher temperatures.

A likely scenario was that they realized that a lot of heat was lost in an open fire, resulting in the idea of confining the fires and trapping the heat. This resulted in the development of kilns.

 

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A native Native American kiln.

This drawing shows small rocks below the pottery. If these rocks were limestone, a very common rock, the fire would have converted some of the smaller pieces to quicklime because the temperatures at the bottom of the kiln would now exceed 1650°F.

After experimenting with this powder (and getting burned a few times because the quicklime reacted violently with the moisture on their skin), they learned how to handle it and ultimately make lime mortar.

Here are the steps in this process and an estimate of the amount of time involved to mass produce lime in 7000 BC (the 1st known use of lime):

  1. Limestone rocks would need to be reduced in size to 1 to 2.5 inches. This would probably take several days and they would probably have used stronger rocks as hammers.
  1. Place the limestone rocks with fuel into the kiln; this would take about 1 day.
  1. Heat the limestone in the kiln for 3 to 7 days. They would need to maintain the temperatures above 1650°F by adding more fuel as needed. The kiln would require constant supervision. This process would require a lot of fuel. Note: The first kilns existed before there were steel axes, so if they were using wood as fuel, gathering the necessary amount of wood would be a challenge in itself. Using dung, peat, or coal as fuel would also create a challenge in gathering the needed amount.
  1. The quicklime this process created would need to cool and then the kiln could be unloaded. This would take about 1 additional day and it would be dirty, dangerous, and difficult work.
  1. Next the quicklime had to be protected from the air and exposed to water to complete the slaking process. This could take days to complete.
  1. The resultant lime putty was now ready to use as a mortar or as a cement.

The process would likely have evolved over several centuries because of the multiple steps that had to be mastered to complete the process. Ultimately, with great perseverance, our early ancestors were able to perfect the process for making lime mortar.

In our next article, I will finalize my quest of when concrete started and then in Part 6 we will look at how the Romans took cement to a whole new level.

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