How Concrete Started—Part 3: GypsumHow Concrete Started—Part 3: Gypsum
In Part Three of this six-part series, concrete historian Luke Snell takes us another step closer to the development of modern cement with the theory that the first man-made cement was gypsum.
November 23, 2020
In the first two articles, we explored the first two cements or mortars used (Part 1, mud and Part 2, tar) and the first two masonry units (sun-dried brick and fired brick). In this article we investigate the likely accidental discovery of powdered gypsum used as mortar, although its potential use was not realized until centuries later.
To contain fires, early man likely built fire rings with stones. They would have witnessed the devastation caused by forest fires, uncontrolled fires, and campfires that were not properly contained. They knew this was a necessary step as they learned how to control and use fire safely.
Some of the stones used to make a fire ring would likely be gypsum. Gypsum is a common sedimentary rock formed from ancient oceans. It is in thick layers and can be found at or near the earth’s surface.
Typical gypsum stone.
Chemically, gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. Early man may have used gypsum because it was soft and could be easily carved. Some of the lower quality gypsum rocks that they did not use for carving would likely have ended in fire circles.
At temperatures around 300 F, well within the range of a campfire, part of the water is driven off from the gypsum molecule:
CaSO4·2H2O → CaSO4·1.5 H2O + 0.5 H2O
The water is released as steam and the gypsum turns into a dry powder. When that dry powder comes in contact with water, it becomes a mortar and that will ultimately revert back to gypsum. This process is quick, the mortar sets in just a few minutes and provides a weak bond that can hold blocks, stones, or aggregates together.
Since 300 F is easily achieved in the stones of a fire ring, logically some parts of the gypsum rocks would have water driven off and the above chemical reaction would have occurred. If this was followed by exposure to water (either rain or water used to put out the fire), they would have witnessed the first man made cement.
By taking small gypsum rocks, they could easily repeat this process and be able to recreate this cement. Some of the first uses of this material were likely as a coating of walls (plaster) and mortar for sun dried bricks. Early men could have also made useful household products or figurines with it.
Products made with this new gypsum powder were not very strong or durable. When in contact with water, it loses strength and breaks rather easily. For these reasons, archeologist have not found any evidence of mankind using gypsum until the construction of the pyramids.
Egyptians used gypsum as a mortar and as a plaster while building the pyramids in 2600 B.C. Egyptologist suggest that pyramid builders did not need mortar to hold the massive stone blocks together since they were cut so that the gaps were less than 0.02 inches and gravity would hold the blocks together. It was speculated that the use of the gypsum mortar was because there was an abundance of gypsum in the area and that the gypsum mortar would “butter” the joints and make it easier to push the blocks into place. At this time in history, few tools had been developed, thus these blocks having an average weight of 2.5 tons had to put into place by brute force. By buttering the joints, it was thought it would be much easier to push these blocks into place.
This photo of the pyramids at Giza taken in 1858 shows the white gypsum mortar between the blocks.
The mortar used for construction of the Egyptian pyramids was analyzed in the 1920s and found to be gypsum with small amounts of limestone and sand. The gypsum rocks in this area of Egypt have about the same percent of sand and limestone, so it was logical that the gypsum mortar was processed from the local gypsum rocks.
In a simple laboratory experiment using concrete bricks, the force of moving a brick was reduced by over 50% when the sliding surface was “buttered” or had a fresh gypsum mortar between the bricks. The speculation that buttering the joints of large rock masonry units to ease construction appears to be reasonable.
Egyptians also used the gypsum as a plaster on the interior walls of the pyramids. It provided smooth writing or drawing surfaces that has lasted over 4600 years.
Egyptian paintings inside one of the pyramids on a gypsum plastered wall.
Early man learned there were two problems with gypsum cement. It was not particularly strong. And when wet, it would lose strength entirely and easily break. Another problem with gypsum is its short pot-life. It sets in just a few minutes (my experience is that plaster starts to set in 10 to 15 minutes). This does not allow much time to work with the mortar during construction.
When using this gypsum for interior work, such as the pyramid walls where there is virtually no moisture, setting times doesn’t pose a problem. It bonds well to the walls and provides the needed smooth surface for writing or painting.
When used to butter the joints between blocks, gypsum must be applied just before the final placement of the blocks. This provides the lubricant needed for the few minutes needed to move the block into place.
This gypsum cement probably had limited use at the time of its discovery. Using mud as a mortar was quite effective and required less preparation.Gypsum used as a plaster did not become common until the late middle ages. Because it could easily be shaped and carved when set, it was used to decorate many important buildings such as palaces and churches. Today gypsum is widely used as wall boards in most residential construction. Most of us have probably made crafts or molds with plaster of paris, a very pure form of gypsum cement.
Although gypsum may have found limited use when discovered, it reinforces the idea that we can use heat to transform clay or stone into useful products.
In the next article we will discuss how our ancestors continued their search for better and more robust cement.