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Geopolymer Concrete, Egyptian Pyramids, and a New Way Forward for Sustainable Masonry

The Great Pyramids of Giza - left to right, the great pyramid of Khufu, 481 feet; the Pyramid of Khafre 448 feet; the pyramid of Menkaure 215 feet; the pyramids of Queens. Image © David Holt used with permission of Creative Commons license.

Spoiler alert: We may be wrong about how the ancient Egyptians built the Great Pyramids. Decades of schoolchildren are taught the prevailing theory - the pyramids were constructed from enormous blocks of solid stone, cut by hand from far away quarries and hauled across the searing desert sands. We imagine - thanks in large part to Cecile B. DeMille - thousands of shirtless, sweating slaves harnessed to thick hemp ropes, dragging enormous square blocks of stone up steep ramps. The feat seems so incredible that some wonder whether the Egyptians had help from other planets. Always a rational voice in the room, Neil deGrasse Tyson counters, “just because you can't figure out how ancient civilizations built stuff, doesn't mean they got help from aliens.”

Figuring out how the pyramids were built has interesting applications beyond Egyptology. Today’s building materials do not have an expected lifespan anywhere near 4,000 years. And many of our modern construction processes consume so much energy and emit so much CO2 that we’re quickly destroying the very world we’re working to build. The Egyptians seemed to know something we don’t about using locally-sourced materials to construct extraordinarily durable buildings without the huge environmental footprint so common today. Did the Egyptians use their minds as much as their muscle, and if so, what can we learn from them?

The skepticism Tyson addresses comes from a logical place. Despite the common teachings of the building of the pyramids at Giza, the feat of construction seems almost implausible. The Great Pyramid of Khufu was the tallest man made structure on earth for over 3,800 years - 16 times as long as our country has existed - until the construction of the Lincoln Cathedral in England. When built, the pyramid was 756 feet long on each side, 481 feet high, and composed of 2.3 million stones weighing on average nearly three tons each. Many of the joints between block are so accurate that a human hair cannot be passed between adjoining blocks.

Cecile B. DeMille’s 1956 film The Ten Commandments, while not specifically about the construction of the Great Pyramids, has contributed to the common image in many of our minds explaining the construction of the pyramids. In the 1980s, a French materials scientist named Joseph Davidovits proposed a very different scenario.

According to what we’ve been taught, quarried stone blocks weighing several tons were hauled to the pyramids, before the invention of the wheel. They were quarried out of the hillside with tools made of copper, a soft metal. And a city’s worth of laborers were housed and worked in a cramped area for decades. It seems so difficult to imagine, much less believe. And little evidence exists to support this idea - no copper tools have been found around the site, no evidence remains from housing that many laborers, and no clear hieroglyphs exist documenting the quarrying, transportation, or ramp-lifting of these blocks.

In the 1980s, a French materials scientist named Joseph Davidovits proposed a different theory - the Egyptians didn’t haul the blocks to the pyramids but rather made the blocks one at a time in place on the pyramids. Davidovits suggested that the blocks were formed by pouring an ancient concrete - he called it geopolymer - into wooden molds. A fraction of the laborers would be needed to haul sacks of moist geopolymer concrete to wooden forms placed exactly where each block was needed. Joints between poured concrete block would always be perfectly accurate as a compacted moist mixture hardens against neighboring blocks. Davidovits suggested that the geopolymer concrete was made from crushed limestone, clay, water, and lime, a highly alkaline (the opposite of acidic) activator that caused the crushed limestone mixture to reconstitute into a man-made stone.

Needless to say, Davidovits’s theory caused quite a stir among Egyptologists, historians, materials science researchers, and anyone who cared that a well-established explanation for the construction of something as iconic as an Egyptian pyramid was being turned on its head. Not only that, but if the Egyptians cast block in place from an early form of concrete, many established theories assigning the invention of mass produced concrete to the Romans would be off by a few thousand years.

One would imagine that modern scientists with electron microscopes could prove in short order whether Davidovits was correct or crazy. Enter Michel Barsoum, professor of materials science at Drexel University. Barsoum, a native of Egypt, never meant to get into the study of the pyramids but was amazed to hear Davidovits’s theory. Barsoum was more amazed to find that no one had proved - or disproved - the idea.

A gash in the side of one of the pyramids built by Senefru - the father of Khufu, who built the Great Pyramid - shows a combination of what appears to be irregularly cut quarried limestone blocks surrounded by tight jointed, cast-in-place geopolymer blocks. Image © Michel Barsoum, used with permission.

Barsoum, along with a graduate student named Adrish Ganguly, began studying samples from the inner and outer casings of the Pyramids. What they thought would be a months long study turned into a 5 year odyssey. In the end, they disproved some of Davidovits’s assumptions but proved his overall theory.

Barsoum believes that the Egyptians did cast a small but significant portion of the block in the pyramids. His electron microscope analysis indicates the Egyptians didn’t use clay in the geopolymer mixture, as Davidovits proposed, but rather Diatomaceous earth, a naturally occurring, commonly found soft sedimentary rock formed from the fossilized remains of algae. And Barsoum importantly disagrees with Davidovits by suggesting that not all the blocks were cast in place geopolymer. Rather, Barsoum suggests that the Egyptians used both man-made cast block along with limestone block quarried and hauled to the site in the way our traditional explanation proposes. Barsoum believes that only the exterior casing blocks and the blocks at the higher levels of the pyramids were cast geopolymer blocks. This makes sense - the casing block were visible, so cast-in-place block with extremely accurate “joints” would be appropriate to exterior application. And the block at higher levels of the pyramids were harder and harder to get to for quarried blocks hauled up ramps - replacing these with cast-in-place geopolymer blocks made life a lot easier.

A ground level block in front of the Great Pyramid of Khufu includes a irregular lip at the bottom that would have been very hard, and somewhat pointless, to carve. This lip indicates that the block was cast in place -  the material in the lip having slid out under the temporary wooden mold before hardening. Barsoum analyzed a piece of material from the bottom lip and says he did not find smoking gun evidence. “The only logical conclusion is that after 5000 years, the binding phase has basically been washed away. Solution? Get samples from the core of that block. Easier said than done.” Image © Michel Barsoum, used with permission.

Linn Hobbs, professor of materials science at the Massachusetts Institute of Technology, has also added to Davidovits’s original theory and Barsoum’s corroborating research. Hobbs’s students have reverse engineered a geopolymer concrete made from crushed limestone, kaolinite, silica, and natron salts, a substance found in the evaporated remains of saline lake beds. The Egyptians used natron salts for mummification. When exposed to water, natron salts become alkaline, a perfect activator to make a geopolymer reaction.

As predicted, new theories that suggest that even a small portion of the stones in the Pyramids at Giza were man made blocks formed from an early form of concrete have erupted into a firestorm of resistance and vitriol, most notably from those with the most to lose when an established theory is pulled apart. As much as Barsoum assumed that solid materials analysis could indisputably prove how some of the pyramid’s block were made, the debate still rages on.

Separating the debate from the historical discussion can shed important light on how we can improve today’s construction materials by exploring what the Egyptians might have done. Just the idea of an ancient form of geopolymer concrete masonry that has lasted 4,000 years can forever change the way we build today.


Cement factory in China. The production of cement alone is responsible for 6% of the world’s CO2 emissions. Image ©Jonathan Kos-Read, used with permission of Creative Commons license.

Concrete is the most voluminous material made by all mankind. It’s used all around the world in roads, bridges, dams, and buildings. The key binding ingredient in today’s concrete - Portland cement - has a terrible carbon footprint. We make so much Portland cement that it’s alone responsible for 6% of all the world’s CO2 output.

Portland cement was invented in England in the mid 18th century and is made by superheating limestone and a few other ingredients in giant kilns. The enormous CO2 footprint emerges in two ways. First, lots of fossil fuels are required to achieve kilning temperature above 2,000 degrees Fahrenheit. Second, the chemical reaction that produces Portland cement involves baking CO2 out of the limestone, CO2 that was originally sequestered in the skeletal fragments of marine organisms that formed the limestone. The CO2 emissions from the production of Portland cement are so significant that producing a pound of Portland cement emits almost a pound of CO2 into the atmosphere. Billions of tons of Portland cement are produced every year. The math is downright scary.

Bahia Honda Bridge in the Florida Keys. The reinforced concrete deck was installed 1938 and abandoned 34 years later. Image © Phil’s 1stPix , used with permission of Creative Commons license

Bahia Honda Bridge in the Florida Keys. The reinforced concrete deck was installed 1938 and abandoned 34 years later. Image ©Phil’s 1stPix, used with permission of Creative Commons license

And concrete made with Portland cement isn’t nearly as durable as its unbelievable environmental footprint might warrant. Concrete bridges are often taken out of service after only 50 years, due in part to harsh conditions like road salt, heavy truck traffic, and freeze-thaw cycles. While the relatively stable environment of the Giza pyramids avoids many of the harsh condition of today’s urban built environment, the 4,000 year durability of the structure indicates the expanded material lifespan possible with geopolymer concrete. When coupled with a much smaller carbon footprint - geopolymer concretes like those the Egyptians likely pioneered have a tenth the carbon footprint of Portland cement based concretes - geopolymers offer a compelling alternative to today’s status quo.

Geopolymer concrete is significantly different from Portland cement based concrete. To simplify the science, Portland cement is akin to a strong glue whereas a geopolymer reaction is akin to a two-part epoxy. Portland cement glues together the other ingredients in concrete - rock and sand. Portland cement can glue together other things, like fibrous paper in the form of papercrete. That’s one of the reasons Portland cement is so popular - it’s so reactive that it can bind together all kinds of aggregates to form relatively strong building materials. But that high reactivity comes at a giant environmental cost.

Geopolymer reactions, on the other hand, require two parts - a source of alumina silicates as well as an alkali activator.  The former - the alumina silicates - is often found in volcanic ash. The latter - the alkali activator - is often found in lime. When the two are combined, a chemical reaction results in the creation of a strong concrete. Interestingly, while the process of creating the structural bonds in Portland cement is different from that of geopolymers, the final product can be near identical - something called calcium-silicate hydrate or CSH.

The ceiling of the Pantheon in Rome — the largest unreinforced concrete dome in the world — still standing 2,000 years later. Image ©Biker Jun used with permission of Creative Commons license.

The Romans are often cited as inventing concrete, and they surely perfected its use. The Pantheon in Rome is to this day the largest unreinforced concrete dome, still standing 2,000 years later. The Romans couldn’t have made a concrete of the type we make today - they didn’t have kilns capable of super heating limestone to 2,000+ degrees Fahrenheit. Rather, the Romans pioneered a form of geopolymer concrete. They combined volcanic ash mined from sources like the island of Pozzollo with lime made from kilning limestone at relatively low temperature to make a very strong concrete, much of which is still around.

Today, many new forms of geopolymer concretes are being explored. The ash left over from burning coal to make electricity - called fly ash - shares many of the chemical properties of volcanic ash and serves as a great source of alumina silicates for a geopolymer reaction. CalStar is making non-structural facing bricks from fly ash, harnessing the benefits of geopolymers to reduce the embodied energy of traditional bricks. Ceratech is making concrete without Portland cement by combining fly ash with alkali activators to create a high strength geopolymer concrete with significantly reduced CO2 emissions.

However, fly ash - today’s version of the Roman’s pozzolanic ash - comes with its own risks. Fly ash contains significant levels of heavy metals left over from the burning of coal, and fly ash is only available where coal is burned for electricity. Most importantly, there’s not enough fly ash on the planet to replace the Portland cement we produce. What if there were a more common source of alumina silicates than the Romans’ volcanic ash or today’s fly ash? The Egyptians seemed to have found just that.


The Pyramids of Giza, as seen from the International Space Station. The pyramids are so large that they’re clearly visible in a photograph taken with a hand-held consumer level digital camera from space. Their size is also overwhelming compared to the structures of modern Cairo. Public domain image courtesy of NASA / ISS Crew 032.

The Pyramids of Giza, as seen from the International Space Station. The pyramids are so large that they’re clearly visible in a photograph taken with a hand-held consumer level digital camera from space. Their size is also overwhelming compared to the structures of modern Cairo. Public domain image courtesy of NASA / ISS Crew 032.

Always ahead of their time, the ancient Egyptian’s command of materials science may have allowed them to create man-made stone from little more than raw earth. While clear evidence exists of volcanic activity in Egypt’s long history, it’s unlikely that significant amounts of volcanic ash existed for the ancient Egyptians to build that quantity of stone. And the materials science research from Barsoum, Hobbs, and others doesn’t indicate ash as the source of the Egyptian’s alumina silicates, but rather locally sourced earth - Diatomaceous earth, kaolins, clays, and limestone - activated with an alkali material - natron salts and lime. This means that the Egyptians appear to have pioneered a geopolymer concrete that lasted throughout the history of modern humanity made from abundant common earthen materials found nearly everywhere on the planet. Compare that to the concrete we make that lasts half a century and comes with a disastrous carbon footprint.

Imagine how we could revolutionize today’s concrete masonry industry by re-discovering the Egyptian’s formula. Low cost, sustainable, resilient, and highly durable masonry could be produced nearly everywhere on the planet from materials sourced locally, all without ultra-high embodied energy binders like Portland cement.

Watershed Materials, with the help of the National Science Foundation, has been exploring just that. Two phases of SBIR grants have been applied towards creating durable concrete masonry with zero Portland cement from the geopolymerization of alumina silicates found naturally in common earthen materials. If we’re successful, we may be able to revive part of the science that allowed the Egyptians to make man-made stones so durable that they’ve not only lasted for over 4,000 years but have also fooled modern historians by appearing identical to geologically formed, quarried rock.

Watershed Materials has developed the first prototype of a new masonry block machine that applies intense compressive force to allow the interparticle contact necessary for geopolymerization of common earthen materials of relatively low reactivity. Along with the design of a new machine for producing sustainable masonry, Watershed Materials is developing mix designs to create strong durable geopolymer masonry from common clays and earthen aggregates found nearly everywhere across the planet. 

Watershed Materials’ research and development applies specifically to masonry - the science may not apply to the poured concrete used in roads, bridges, and dams. However, concrete masonry blocks - otherwise known as cinder blocks - are one of the most common building materials used around the world. Tens of billions are produced every year. Finding a more sustainable alternative to concrete masonry - one that uses the type of geopolymers pioneered by the ancient Egyptians in place of Portland cement - would offset enormous amounts of CO2 emissions and would allow developed and developing economies around the world to produce durable, resilient masonry from locally sourced, inexpensive earthen materials.

While we may have been wrong about how the ancient Egyptians built the pyramids, learning the right answer has implications for modern materials science and a new way forward towards replacing the most common building materials on earth with a far more durable and sustainable alternative.

This post was originally published on Medium.