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.

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.

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.