Geopolymerisation and the pyramids

One of the themes of May’s SRCR ’11 conference is “Developments in geopolymers as alternatives to conventional cements”. Geopolymer is a term covering a class of synthetic aluminosilicate materials with potential use in a number of areas, essentially as a replacement for Portland cement and for advanced high-tech composites, ceramic applications or as a form of cast stone.

Minerals Engineering has published a number of papers on geopolymerisation over the years, and, far from being a mundane subject, it is a field of great scientific potential, with some fascinating implications, which bring together two of my interests, science and history.

I am fairly sure that the first paper on this topic published in Minerals Engineering was co-authored by my two old friends Jannie van Deventer and Leon Lorenzen, who were then at the University of Stellenbosch, but are now pursuing successful careers in Australia.

The paper, by Jannie, Leon and J.G.S. Van Jaarveld, entitled “The potential use of geopolymeric materials to immobilise toxic metals: Part I. Theory and applications” is an excellent review of the science of geolpolymeric materials, but what hit me between the eyes when I read it all those years ago was a couple of paragraphs describing the ideas of Davidovits, who postulated that the pyramids of Egypt were not built by the means previously thought, but were cast in place and allowed to set, creating an artificial zeolite rock. His ideas were published in a book (The Pyramids; An Enigma Solved, Hippocrene Books, Inc, New York, 1988). Unfortunately I did not follow this up, as at the time I thought it was just another crackpot idea, such as von Daniken’s 1968 book Chariot of the Gods, which proposed that the pyramids were built by aliens, or the later, and more believable, Fingerprints of the Gods by Graham Hancock, which proposed the building by a long-gone ancient civilisation, but which was also deeply flawed, being based on very selective “research”. I was then unaware that Joseph Davidovits was a respected materials scientist, who first applied the name geopolymer to these materials in the 1970s.

Barbara and me at Giza in 1995

Having visited the pyramids at Giza a couple of times, I am fascinated by not only their sheer size but also the precision in construction, by what was essentially a civilisation only just emerging from the stone age. There have been many hypotheses about the construction techniques, which seem to have developed over time, later pyramids not being built in the same way as earlier ones. Most of the construction hypotheses are based on the idea that huge stones were carved with copper chisels from stone quarries, and these blocks were then dragged and lifted into position. Disagreements chiefly concern the methods used to move and place the stones.

Davidovits claimed that the blocks of the pyramid are not carved stone, but mostly a form of limestone concrete and that they were "cast", as with modern concrete. According to this hypothesis, soft limestone with a high kaolinite content was quarried in the wadi on the south of the Giza Plateau. The limestone was then dissolved in large, Nile-fed pools until it became a watery slurry. Lime (found in the ash of cooking fires) and natron (also used by the Egyptians in mummification) was mixed in. The pools were then left to evaporate, leaving behind a moist, clay-like mixture. This wet "concrete" would be carried to the construction site where it would be packed into reusable wooden moulds and in a few days would undergo a chemical reaction similar to the "setting" of concrete. New blocks, he suggests, could be cast in place, on top of and pressed against the old blocks. 

In 1979, at the second International Congress of Egyptologists, Grenoble, France, Davidovits suggested that the pyramid blocks were cast as concrete, instead of carved. Such a theory was greatly disruptive to the orthodox theory and his research was fiercely opposed by some experts (geologists and Egyptologists).

My interest in this has been reawakened by a fairly recent peer-reviewed paper being drawn to my attention (M. W. Barsoum, A. Ganguly, G. Hug (2006). Microstructural Evidence of Reconstituted Limestone Blocks in the Great Pyramids of Egypt. Journal of the American Ceramic Society 89 (12), 3788–3796). The findings of Michel Barsoum and his colleagues at Drexel University, USA support Davidovits' hypothesis. They claim to have found particles and air cavities in pyramid limestone that do not occur in natural limestone. After extensive scanning electron microscope observations and other testing, they finally began to draw some conclusions about the pyramids. They found that the tiniest structures within the inner and outer casing stones were indeed consistent with a reconstituted limestone. The cement binding the limestone aggregate was either silicon dioxide or a calcium and magnesium-rich silicate mineral.

The stones also had a high water content, which is unusual for the normally dry, natural limestone found on the Giza plateau, and the cementing phases, in both the inner and outer casing stones, were amorphous, their atoms not being arranged in a regular and periodic array. Sedimentary rocks such as limestone are seldom, if ever, amorphous.

More startlingly, Barsoum and another of his graduate students, Aaron Sakulich, recently discovered the presence of silicon dioxide nanoscale spheres (with diameters only billionths of a meter across) in one of the samples. This discovery further confirms that these blocks are not natural limestone.

Egyptologists are consistently confronted by unanswered questions: How is it possible that some of the blocks are so perfectly matched that not even a human hair can be inserted between them? Why, despite the existence of millions of tons of stone, carved presumably with copper chisels, has not one copper chisel ever been found on the Giza Plateau? At the end of their most recent paper reporting these findings, the researchers reflect that it is “ironic, sublime and truly humbling” that this 4,500-year-old limestone is so true to the original that it has misled generations of Egyptologists and geologists and, “because the ancient Egyptians were the original-albeit unknowing-nanotechnologists.”

To counter this, Dipayan Jana, a petrographer, made a presentation to the ICMA (International Cement Microscopy Association) in 2007 and gave a paper in which he discusses Davidovits' and Barsoum's work and concludes "we are far from accepting even as a remote possibility of a “manmade” origin of pyramid stones."

So there is still much controversy, but what a fascinating subject this is. I would like to know what other researchers in the field of geopolymerisation have to add to this, and hopefully talking to a few of them at the conference in May.