Thursday, February 07, 2008

Ancient Egyptian Cement

Blogging on Peer-Reviewed Research
Barsoum et al., in their paper “Microstructural Evidence of Reconstituted Limestone Blocks in the Great Pyramids of Egypt” provide microstructural evidence that some of the blocks used to build the pyramids were made from a type of cement.

It should be noted that by “cement”, I am not talking about the modern cement that we all know and love. That sort of cement is called Portland Cement, as it was invented in an English town named after the capital of Oregon. It is made by first decarbonating calcite, in the reaction CaCO3 to CaO + CO2. As basic thermodynamics dictates, the side of the equation with higher entropy is stable at higher pressures, and gasses have higher entropy than crystals do. So all carbonate minerals will eventually disassociate into CO2 plus oxide. As explained at the Green Gabbro protolith, this reaction releases carbon dioxide, and any volatile toxic elements, into the surrounding atmosphere.

In Portland cement, this CaO is mixed with SiO2. CaO is a fairly unstable oxide, so when water is added, it reacts with the SiO2 to form an amorphous hydrated calcium silicate, which is what cement is. That is not what the Egyptians used.

The Egyptian cement is more of a synthetic caliche. It is made mostly of calcite and dolomite, and not calcium silicates. And the grain boundary cements appear to be precipitates, mostly carbonates So there was little, if any lime involved.

But in many ways the papers was very frustrating. The authors appeared to have no geologic or mineralogical background. As a result, the spent lots of time describing methods that are standard in microanalysis, and their descriptions of mineralogical associations were maddening in that what they chose to describe was sometimes extraneous, while standard textural information was not presented. The complete absence of jargon was also surprisingly annoying. Usually, I’m not a fan of jargon, but discipline specific language means that you have to actually look for the things that people expect you to find.

In the end, I have no problem with their conclusions, but the way in which they arrived at them seemed to be maddeningly circuitous. It is a shame they didn’t have a carbonate sedimentologist on the paper, who could have described the natural stone textures as well as the artificial ones.

Also, their quoted excitation volume seems to be suspiciously small, especially for a carbonate.

Reference:
M. W. Barsoum, A. Ganguly, and G. Hug. 2006 Microstructural Evidence of Reconstituted Limestone Blocks in the Great Pyramids of Egypt. J. Am. Ceram. Soc. 89 (12) 3788-3796.

4 comments:

andrew said...

Maybe Barsoum's white paper makes things clearer.

James said...

Hmmmmm... deliberate mistake to check whether readers are paying attention properly, I guess.

Portland cement WAS invented in England, in the early 19th century, but in Yorkshire not in Portland, Dorset - which was incidentally in existence a long long time before Oregon had a name let alone a capital. The cement was named for the supposed resemblance between a mortar made from it and the oolitic limestone - Portland Stone - which was (and is) quarried from the Isle of Portland.

Dr. Lemming said...

When was Dorset settled? Major Robers, the forst to use the name "Oregon" in print, claims that

"...from the Great Lakes towards the Head of the Mississippi, and from thence to the River called by the Indians Ouragon, which flows..."

Oregon, which was more or less unglaciated, is one place with significant pre-clovis remains (e.g.). I don't know if their name would have died out when they did, or was adopted by later indians.

14000 years ago, was the English Portland under ice or part of a barren inhospitable outwash plain?

As for the misattribution of cement origin, it is the sort of mistake that Americans are likely to make or miss during peer review.

Anonymous said...

The Capital of Oregon is Salem NOT Portland.