A follow-up letter to the New Scientist article about the concrete-in-the-Great-Pyramid speculation (9th December 2006, p.6) has appeared in the most recent issue of New Scientist on the Letters page (27th Janaury 2007, p.21). The letter challenges the claim that the discovery, if accepted, would put the earliest date for concrete at 2500 years before the previously recorded first use. The letter writer, Stan Wild (Craven Arms, Shorpshire, UK) writes: "Concrete is known to be much older than this. One of the earliest examples is a hut floor dating from 5600BC in what is now Serbia. It consisted of a mixture of red lime, sand and gravel." He goes on to discuss other early uses of mixtures that could be labled "concrete" before this date as well.
The letter is published in full on the New Scientist website at the above address.