Dr. Michel Barsoum, Professor of Materials Engineering at Drexel University in Philadephia, and colleagues, materials science and engineering alumnus Dr. Adrish Ganguly and Dr. Gilles Hug of the National Center for Scientific Research in France, published a peer-reviewed paper in the December 1, 2006 edition of the Journal of the American Ceramic Society stating that the Great Pyramids of Giza are constructed with a combination of carved stones and the first blocks of limestone-based concrete cast by any civilization. Barsoum and his team's paper challenges the longstanding belief that the pyramids were constructed entirely of limestone blocks cut to shape in nearby quarries using copper tools and transported to the pyramid sites, hauled up ramps and hoisted into place with the help of wedges and levers. Barsoum and his team posit that, although the majority of the stones were carved and hoisted into place, crucial parts were not. The paper argues that ancient builders cast the blocks of the outer and inner casings of the pyramids and, most likely, the upper parts of the pyramids using a limestone concrete, called a geopolymer. To arrive at their findings, Barsoum and his colleagues analyzed more than 1,000 micrographs of stones from the pyramids and their vicinity, over a two-year period. To construct the pyramids only with cast stone would have required an unattainable amount of wood and fuel to heat lime-used to reconstitute the stone - to 900 degrees Celsius. The hybrid technique discovered by the team provides answers to how the pyramids were constructed with such precision. The findings also answer how steep ramps could have extended to the summit of the pyramids; builders could cast blocks on site without having to transport stones great distances. By using cast blocks, builders would have been able to level the pyramids' bases to within an inch with relative ease. Finally, builders were able to maintain precisely the angles of the pyramids so that the four planes of each arrived at a peak. The team's research also discusses how the geopolymer used for the pyramids' construction could be utilized today as a form of concrete to provide developing nations a way to build aesthetically pleasing structures with inexpensive and easily accessible materials. According to Barsoum, the raw materials used to produce the concrete in the pyramids-lime, limestone and diatomaceous earth-can be found worldwide and are affordable enough to be important construction materials for developing countries. Additionally, this concrete would reduce pollution and outlast Portland cement, the most common type of modern cement. Portland cement injects a large amount of carbon dioxide into the atmosphere and has a lifespan of about 150 years. If widely used, this geopolymer-based concrete would reduce that amount by 90 percent and presumably last significantly longer.