Great Pyramid at Giza

The earliest known mortar was used by the ancient Egyptians and was made from gypsum. This form was essentially a mixture of plaster and sand and was quite soft. The Egyptians mixed limestone and gypsum together to make the earliest known mortar for use as a building material. It was used to fill the joints between the facing stones made of brilliant white polished limestone on the great pyramid at Giza. This remarkable structure is the only remaining member of the Ancient Wonders of the World.

Yet, this mixture was not very strong. Why was it used? Luke M. Snell and Billie G. Snell have provided a theory that this mixture reduced the sliding friction by about 50% between the large heavy stones and allowed them more readily to be set into place. The Eqyptians didn't need the mortar for strength.

The particular mixture of gypsum, limestone and sand used by the Egyptians appears to have the same composition as raw gypsum mined in the area. Thus, the Egpytians probably just ground up gypsum from local deposits and used it between the stones for lubrication, bonding or both.

Port Cosa

The Romans discovered that if they mixed lime with pozzolana, an often red clay found in many parts of Italy, it formed a substance as hard as natural stone. Pozzulana is a fine, sandy volcanic ash first dug at Pozzuli near Vesuvius. Vitruvius speaks of four types of pozzolana with black, white, gray and red colors. They could make it into any size or shape and even form it under water as was the case with the Roman port at Cosa where three piers are still visible. Many of these Roman creations are still standing with some still being used over 2,000 years later. Notably, pozzolan is used today to make stronger and more alkali-resistant cements. The use of pozzolan in making cement was lost in the dark ages along with the process of cement making and had to be rediscovered much later.

In 1824, the Englishman, Joseph Aspidin (1788-1855) invented Portland cement. He named it Portland cement because it resembles a natural stone found on the Isle of Portland, Dorset. Portland stone is limestone from the Jurassic period.

He created his cement by burning a mixture of ground limestone and clay to form clinker. The clinker can be easily transported as is. To make the final product, the clinker is ground up and a few percent gypsum added. This final mixture contains oxides of calcium (from the limestone), silicon and aluminum (from the clay). The gypsum (CaSO₄·2H₂O) provides a source of sulfate. Gypsum is the primary material in drywall.

Cement Kiln

Today, cement is made from limestone mixed with clay or sand with small amounts of other materials such as bauxite (aluminum ore) and iron ore added to provide specific properties. The raw materials are finely ground, mixed and fed into a rotary kiln, which is the largest piece of moving machinery in the world. Temperatures in the kiln reach 2700°F (1480°C). As the kiln rotates, the materials move to progressively higher temperatures and undergo complex chemical reactions. First, limestone (CaCO₃ is calcined into lime (CaO) by removal of carbon dioxide (CO₂). Next, calcium oxide reacts with the silicates in the clay to form dicalcium silicate (Ca₂SiO₄ and tricalcium silicate (Ca₃SiO₅. With bauxite and iron ore present, tricalcium aluminate (Ca₃Al₂O₆) and tetracalcium aluminoferrite (Ca₄Al₂Fe₂O₁₀ are also formed. The final reactive ingredient is the small amount of gypsum added after the kiln clinker has been ground to a powder. About one-half of the weight of portland cement is tricalcium silicate.

Portland Cement

Adding an aggregate such as sand or small stones to cement creates concrete, which results in a strong building material. Adding fly ash, the residual left from burning garbage, will act as a retardant and slow down the time it takes for the cement to harden. This is important when larger structures are made of concrete, such as a dam. When the concrete hardens heat is created. As the material cools, it shrinks and will crack if the cooling is too rapid. The heat also can drive off the water resulting in premature drying and an incomplete chemical reaction so that the concrete is much weaker.

The water reacts with the various anhydrates in the cement in a chemical reaction, hydration. This is not a drying process but an incorporation of the water into the material. The reaction of water with cement is strongly exothermic, producing a great deal of heat. The reaction also releases hydroxide ions, which make the material very alkaline. Do not put your hands into freshly mixed, fluid concrete because the combination of heat and alkalinity can dissolve your skin quite readily. Here is the reaction of water with tricalcium silicate.

NaCl or salt, when added to cement results in the production of concrete that will degrade in a short length of time. Several disasters have occurred because the concrete was made of sand from a beach that contained a lot of salt.

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Contents copyright 2006 by Dr. A. V. Persson and ParaComp, Inc. All rights reserved.

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