posted 07-07-99 11:55 AM           

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Cement is a baked mix of clay and limestone which is chemically a complex mix of calcium silicates. Although known to the ancient egyptians and mastered by the Romans it was not used in medieval castles.
Medieval masons used lime mortars, chemically calcium hydroxide and an aggregate, usually sand but I have seen shells used. The calcium hydroxide was produced by baking limestone to produce quicklime (calcium oxide) which was then slaked (soaked in water) to make slaked lime (calcium hydroxide). The lime mortars harden as the dry and some of the calcium hydroxide is converted to calcium carbonate as it reacts to carbon dioxide in the air. Slaked lime was also used for whitewash and plasters. Since the slaked lime has a limited life castles may well have stores of the caustic quicklime readily available for repairs which could also have been used as a chemical weapon to be dropped onto besiegers.
Both cement and lime mortars have low tensile strength, which means that they don't stick things together very well. The main function of building mortars was to level the courses of stones up to ensure stability. Extremely solid walls can be built with no mortar at all if the stones are finely tooled so that the don't slip off each other (The Inca's were extremely good at this) but fine stone masons are much more expensive that mortar.

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Neurotics build castles in the air, psychotics live in them,
Psychiatrists charge the rent, art therapists do the interior design
and nurses clean out the garderobes!