Calcareous clays, that is clays with appreciable quantities of calcium carbonate, are more properly considered as varieties of marl, and as such they are not normally very good for brickmaking. Yet in many countries such marly deposits are the only raw materials readily available in quantity. This article from the Egyptian National Research Centre at Dokki, near Cairo, describes successful tests on bricks made with a marly clay taken from the extensive deposits near the Bakr oil field in the Gulf of Suez. The material contained up to 30 per cent by weight of calcite (calcium carbonate). The benefits of extrusion forming are described, as well as the advantages of prepugging and repugging. Because of the high plasticity of the material, ordinary desert-type sand could be added with consequent advantages, including the possibility that the sand grains provided an exit path during firing for the escape of carbon dioxide as the calcium carbonate broke down. Controlled burning at a slower rate was also an advantage. The result was a brick of acceptable quality and crushing strength.
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