Aluminous Cement
Aluminous cement is also known as ‘calcium aluminate cement’, ‘high alumina cement’, and by its original French name, ‘ciment fondu’. Unlike traditional cements which are comprised largely of calcium silicates, aluminous cements are made with calcium aluminates. Aluminous cements were invented in France in the early 20th century, and patented in 1908. Originally, aluminous cements were invented as alternatives to existing types of cement which did not have good sulfate resistance, and would often expand and distort when they came into contact with the ground and ground water.
However soon after their invention it was discovered that over and above being resistant to sulfates, aluminous cements also develop strength very quickly, have a high chemical resistance, and some blends maintain integrity at high temperatures as well. Of course, the trade off for these high performance specifications is price, and aluminous cements are a great deal more expensive than ordinary Portland cement.
Uses of Aluminous Cement
Aluminous cement is only used in a very small number of applications, due to its special qualities and its high cost. The sort of construction projects which require the use of aluminous cement include those which will be exposed to high chemical levels, or high temperatures, or applications which require fast high strength development. Aluminous cement is also therefore quite often used as a component of various specialist construction concretes which are used to construct buildings and other structures which will be exposed to concentrated chemicals or extreme temperatures.
Making Aluminous Cements
Aluminous cements are made in a process similar to that by which Portland Cement is made, however instead of using limestone and silica as the primary raw materials, limestone and bauxite are commonly used instead. Refined alumina is also used in the case of white aluminous cement or refractory (high temperature) cements.
At the beginning of the manufacture of aluminous cement, the raw materials are transported from various silos by use of a conveyor belt, which carries them to a raw mill. The purpose of the raw mill is to grind down the chunks of raw material into small pieces which are all of a similar size. It is important that the materials be of similar size so that when they melt in the kiln, they do so evenly.
Once they have been thoroughly ground up in the raw mill, the raw materials are sent on to the blender, where they are mixed together. The blended raw mix is analyzed carefully at this stage to ensure that the correct portions of materials are present. Perhaps the most important aspect of any form of cement production is controlling the relative ratio of raw materials in the raw mix. If the raw materials are present in the wrong proportions when the raw mix enters the kiln, the resulting product will be faulty.
Once blended, the raw mix is then sent through into the kiln where it is melted and where calcium aluminates form in the extreme temperatures. The result of the firing process are small pebble looking nodules of material which are called ‘clinker’.
In order to transform the clinker into cement, it must be ground thoroughly until it is a very fine powder. This is carried out inside a rotating cylinder where the clinker is mixed with hard balls, and gradually broken down to a very fine powder. This powder is aluminous cement, and is ready for use.
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