The material aircrete
The material aircrete (Autoclaved Aerated Concrete, AAC) and how to make it
The construction material aircrete consists out of 60 % to 85 % of air by volume. The solid material part is a crystalline binder, which is called Tobermorite by mineralogists. Besides the binding phase Tobermorite you find grains of Quartz and in minor amounts some other minerals. The chemical composition of Tobermorite shows siliciumdioxide, calciumoxide and H2O (water). It is Tobermorite which provides the high compressive strength of aircrete in spite of the high proportion of pores in this construction material. The photograph shows Tobermorite crystals inside of an air pore. The picture was taken with a scanning electron microscope.
The essential raw materials used to make aircrete are lime, sand of Quartz, and water. In most cases there will be also used cement, Gypsum or Anhydrite and sometimes, as a substitution to Quartz, fly ash and ground metallurgy slag. These latest ones and the Quartz sand provide siliciumdioxide (silica) for the formation of Tobermorite. Calciumoxide is given by the lime. If there will be used cement too, then by this way both calciumoxide and siliciumdioxide will be put into the mixture. So, all together with water, the main raw materials to produce aircrete are provided.
But, how put the many many pores inside of this construction material? Well, it is quite similar to baking a cake. As gas forming agent aluminium powder or paste is taken. The very tiny aluminium flakes produce a lot of small bubbles of Hydrogen in the slurry of the raw material mixture. This way the mixture rises in its moulds until all aluminium has reacted and the desired volume is reached. You need only a very small amount of aluminium powder.
The bubbles formed are mostly about 1 mm in diameter. Hydrogen is a very volatile gas, so it is replaced rapidly by air.
The mixture is getting hard soon after expanding process has ended. It also increases in temperature up to over 80°C. After a certain time of setting the material is hard enough to be cut into pieces of desired format. A machine with wires arranged at certain distances makes this step of production process.
Leaving the material at room temperature and in air would not result in sufficient strength, it is necessary to cure the material in saturated steam at high pressure. This last step in process of producing aircrete has to be done in autoclaves at 180 to 200°C. After some hours aircrete is finished. All the calciumoxide for the formation of Tobermorite is consumed, only a residual of siliciumdioxide remains and can be found as small grains of Quartz.