THE SVANHOLM GROUP OF COMPANIES

PRODUCT Autoclaved Aerated Concrete AAC is manufactured from Silica Sand, Cement, Lime and Water. It is aerated with help of Aluminium Powder and it is Autoclaved. The Autoclaving is an important feature that saves energy and raw material and gives superior properties. Sand The ideal sand consists to 100% of SiO2. A chemical analysis of the sand should be made and a minimum of 80% is desirable. 90% and up is preferred. During the autoclaving the lime in the binder reacts with water and silica to form calcium silicate hydrates. The preferred binding material is 11Å-Tobermorite. Cement Portland cement is a finely ground mixture of man-made minerals. These minerals can combine with water to form crystals of various mineral hydrates and these mineral hydrates bind the aggregate grains together. The cement is manufactured by a high-temperature process that is very energy consuming and the cement is the sole binder in ordinary concrete products. Lime The main binding agent in AAC is usually quicklime. Quicklime is slightly less expensive to produce, but this is not so important. Important is that in the autoclave a part of the silica will combine with the lime so that half of the mineral hydrates comes not from the binder but from the aggregate. Thus considerable savings in cost and energy are possible. The autoclave will also consume energy but at a very much lower temperature than that of a cement or lime kiln so the total energy expenditure is low. Water The water is used for two purposes, for generating steam and in the process. Ordinary drinking water is practically always suitable. Most of the waste water will be possible to reuse in the process. Aluminium powder This product is aerated by aluminium powder. When the aluminium is mixed together with the other ingredients it will develop hydrogen gas (H2) which will raise to the surface together with the solids. It is during this process that the specific structure of AAC is obtained. Autoclaving The matrix of AAC is formed during autoclaving. The preferred binding material is 11Å-tobermorite. This material is formed from lime, silica and water. It is possible to make foamed or aerated concrete without autoclaving but the properties will be very bad. Of course no coarse aggregates can be used and the formula will contain mostly cement. The strength will not be very high but perhaps sufficient. Shrinkage on drying will, however be excessive. As a rule such products will start cracking by themselves when they approach equilibrium moisture content after a couple of years. A mixture of mostly cement and water shrink considerably when setting but will shrink more on drying. In ordinary concrete products, this is no problem since there is so much hard non-shrinking aggregate in the mix, but in foamed or aerated concrete the aggregate is air and if manufactured without autoclaving, drying shrinkage will cause trouble. The calcium silicate hydrates formed during autoclaving are more stable than these formed from cement at room temperature. Autoclaved products have low shrinkage and give no problems. Autoclaving is a relatively fast process. Cement will need four weeks to attain final strength at room temperature whereas autoclaving takes much less than a day. Properties It is possible to vary the properties of AAC to a certain extent by producing AAC of different densities. As the density alters the compressive strength and the thermal conductivity are also changed. Density Normally the density of AAC varies between 400 - 600 kg/m3. Ordinary cement based concrete blocks have approximately four times the density of AAC. Compressive strength The compressive strength of a light weight material will be less than of a heavy weight. The compressive strength of AAC is sufficient to allow building of three-story residential buildings with on site poured concrete floors, with AAC as loadbearing and insulating single skin walls. This was for many years a very popular type of construction in Sweden. Thermal conductivity AAC insulates against heat or cold more than ten times as much as ordinary concrete does. AAC is used in the most varying climates and, if proper construction methods are used, Frost, Damp or Pests make no harm. AAC is especially resistant against fires and is commonly used in fire retarding walls. Fire rating tests vary from country to country but as illustration we can mention that in England, a loadbearing wall that shall keep fire out for two hours may be constructed in 4” AAC. If the wall is not loadbearing it will be considered to stand the fire for four hours. (British Standard 476, Part 1) The AAC Block AAC can be made into many types of products and we have elected to manufacture precision blocks and mini panels. The length of the precision block is 24”, the height is 8” and the thickness 4, 6, 8, 10 or 12”. The length of the mini panel is 24”, the height is 32” and the thickness 4, 6, 8, 10 or 12”. The precision block is not laid in ordinary mortar. Instead a special thin joint compound is used. This comes as a dry mix in sacks and shall be mixed with water according to the instructions on the sacks. A special trowel or scoop with teeth is used to lay on the jointing material, which is usually but not quite correctly referred to as “glue”. The resulting joint is only 1/8” thick. The mini panel is also laid in thin joint mortar in the same way as the precision block. To be able to lift the mini panel in place you use a special design light weight crane. Because of the even joints, which are achieved with scoop and thin joint mortar, no special masonry skill is needed. Any worker can quickly learn to lay excellent walls. In Sweden a team of a block layer and a helper are expected to erect at least 40 sq ft of wall in one hour. The thin joint mortar is in itself more expensive than ordinary mortar but since the joints are thin and there is very little waste, the cost for mortar is much less than when normal blocks are used. AAC is more a “Carpenters material” than a masons. It can easily be shaped with ordinary wood-working tools and it is quite usual to use nails. It is standard practice to let openings and gables have irregular shape and to later give final shape with the saw. Ducts for electricity, water etc. are cut with simple tools. Larger holes for junction boxes are made with Brace and bit.