Structure and properties of the fiber cement
In its structure the fiber cement board is not uniform monolithic material, and it has special regular structure. This structure provides the material with the increased mechanical impact strength and fracture strength. The fiber cement gets such structure thanks to the principle of its manufacture.
Suspension consisting of cement, fibers of cellulose, grains of sand and water sediments on a surface of a wide tape made of fabric through which liquid leaves solution. This process creates a film with thickness of from 0,25 to 0,40 mm on the surface of fabric. Further such films are overlapped against each other and pressed in a sheet of the fiber cement to form a board with the necessary thickness. As a result the sheet 8 mm thick will contain from 20 to 30 or even more such layers.
The cement and fibrous layers obtained by means of sedimentation of the suspension on fabric are non-uniform in composition - one their side is richer with fibers, than another. It is the result of filtration on fabric which passes the ultimate particles of mix through the openings, but detains the cellulose fibers which usually have length from 2,5 to 3 mm. At some point the fibers cover the openings of a sieve and small particles of suspension sediment already on them. This ability of fibers to form the lower layer is crucial for the organization of structure of future sheet of the fiber cement. As a result the part of the film which is formed first is much richer with fibers. The layers sedimenting afterwards are much poorer in fibers in comparison with lower layers adjacent to the filter fabric.
Then the layers formed in Hatschek machine are overlapped against each other to get the needed thickness of a sheet of the fiber cement. As the top and lower surfaces of each fiber cement film are differently rich with cellulose fibers (the top surface contains more particles of cement and mineral filler — sand, and the lower surface contains more fibers), the structure of the formed sheet turns out layered, similar to internal structure of a tree. Such structure gives to material the special strength and ability to withstand loads of bending. In cases when monolithic material just breaks when bending or at the localized impact, the fiber cement will withstand and even return a primary form after removal of loading due to elasticity of the layered structure. Such internal structure of the fiber cement has also some minuses which, however, successfully are solved by means of applying special techniques and means.
For example, because of hydrodynamic processes at suspension sedimentation Hatschek machine has a tendency to level fibers in the direction of the movement of the filtering tape, as if "drewing out" them from solution. It turns out that the subsided cement and fibrous film is stronger in the longitudinal direction, than in cross direction. The majority of models of Hatschek machines have the devices of determination of more uniform orientation for fibers helping to compensate this shortcoming. However, usage only of one this device doesn't give absolute efficiency, can't solve completely a problem - the obtained sheet of the fiber cement would have quite noticeable difference between values of longitudinal and cross strength.
The engineering solution of this problem was found. Combination of 20-30 fiber cement films in one sheet takes a lot of time therefore the majority of Hatschek machines have several tapes of formation of films, creating 3 or even 6 layers of future sheet at the same time. In such cases the devices of fiber orientation on each of these 3 or 6 tapes place them in the opposite direction against the previous layer. It gives more guarantee that the sheet will be made with symmetric mechanical properties in all directions. Other feature of the layered structure of the material is that the fiber cement boards are more moisture permeable from the sheet end faces. This is because the regular layers create kind of network of fibers paralleled to layers of the board. However, and this problem is solved by coating a special hydrophobic covering, with the subsequent coating with several coats of paint that successfully protect end faces and surface of the fiber cement boards from moisture. Processing of end faces of the fiber cement panels in plant conditions gives reliable protection for many years to come.
The technology developed at the beginning of the 20th century by Ludwig Hatschek for manufacture of the roofing materials develops and is modernized constantly, allowing to manufacture modern asbestos-free durable material nowadays - fiber cement panels and siding.