Magnesia binder
Magnesia binder is a binder used in construction . It consists mainly of finely ground caustic magnesia and is mainly used to make magnesia screed .
Raw materials and manufacture
The raw materials magnesite (MgCO 3 ) and dolomite (CaMg [CO 3 ] 2 ) are used to produce magnesia binders . The raw material is crushed and fired at temperatures between 800 and 900 ° C. When burning, the magnesite releases parts as carbon dioxide and caustic magnesia is formed. The caustic (corrosive) magnesia is reactive with water and must consist mainly of magnesium oxide with more than M-%, but can also contain secondary components such as carbonate, silica, aluminum oxide or iron oxide.
The burning of dolomite proceeds as follows:
Burning magnesite goes as follows:
The caustic magnesia is mixed with salt solutions of divalent metals to form a binder.
In 1867, the French physicist Stanislas Sorel discovered that caustic magnesia solidified into a stone-like mass with salt solutions made from divalent metals. This is why magnesia binder was also called Sorel cement in the past . This term is misleading because magnesia binder does not harden hydraulically like cement and has different properties.
Magnesia binder is usually made with magnesium chloride solution (MgCl 2 ), which is traded as a solid salt or aqueous solution. However, it can also be produced with various other salt solutions such as MgSO 4 , CaCl 2 or ZnCl 2 .
During the hardening process, complex basic magnesium salts are formed, such as Mg 2 (OH) 3 Cl ∙ 4H 2 O. At first a gelatinous mass is formed, from which needle-shaped crystals gradually separate. In addition, Mg (OH) 2 is formed from the absorption of CO 2 from the air - analogous to the hardening of lime - MgCO 3 .
The solidification of the magnesia dust may begin no earlier than 30 minutes after it has been mixed and should end no later than 5 hours after it has been mixed. The result is a marble-like mass that can be polished.
The mixing ratio of anhydrous magnesium chloride to magnesium oxide should be between 1: 2 and 1: 3.5. If there is an excess of the hygroscopic magnesium chloride, the hardened mixture tends to become saturated. On the other hand, too much magnesium oxide leads to a porous end product. When using MgSO 4 instead of MgCl 2 , there is no risk of moisture penetration, but only lower strengths can be achieved.
Magnesia binder is usually mixed with fillers to make magnesia mortar. Magnesia binder can be mixed with inorganic aggregates such as sand, pumice or corundum , as well as with organic aggregates such as sawdust , cork , paper , rubber or textile fibers. Color pigments are usually added.
In the post-war period, magnesia binder with sawdust was used as screed, so-called stone wood floors . Stone wood was used particularly in residential construction, but also for commercial purposes.
Properties and use
Magnesia binder can be used in screed mortars for the production of magnesia screed according to DIN 18560.
Positive properties are (depending on the filler):
- warm to the feet
- resilient
- tough
- resistant to shock and impact
- soundproofing
- non-skid
- not dusty
- resistant to gasoline and benzene
- electrically conductive
- low shrinkage
Due to its properties, it is used for dissipative substrates.
A major disadvantage of magnesia screed is that it is not moisture-resistant and is therefore not suitable for damp rooms or outdoors. In addition, due to the MgCl 2 solution, it contains highly corrosive chloride ions that can cause problematic corrosion in reinforced concrete components . Therefore, when handling magnesia binders, care must always be taken to equip metal parts with corrosion protection. In the case of reinforced concrete components, a barrier layer must be built in. Magnesia binders are not permitted in the vicinity of prestressed concrete components.
Another use of magnesia binder is in the production of wood wool lightweight panels from binder and wood flour or chips. When magnesite is burned at higher temperatures around 1600 ° C, sintered MgO is also formed, which is used for the production of high-refractory bricks.
Norms
- DIN EN 14016-1: Binders for magnesia screeds - Caustic magnesia and magnesium chloride - Part 1: Terms and requirements
- DIN EN 14016-2: Binders for magnesia screeds - Caustic magnesia and magnesium chloride - Part 2: Test method
- DIN 18560: Screeds in construction - Part 1: General requirements, testing and execution
literature
- Günter Neroth and Dieter Vollenschaar: Wendehorst building materials science - basics - building materials - surface protection , 27th edition. 2011, p. 305 f.
- Wilhelm Scholz, Harald Knoblauch and Wolfram Hiese: Knowledge of building materials . 2007.