Gypsum can be used as a raw material, mineralizer, retarder and activator in cement plants. Among the components, SO2 is used to balance the sulfur-alkali ratio in the calcining of clinker and improve the calcining operation and the service life of refractory materials. Roles of gypsum:
1. As a raw material, it can be used in the production of CSA cement, slag-gypsum cement, gypsum bauxite expansive cement, etc.;
2. As a mineralizer, it can reduce the calcining temperature and save coals;
3. As a cement retarder, it makes the cement setting time in line with national standards and user requirements by its retarding effect;
4. As a sulfate activator, it can stimulate the activity of industrial waste residues such as flyash and slag and improve the strength of flyash cement and slag cement;
5. An appropriate amount of gypsum can improve the cement performance, such as strength, shrinkage and corrosion resistance;
6. Adjust the sulfur alkalization in clinker to reduce the possibility of crust and blockage and improve the compatibility with concrete.
Gypsum makes up only about 3% or less of cement, but it plays an important role in cement. The application of gypsum in cement is mainly to delay the setting time of cement, which is beneficial to the mixing, transportation and construction of concrete. If there is no gypsum in the cement, the concrete will set quickly in the mixing process, resulting in the failure of stirring and construction.
The reason why fast setting occurs in the cement without gypsum after mixing with water is mainly because the C3A in the clinker dissolves rapidly in water and quickly forms calcium aluminate hydrate, which makes the cement paste set fast. In order to avoid this abnormal fast setting phenomenon, it generally needs to add an appropriate amount of gypsum in the cement to adjust the setting time of cement.
The delayed coagulation on cement clinker produced by mixing a moderate amount of gypsum is generally thought that when cement is hydrated, gypsum reacts quickly with C3A and Ca (HO) 2 to form water-insoluble calcium sulfoaluminate hydrates (that is ettringite C3A•3CaSO4•Ca (HO) 2), which can form an coating layer on the surface of C3A particles, prevent further hydration of C3A and reduce the solubility of aluminate in the solution, so that the hydration products of calcium aluminate cannot be separated. In this way, the setting time will be determined not by C3A, but by the increase of the concentration of the slower-reacting C3S colloidal solution itself, thus delaying the setting time of cement. Gypsum makes up only about 3% or less of cement, but it plays an important role in cement. The effects of gypsum on cement are analyzed from the following two aspects: flash setting and false setting.
The main chemical component of Gypsum is calcium sulfate, whose main chemical components are 32,5% CaO, 46.6% SO3, 20.9% H2O+, and the components don't change much. There are often clay, organic matter and other mechanical mixtures in it, and sometimes it contains SiO2, Al2O3, Fe2O3, MgO, Na2O, CO2, Cl and other impurities.
There are four stages of discharging crystal water when gypsum is heated:
(1) At 105-180℃, 1 water molecule is firstly discharged, then half of water molecule is immediately discharged, and gypsum is transformed into calcined gypsum Ca[SO4]•0.5H2O, which is also known as plaster of paris or hemihydrate gypsum.
(2). At 200 - 220 ℃, the rest half of water molecule is discharged, and the calcined gypsum is converted into type Ⅲ anhydrite Ca [SO4]•εH2O (0.06<ε<0.11).
(3). At about 350 ℃, Ca [SO4]•εH2O (0.06<ε<0.11) is converted into type Ⅱ anhydrite Ca (SO4).
(4). At 1120 ℃, the type Ⅱ anhydrite Ca (SO4) is further converted into typeⅠanhydrite. The melting temperature is 1450℃.
When there is no gypsum in cement, the tricalcium aluminate (C3A) in cement will react quickly with water, and tricalcium silicate (C3S) will also have a significant hydration effect. If the content of C3A is high and the specific surface area of cement is large, it is likely to produce flash setting. The hydration process is accompanied by significant heat release and is followed by hardening of the cement paste, which is known as flash setting. The flash setting makes the fresh concrete lose its plasticity, which is very unfavorable to the concrete production, therefore, appropriate amount of gypsum should be added in cement grinding. When the content of C3A is high, more gypsum should be added, but too much gypsum will produce adverse effects and lead to false setting even poor stability of cement.
False set is caused by the formation of gypsum after the re-hydration of hemihydrate gypsum or soluble anhydrite. The dissolution of hemihydrous gypsum causes the solution to be supersaturated, resulting in the precipitation of gypsum and the hardening of the slurry. But this structure can be destroyed by stirring again and the cement will return to normal setting, so there is an essential difference between false set and flash set.
There is also an abnormal type of setting called air setting, which can happen when cement is exposed to certain wet conditions. This setting often occurs in cement storage caverns and can result in cement mass. The setting occurs because the sulfuric acid alkali in cement absorbs water. It is difficult to disperse the cement mass produced by this kind of setting, which also has a negative effect on the properties of cement concrete.