Desulfurisation gypsum is an industrial by-product obtained from the desulfurisation and purification of coal-fired flue gas. It is produced by grinding quicklime or limestone and reacting it with sulphur dioxide in the flue gas to form calcium sulfate dihydrate with fine particles, high water content and a large specific gravity. Its fineness generally ranges from 40 to 60 um. (The fineness is greater than 160 is 0.7%, the fineness between 160 and 180 mesh is 4.23%, the fineness between 180 and 200 is 83.53%, the fineness between 200 and 250 is 9.03%, and the fineness less than 250 is 2.4%).
The particle size curve is narrow and lean, so when plastering plaster is prepared using desulphurised building plaster alone, it will result in problems such as poor slurry and workability, poor bonding properties, low water retention, serious segregation and delamination, and a large apparent density of the product. In other words, during the production of desulphurised building plaster, the particle size distribution of desulphurised gypsum can be changed using different calcining equipment and processes, but they all tend towards finer direction, while the construction gypsum required for preparing plastering gypsum is not the finer the better. The fineness of desulfurized construction gypsum is fine, the specific surface area is large, the particle size range is narrow, the water consumption of the standard consistency is high, the thixotropy and workability are poor, the anti-sagging property and construction operability are also poor, and the surface finish of gypsum products is not good.
Therefore, sufficient attention should be paid to the particle characteristics of desulfurisation gypsum, and if necessary, further process treatments such as grinding, compounding, or recrystallisation should be carried out. The plaster gypsum products we need also need to pay attention to the relevant properties in construction and application, such as workability, sag resistance, crack resistance, prevention of bleeding and delamination, the uniform dispersion of the slurry during mixing (whether there is agglomeration), and other construction and operational properties. A good plaster gypsum must be a product with good technical properties and construction and application properties.
Precautions for preparing plastering gypsum
Most plastering gypsum is made from a mixture of semi-hydrated gypsum and anhydrite, with the addition of retarders, water retention agents and other admixtures, as well as other aggregates.
When preparing plastering gypsum, it can be found that different gypsum raw materials, different calcination conditions, different finenesses, and different water-gypsum ratios will all affect the stability of the product. The stability of plastering gypsum products is currently the most important issue with plastering gypsum.
It is very important for us to obtain construction plaster with stable properties. This is because gypsum is prone to phase mixing during calcination, which has a significant effect on the setting time of the plaster.
In actual production, if the retarder cannot properly adjust the setting time (sometimes no matter how much retarder is added, it cannot extend the setting time of the plaster to the required time), it is necessary to first check whether the construction plaster powder contains a large amount of type III anhydrite or under-fired dihydrate gypsum.
Type III anhydrite is also known as dehydrated hemihydrate gypsum. Its crystal phase structure is the same as that of the original hemihydrate gypsum. Type III anhydrite is unstable and will quickly hydrate to hemihydrate gypsum when it comes into contact with moisture in the air. This unstable phase is unavoidable when producing hemihydrate gypsum, but it can be converted to hemihydrate gypsum through an aging process. If this is not taken into account, the use of semi-hydrated gypsum to prepare plastering gypsum is likely to result in unstable performance. The main manifestation of this is that when the same amount of retarder is used, the setting time of plastering gypsum varies from batch to batch. The reason for this is that the type III anhydrite in the semi-hydrated gypsum plays a role in accelerating the setting process during the hydration of the gypsum. When the proportion of type III anhydrite in the semi-hydrated gypsum is different, the effect of accelerating the setting process is also different. This is the main reason why many manufacturers cannot obtain a product with stable performance when using semi-hydrated gypsum to prepare plastering gypsum.