Due to the complex mineralized components of different cements and the variety of additives, the technical effects of additives for different types of cement are quite different. Therefore, additive has the problem of compatibility (adaptability) to different cement. This is a technical problem that has puzzled the builders for a long time.
Gypsum is used as a coagulant of cement, and its dosage basically matches the content of C3A in cement. After adding water, a certain amount of ettringite is formed in the gypsum cement, which is adsorbed in C3A to control the hydration of C3A and play a role in regulating the setting time of cement. Gypsum dihydrate (CaSO4. H2O) has the best water solubility, so dihydrate gypsum is often used in cement production. However, gypsum is often grinded with cement clinker in cement production. When the grinding temperature is too high, a large amount of dihydrate gypsum will be transformed into hemihydrate gypsum (caso4. 1/2 H2O) or anhydrous gypsum (CaSO4), namely anhydrite. Some cement plants will also directly use anhydrite or some industrial waste gypsum, such as fluorogypsum, desulfurization gypsum, phosphogypsum. Anhydrite and the above-mentioned waste gypsum have poor water solubility and slow dissolution in water. Retarders such as calcium lignosulphonate or calcium sugar with high cost performance are usually added into the additives, and the addition of these water reducer will affect the solubility of gypsum. Because gypsum can not be dissolved quickly, C3A in gypsum cement will be hydrated rapidly, and a large number of calcium aluminate crystals will be produced, which will cause false setting of concrete.
In order to prevent false setting of cement mixed with anhydrite cement or other gypsum with poor water solubility, it is better not to use water reducer, such as lignite, sodium lignosulphonate and calcium sugar, which affect the dissolution of gypsum. The results show that it is effective to control the dosage of water reducer. It can be mixed with a large amount of additives which can supplement SO3 in cement, and also can control false setting.
