Sulphoaluminate cement was developed and popularized by Wang Yanmou and Su Muzhen and other cement scientists in the early 1980s. After more than 30 years of research, it has been known as the third series of cement besides Portland cement and aluminate cement in China. Because of its main component 3CaO · 3Al2O3 · CaSO4 (anhydrous calcium sulphoaluminate, C4AS̅), the cement has excellent characteristics of early strength, high strength, frost resistance, impermeability, corrosion resistance and low alkalinity, so it is widely used in winter construction, hydraulic engineering and repair and other special projects.
The early strength and rapid hardening characteristics of sulphoaluminate cement are due to the rapid reaction of anhydrous calcium sulphoaluminate with gypsum and water to form ettringite. As a new variety in the history of world cement development, it shows a very optimistic development prospect today. However, due to the rapid slump loss, difficult control of setting time and uneven slurry of sulphoaluminate cement concrete, it is difficult to meet the construction requirements of practical projects, which seriously hinders the popularization and application of sulphoaluminate cement.
Sulphoaluminate cement is made of clinker containing C4A3S and 2CaO · SiO2 (dicalcium silicate, C2S) that is mixed with CaSO4 · 2H2O (dihydrate gypsum, CS̅H2) or CaSO4. The difference in composition of sulphoaluminate cement is mainly due to the different amount of gypsum. To study the hydration process of sulphoaluminate cement is essentially to study the chemical changes in the Quaternary System of C4A3S̅-C2S-CS̅H2-H2O. Its hydration products are AFt, C-S-H gel and Al2O3 · xH2O (aluminum glue, AHx-gel), etc. The main product of sulphoaluminate cement hydration is AFt, and the form of aft is different from various hydration conditions and ages. In the presence of a large amount of water, the sulphoaluminate cement will hydration quickly to form acicular AFt. In the sulphoaluminate cement paste with gypsum quality less than 15%, rod-shaped AFt will be formed. When the quality of gypsum is 25-45%, dense short columnar aft will be formed. Under appropriate conditions, tubular AFt will also be formed.
When the gypsum is insufficient and the reaction can not reach equilibrium, 3CaO · Al2O3 · CaSO4 · 12H2O (calcium sulphoaluminate hydrate) will be formed. As the hydration of clinker in cement paste is difficult to reach equilibrium, a small amount of hydrated calcium sulphoaluminate often exists in sulphoaluminate cement paste in addition to the above hydration products. The CH generated after hydration of dihydrate gypsum will react with other hydration products to form new compounds.
After contacting with water, the cement particles begin to react rapidly. At the final setting of cement paste, the degree of hydration can reach 50-60%, and about 70% after one day. Then the hydration gradually slows down, and the final hydration degree can reach about 80%. When anhydrous calcium sulphoaluminate is hydrated with a large amount of gypsum, the gypsum has been consumed about 70% at the initial setting. Since then, the content of CaSO4 remains unchanged, and the amount of aft does not increase any more. Anhydrous calcium sulphoaluminate continues to hydrate to form monosulfide calcium sulphoaluminate (AFm). However, from the 7th day, the content of AFt decreases gradually while the content of AFm increases, which indicates that AFt dehydrates and transforms into AFm. This is the reason why the strength of sulphoaluminate cement no longer increases or even shrinks slightly at the later stage.
