1. Fast-setting agents primarily composed of aluminium sulfate
The main component responsible for the fast-setting action in this agent is aluminium ions. However, due to the low solubility of aluminium sulfate, the solution is unstable, and it generally has the drawbacks of high dosage, low early strength, and significant loss of strength in the later stages.
2. Fast-setting agents primarily composed of aluminium sulfate and sodium aluminate
Aluminium sulfate and sodium aluminate can react under certain conditions to produce polymeric aluminium sulfate, which increases the aluminium ion content in the solution. However, the stability of polymeric aluminium sulfate is poor, and currently, stabilisers and pH adjustment methods are mainly used to control the hydrolysis of aluminium ions, extending their storage period. This type of fast-setting agent has a minimal impact on the later strength of cement and is well-suited for different types of cement. However, fundamentally, the addition of sodium aluminate inevitably introduces sodium ions, which contradicts the trend of developing fast-setting agents towards being alkali-free.
3. Fast-setting agents primarily composed of aluminium sulfate and aluminium hydroxide
Aluminium sulfate and aluminium hydroxide can directly react to produce polymeric aluminium sulfate, simplifying the reaction process of aluminium sulfate and sodium aluminate, which first generates aluminium hydroxide and then polymeric aluminium sulfate, while maximally introducing aluminium ions. The advantage is that it does not introduce alkali metal ions, but the disadvantage is that the proportion of aluminium hydroxide used is relatively large, leading to higher costs, and this type of fast-setting agent has poor stability, is prone to precipitation and crystallisation, and is not conducive to long-term storage and use.
4. Fast-setting agents prepared by combining aluminium sulfate with other components
Aluminium sulfate can also be combined with sodium fluoride, magnesium sulfate, etc., to form fast-setting agent products. However, both sodium fluoride and magnesium sulfate have certain drawbacks. For instance, sodium fluoride can promote the formation of cement hydration products, shorten the setting time of cement, and enhance the strength of concrete, while also acting as a complexing agent that can form a stable complex system with aluminium sulfate, increasing the stability of aluminium ions in aqueous solution. However, the drawback is that it introduces alkali metal ions. Magnesium sulfate can enhance the early strength of concrete and improve the adaptability of fast-setting agents to cement, but excessive dosage can lead to an increased possibility of generating secondary ettringite due to the introduction of excessive sulfate ions, reducing the durability of concrete.
