A concrete expanding agent is an admixture mixed with cement and water that undergoes a hydration reaction to form ettringite or calcium hydroxide, causing the concrete to expand to a specific volume. Expanding agents into ordinary concrete can prepare shrinkage-compensating concrete and self-stressing concrete, so expansion agents are widely used.

The essence of concrete water permeability and seepage is due to the cracking of the concrete itself and the existence of connecting holes. To make concrete impermeable, new types of expanding cement are used to improve the compactness of the concrete structure itself – impermeability is an essential method of preparing waterproof concrete. As a separate expanding agent, it is separated from the expanding cement and sold directly on the market. The expanding agent is added to the Portland cement as an expansion component, making it more flexible in application.

Types, performance requirements, and application scope of expanding agents

The expanding agent’s chemical composition or expansion product can be divided into the following types: calcium sulfoaluminate, calcium oxide, magnesium oxide, iron oxide, air-entraining agent, and composite expanding agents.

The component of the expanding agent is calcium sulfoaluminate hydrate (ettringite) or calcium hydroxide, which produces an expansion effect due to its chemical reaction in concrete. Under the constraints of steel bars, this expansion is converted into compressive stress, thereby reducing or eliminating the volume reduction during dry shrinkage and condensation of concrete and improving the quality of concrete. Meanwhile, the generated crystals, such as ettringite, can fill and block the capillary pores of concrete and improve its anti-permeability.

Concrete (mortar) containing calcium sulfoaluminate and calcium sulfoaluminate-calcium oxide expanding agents shall not be used in projects where the long-term ambient temperature is above 80°C; it should not use concrete (mortar) containing calcium oxide expansion agents with seawater or corrosive water.

Concrete mixed with expanding agents is suitable for reinforced and filled concrete projects; the maximum internal temperature of large-volume concrete mixed with expanding agents should comply with the relevant standards, and the temperature difference between the inside and outside of the concrete should be less than 25°C.