1. Calcium oxide concrete expanding agent
Calcium oxide concrete expanding agent uses CaO as the expansion source and is made from ordinary lime and stearic acid ground together in a particular proportion. Stearic acid is added to lime (calcium oxide) during grinding. On the one hand, it acts as a grinding aid. On the other hand, during the grinding process of the ball mill, stearic acid adheres to the surface of the lime to form a stearic acid film, which acts as a repellent. The water isolation effect prevents CaO from immediately interacting with water. Instead, the membrane gradually ruptures during the hydration process, which delays the hydration rate of CaO and thereby controls the expansion rate. Calcium oxide expanding agent is mainly used for equipment to make grouting materials. They are used in foundation grouting of large equipment and anchor screw grouting to reduce concrete shrinkage, increase volume stability and improve strength. This type of material has been used extensively in installation projects.
2. Magnesium oxide expanding agent
The magnesium oxide expanding agent commonly used in modern concrete is generally obtained by grinding calcined dolomite at 800°C to 900°C. Magnesium oxide hydrates to form magnesium hydroxide crystals (brucite). The volume can be increased by 94% to 124%, which can cause concrete expansion. Adding calcium oxide into concrete at 5% to 9% of the weight of cement can achieve the required expansion performance.
3. Iron oxide expanding agent
Iron oxide expanding agent adds an appropriate amount of oxidant (such as per chromate, permanganate, etc.) to iron powder and causes volume expansion due to the oxidation of metallic iron (accounting for more than 10%) in the iron powder. The main advantages of this type of expanding agent are good heat resistance and stable early expansion. It is suitable for dry and high-temperature environments, such as filling the foot screws in hot workshops, repairing floor bases, and caulking. This type of expanding agent is used when concrete or mortar should be mixed at the construction site and used immediately.
4. Air entraining expanding agent
An air-entraining expanding agent is also called an aluminum powder expanding agent; a dispersant is added to the metal aluminum powder, first stirred evenly with cement, and then prepared into concrete. When water is added and mixed, hydrogen bubbles are generated, and the volume expands. This type of expanding agent is also called an aluminum powder expanding agent. Although expanding agent has been used for a long time, the expansion begins 10 minutes after adding water and reaches 90% of the maximum expansion rate in about 4 hours due to the early onset of expansion and the short duration. The concrete is still in a plastic state, so under restrictive conditions, it cannot play a compensation role and can only fill holes and gaps. In addition, it has shortcomings, such as low strength and sizeable dry shrinkage, so it is rarely used in engineering. Air-entraining expanding agent also include hydrogen peroxide and other gas-generating agents. Their expansion principles and properties are the same as aluminum powder expansion agents.
5. Composite concrete expanding agent
Composite concrete expanding agent refers to a concrete compound admixture composed of calcium sulfoaluminate or calcium oxide concrete and chemical admixtures. It has the properties of a concrete growing agent and other concrete chemical admixtures.
The following matters should be paid attention to when using concrete expanding agents.
(1) Expansive concrete (mortar) prepared by adding calcium sulfoaluminate expanding agent shall not be used in projects where the environmental mixing degree is above 80℃ for a long time.
(2) Filled expanded concrete (mortar) mixed with iron filings expanding agent shall not be used in projects with stray current and parts in contact with aluminum and magnesium materials.
(3) The mixed proportion design of expanded concrete is the same as that of ordinary concrete. When calculating the mix ratio, the cement dosage is the sum of the actual cement and expanding agent dosage in 1m³ of concrete. The mass of the iron filings expanding agent is not included in the cement dosage.
(4) Expanded concrete (mortar) should be stirred mechanically and evenly. Generally, the mixing time of ordinary concrete (mortar) is longer than that of regular concrete (mortar) by more than 30 seconds.
(5) Expansive concrete (mortar) must be cured in a humid state for more than 14 days or cured with a spray curing agent.
(6) For concrete mixed with calcium sulfoaluminate or calcium oxide expanding agent, it is not advisable to use chlorine salt admixtures simultaneously.
