High-alumina cement (HAC) is a special cement made from bauxite and limestone and calcined at high temperature. Due to its excellent high temperature resistance, corrosion resistance and strength, high-alumina cement is widely used in construction projects in extreme environments, such as high-temperature furnaces, chemical equipment, bridges, etc. However, the construction process of high-alumina cement also has its special requirements and precautions. Reasonable use and construction skills can ensure the maximization of its performance. This article will introduce the precautions and construction skills of high-alumina cement in detail.

1. Precautions for the use of high-alumina cement
Water-cement ratio control

The hydration reaction of high-alumina cement is more intense than that of ordinary cement, and it releases a lot of heat. Therefore, the water-cement ratio must be strictly controlled when using it. Under normal circumstances, the water-cement ratio should be controlled between 0.25-0.30. Excessive water will reduce the strength of cement and even affect its fire resistance and corrosion resistance. Water should be added in appropriate amounts according to the specific situation during construction.

Preventing early cracking

Due to the high heat of hydration reaction of high-aluminum cement, concrete is prone to cracks during solidification, especially when poured over a large area. Therefore, insulation measures should be taken during construction to ensure that the temperature is balanced during the hardening process of cement to prevent cracking caused by excessive temperature differences. Common insulation methods include covering with insulation film and using insulation materials.

Appropriate mixing time

The hydration reaction of high-aluminum cement is relatively rapid, so the time needs to be controlled during the mixing process. If the mixing time is too short, it may cause uneven concrete and affect the strength; if it is too long, it may affect the performance of concrete due to excessive hydration. Therefore, it is recommended to mix in strict accordance with the technical specifications of the cement plant, generally controlled within 2-3 minutes.

Avoid mixing with other cements

The chemical composition of high-aluminum cement is special. Its mixing with other types of cement (such as ordinary Portland cement, slag cement, etc.) may affect its performance, especially its resistance to high temperature and chemical corrosion. Therefore, it should be avoided to mix with other cements, especially in high temperature or corrosive environments.

Influence of ambient temperature

High-aluminum cement is sensitive to ambient temperature. Under low temperature conditions, the hydration reaction of cement is slow, which may affect the growth of strength; under high temperature conditions, it may cause cement to solidify prematurely and affect the construction quality. Therefore, it is necessary to ensure that the ambient temperature is suitable during construction. The construction temperature is usually recommended to be 5°C to 35°C.

Storage and transportation

High-aluminum cement is sensitive to humid environments and is easy to absorb moisture and agglomerate. Therefore, it should be avoided as much as possible during storage and transportation. Cement should be placed in a dry and ventilated environment during storage, and attention should be paid to shielding from rain. During transportation, it is best to use sealed packaging to prevent the cement from being affected by moisture.

2. Construction skills of high-aluminum cement
Accurate batching

The use effect of high-aluminum cement is closely related to the batching. Therefore, before construction, concrete should be accurately prepared according to design requirements and engineering characteristics. When batching, pay attention to the ratio of high-aluminum cement to aggregate and admixture to ensure the workability, strength and durability of concrete.

Mix evenly

The hydration reaction of high-aluminum cement is relatively complex. During construction, it should be ensured that cement is fully mixed with water and aggregate to avoid cement lumps and uneven mixtures. Use appropriate mixing equipment and pay attention to observation during the mixing process to ensure the uniformity and workability of the concrete.

Layered pouring

When using high-alumina cement for pouring, layered pouring should be adopted. The thickness of each pouring should be controlled between 20-30 cm to avoid pouring too much at one time, resulting in excessive temperature differences and cracks. Each layer should be vibrated or compacted after pouring to ensure the density of the concrete.

Post-construction maintenance

The hydration reaction of high-alumina cement is more intense, and a longer maintenance period is required to achieve optimal performance. Therefore, proper maintenance should be carried out after the construction is completed. Common maintenance methods include water spraying maintenance, covering with wet cloth maintenance or using curing agents. The maintenance time should last at least 7 days to ensure that the strength of the cement gradually increases.

Selection and cleaning of construction tools

High-alumina cement has a short setting time and hardens quickly, so the construction tools should be cleaned in time after use. The mixers, pouring tools, etc. used need to be cleaned in time to avoid cement agglomeration and affect subsequent construction. In addition, it is recommended to choose tool materials that are not easy to bond cement, such as stainless steel or plastic tools, to improve work efficiency.

Test of fire resistance

The fire resistance of high alumina cement is one of its major advantages, especially in high temperature environments. During the construction process, fire resistance tests should be carried out according to the project requirements to ensure that the high temperature performance of the construction materials meets the standard requirements. The test can use the furnace temperature test method to expose the cement sample to high temperature and detect its compressive strength and stability at high temperature.

Avoid excessive use of admixtures

High alumina cement has poor compatibility with admixtures and should be added with caution when used. Excessive or inappropriate admixtures may affect the hydration reaction of cement and reduce its strength or fire resistance. It is recommended to conduct small-scale experiments when using admixtures to ensure that the admixtures are compatible with high alumina cement.

3. Summary
As a special cement material, high alumina cement has excellent high temperature resistance and corrosion resistance, which makes it irreplaceable in extreme environments. However, during construction, attention must be paid to key factors such as water-cement ratio, temperature control, and mixing time, and operations must be strictly followed to ensure the best performance of cement. In addition, reasonable construction techniques and maintenance measures are essential to ensure the quality of the project. Therefore, construction personnel must understand the characteristics of high-alumina cement, follow specifications and operating guidelines, and ensure that every link in the construction process meets the requirements, thereby ensuring the long-term stability and safety of the project.