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Choice of Mixing Tools
Mixing is one of the important factors affecting the hardness of high alumina cement. Choosing the right mixing tool can significantly improve the hardness of high alumina cement.
Typically, methods such as vibrators, pneumatic stirrers, manual mixing, and mechanical mixing can be used to mix high alumina cement. Among them, mechanical mixing is the most commonly used method because it can more evenly mix high alumina cement and other raw materials. If the effects of manual mixing and vibrators are not satisfactory, consider using mechanical mixing.
Adding Hardeners
You can add some hardeners to improve the hardness of high alumina cement. In general, an addition of no more than 2% of the weight of high alumina cement is sufficient to produce a certain hardening effect.
Common hardeners include silicates, chromium salts, aluminate salts, hydroxyethyl cellulose, and one-step molding hop. Among them, silicate hardeners are the most widely used at present. They can make the phase structure of high alumina cement denser and more uniform, thereby improving hardness.
Controlling Water to Cement Ratio
The water to cement ratio also affects the hardness of high alumina cement. The smaller the water to cement ratio, the higher the hardness of high alumina cement. Therefore, when preparing high alumina cement, it is necessary to reasonably control the water to cement ratio to achieve the best hardness effect.
Increasing Heat Treatment Temperature
High-temperature treatment is one of the effective methods to improve the hardness of high alumina cement. Generally speaking, the hardness of high alumina cement gradually increases with the increase of heat treatment temperature. Usually, processing at above 250℃ has a better effect.
High alumina cement, known for its quick setting and early strength characteristics, was initially used in many defense engineering and rapid repair projects. Later, its application extended to industrial and residential buildings. However, due to the phase transformation of the hydration product of high alumina cement, the void ratio of the hardened cement stone increases, leading to a strength degradation issue in later stages. This makes people very cautious when using high alumina cement in structural engineering. Later, with intensive and systematic research on its hydration mechanism and long-term durability by many researchers, it is believed that taking some technical measures can well solve the strength degradation problem, thus allowing its use in structural engineering with confidence. Currently, the main measures to control strength degradation are listed below.
Control the Water-Cement Ratio and Cement Content in Concrete
When used to produce concrete, the water-cement ratio is less than 0.4, and the cement content is more than 400kg/m3. In this system with a relatively small water-cement ratio and large cement content, some unhydrated cement can be retained, which reacts with free water released during the phase transformation of the hydration products, forming microcrystals that fill the pores in the concrete, reducing strength loss, and thus compensating for later strength.
Composite with Other Materials
Adding some calcium or silicon fillers to high alumina cement, or using aluminate and silicate composites, can minimize or avoid the formation of metastable hydration products CAH10 and C2AH8, thereby solving the strength degradation problem.
Pay Attention to the Usage Environment
At a normal temperature (20℃), CAH10 and C2AH8 are converted into C3AH6 very slowly; it only converts faster under the hydrothermal conditions of 30~50℃. Therefore, if high temperature and humidity can simultaneously be avoided during use, the strength degradation problem can also be well controlled.
In conclusion, the hardness of high alumina cement is influenced by multiple factors. To improve the hardness of high alumina cement, you can choose the correct mixing tools, add hardeners, control the water-cement ratio, and increase the heat treatment temperature. The strength degradation of high alumina cement can be addressed by controlling the water-cement ratio and cement content in concrete, combining it with other materials, and paying attention to the usage environment. At the same time, it is necessary to determine the best plan according to the actual situation and requirements to achieve the best effect.
