Alumina cement: features and application

Alumina cement is a very special type, which in its properties is very different from any related material. Before deciding to buy this expensive raw material, you need to take into account all the features, as well as familiarize yourself with the areas of application of the product.

The first thing that distinguishes alumina cement from all others is the ability to harden extremely quickly in air or in water. To achieve this effect, the raw materials are processed in a special way, fired, and crushed. So, the initial raw material is necessarily soils enriched with aluminum, and they are supplemented with alumina. It is because of the special raw materials that the second name of alumina cement has gone - aluminate.

As mentioned above, alumina cement has a much shorter setting time than other types. This type is grabbed within 45 minutes after application. The final hardening occurs after 10 hours. In some cases, it becomes necessary to speed up an already fleeting process. Then gypsum is added to the original composition, getting a new variety - the gypsum-alumina version. It is characterized only by a faster setting and hardening period with full preservation of high strength characteristics.

All the positive properties of alumina cement can be combined into a large list.

• Excellent strength characteristics. Even under water, the material will be resistant to chemical and mechanical external influences. It does not corrode, it is not afraid of extremely low temperatures. All this opens up tremendous opportunities for its use.
• High speed of setting and hardening. This is especially true if you want to build any structure as soon as possible (for example, in three days).

• Immunity to aggressive components of the external environment. We are talking about all kinds of chemical compounds that affect the finished cement structure for a long time, for example: hard sulfite-containing water during mining operations, toxic gases, extreme heating.
• Excellent adhesion to all kinds of materials. An example is, for example, metal reinforcement, which is often used to seal blocks of alumina cement.

• Resistant to open fire. There is no need to fear that the cement will dry out and crumble. It perfectly withstands both exposure to high temperatures and a direct fire stream.
• Can be used as an additive to conventional cement. This is important when you need to make the structure frost-resistant, while saving money. On the basis of alumina raw materials, rapidly expanding and non-shrinking cement mixtures are made, which are used in industrial construction or during urgent repair work.

There are alumina options and disadvantages.

• The first and foremost is the high cost of producing the material.It is important here not only equipment, which should be super-strong and have increased power, but also strict adherence to technology, maintaining temperature conditions during firing and other nuances.

• The second disadvantage is associated with the advantage of the mixture. Due to the fact that the alumina variety generates heat when solidifying, it is not suitable for pouring large areas: cement may not solidify properly and collapse, but in one hundred percent of cases it will greatly lose its strength characteristics. You cannot pour such cement even in extreme heat, when the thermometer shows a temperature of over 30 degrees. It is also fraught with loss of strength.

• Finally, despite the high resistance of the alumina version to acids, toxic liquids and gases, it is absolutely unable to withstand the negative effects of alkalis, therefore it cannot be used in alkaline environments.

Alumina cement is divided into two large groups: expanding and mixed. The peculiarity of the expanding material lies in the ability of the raw material to increase during the hardening process. The changes will not be noticeable with the eye, however, this has a positive effect on the resulting density of the monolithic cement block. Expansion occurs within 0.002-0.005% of the original volume.

Mixed samples are made mainly in order to reduce the cost and, accordingly, the price of the product., however, in some cases, additives provide additional characteristics. So, for example, gypsum guarantees a higher setting rate, while the cost of cement increases. Slags and other active mineral additives, on the contrary, increase the setting time, but the price for such a mixed cement is noticeably lower.

The technical characteristics of alumina cement fluctuate depending on which brand it belongs to. According to GOST 969-91, developed back in the 70s, according to its strength, such cement is divided into GC-40, GC-50 and GC-60. Also, the proportions of certain substances in the composition depend on what properties need to be achieved and in what area the cement will be used. It makes no sense to give here the chemical formulas of the substances that make up the cement, but for comparison, it is worth saying that ordinary alumina cement contains from 35% to 55% of bauxite, while high-alumina refractory cement contains from 75% to 82%. As you can see, the difference is significant.

With regard to technical properties, although alumina cement is a fast-setting option, this should not affect the speed of its setting. According to the rules and regulations, it should be at least 30 minutes, and full curing occurs after 12 hours after application (maximum). Since the material has a special crystalline structure (all crystals in the substance are large), it is not very susceptible to deformation changes, and therefore we can confidently speak of its non-shrinkage and relatively small mass.

Variants differ in characteristics and depending on the method of their production. In total, only two methods are presented: melting and sintering.

Each of them has its own specifics.

• Scientifically, the first method is called the method of melting the raw material mixture. It involves several stages, each of which deserves close attention. First you need to prepare the raw materials. After that, the raw cement mixture is melted and gradually cooled, closely monitoring the temperature indicators to ensure the best strength characteristics. Finally, the obtained high-strength slag is crushed and ground to obtain alumina cement.

• With the sintering method, everything happens the other way around: first, the raw materials are crushed and crushed, and only then they are fired. This is fraught with the fact that the cement obtained in this way is not as strong as in the first method of production, but the second option is less laborious.

The technical parameters of the alumina cement composition can vary within a fairly wide range. (this also applies to the chemical formulas of a substance), but this should not significantly affect its main characteristics, such as the speed of solidification, strength, moisture resistance, resistance to deformation. If the technology was not followed during the manufacture, and some of the listed characteristics are lost, then the material is considered defective and is not subject to further use.

Alumina cement has a huge range of purposes for which it can be used. Most often, it is chosen for emergency work or for cocking structures underground or water, but this list is not limited to.

• If the bridge structure has been damaged, then it can be successfully restored using an alumina variety due to the water resistance of the material and its ability to quickly set and harden without compromising strength even in water.

• It so happens that a structure needs to be erected in a short time, and it is necessary that it gain strength in the first two days after foundation. Here, again, the best option is alumina.

• Since HC is resistant to all kinds of chemicals (with the exception of alkalis), it is suitable for construction in conditions of high sulfate content in the environment (most often in water).

• Due to its resistance to all kinds of corrosive processes, this type is suitable not only for fixing reinforcement, but also for anchors.
• When isolating oil wells, alumina (more often high-alumina) cements are used, since they solidify even when mixed with oil products.

• Since alumina cement has a low weight, it is excellent for sealing gaps, holes, holes in sea vessels, and due to the high strength of the raw material, such a "patch" will last for a long time.
• If you need to lay the foundation in soil with a high groundwater content, then any of the GC brands is perfect.

• The alumina variety is used not only for the construction of buildings and structures and the embedding of something. Containers are cast from it, in which it is planned to transport highly toxic substances, or if they must be located in aggressive environmental conditions.
• During the manufacture of refractory concrete, when the heating temperature is planned at the level of 1600-1700 degrees, alumina cement is added to the composition.

If you plan to use such cement at home (for example, for the manufacture of hydro-resistant plaster or construction), then you must follow the instructions for working with it.

Waterproof plaster with the addition of alumina cement is used in many areas:

• for sealing cracks in water pipes;
• wall decoration in underground rooms;
• sealing of pipeline connections;
• repair of swimming pools and showers.

Since every person living in a private house may be faced with the need to use the alumina option, below is an instruction on how to work with it correctly.

• It should be borne in mind that the best way to work with this type of cement is to use a concrete mixer. It is not possible to mix the mixture so well and quickly by hand.

• Freshly purchased cement can be used immediately. If the mixture has lain down a little, or the shelf life is almost over, then it will be necessary to sift the cement first. To do this, you need to use a special vibrating sieve. The mixture is placed in it using a construction paddle auger and sieved.This loosens the cement mixture and prepares it for further use.

• It is necessary to take into account the higher viscosity of alumina cement compared to other types. Therefore, the mixing of the cement slurry is carried out for a longer time. If in normal cases it takes an hour or an hour and a half, then in cases with alumina varieties - 2-3 hours. It is not recommended to stir the solution longer, as it will begin to set, and it may be difficult to apply it.

• Keep in mind that the concrete mixer must be cleaned immediately, since later, when this ultra-strong cement hardens, the washing procedure will require a lot of effort and time, not to mention the fact that sometimes it is not possible to clean the concrete mixer at all.

• If you plan to work with alumina options in the winter, then it is worth keeping in mind a number of nuances. Since the material actively generates heat during the hardening process, all measures for dilution and application of the mixture will differ from those when working with ordinary cement mortars. Depending on how many percent of water is in the mixture, its temperature can reach 100 degrees, and therefore you need to work extremely carefully, not forgetting about safety precautions.

• If work is carried out with concrete that contains alumina cement in the composition, then you need to ensure that its temperature remains at the level of 10-15 degrees and in no case rises higher, otherwise the concrete will begin to freeze even before you have time apply.

As mentioned above, according to GOST, three brands of this variety are distinguished: GC-40, GC-50 and GC-60, each of which differs from the other in a number of characteristics. They all have the same setting and hardening times, but their strength varies greatly. Even at an early age, the mixtures gain strength: GC-40 - 2.5 MPa in a day and 40 MPa in three days; GC-50 - 27.4 MPa in a day and 50 MPa in three days; GC-60 - 32.4 MPa in a day (which is almost identical to the strength of cement grade GC-40 after three days) and 60 MPa on the third day.

Each of the brands perfectly interacts with other substances: set retarders or accelerators.

• Retarders include borax, calcium chloride, boric acid, citric acid, sodium gluconate, and others.
• Accelerators are triethanolamine, lithium carbonate, Portland cement, gypsum, lime and others.

In addition to ordinary alumina cement, high-alumina variants of the first, second and third categories are distinguished by the content of aluminum oxide. Their marking is, respectively, VHC I, VHC II and VHC III. Depending on what strength is expected on the third day after use, the marking is supplemented with numbers.

There are the following options:

• VHC I-35;
• VHC II-25;
• VHC II-35;
• VHC III-25.

The higher the percentage of aluminum oxide in the composition, the stronger the finished cement is. For a high-alumina solution of the first category, the content of aluminum oxide in the composition must be at least 60%, for the second category - at least 70%, for the third - at least 80%. The setting period for these samples is also slightly different. The minimum threshold is 30 minutes, while complete solidification should occur in less than 12 hours for VHC I-35 and in 15 hours for VHC of the second and third categories.

Ordinary alumina cement does not possess fire-resistant qualities, and VHC of all categories must withstand high temperatures. Fire resistance standards start at 1580 degrees and go up to 1750 degrees for VHC III-25.

In conclusion, it is necessary to give advice on how to distinguish genuine from fake cement.Alumina and especially high-alumina refractory options are quite expensive, so you can often come across a counterfeit in this market. According to statistics, about 40% of the cement on the Russian market is counterfeit.

There are a number of guidelines to help you spot the catch right away.

• The most obvious rule is to buy cement from proven, reliable suppliers. Well-established firms include Gorkal, Secar, Ciment Fondu, Cimsa Icidac and a few others.

• To dispel final doubts, you need to ask the seller to show the sanitary and epidemiological conclusion. It states that the material is absolutely safe for human health. Some unscrupulous manufacturers add radioactive substances to cement mixtures. Although present in small quantities, they can cause significant harm to health. The norm for the content of natural radionuclides is up to 370 Bq / kg.

• If, after checking this conclusion, doubts remain, we advise you to verify the address of the authority that issued the sanitary and epidemiological conclusion. On the packaging and on the conclusion itself, this address must be the same.
• Check the weight of the bag in accordance with GOST. It should be equal to 49-51 kg and in no case go beyond these limits.

• Having chosen the composition, first buy one bag for a sample. At home, knead the cement, and if you evaluate it as of high quality, you will not find any foreign additives in it in the form of crushed stone or sand, then this means that it is of high quality.
• Finally, pay attention to the expiration date. It is extremely small - only 60 days from the date of packaging. Be sure to take this criterion into account when choosing, otherwise you risk buying a material whose performance will be many times worse than expected.

See below for more details.