In concrete on liquid glass, the binder is an aqueous solution of sodium silicate at Na 2 O* nSiO 2 * mH 2 O, which, as a result of physicochemical interaction with sodium silicofluoride or other additives (hardening reagents), decomposes with the release of Si(0H) 4, coagulates and glues aggregate grains together into a monolithic conglomerate. Liquid glass has high adhesive properties in relation to all materials used in the refractory industry. Its adhesive ability is 3-5 times higher than cement, which ensures the production of high-quality heat-resistant concrete based on it.

Unlike concrete with hydraulic binders, hardening of concrete occurs not as a result of hydration of minerals, but as a result of the formation of colloidal glue Si (OH) 4, which acquires maximum strength after drying and recrystallization into Si0 2 with the release of water. Concrete hardens in air-dry conditions at an air temperature of at least 15 °C. At lower temperatures, the hardening process practically does not occur; the most favorable hardening temperatures are 25–50 °C. The most satisfactory properties are possessed by liquid glass, in which the silica module (molar ratio of SiO 2 and Na 2 O) ranges from 2.5 to 3. The silica module is also called the glass module. The process of setting and hardening of concrete occurs only at the moment of separation of silica gel from the colloidal solution:

The setting and hardening of concrete on liquid glass with the addition of sodium silicofluoride or other hardening reagents is a complex colloidal adsorption process caused by the colloidal chemical interaction of the hardening reagent with alkali sodium silicate. In a simplified form, the chemical interaction of sodium silicofluoride with alkali sodium silicate, whose silicate modulus is two, can be expressed by the following scheme:

Na 2 SiF 6 + 2 (Na 2 O * 2SiO 2) + 10H 2 O = 5Si (OH) 4 + 6NaF;

Sodium silicofluoride, due to its low solubility in water (0.6%), reacts with liquid glass slowly.

The process of setting and hardening depending on the amount of added silicofluoride, temperature and modulus liquid glass starts after 30-60 minutes. During this time, the freshly prepared mass is quite plastic and well shaped. The amount of sodium fluoride silico should ensure normal setting and hardening times for concrete, as well as the required strength of concrete at the time of formwork. It should not be forgotten that sodium fluoride is a highly active flux that reduces the fire-resistant properties of concrete based on liquid glass.

In addition to sodium silicofluoride, nepheline sludge, ferrochrome slag, and calcined serpentinite are sometimes used to harden concrete on liquid glass, which is also used as a filler to produce refractory concrete with faster hardening times (10-30 minutes).

When heating hardened liquid glass with the addition of sodium silicofluoride, the main part of the moisture (80%) is removed at 100 °C; when heated to 200 °C, another 12% of the moisture is removed. Residual moisture (8%) is removed by heating to 300 °C, due to dehydration of helium silicic acid during crystallization of Si0 2. As a result of the removal of moisture in concrete, shrinkage is observed, which, with the correct selection of concrete composition, does not exceed 0.8%, and when using concrete with finely ground magnesite, 0.25%.

Heating to 800–900 °C leads to partial sintering of concrete. With the introduction of refractory finely ground additives, concrete sintering occurs at more high temperatures, its fire resistance increases.

To prepare finely ground additives, chamotte, magnesite, chromite, chromomagnesite, quartz, dunite, serpentinite, talc, andesite, diabase, etc. are used. The degree of grinding of all types of additives must be such that at least 50% of the mass of the material passes through a 0.09 mm sieve (4900 holes/cm2).

The choice of one type of additive or another depends on the required fire resistance of the concrete and the service conditions of the lining. The use of finely ground magnesite and chromium magnesite increases the fire resistance to the greatest extent.

The lower the density of liquid glass, the lower the strength of concrete, for example, when using liquid glass with a density of 1.25, the tensile strength is only 50% of the compressive strength of dried concrete (25-30 N/mm2) prepared with liquid glass with a density of 1. 36 g/cm 3 .

As the consumption of liquid glass increases, the amount of water in concrete increases, as a result of which its porosity increases and its strength decreases. Thus, when the liquid glass content increases from 400 to 500 kg per 1 m 3 of concrete, the compressive strength decreases in proportion to the Na 2 O content.

As a result of firing, the compressive strength of concrete changes insignificantly compared to the strength of dried concrete. Heating to 300–400 °C causes strengthening of its structure due to dehydration of the gel; at 400–600 °C there is some decrease in strength; with an increase in temperature to 800–1000 °C, the strength for most compositions does not change or increases slightly.

Types of finely ground additives affect the strength of concrete when heated. It is highest for concrete with finely ground magnesite and fireclay additives. The addition of finely ground quartzite significantly reduces the strength due to its modification transformation at 575 °C.

The degree and methods of its compaction have a great influence on the strength of concrete. To ensure the mobility of concrete during compaction by vibration, at least 16% of liquid glass from the total mass of concrete must be added to concrete with fireclay fillers. It is impossible to reduce the consumption of liquid glass with this compaction method, since concrete has high viscosity and is not compacted by vibration.

To obtain high-strength, non-shrinkable concrete with a liquid glass content of 10-14%, it is necessary to use compaction with pneumatic tampers. In this case, the size of the aggregate in concrete should not exceed 5 mm, since coarsening leads to crushing by tamping and a decrease in the strength of concrete.

When using compaction of semi-dry mixtures, the compressive strength of concrete on liquid glass increases by 1.5-2 times. At the same time, shrinkage during the drying and heating process is almost not observed; this is of great importance when lining induction melting furnaces for aluminum melting.

Increasing the sodium fluoride content in concrete reduces fire resistance and strength at high temperatures, since it is a strong flux.

The highest application temperature is for liquid glass concrete with finely ground additives and fillers from broken magnesite bricks (1300-1400 °C). Such concrete begins to soften under a load of 0.2 N/mm 2 at 1250-1300 °C and collapses at 1400-1450 °C.

Liquid glass concrete with finely ground magnesite and fireclay fillers is widely used in induction furnaces for melting aluminum. This concrete has high heat resistance and is resistant to the reducing action of molten aluminum due to the fact that the fireclay grains in this concrete are covered with a shell of magnesite cement stone.

Refractory concrete, as the name suggests, is used where the structure may experience significant temperature loads. The properties of this material allow it to withstand heating to high temperatures without loss of strength, and therefore it is indispensable when arranging chimneys, laying stoves, etc. And for ordinary structures, resistance to fire will not be superfluous.

What groups are refractory concretes divided into, what is included in their composition and how to prepare such a solution yourself - we will tell you in our article.

By adding various components to the solution, you can greatly increase its resistance to high temperatures.

Material overview

Concrete and reinforced concrete themselves are quite strong and fire-resistant materials. This can also be confirmed by a process such as diamond drilling of holes in concrete: even with significant heating from friction, the frozen solution does not melt and does not lose its properties.

IN various ovens parts based on fire-resistant cement are actively used

However, the low thermal conductivity of concrete “triggers” only during short-term heating. If, through prolonged exposure, the structure is brought to 250 0C, it will begin to collapse, and at 200 0C it will lose its strength by 25-30%. This can lead to the most dire consequences, and therefore in some cases it is recommended to use fire-resistant and heat-resistant compounds.

Based on their properties, concretes are divided into several groups. Their brief characteristics can be seen in the table:

Note!
Heat-resistant and fire-resistant compositions with a density of less than 1500 kg/m3 are classified as lightweight concrete.

The instructions recommend using such materials wherever the structure experiences periodic or constant exposure to high temperatures. Also, the use of heat-resistant mixtures is justified if the destruction of load-bearing elements in a fire can lead to tragic consequences (load-bearing foundations of workshops, residential and public buildings and so on.).

Factory-produced mixture packaging

Manufacturing methodComposition features

For laying stoves and fireplaces, arranging chimneys and solving similar problems, we may need a material that can withstand heating up to 1000 - 1200 0C without loss of strength. The price of ready-made factory mixtures is quite high, so you can try to make the solution yourself.

Consequences of exposure to high temperature flames

To understand what substances should be added as modifiers, it is worth understanding what happens to hardened cement during combustion:

• As you know, water, which reacts with the granules of the material, is largely responsible for the hardening of cement in concrete.
• As the temperature rises, the bulk of the liquid evaporates, dehydration of the cement occurs, and it loses its strength.
• This process is irreversible, therefore it will not be possible to restore the properties of the material at least partially.

Therefore, to avoid concrete deterioration, we need to keep the water inside by adding cementitious additives.

This role is usually played by:

• Portland cement/slag Portland cement.
• Periclase cement.
• High alumina cement.
• Liquid glass.

Cement, alumina, liquid glass, etc. promote water retention

In addition, to improve heat resistance, finely ground additives are introduced into the material:

• Broken bricks (magnesite, dolomite, fireclay).
• Pumice.
• Chromite ores.
• Blast furnace slag (ground and granulated).
• Expanded clay.
• Ash.

Fragments of refractory bricks, blast furnace slag and fragments of durable rocks: diabase, basalt, tuff, etc. are also used as filler. Light fire-resistant solutions are made with perlite or vermiculite.

Note!
Filling with crushed gravel from dense rocks makes it almost impossible to process the hardened mortar.
So, if necessary, cutting reinforced concrete with diamond wheels or drilling using similar tools is used.

Independent production

Make your own fireproof concrete mixtures quite possible.

To ensure acceptable quality, you should follow the following algorithm:

• In a concrete mixer, mix three parts gravel (crushed basalt or tuff), two parts sand, two parts refractory cement and half a part lime.

Mix all dry ingredients

• To improve heat resistance, you can add 0.25 parts of finely ground substances - ash, blast furnace slag or pumice.
• Add water in small portions, bringing the solution to the optimal consistency.

In any case, we proceed like this:

Plastic mold for concrete furnace elements

• We make fairly strong formwork from plywood, plastic or metal.
• We pour the solution into the formwork, trying not to make gaps or voids.
• Carefully compact the material, removing all air bubbles.

Note!
Prolonged vibration treatment causes the gravel filler to settle to the bottom of the formwork.
That is why it takes a very short time to compact the solution.

Remove excess solution with a trowel.

After this, we move on to drying the material:

• Fire-resistant concretes are more sensitive to hydration conditions. The presence of lime in their composition allows long time maintain an elevated temperature inside the mixture, which ensures effective set strength of concrete products.
• To prevent this process from slowing down, it is necessary to carefully cover the formwork, minimizing heat loss and reducing the rate of water evaporation.

In principle, the technology allows the formwork to be dismantled immediately after the mixture has cooled. However, to ensure maximum mechanical characteristics, experts recommend keeping the solution in the mold for at least three days, and after dismantling it, moistening all surfaces for another three to four days in a row.

Photo finished part, cast in formwork

If we are talking about small volumes (for example, for building a chimney or laying a fireplace), then anyone can make fireproof concrete with their own hands. To master the technique, it will be enough to purchase the necessary components, as well as follow the tips given in the video in this article.

The need to use fire-resistant materials quite often arises during the construction of facilities. In the future, this allows you to protect structures and people from the unpleasant consequences of accidental fires. One of these materials is heat-resistant concrete, which can withstand high temperatures up to 1000 °C. At the same time, he retains useful qualities and does not lose shape.

Classification

There are several types heat-resistant concrete, which is also called fire-resistant or heat-resistant. The material contains special fire-resistant additives. The main binding component in the production of heat-resistant concrete is Portland cement. The following can be used as fillers: blast furnace slag, rock screenings (diabase, andesite, porous rocks of volcanic origin, diorite, artificial fillers), blast furnace slag.

The material is divided into separate classes according to:

1. Structure (heavy, light, porous).
2. Purpose (thermal insulation, structural).
3. The nature of the fillers.
4. The binder components used.

Specifications

Fire-resistant concrete prepared using Portland cement as a binder has a classic strength index. When conducting a compression test, the limit values ​​are in the range from 200 to 600 MPa/cm2.

Manifestations of thermal stability are observed when temperatures reach no more than 500 °C. Prolonged exposure to an open flame or prolonged contact with hot surfaces significantly reduces the strength properties of cement and often causes defects.

The most fire-resistant concretes prepared on the basis of alumina are able to withstand any household temperatures. Aluminous coatings saturated in composition are characterized by thermal stability of about 1600 °C and higher. A gradual increase in temperature leads to in this case to an increase in heat resistance, since the cement mass is converted into a ceramic state.

However, despite its high resistance to elevated temperatures, aluminous refractory concrete has relatively low strength. The material made using such components can withstand mechanical pressure of up to 25-35 MPa/cm2.

Primarily, refractory material is used in the manufacture of thermal structures, industrial furnaces and household use, foundations, collectors, combustion chambers. However, it cannot be said that such concrete is used only in structures that are susceptible to thermal influences.

Thus, the specific composition of refractory concrete contributes to its widespread use in chemical industry, in the production of building materials, to meet the needs of the energy sector.

Heat-resistant material is used in the construction of floors, floating structures, and purlin bridges. They give preference to this construction basis due to the need to lighten structures, taking into account high strength and reliability indicators. The refractory composition makes it possible to reduce the weight of structures by approximately 40%. This is explained by the use of a significant volume of porous fillers in the mixture.

Preparation of the composition

How to create fireproof concrete by making your own mixture? For this purpose water is used, binders and various heat-resistant fillers. The manufacturing process has its own distinctive features. The components used must be of particular purity. In addition, clogging of refractory and refractory components with sand, limestone or granite is eliminated.

Making such mistakes in production technology often leads to rapid destruction of the material.

Manufacturing techniques

There are several ways to produce heat-resistant concrete with your own hands. First of all, you can obtain the material using a ready-made dry mixture, which has all the necessary components. A more complex option involves mixing the components yourself in the required proportions.

The optimal solution is to use the first method, since the best components are used in the production of heat-resistant mixtures in the factory. In addition, in this case, manufacturing technology is carefully observed. As a result, the consumer gets the opportunity to use a ready-to-use mixture highest quality. You just need to add solvent or water.

When making it yourself, in order for the material to acquire fire-resistant properties, it is advisable to add following components fine grinding: andesite, fireclay, chromite ore, magnesite cement. The result correct selection ingredients and proportions becomes a material that can withstand elevated temperatures without collapsing.

Tools and materials

By resorting to doing it yourself, you can save significantly by refusing the services of masters. However, before you start making the mixture, it is recommended to prepare the necessary tools and materials. Here you will need the following:

• equipment for mixing concrete components;
• spatula-trowel;
• wheelbarrow for transporting materials;
• shovel;
• water spray;
• wooden formwork, casting molds;
• sand, gravel, slaked lime, heat-resistant components;
• Portland cement.

Manufacturing Features

When producing refractory cement, pre-prepared dry components are placed in a concrete mixer (cement-sand ratio is 1:4). After forming a homogeneous mixture, water is added in the amount necessary to achieve a dough-like consistency. Since refractory building bases have specific viscosity characteristics and quickly harden by adding water, it is better to follow the recommendations of the cement manufacturer.

The finished mixture is distributed into molds, poured into formwork or used as a binding material when laying refractory bricks. When using aluminous fillers, after adding water they act extremely quickly, which avoids premature setting of the solution.

If it is necessary to prepare small volumes of mortar using Portland cement, the components can be mixed manually. It is convenient to use wide containers for this - deep basins, bathtubs, troughs.

Heat-resistant concrete is an artificial stone material necessary for industrial units exposed to heat. building structures, boiler linings. The purpose of the material for industrial furnaces is explained by its performance qualities.

Classification of refractory material

Heat-resistant concrete is divided into subtypes. The category of a substance is determined by the type of binder used. There are several types of binder raw materials:

• Portland cement, created by combined, dry, wet method. A refractory composition with the addition of Portland cement is a high-quality building material;
• Slag Portland cement is a heat-resistant material used in the process of laying foundations and building walls. A non-shrinkable heat-resistant concrete mixture based on slag Portland cement has increased heat resistance and water resistance;
• Liquid glass is a binding element that consists of water and silicate salts. Fireproof concrete with the addition of liquid glass is a godsend in the construction of residential buildings;
• Aluminous cement is a deformation-resistant material with a coarse crystalline structure. It is customary to use mortar for fireplace stoves in the construction of cottages.

To increase the strength and reliability of the building foundation, professionals use unique additives in the form of granulated slag and chromite ore. When adding a special binder to a composition prepared on the basis of Portland cement, the fineness of grinding is taken into account. Sieve 009 should pass no more than 70% of the substance. For the production of concrete based on the binder component of liquid glass, the grinding fineness should be such that sieve 009 would pass no more than 50%. In the process of manufacturing any types of heat-resistant concrete, GOST is taken into account.

Properly selected components of heat-resistant concrete contribute to the durability of the building, the construction of which involved the use of raw materials such as heat-resistant concrete. The building material can be used at high temperatures. The price of a material is determined by several factors - its type, quality, quantity. To buy refractory concrete for stoves and fireplaces wisely, you should familiarize yourself with the rules for their selection.

Born in England in 1961, lives in Montreal, Canada. Member of the Stove Makers Association North America. He has been involved in the furnace business for more than 20 years and specializes mainly in the construction of Finnish counterflow furnaces in various options. Area of ​​interest: non-standard cladding made of antique bricks, Art Deco design, history of stove making. In filling its website www.pyromasse.ca adheres to the “open source” policy.

Translation: 02/12/2011

Refractory concrete for furnaces - preparation on site

Selecting a mixture to prepare refractory concrete suitable for use in a furnace can be difficult. The following requirements are imposed on it: high density, large grains and good resistance to thermal shock. The refractory concrete used here is Mount Savages Heatcrete 24 ESC (24 f. extra strength course). The article describes the formation, pouring, and demolding of four concrete modules used in the construction of an indirect oven oven. The article outlines the methods of routine work on site. The equipment and techniques in a workshop setting can, of course, be much better.

There are 4 forms to fill. From top to bottom, clockwise. Hearth, back plate, top plate and furnace lintel. The hearth mold is dark, as it is made from plywood used for molding work. Once the molds are assembled, they should be sealed to prevent water from evaporating during the reaction and allowing the castings to be easily removed. The molds can be coated with polyethylene, or treated with vegetable fat and silicone. Both methods are suitable, the method using vegetable fat is described here. Polyethylene gives the completed modules a shiny surface type finish that is easy to clean. This sheen, however, can significantly hinder the removal of mechanically bound water during heating. The surface of modules from molds treated with fat is much more porous.

Before pouring refractory concrete, all forms are compacted. Silicone is applied to all joints. The surfaces of the molds are thoroughly coated with vegetable fat.

A strip of ceramic paper is placed at the base of the mold for the discharge bridge. This will form a recess where the same strip will be placed when the jumper is installed. The paper should be covered with a strip of polyethylene to stop the mixture from being absorbed as the mold vibrates.

The mixture should be perfectly mixed in a mechanical mixer. A large number of the mixtures are almost impossible to mix by hand. Manufacturers recommend a certain amount of water. One and three quarters of a gallon (7.7 L) of water per 50 lb (22.5 kg) bag of mixture seems to be too little. Although after thorough stirring the mixture vibrates well into place. Even a small excess of water can significantly damage the finished modules.

The water used must be clean. Both the water and the dry mixture must be relatively warm during mixing, and kept warm before and during reaction and after pouring. 15-20 C is optimal. If you have to pour at a low temperature and heat the materials, it is important not to overheat, otherwise the mixture will begin to set before it is laid.

Because the mixture is so tough, it is important to work quickly. Fireproof concrete is placed in a mold. It is better to overfill the form and remove the excess than to underfill it and add more later. The concrete must be placed into the mold with a trowel before it is vibrated. The images show refractory concrete after vibrating for one minute. Although the mixture seemed too dry up until this point, it filled the molds perfectly after vibrating once.

Vibration placement of refractory concrete using a hammer drill. Video, 11 sec.

Vibration laying, removal of air bubbles. Video, 12 sec.

The shapes are nailed to a sheet of plywood that rests on top of another sheet of plywood. This makes vibration more effective, especially when working on concrete floor. Vibration is carried out with a chipper or hammer drill. By placing a drill bit into the wood portion of the mold, the mold vibrates, causing the concrete to shrink and trapped air bubbles to float to the surface.

These three refractory concrete molds are designed so that the middle and two inner surfaces of the outer parts cannot be easily vibrated, and Special attention attention should be paid to vibrating these particular parts.

Vibrating sets the refractory concrete and removes air, but it also causes the coarse grains to settle toward the base of the form, pushing the finer grains upward. Since this leads to inhomogeneity of the composition, the mold should not vibrate longer than necessary.

The outer surfaces of the modules facing the fire should be left rough and not troweled. After pouring, the molds should be tightly covered with plastic and all air removed from underneath by smoothing it by hand. It’s good to shoot the plastic to the forms with a stapler so that the corners are not lifted by treacherous night winds.

Forms lined with polyethylene.

The same forms, filled with concrete and covered with polyethylene.

Aging greatly affects the strength of the finished product. The workspace should be warm when holding. The exothermic hydraulic setting reaction of refractory concrete will begin several hours after pouring, depending on the amount of water and the temperature of the materials. The reaction will make the product quite hot as it continues for several hours. It is important that the product is carefully covered to prevent water loss through evaporation during the reaction. Although I remove and use the modules a day after pouring, once they have cooled, it is best to leave them in their molds for an additional two days. If they are taken out every other day, it is best to keep them damp for a few days

The jumper floats during an exothermic reaction. Video, 18 sec.

When making molds for refractory concrete, you need to work precisely. The surfaces of the modules sealed with 1/8 inch (3 mm) ceramic paper must be straight and square to work properly.

The inner surface of the oven hearth was cast into a lightly greased wooden mold. It is probably preferable to cast it in polyethylene as this will provide a smoother surface that is less permeable to water and easier to clean.