Source: Archipedia

The article deals with the classification and properties of refractory concretes.

 

Refractory concrete

Refractory concretes are mixtures of refractory aggregates and cements, which when solidified, turn into a stone-like material capable of maintaining the specified mechanical properties under prolonged exposure to high temperatures. Recently, the refractory industry produces an increasing number of non-ignition refractory products. They can be considered as refractory concretes on the basis that, by analogy with conventional concretes, they consist of a refractory filler, inert at normal temperatures, and a binder of mineral or organic origin.

Refractory concrete different from ordinary concrete, first, fire resistance and sufficient strength in the conditions of service at high temperatures; second, their operational properties they acquire in the process, when exposed to high temperatures. Refractories of this type are widely used because there is no complex and expensive technological process in the technology of their production — firing. Refractory concretes are made in the form of large blocks or monolithic lining structures, which allows to industrialize the construction and repair of industrial furnaces. Refractory concretes have some advantages over baked refractory products:

1) in the monolithic concrete lining completely no seams, and in the case of large concrete blocks the number of seams is significantly reduced;

2) firing of traditional refractory products, as a rule, occurs in an oxidizing medium and the phase composition of the burned products is characterized by the oxide forms of certain components, respectively. These refractories are used in most cases in the reducing medium at temperatures at which the oxide forms become unstable. Therefore, in the burnt products of any type in service conditions there are changes in the phase composition, often accompanied by changes in the volume of minerals, which leads to a loss of strength of products. In refractory concretes, the change in phase composition occurs only in the inert filler;

3) in the manufacture of firing products, minerals are crystallized from the liquid phase formed at high temperatures. In service conditions, the reverse process is observed-the dissolution of these minerals in the liquid phase. Since the specific volumes of the substance in the liquid and solid States are different (the volume of the melt of oxide substances is about 10% larger than the volume of the solid substance), the crystallization of minerals is accompanied by submicroscopic porosity, which causes an increase in the free energy of the refractory and, consequently, its increased reactivity.

In refractory concretes, this phenomenon is absent.

Refractory concretes are always more heat-resistant and less heat-conductive than the products that they have been burnt according to their chemical composition. However, refractory concretes are always less durable, especially to abrasion.

Refractory concretes must: harden quickly enough at normal temperatures; not sharply lose strength when heated to decomposition temperatures of hardening products, and then increase it at higher temperatures as a result of partial sintering; have sufficient thermal resistance and fire resistance; have a small shrinkage during drying and firing, a sufficiently high deformation temperature under load.

Thus, only the first two requirements are concrete-specific. The rest are common for any type of refractory.

In the technology of refractory concrete terminology used is somewhat different from the terminology used in the field of refractory ceramics.

Refractory powders, divided into fractions, used for the production of refractory concretes, called a filler (large, small, thin). Refractory powders containing all the fractions necessary for the production of concrete, and dry binders are called dry concrete mixtures. Mixtures with water or liquid binders called concrete mixtures. Refractory concretes are classified according to the mud of their products, by the type of binders and inert fillers used in their production.

Product type:

1. non-burning products;

2. large block;

3. monolithic linings from printed or molded masses.

In view of the used binders are distinguished:

By type of filler refractory concretes distinguish:

1. silica (actually silica, quartz etc.);

2. corundum;

The variety of concrete in the composition of the filler is large.

Any refractory non-shrinkable material can be a filler.

Fillers are obtained by crushing and sieving on the fraction of the refractory source material. Fine-grained filler is obtained in ball and tube mills. The concrete mix is prepared in a conventional mixer.

In the monolithic construction of the concrete stack with inertial vibrators, and the blocks are molded by vibratory plates.

Depending on the compressive strength of the concrete is divided into grades 100, 150, 200, 250, 300 and 400. The loss of strength of refractory concrete when heated to certain temperatures due to the decomposition of the binder is determined by the ratio of the strength of concrete after heating to the strength of the concrete before heating. The greatest loss of strength of concrete is observed at a temperature of 900 to 1100°C. above this temperature, the sintering of concrete components and again the increase in strength.

The process of forming the structure of refractory concrete can be conventionally considered as consisting of three successive interrelated processes:

1) hardening -.process occurring at low temperatures (up to 300°C);

2) softening (or hardening) — processes occurring at medium temperatures (about 300-1100°C);

3) sintering-a process that occurs at high temperatures (>1000 °C).

The joint study of these processes allows to choose the optimal compositions of bundles and to determine the most efficient technology that provides high properties of refractory concretes at different temperatures in operation.

The process of concrete hardening is caused by chemical interaction of components, recrystallization of chemical compounds or their hydration. The first and second processes are typical for air-hardening binders, the last-for hydraulic binders.

Softening of concrete structure on hydraulic binders in the range of average temperatures is primarily due to dehydration and decomposition of calcium hydrosilicates. The processes of decomposition of the bunch are observed in most concrete on air-hardening binders (liquid-glass, magnesium, sulfate, etc.).

Recently, concrete on phosphate bonds has become widespread. This is due to the fact that they have a sufficiently high strength at temperatures of 400-1000°C, i.e. in the temperature range in which the strength of conventional concrete is low.

Bundles for refractory concrete. At present, a number of binders are known on the basis of orthophosphate acid (H3PO4): aluminophosphate (cafe.), magnesium, calcium, chromium, iron, zirconium phosphate.

The most widespread in the production of refractory concretes are aluminum-phosphate and magnesium-phosphate bonds.

Alumophosphate bundles represent colloidal solutions of alumophosphates obtained as a result of the interaction of alumina hydrate with diluted phosphoric acid. There are three types of aluminum-phosphate bonds depending on the degree of hydrogen substitution with cations:

1.Unsemescerove alumophosphate solution of Al(H2PO4)3. It is prepared from a mixture of 14% alumina hydrate Al (OH)3 (a semi-product of the production of alumina of GO and ΓΟ brands) and 86% technical 60% phosphoric acid. The density of the solution is 1.54-1.55 g / cm3.

2.A solution of single-substituted alumophosphate Al (HSO4)3 is prepared from a mixture of 21% alumina hydrate and 79% technical 50% phosphoric acid. The density of the solution 1,49-1,51 " g / cm3.

3.A solution of Al3(PO4)3 tri-substituted aluminophosphate is prepared from a mixture of 22% alumina hydrate and 78% technical 50% phosphoric acid.

These solutions are prepared at the site of production of refractory concretes. To do this, the technical alumina hydrate is ground in ball mills to obtain particles smaller than 60 microns and poured into an acid-resistant reactor with dilute phosphoric acid, continuously stirring. The solution can be stored for up to two months.

Manifestatie bundles prepared similarly alumophosphates.

As a filler, it is recommended to use only high-refractory materials: corundum, battle corundum and high-alumina refractories, chromite and chromomagnesite. The grain composition of the filler is selected based on the General requirements of the technology of concrete and refractories.

The field of application of refractory concrete is quite extensive. For example, concrete on Portland cement can be used for installation of walls and vaults of the zone of heating and cooling of tunnel furnaces for the production of ceramics, in furnaces of flameless combustion of oil refineries, in furnaces of steam boilers. The concrete on alumina and high alumina cement with the fireclay is used for insulating coolers on the arches of the furnaces, and the concrete on periklutos the cement in separate units, open-hearth furnaces. Refractory concretes on phosphate bundles are used as lining of blast furnace air heaters, front walls of vertical channels of open-hearth furnaces, induction furnaces for melting silver, zinc, copper and aluminum alloys, etc.