Refractory mortar

Refractory mortar is a joint material used for laying shaped refractory products. It is composed of refractory powder, water or liquid binder and admixtures (such as dispersants, plasticizers, stabilizers or water retention agents). It is a kind of paste slurry containing high concentration of solid particles (or thick suspension), with Bingham fluid characteristics. The solid/liquid mass ratio is about (70~75)/(30~25), while the solid/liquid volume ratio is about (35~50)/(65~50) depending on the specific gravity of the refractory powder. It is usually applied with a spatula.

Generally speaking, the chemical properties of refractory mortar should be similar to the chemical properties of the masonry refractory products, so the classification of refractory products is the same, there are siliceous, semi-siliceous, clayey, high alumina, rigid jade, zirconite, magnesia, magnesia aluminum, magnesia chrome, carbon and/or silicon carbide mud. According to the properties of the mixture added, it can be further divided into drainage mud (mixed with water and liquid chemical binder) and undrainage mud (mixed with liquid organic binder). But now also does not add the liquid mixture liquid dry type fireclay to make the joint material, called the dry fireclay. In addition, according to the function of refractory mud can be divided into thermal insulation refractory mud, shrinkage resistant refractory mud and buffer refractory mud.

(1) Siliceous refractory mud

Siliceous refractory mud is made up of silica powder, silica brick powder, plastic clay, binder and additive. The main crystalline phase of silica powder is -quartz. During heating, -quartz will produce crystal transformation and unstable volume. The silica brick powder is prepared by grinding the high temperature calcined silica brick or waste silica brick. Therefore, the reasonable combination of the silica brick powder and the silica powder can control the load-softening temperature and the linear change rate of the heating process of the silica refractory mud, and the silica mud can be prepared in accordance with the requirements.

The working property and binding strength of silicic refractory mortar are adjusted by binders and additives, just as the mineralizing agents and binders such as lime milk, iron scale and lignin sulfonate are used in the production of silicon bricks to adjust their working and physical properties. Chemical binders, such as water glass and phosphate solution, can also be used as binders for silica refractories to improve the bonding strength after firing.

The size range of siliceous refractory mud is 0~0.5mm, of which 0.5-0.074mm accounts for 40% and less than 0.074mm accounts for 60%. Now, to improve the operation and sintering property of silica refractory mud, a small amount of high-purity silicon oxide micro powder (soot silicon, SiO2 not less than 98%, particle size less than 1 m) can also be added.

Siliceous refractory mud is mainly used for laying blast furnace hot air furnace, coke oven carbonization chamber, glass melting pool and rotating top, etc. The physical and chemical properties of kiln and kiln are different with their different parts. For example, the SiO2 content of silicon mud used in blast furnace hot air furnace is greater than 94%, the initial softening temperature of 0.2MPa under load is not less than 1600℃, the SiO2 content of silicon mud used in glass kiln is 94%~96%, the initial softening temperature of silicon mud used in coke oven is 1600~1620℃, and the SiO2 content of silicon mud used in coke oven is 85%~92%. The initial temperature of load softening was 1420~1500℃.

(2) Aluminum silicate refractory mud

Aluminosilicate refractories include clayey, mullite and high alumina refractories. Depending on the material, this type of refractory mortar is made of clay clinker, sintered or fused mullite, high alumina clinker, sintered or fused corundum powder and is prepared with soft clay, binder and admixture. Generally, aluminum silicate refractory mud is added with soft clay (plastic) clay. The purpose of adding soft clay is to improve its working property, such as spreading and adhesion. Silicon oxide powder (silica fume) can also be used to replace soft clay to improve performance and reduce sintering temperature of refractory slurry.

Generally, no binder is added in aluminosilicate refractory mud, but it must be added in order to improve the bonding strength. The binder used has two series :(A) silicate series, namely sodium silicate with different modulus (SiO2/Na2O); (B) Phosphate series, such as acidic aluminum phosphate, polyphosphate, etc. The amount of binder can be adjusted according to the use requirement. Generally speaking, with the increase of the amount of binder added, the bond strength after burning also increases, but the strength under high temperature is opposite, this is because the addition of binder will lead to the increase of liquid content in the mud under high temperature.

Additives in aluminosilicate mud include water reducer, stabilizer, plasticizer and shrinkage inhibitor, etc. Water-reducing agent (water-reducing agent) can use polyphosphate, sodium polyacrylate, methylene naphthalene sulfonate, etc. Stabilizer is to prevent the mud in the static process of the preparation of solid liquid separation (solid powder precipitation), the general use of organic polymer compounds as stabilizer, such as methyl cellulose, carboxymethyl fiber, etc., but also by adding acid or alkali to adjust the pH value of mud to obtain a stable suspension (mud). Plasticizers are added to improve the spread of mud and prevent it from flowing. Generally, plastic clay with higher water absorption rate is used as plasticizer, and some organic polymer compounds can also be used to improve the plasticity. Shrinkproof agent is the additive to prevent shrinkage of refractory mud by sintering at high temperature. In aluminum silicate refractory mud, blue crystal powder and quartz powder are generally used as shrinkproof agent. The size range of aluminum silicate mud is generally 0~0.5mm, 0.5~0.074mm accounts for 50%, less than 0.074mm accounts for 50%, the size composition can also be adjusted according to the use requirements.

Aluminosilicate refractories are stream refractories and should not be used in the construction of carbon bricks because of the destructive effect of water vapour volatilized from stream refractories in the oven and service on the brickwork. Therefore, non - drainage refractory mud must be used. Non-drainage aluminosilicate refractory mud is combined with liquid organic binder, most of which are made of liquid phenolic resin, and the viscosity of phenolic resin is adjusted by anhydrous ethanol. Non-drainage aluminosilicate refractory mortar is mainly used as joint material for refractory masonry in hearth and hearth of blast furnace.

(3) Alkaline refractory mud

Basic refractories are used for the construction of basic refractories joint materials, including magnesia, magnesia aluminum, magnesia chromium and magnesia silica refractories, etc. Alkaline refractory mud is made of sintered or fused magnesia, or synthetic raw materials (such as magnesia alumina spinel, magnesia chrome spinel), or waste magnesia brick, waste magnesia chrome brick and other powder made after breaking and crushing, plus binder and additive. The powder size composition of the prepared mud is similar to that of other fire-resistant slurries. It is made up of 0.5~0.074mm and less than 0.074mm, and its mix ratio is (70~75) :(25~30).

Alkaline refractory mud contains MgO, and when MgO is in contact with water, it is easy to produce hydration reaction to produce Mg (OH) 2, resulting in volume expansion, and after heating, it will be dehydrated and decomposed into MgO. Therefore, in use, alkaline mud mixed with water is prone to burst and lose its binding strength. Therefore, alkaline refractory mud cannot be prepared by water alone, but must be prepared by liquid chemical binder that can react with magnesite (MgO) to produce compound salt or non-drainage organic binder. On the other hand, it must be noted that alkaline refractory mud can not be used with acidic chemical binder (such as phosphoric acid, aluminum phosphate), because the acidic chemical binder and MgO reaction rate is very fast, there will be transient coagulation phenomenon, the mud will lose the performance and can not be used.

The chemical binder used in alkaline refractory mud includes magnesium chloride (brine), magnesium sulfate, sodium tripolyphosphate, sodium hexametaphosphate, sodium silicate (sodium silicate), etc. The alkaline refractory mud concocted by the aqueous solution of these compounds has a certain working time, and has a certain binding strength after solidification. But brine and sodium polyphosphate binders are more commonly used. Non-drainage alkaline refractory mortar is prepared with liquid phenolic resin and its performance is regulated by anhydrous ethanol.

There is no unified requirement for the physical properties of alkaline refractory mud, but it is prepared according to the use requirements, and the content of its main chemical composition is stipulated in the industry or enterprise standard.