Don’t You Know What Refractory Concrete Is? You’ll Understand It After Reading This!

Refractory concrete refers to a special‑purpose concrete that maintains its physical and mechanical properties when operating at temperatures ranging from 900 °C to 1600 °C. These properties include high refractoriness, good thermal stability, high load‑softening point and low shrinkage at high temperatures.

Refractory concrete is mainly composed of heat‑resistant aggregates, cementitious materials, special admixtures and water. Heat‑resistant aggregates consist of crushed fire‑clay bricks, chamotte, bauxite clinker and crushed high‑alumina refractory bricks, etc. Fine aggregates such as magnesia sand, fly‑ash‑rich crushed high‑alumina bricks, crushed magnesia refractory bricks and corundum sand serve as the skeleton structure.

Cementitious materials play the role of binding, and mainly include Portland cement, ordinary Portland cement, slag Portland cement, high‑alumina cement and low‑calcium aluminate cement. Water‑reducing agents are commonly used special admixtures to reduce water consumption during concrete mixing.

It is a monolithic refractory prepared from refractory aggregates, fine powders and binders, mixed with water or other liquids. It is directly used without pre‑firing, and is also known as refractory castables. It can be classified into:

① Ordinary refractory concrete. Its aggregates may be high‑alumina, fire‑clay, silica, basic materials (magnesia sand, chromite, dolomite, etc.) or special‑grade materials (carbon, silicon carbide, zircon, etc.). Combinations of several refractory aggregates are also acceptable.

② Insulating refractory concrete. It is mainly formulated with lightweight refractory aggregates. The lightweight aggregates include expanded perlite, vermiculite, ceramsite, porous chamotte, hollow alumina spheres, etc. Mixtures of multiple lightweight refractory aggregates or blends with conventional refractory aggregates can also be used.The binders for refractory concrete include high‑alumina cement, phosphate binders, water‑glass binders, fire‑clay and so on.

Refractory concrete is an unfired product. It features simple production process and energy‑saving advantages. It can be shaped as required and delivers better integrity than brick‑built linings. It is suitable for mechanized construction and can often extend the service life of furnace linings when properly applied.Refractory concrete is mainly used in industrial furnaces for metallurgy, petroleum, chemical, building materials, machinery and other industries. Its general service temperature ranges from 1300 °C to 1600 °C.Refractory concrete with a service temperature below 900 °C is defined as heat‑resistant concrete, which is primarily applied to structures such as foundations of thermal equipment, chimneys and flues.

 Superior performance compared with refractory bricks:Its refractoriness is close to that of refractory bricks of the same material. However, as refractory concrete (castable) is unsintered, it exhibits considerable shrinkage during the first heating cycle, resulting in a slightly lower load‑softening point than refractory bricks. Even so, its overall performance is superior to refractory bricks.

 High compressive strength:Benefiting from the effect of low‑temperature binders, refractory concrete possesses high cold‑crushing strength. Meanwhile, thanks to the excellent integrity of the monolithic lining, the furnace achieves good gas‑tightness and high deformation resistance. The outer steel furnace shell can even be omitted. It shows better resistance to mechanical vibration and impact than brick‑built linings. For instance, when applied to the upper side‑wall of soaking pits where heavy mechanical wear and collision occur, its service life is several times longer than that of brick linings.

 Good thermal stability:It features good thermal stability. Most or all of its aggregates are clinkers. The expansion of aggregates counteracts the shrinkage of binders. Consequently, the thermal expansion of the monolithic lining is relatively lower than that of bricks, along with reduced thermal stress. Besides, various network‑like, acicular and chain‑like crystalline phases exist within its structure, granting strong resistance to low‑temperature thermal stress. For example, when used for casting the door and lid of soaking pits, the service life can be extended to one and a half years.

 Simple production process:It has a simple production process and eliminates the complicated brick‑making procedures. Various precast blocks can be manufactured, and mechanized construction is available, which greatly accelerates furnace‑building speed. The construction efficiency is more than ten times higher than bricklaying. Waste bricks and other scraps can also be used as aggregates to realize waste recycling.

The basic parameters for the mix‑proportion design of refractory concrete are largely similar to those of ordinary concrete, which mainly include: dosage of cementitious materials, water‑cement ratio (water‑binder ratio), dosage of admixtures, aggregate gradation and sand ratio.

Under normal conditions, the refractoriness of concrete aggregates is higher than that of cementitious materials. When the content of cementitious materials exceeds a certain range, the load‑softening point of concrete decreases and the residual deformation increases with the rising dosage of cementitious materials.

Therefore, to improve the high‑temperature performance of refractory concrete, the dosage of cementitious materials shall be minimized on the premise of satisfying workability for construction and cold‑state strength of concrete.

For cement‑bonded refractory concrete under different service conditions, the cement dosage can be around 15 %. For cement‑bonded refractory concrete that requires high load‑softening point and refractoriness yet low cold‑state strength, the cement content should be controlled at approximately 12 %.

Variations in the water‑cement ratio of cement‑bonded refractory concrete exert a remarkable influence on its strength and residual deformation. An increase in the water‑cement ratio leads to strength reduction of concrete, and this effect is even more pronounced for refractory concrete. Therefore, water consumption and the water‑cement ratio shall be reduced as much as possible when preparing refractory concrete. Generally, the slump of concrete mixture should be less than 2 cm, and stiff concrete is preferred.

For refractory concrete bonded with water glass, the modulus of water glass shall be controlled within 2.6‑2.8, with a relative density of 1.36‑1.40. The dosage of sodium fluorosilicate hardening accelerator is generally 10 %‑12 % of the water‑glass mass. For refractory concrete using phosphoric acid as the binder, the concentration of phosphoric acid is normally 50 %.

Adding an appropriate amount of admixtures into refractory concrete can significantly improve its high‑temperature performance, enhance the workability of concrete mixture and save cement consumption.

Test results indicate that for refractory concrete with low requirements on cold‑state strength, the admixture dosage should be higher, generally ranging from 30 % to 100 % of cement mass, and can reach up to 300 %.

Aggregates account for approximately 80 % of the total mass of refractory concrete mixture. Optimizing aggregate gradation brings favorable effects on improving the density and high‑temperature properties of refractory concrete.

When selecting aggregates, attention shall be paid to the type and refractoriness of aggregates to ensure compatibility with cementitious materials, and suitable particle sizes shall also be selected. Excessively large particle size or excessive dosage will result in poor workability of concrete mixture and difficult forming, which reduces concrete density and tends to cause delamination and spalling under high‑temperature conditions. The sand ratio should be controlled at around 45 %.

For the mix‑proportion design of refractory concrete, empirical mix proportions shall be adopted as the initial mix proportions. A suitable mix proportion shall be determined through trial mixing and adjustment. If the calculation method is used for mix‑proportion selection, the complete process covers calculation, trial formulation and final confirmation of the mix proportion.

 Determination of Mix Proportion

There are generally two methods to determine the mix proportion of refractory concrete. First, in accordance with raw‑material requirements specified in design drawings or design notices, construction can be carried out with this mix proportion once trial mixing meets construction workability requirements. Second, if only the grade and technical requirements of refractory concrete are specified in design drawings, the construction unit shall determine the construction mix proportion following relevant national codes and standards in accordance with the procedures below.

Firstly, the test department shall propose the tentative mix proportion and technical requirements for raw materials. After sample testing meets design specifications, the material‑supply mix proportion shall be submitted to the procurement department for raw‑material purchasing. Secondly, the material‑testing department shall formulate the construction mix proportion and conduct on‑site trial mixing. Construction shall proceed with this mix proportion if construction workability requirements are satisfied.

 Permissible Errors for Refractory Concrete Batching

To ensure that all technical performances of refractory concrete satisfy design requirements, the weighing of each constituent material must be strictly controlled. The permissible errors for batching refractory concrete are roughly the same as those for ordinary cement concrete. Specific requirements are as follows:

1. ±1 % for cement and fine powders;
2. ±3 % for refractory aggregates;
3. ±1 % for water and various liquid binders.