How does the external temperature influence the setting time of the refractory castable?

In addition to the installed thickness, the type of castable and the demoulding time, the external temperature is a variable to take in consideration at the time of defining when to demould the lining. We analyze how the temperature influences the setting time of the refractory castable.

In many occasions, we are in a hurry to remove the mould after the installation of a refractory castable.
The necessity to run can be disastrous if the consequences are not well measured. A hidden, non-visible crack - caused by incorrect setting or unsupported stress during demoulding - which becomes apparent during heating, can be the metal exit point from a furnace.
We set a minimum waiting time of 16 hours before demoulding a refractory concrete with aluminium cement. However, this time may vary depending on different variables, including the external temperature.

Go to: Conclusions

A hydraulic refractory concrete is a mixture of aggregates and an aluminous cement. The main constituent of aluminous cements is calcium aluminate (CA). For the manufacture of aluminous cements, limestone or lime and bauxite or alumina are used and, depending on the process followed, fused and sintered aluminous cements are obtained.

In cement chemistry, the following nomenclature is used: CaO (C), Al2O3 ( A), SiO2 (S) and H2O (H).

Aluminium cement reacts with water to form a crystalline hydrated compound, which increases its strength as the reaction progresses, although for a shorter time than its cousins, Portland cements (S-C) . This reaction is called cement hydration and the compounds formed are known as hydrates. This reaction is exothermic, generates heat, and is what is measured in the laboratory for the setting controls of the concretes, with a device called EXO. The hardening developed through this hydration is known as hydraulic setting.

The formation of hydrated calcium aluminates is influenced by the temperature during hydration.

  • Up to 20ºC CAH10 is formed
  • 20-35ºC forms C2AH8, C3AH6, AH3
  • Over 35ºC C3AH6, AH3 is formed

Each hydrate formed differs considerably in the combined water ratio and density.

  • CAH10 is 53.5% water, density 1.71 g/cc
  • C2AH8 is 40.3% water, density 1.95 g/cc
  • C3AH6 is 28.6% water, density 2.52 g/cc
  • AH3 is 34.6% water, density 2.42 g/cc

So, the CAH10 hydrate formed in winter or low temperature:

  • presents a higher water of crystallization
  • causes a very slow exothermic reaction that makes: the concrete stay "alive" longer and the initial setting point is extended in time.
  • the CAH10 phase has a clearly lower mechanical resistance than the phases formed at higher temperatures.
  • A slight increase in temperature causes the reaction: 3CAH10 -> C3AH6 + 2AH3 + 18H … that is, C3AH6 and AH3 are formed. Both compounds have a higher density, favouring an increase in porosity which reduces their resistance.
    On the contrary, C3AH6 - AH3 hydrates formed in summer or at high temperature,
  • Present less water of crystallization
  • The exothermic reaction is faster and makes the newly formed concrete have a very short life time.
  • The phases formed have higher mechanical resistance.

On the other hand, the dehydration temperature differs between the hydrates and must be taken into consideration when drying

Hidrato Temperatura Deshidratación (°C)
     CAH10 100 ~ 130
     C2AH8 170 ~ 195
     C3AH6 240 ~ 370
      AH3 210 ~ 300
       AH 480 ~ 565
     C4A3H3 565 - 620

CONCLUSION on castable setting

  • There are three variables to take into consideration when demoulding a castable: installed thickness, demoulding time and external application temperature.
  • In winter, the formation of micro-cracks in the concrete, imperceptible to the human eye, may be a reality, hence the need for extreme precautions.
  • In summer, on the other hand, the natural formation of phases C3AH6 and AH3 makes it safer to remove the mould.
  • In any case, respect a minimum time for demoulding of 16 hours. If this is not possible, consult INSERTEC about the use of accelerators and/or retardants, which will depend on the ambient temperature, temperature of the concrete, quantity to be installed and other installation parameters.