After building of a new refractory-lined industrial furnace or equipment, or refractory maintenance or relining of existing equipment, a necessary step is the start-up of the operation, which usually involves heating-up the unit in a controllable way, in order to prevent spall

Spall

Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure...

ing or shortening of the materials' predicted lifetime.

Refractory castable and water removal

The most well-known problem that can be avoided with proper heat-up is the dry-out spalling, which is often the result of excessive pressurization of entrapped steam after heating above the ebullition point of water.
Similarly to the Portland concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 used in civil engineering, water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 is added to the refractory castables to provide workability and allow molding, pumping, shotcreting or other forms of placement.
Water exists within castables in free or combined forms: while free water remains in the pores without reaction with the materials' other constituents, combined water is present usually in the hydrated compounds of cement

Cement

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

. As a result, the energy needed for their removal is different and, while free water leaves for the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 at higher rates from about 100 degrees Celsius, temperatures at the range of 150 to 300 degrees Celsius may be necessary to remove the combined water.
Because of the technological trend to reduce alkali

Alkali

In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

 content from the majority of refractories, the amount of combined water present in the moment of heat-up was reduced in the last years. On the other hand, recent developments which led to increases in mechanical strength, thermal shock resistance, erosion resistance, etc., also led to a reduction in permeability. Such permeability reduction caused an increase in vapor entrapment, which can lead to explosive spalling.

Microstructural engineering to avoid explosive spalling

Several efforts can be carried out to increase the materials' ability to withstand a more aggressive dry-out. They involve incorporating low melting point fibers, such as polypropylene, or oxidizing metallic powders (e.g. aluminium powder) to provide permeable paths to deliver the vapor to the atmosphere, thereby relieving the internal pressure. Another option is to mechanically reinforce the microstructure so the material can dissipate enough energy with the beginning of crack growth, avoiding explosion.

Thermal shock

When the material goes from a thermal configuration to another one, the resulting transient temperature fields will lead to variation in the thermal expansion condition at adjacent places. Therefore, thermal stresses may result in thermal shock crack propagation, which can shorten the lifetime expectancy for the material.

Heat-up methods

The technology chosen to heat-up a given equipment should account for the following features:

1. Thermal stability of the heat source

2. Thermal homogeneity within all the equipment

3. Safety of operation

4. Reproducibility

Reproducibility

Reproducibility is the ability of an experiment or study to be accurately reproduced, or replicated, by someone else working independently...

The most used method is based on the convective heat transfer

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

.