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 used in civil engineering, water 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. As a result, the energy needed for their removal is different and, while free water leaves for the atmosphere 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 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.
Understanding the effects of different heating rates on green concrete structures is of primary importance to engineers and industry, particularly to avoid the occurrence of an explosive spalling event. If an explosive spalling occurs, projectiles of reasonable mass (1-10 kg) can be thrust violently over many metres, having safety implication and rendering the refractory structure unfit for service. Repairs will then be required resulting in significant costs to industry.