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Due to its low thermal conductivity, a layer of concrete is frequently used for fireproofing of steel structures. However, concrete itself may be damaged by fire.Up to about 300 ░C, the concrete undergoes normal thermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 ░C, the major structural changes are carbonation and coarsening of pores. At 573 ░C, quartz undergoes rapid expansion due to Phase transition, and at 900 ░C calcite starts shrinking due to decomposition. At 450-550 ░C the cement hydrate decomposes, yielding calcium oxide. Calcium carbonate decomposes at about 600 ░C. Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonation.Concrete exposed to up to 100 ░C is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 ░C (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 ░C the concrete will turn light grey, and over approximately 1000 ░C it turns yellow-brown One rule of thumb is to consider all pink colored concrete as damaged, and to be removed.Fire will expose the concrete to gasses and liquids that can be harmful to the concrete, among other salts and acids that occur when fire-gasses get in contact with water.
Yes..I did want a concrete lesson..THANKS KAREN !!Just astonished that it's you giving the lesson.....but...knowing how great you are at DIY..I should not be so surprised really !!LOL...some of us have ponds or bird baths....ewe have a cement mixer !! Your talents are well cemented !!.......*groan*..sorry !! 
Several years have passed since this thread was posted.Concrete can be melted by using an "iron lance" which burns iron rods with oxygen to produce enough heat to melt concrete.It consists of a cylinder holding iron rods or wire through which oxygen is pumped. An acetylene torch is used to ignite the open end so the iron combusts. Forced air blows the very hot flame outwards.It was used in the late 1940s to help dismantle concrete bunkers used in the war.I'm unable to find a wiki article on this device so I'm relying on memory for these facts.
A bit off-subject: but I see that Rossignol is melting basalt (the most abundant crustal volcanic rock) and manufacturing basalt fibers (which I assume are a glass fiber) which they use to manufacture skis.
"The hotest fires tend to be those where oxygen is restricted. "What?