Does concrete melt?

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qed

Senior Member
9-11 Debunker, Steven Dutch (Natural and Applied Sciences, of the University of Wisconsin), has this to say

If the World Trade Center was hot enough to melt steel, where's all the molten concrete? Iron melts around 1500o C but so do many of the silicate minerals in concrete, and a mixture of silicate minerals would melt at a temperature lower than any of the individual minerals (I'm a geologist - I get paid to know about stuff like that). The fine particle size of the concrete dust would facilitate melting. So why wasn't there a huge puddle of molten concrete at Ground Zero? (There was some, but about what you'd expect from a large fire; certainly not what you'd expect from something hot enough to melt large amounts of steel.)
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  • Does concrete melt?
 

Landru

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9-11 Debunker, Steven Dutch (Natural and Applied Sciences, of the University of Wisconsin), has this to say

If the World Trade Center was hot enough to melt steel, where's all the molten concrete? Iron melts around 1500o C but so do many of the silicate minerals in concrete, and a mixture of silicate minerals would melt at a temperature lower than any of the individual minerals (I'm a geologist - I get paid to know about stuff like that). The fine particle size of the concrete dust would facilitate melting. So why wasn't there a huge puddle of molten concrete at Ground Zero? (There was some, but about what you'd expect from a large fire; certainly not what you'd expect from something hot enough to melt large amounts of steel.)
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  • Does concrete melt?

Your premise is that the WTC fires were hot enough to melt steel and therefore should melt concrete. Please provide evidence to support this but before you do please read the 7 pages of this other thread. https://www.metabunk.org/threads/molten-and-glowing-metal.2029/
 
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qed

Senior Member
@Landru

All I am asking is if concrete can melt.

Please stay on topic. I had to post evidence and I got it from a debunking site. I have no premise.
 

Landru

Moderator
Staff member
@Landru

All I am asking is if concrete can melt.

Please stay on topic. I had to post evidence and I got it from a debunking site. I have no premise.

Does concrete melt? No.
http://www.newton.dep.anl.gov/askasci/mats05/mats05054.htm

Concrete does not melt, at least not in the way you may be thinking. Concrete is composed largely of gravel an sand, with Portland cement that holds the sand and gravel together into a solid mass. The sand and gravel will melt, but you will not be doing it in your kitchen oven! A temperature of several thousand degrees is needed, and the result will be much the same as the lava that comes out of volcanos. After all, gravel and sand are just rock, as is molten lava. The Portland cement in concrete, is a mixture of various hydrates and silicates of calcium, aluminum and other elements. It too is a "rocky" material that will not melt at any practical temperature, either.

Regards,
Robert Wilson
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Mr. Wilson's credentials

http://www.newton.dep.anl.gov/scicorps/wilson_r.htm

You could have done this with a Google search.
 

qed

Senior Member
I did do a Google search "does concrete melt" and that was what I got. Perhaps we should start a Debunked person thread on Steven Dutch.
 

qed

Senior Member
What is the melting point of concrete?


Concrete does not have a melting point, even at very high temperatures. Instead, it decomposes. Some of the components of concrete will vaporize, but other components will remain solid even at very high temperatures.
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qed

Senior Member
Molten concrete is one of the indicators that is pointed out in the exotic accelerants section of the NFPA investigation guidelines.

Interesting. Can you give us the quote please because there appears to be doubt on this issue?
 

Jazzy

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Any competent scientist knows that concrete could not possibly melt, because it is composed of interlocked silicate crystals with WATER as part of their structure. Plaster is another material using water of crystallization. That won't melt either.

If you heat concrete it will eventually explode, as the vibrational energy of the water molecules in the concrete becomes greater than the molecular binding energy in the concrete, and the water escapes as steam, leaving a dehydrated silicate powder. At this point you would know the concrete was destroyed.

Most of the remainder will eventually fuse, if you raise its temperature further, but this isn't any longer concrete, as we know it (Jim)...
 
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qed

Senior Member
@Jazzy

Concrete is a very complicated mixture of different metal oxides, hydroxides, and silicates (many of which form extensive, interpenetrating networks), mixed with a filler material such as gravel or rock. It does not maintain its chemical identity when heated. If concrete is heated to a high enough temperature, the hydroxides decompose to form oxides and water; the water is quickly lost as the vapor. The remaining metal oxides are quite refractory; they remain solid at very high temperatures. The rock components of concrete will decompose or melt at differing temperatures depending on their mineral composition.

So the short answer to your question is that concrete will decompose rather then melt when heated, and the clinker that remains after it cools back down will unmistakably not be concrete.
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I still don't quite see why we don't get "lava"? (Not enough granites left to melt?)
 

Jazzy

Closed Account
I still don't quite see why we don't get "lava"? (Not enough granites left to melt?)
It is rare for granites to be present in concrete. They are strong and hard, and expensive to crush. Flint stones are another water-of-crystallization product (found in chalk beds) which make great concrete. That's the usual material. Where I am they use pumice, or slightly less-aerated, er, lava. But volcanic islands don't normally have chalk in their soils.
 

Mick West

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Yes. Molten concrete is one of the indicators that is pointed out in the exotic accelerants section of the NFPA investigation guidelines.

Seems like they are talking hypothetically though, rather than from experience. What does "melted concrete" look like?


NFPA 921 (2004 Editon)
22.2.4* Exotic Accelerants. Mixtures of fuels and Class 3 or
Class 4 oxidizers (see NFPA 430, Code for the Storage of Liquid and
Solid Oxidizers) may produce an exceedingly hot fire and may
be used to start or accelerate a fire. Thermite mixtures also
produce exceedingly hot fires. Such accelerants generally
leave residues that may be visually or chemically identifiable.

22.2.4.1 Exotic accelerants have been hypothesized as having
been used to start or accelerate some rapidly growing fires and
were referred to in these particular instances as high temperature
accelerants (HTA). Indicators of exotic accelerants include
an exceedingly rapid rate of fire growth, brilliant flares (particularly
at the start of the fire), and melted steel or concrete.
A study of 25 fires suspected of being associated with HTAs
during the 1981–1991 period revealed that there was no conclusive
scientific proof of the use of such HTA.

22.2.4.2 In any fire where the rate of fire growth is considered
exceedingly rapid, other reasons for this should be considered in
addition to the use of an accelerant, exotic or otherwise. These
reasons include ventilation, fire suppression tactics, and the type
and configuration of the fuels.
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Jazzy

Closed Account
It's easy enough to test. Take a piece of spalled-off concrete, and, wearing goggles and protective clothing, toast it with a propane burner. I've done it, and lived to tell the tale. Nice bang...

I have never managed to melt it.
 

Mick West

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It's easy enough to test. Take a piece of spalled-off concrete, and, wearing goggles and protective clothing, toast it with a propane burner. I've done it, and lived to tell the tale. Nice bang...

I have never managed to melt it.

Here's some concrete being heated until it fractures (spalls) off chips.

 

Mick West

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This video shows someone cutting concrete and stone with a thermal lance. There's clearly something molten there.

"thermal lance, 17mm diameter and 3 feet long, cutting a stone and refractory concrete blocks"
 
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Jazzy

Closed Account
This video shows someone cutting concrete and stone with a thermal lance. There's clearly something molten there.
A thermal lance is thermite-in-a-rod, so the "something molten" could be iron or metal slag. (Or anything else from which oxygen might be stripped by the burning aluminum).
Most likely, it's alumina, or an alumina-glass mix. Yessir, I keep forgetting it's pure oxygen going in, so only metallic and silicaceous oxides will be coming out.
 
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Tony Szamboti

Active Member
It's easy enough to test. Take a piece of spalled-off concrete, and, wearing goggles and protective clothing, toast it with a propane burner. I've done it, and lived to tell the tale. Nice bang...

I have never managed to melt it.

Oxyacetylene burns much hotter than propane and is likely the only easy to access thing needed to melt some of the ceramic/refractory constituents of concrete.
See http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting
 

Mick West

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@Mick West
  • Does concrete melt?
Yes, no, or I don't know.

Not really. Some of the components of concrete (like rocks, basically) will melt at a high enough temperature. But at that point it's no longer concrete.

Really high temperatures destroy concrete, and what's left over can melt. But there's really no such thing as molten concrete.
 

qed

Senior Member
Consider a block of concrete in a hypothetical oven at 800 degrees c. I leave it for a while.

  • Will I see a puddle of "lava" on the floor?
 

Mick West

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Consider a block of concrete in a hypothetical oven at 800 degrees c. I leave it for a while.

  • Will I see a puddle of "lava" on the floor?

No.

800C is not going to do much for a start. If you continue to heat it then the concrete will break apart. You are then left with a mixture of type of minerals. If you heat that up enough you will get "lava", more like 1800C than 800C though. (Melting point of common sand is around 1720C)

The answers referenced earlier seem quite reasonable:
http://www.newton.dep.anl.gov/askasci/mats05/mats05054.htm
 

qed

Senior Member
While it is some time since I sat an exam in igneous petrology, pulling A. Hall from my bookshelf, I read
  • "The temperature range up to 1500 degree c encompasses all the igneous phenomena likely to occur near the Earth's surface".
Perhaps I misunderstand you?
 

Mick West

Administrator
Staff member
While it is some time since I sat an exam in igneous petrology, pulling A. Hall from my bookshelf, I read
  • "The temperature range up to 1500 degree c encompasses all the igneous phenomena likely to occur near the Earth's surface".
Perhaps I misunderstand you?

I'm just looking stuff up here. Sand is generally silicon dioxide which Wikipedia lists as melting at 1600C to 1725C. I believe it is plastic at a much lower temperature - like with glass blowing.
 

Josh Heuer

Active Member
I'm just looking stuff up here. Sand is generally silicon dioxide which Wikipedia lists as melting at 1600C to 1725C. I believe it is plastic at a much lower temperature - like with glass blowing.
It actually depends on the source. wolfram Alpha lists quartz/silicon dioxide at a melting point of 1427C. And sand varies with location, so it could be different depending on what the sand actually consists of.
 

qed

Senior Member
"Melting and crystallization temperatures of igneous rock in the laboratory vary between 950dC and 1250dC. Natural rocks have considerably lower melting temperatures than their constituent mineral..." A. Hall
 

qed

Senior Member
@Mick West [:sigh:]

I place 100kg of concrete in an oven at 1000C.

I return later.
  • What will I see inside the oven?
I then let the oven cool and determine the mass of the contents of the oven.
  • What will be the mass?
 

Mick West

Administrator
Staff member
@Mick West [:sigh:]

I place 100kg of concrete in an oven at 1000C.

I return later.
  • What will I see inside the oven?
I then let the oven cool and determine the mass of the contents of the oven.
  • What will be the mass?

Interesting questions, and I think largely dependent on the actual composition of the concrete. The key thing is that calcium hydroxide dehydrates at 1000F (538C) which basically means that well before the concrete gets to 1000C, it's going to easily crumble to powder, sand, and aggregate. The mass will be reduced because all the water will disassociate and evaporate (so around 16% reduction in mass from that). The aggregates themselves break down at different temps.

But the internet knows more than I do:

http://info.ornl.gov/sites/publications/files/Pub1043.pdf

If concrete made with Portland cement or blast furnace slag cement is subjected to heat, a number of transformations and reactions occur, even if there is only a moderate increase in temperature.5,6 As aggregate materials normally occupy 65 to 75% of the concrete volume, the behavior of concrete at elevated temperature is strongly influenced by the aggregate type. Commonly used aggregate materials are thermally stable up to 300°C–350°C. Aggregate characteristics of importance to behavior of concrete at elevated temperature include physical properties (e.g., thermal conductivity and thermal expansion), chemical properties (e.g., chemical stability at temperature), and thermal stability/integrity. Aggregate materials may undergo crystal transformations leading to significant increases in volume [e.g., crystalline transformation of α-quartz (trigonal) to β-quartz (hexagonal) between 500 and 650°C with an accompanying increase in volume of ~5.7%]. Some siliceous or calcareous aggregates with some water of constitution exhibit moderate dehydration with increasing temperature that is accompanied by shrinkage (i.e., opal at 373°C exhibits shrinkage of ~13% by volume).7 Most nonsiliceous aggregates are stable up to about 600°C. At higher temperatures, calcareous aggregates (calcite – CaCO3), magnesite (MgCO3), and dolomite (MgCO3/CaCO3) dissociate into an oxide and CO2 (CaO + CO2). Calcium carbonate dissociates completely at 1 atm pressure at 898°C with partial dissociation occurring at temperatures as low as 700°C (Ref. 8). Above 1200°C and up to 1300°C some aggregates, such as igneous rocks (e.g., basalt), show degassing and expansion. Refractory aggregates can be utilized to produce significant improvements in the heat resistance of Portland cement concretes. It has been noted that the thermal stability of aggregates increases in order of gravel, limestone, basalt, and lightweight.9

Apart from the crystalline transformations occurring mainly in the aggregate materials during heating, a number of degradation reactions occur, primarily in the cement paste, that result in a progressive breakdown in the structure of the concrete. An increase in temperature produces significant changes in the chemical composition and microstructure of the hardened Portland cement paste. At low temperatures these reactions mainly take the form of dehydration and water expulsion reactions. Changes in the chemical composition and microstructure of the hardened Portland cement paste occur gradually and continuously over a temperature range from room temperature to 1000°C. At room temperature, between 30 and 60% of the volume of saturated cement paste and between 2 and 10% of the volume of saturated structural concrete are occupied by evaporable water. As the temperature to which the cement paste is subjected increases, evaporable water is driven off until at a temperature of about 105°C all evaporable water will be lost, given a sufficient exposure period. At temperatures above 105°C, the strongly absorbed and chemically combined water (i.e., water of hydration) are gradually lost from the cement paste hydrates, with the dehydration essentially complete at 850°C. Dehydration of the calcium hydroxide is essentially zero up to about 400°C, increases most rapidly around 535°C, and becomes complete at about 600°C (Ref. 10). Figures 1 and 2 indicate the influence of temperature on the ultimate compressive strength and modulus of elasticity of a Portland cement paste (Type I Portland cement; water/cement = 0.33) (Ref. 11).

A good summary of the degradation reactions that occur in Portland cement concrete is provided in
Ref. 4. Upon first heating, substantial water evaporation occurs from the larger pores close to the concrete surface. Then, from 100°C onward, the evaporation proceeds at a faster rate with water being expelled from concrete near the surface as a result of above-atmospheric vapor pressure (i.e., steam flow). At 120°C the expulsion of water physically bound in the smaller pores, or chemically combined, initiates and continues up to about 500°C where the process is essentially complete. From 30°C to 300°C, in conjunction with evaporation, dehydration of the hardened cement paste occurs (first stage) with the maximum rate of dehydration occurring at about 180°C [Tobermorite gel is stable up to a temperature of 150°C (Ref. 12)]. In the temperature range from 450°C to 550°C there is decomposition of the portlandite [i.e., Ca(OH)2 → CaO + H2O) (Ref. 12)]. At 570°C the α → β inversion of quartz takes place with the transformation being endothermic and reversible. A further process of decomposition of the hardened cement paste takes place between 600°C and 700°C with the decomposition of the calcium-silicate- hydrate phases and formation of β-C2S. Between 600°C and 900°C the limestone begins to undergo decarbonation (i.e., CaCO3 → CaO + CO2). The rate of decomposition and the temperature at which it occurs are not only dependent on temperature and pressure, but also by the content of SiO2 present in the limestone. Above 1200°C and up to 1300°C, some components of the concrete begin to melt. Above 1300°C to 1400°C concrete exists in the form of a melt. Apparently liquifaction of the concrete commences with melting of the hardened cement paste followed by melting of the aggregates.13–15 The melting points of aggregates vary greatly. At 1060°C basalt is at the lower limit of all types of rock, with quartzite not melting below 1700°C (Ref. 5).

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http://www.concreteconstruction.net...erature on Hardened Concrete_tcm45-345325.pdf

A number of factors will enter in- to a decision regarding the type of concrete to use under conditions of elevated temperatures. These in- clude the following: length of expo- sure, rate of temperature rise, tem- perature to which the concrete mass will be raised, temperature of concrete at initiation of exposure to high temperature, degree of water saturation of the concrete, age of the concrete, type of aggregate used, type of cement used, aggregate/ce- ment ratio, and loading conditions at time of exposure.

Concrete appears to sustain no appreciable damage when exposed to temperatures up to 400 degrees Fahrenheit. If temperatures above 400 degrees Fahrenheit are to be ex- perienced, it is wise to investigate the exposure conditions and the concrete which will be employed.

The reaction of concrete to ele- vated temperatures will vary with the factors listed previously; but to get some idea of a typical succes- sion of effects as temperature rises, let us trace the laboratory history of an ordinary concrete when it is sub- jected to increasing heat.

The first effects of a slow temper- ature rise in concrete will occur be- tween 200 and 400 degrees Fahren- heit when evaporation of the free moisture contained in the concrete mass occurs; instant exposure can result in spalling through genera- tion of high internal steam pres- sures. As the temperature ap- proaches 500 degrees Fahrenheit, dehydration or loss of the non- evaporable water or water of hydra- tion, begins to take place. The first sizable degradation in compressive strength is usually experienced be- tween 400 and 750 degrees Fahren- heit. At 600 degrees Fahrenheit strength reduction would be in the range of 15 to 40 percent. At 1,000 degrees Fahrenheit reduction in compressive strength would typi- cally range from 55 to 80 percent of its original value. Temperatures in the 1000 degrees Fahrenheit range are critical because calcium hydrox- ide dehydrates at that temperature. Calcium hydroxide is a hydration product of most portland cements, the amount being dependent upon the particular cement being used.

Aggregates also begin to deterio- rate at about 1000 degrees Fahren- heit; for example, quartz expands greatly and suddenly at 1063 de- grees Fahrenheit. Concrete will un- dergo normal expansion up to 300 to 500 degrees Fahrenheit but above this it begins to shrink at an even faster rate. At 700 to 800 degrees Fahrenheit, in addition to great re-
ductions in strength, cracking of re- strained concrete will have rendered the concrete virtually valueless. At around 1,000 degrees Fahrenheit it will usually be possible to break the concrete into pieces with the bare hands.

Naturally, the rate at which the concrete is brought to these tem- peratures and the length of time the temperatures are maintained will have an important bearing on how the physical properties of the con- crete are affected. At approximately 600 degrees Fahrenheit surface hair- line cracks begin to form. At 1,000 degrees Fahrenheit deep cracks have begun to be formed and these increase in size upon cooling of the concrete; they eventually result in the disintegration of the concrete. After the concrete cools there is a further reduction in strength amounting to a loss of about 20 per- cent of the strength of the concrete observed at the l,000-degree tem- perature.
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This is for high performance concrete, which remains solid at 1000C, but shows the weight loss from dehydration:
http://www.ipublishing.co.in/ijcserarticles/twelve/articles/voltwo/EIJCSE3035.pdf
 
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Mick West

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@Mick West [:sigh:]

I place 100kg of concrete in an oven at 1000C.

I return later.
  • What will I see inside the oven?
I then let the oven cool and determine the mass of the contents of the oven.
  • What will be the mass?

Based on the previous, I'd say:
  • What will I see inside the oven? - A crumbling block or heap of very weak concrete
  • What will be the mass? - around 85-90kg
 

qed

Senior Member
I place 100kg of concrete in an oven at 1500C.

I return later.
  • What will I see inside the oven?
1000C = 1832F
1000F = 537C
 
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Mick West

Administrator
Staff member
I place 100kg of concrete in an oven at 1500C.

I return later.
  • What will I see inside the oven?
1000C = 1832F
1000F = 537C

What's with the helpful conversions? Did I get something wrong?

That's some fancy oven you've got there :)

You'd probably see a dirty glassy blob on the bottom of the oven, based on this from the above Oak Ridge National Labs paper:

Above 1200°C and up to 1300°C, some components of the concrete begin to melt. Above 1300°C to 1400°C concrete exists in the form of a melt. Apparently liquifaction of the concrete commences with melting of the hardened cement paste followed by melting of the aggregates.13–15 The melting points of aggregates vary greatly. At 1060°C basalt is at the lower limit of all types of rock, with quartzite not melting below 1700°C
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Maybe you could explain the point you are trying to make?
 

Josh Heuer

Active Member
But it's not actually concrete at that point.

Iron melts. Concrete decomposes, some components of it evaporate, and some melt (at various temperatures).
It's hard to say really...we could get into a debate over what's considered melting.
When iron melts, the oxygen is released, obviously it doesn't melt. So can we say iron doesn't melt because one of its constituents doesn't?
 

Mick West

Administrator
Staff member
It's hard to say really...we could get into a debate over what's considered melting.
When iron melts, the oxygen is released, obviously it doesn't melt. So can we say iron doesn't melt because one of its constituents doesn't?

Oxygen is not a constituent of iron. Iron is an element.

I'd say a handy definition of if something can melt is if it can exist in molten form. Concrete cannot exist in molten form.
 
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