I have been looking at the recent article by Kurt King entitled “WTC Dust Cloud Energetics and Implications”, published in Volume 1 of the 9/11 Scholars Review at: http://www.911scholarsreview.org/current.html
I have started an e-mail exchange about this article with Mr. King and I thought JREFers would be interested in reading some of it so here we go:
Dear Mr. King,
I have read with great interest your recent article entitled “WTC Dust Cloud Energetics and Implications”, (published in Volume 1 of the 9/11 Scholars Review). My interest stems from the fact that I have also carried out research on this topic. I have to say that I have many problems with the physics and chemistry of your article, but for now I would like to focus on one issue: the energy needed to vaporize the moisture in the pulverized concrete, as discussed on page 14 of your article. I would like to review this part of your calculation because it constitutes one of your largest energy terms.
On page 15 of your article you calculate the energy needed to vaporize the water in the WTC concrete using two assumptions:
(i) The concrete contained 1 % water by weight.
(ii)The water was liberated “within a second” at 100 deg C, the boiling point of water, so that the heat of vaporization of the water in the concrete is ~ 540 cal/g.
I would question both of these assumptions…
Concrete is made from Portland cement paste, (300 – 400 kg/m^3), and fine or coarse “aggregate” materials such as sand, gravel, crushed rock, gypsum, silica, etc, (1500 – 2000 kg/m^3). Dry, unhydrated, cement is mostly composed of calcium, iron and aluminum silicates, such as tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite.
In the presence of added water, typically 150 – 200 kg/m^3, up to 24 % of the cement is converted to calcium hydroxide which forms a gel that sets and hardens as it dries. Nevertheless, even after years of exposure to dry air, “dried” concrete retains a portion of the initially added water. This water is believed to be present in three forms: capillary water, gel water and chemically combined water. Capillary water is the most loosely bound form of water in hardened cement, existing as a liquid in the pores and capillaries of the material; this water is partially removed by air-drying at ambient temperatures. By comparison, gel water is not removed by drying cement at room temperature but is lost by heating the material to ~ 110 deg C. Chemically combined water, also known as non-evaporable water, is an integral part of the calcium silicate hydrate - calcium hydroxide gel and is only removed by heating the material to temperatures in the range 450 – 500 deg C. Thermo-gravimetric analysis of concrete shows that most samples loose at least 8 % of their weight as evaporated water upon heating to 600 deg C. Additional weight loss usually occurs by heating samples in the range 600 – 1000 deg C due to CO2 loss from the decomposition of calcium carbonate also present in cement.
These properties of cement and concrete show that there is no experimental basis for the assumption that the WTC concrete contained only 1 % water, it would have been at least eight times higher than this! But this is only the beginning of problems with “WTC Dust Cloud Energetics and Implications”. The diffusion coefficient of capillary water in concrete at temperatures below 100 deg C is reported to be about 10^-9 m^2/s. Now, since it is argued on page 14 of “WTC Dust Cloud Energetics and Implications” that the moisture in the pulverized concrete would have been liberated “within a second”, let’s consider how far water molecules would be able to migrate in concrete in one second. The average distance that a species with diffusion coefficient D migrates in time t is ~ Sqrt[Dt]. Using the value of D noted above we conclude that water molecules would migrate in concrete only about 30 microns in one second. Thus, to escape from the WTC concrete in this time interval, the concrete would need to be pulverized to particles that were 60 microns or smaller.
The most detailed analyses of WTC dust appear to be those reported by P. J. Lioy et al. in Vol. 110, page 703, of Environmental Health Perspectives, J. K. McGee et al. in Vol. 111, page 972, of Environmental Health Perspectives , and by G. P. Meeker in the USGS Report No. 2005–1031. These authors collected samples shortly after 9-11 at several locations within 1 km of ground zero and carried out detailed chemical and particle size analyses; their measurements show that, for samples collected closest (< 400 meters) to ground zero, the particle size distribution was: 16 % greater than 300 microns; 46 % in the range 75 – 300 microns; 38 % less than 75 microns. This shows that capillary water could not have escaped from the majority of the pulverized concrete in the short time interval proposed in “WTC Dust Cloud Energetics and Implications”.
The situation for the gel water and chemically combined water in the WTC concrete would have been even less favorable to the loss of water “within a second” because the pulverized concrete would have to have been at a temperature over 450 deg C. This level of pre-heating might have occurred for concrete in the upper, fire-ravaged floors of WTC 1 & 2, but certainly did not occur for floors in the lower two thirds of the Towers. Nevertheless, “WTC Dust Cloud Energetics and Implications” appears to be saying that all the concrete in the Towers must have been flash heated to at least 100 deg C to account for the observed expansion of the dust cloud. This view assumes that shock heating is the only possible source of energy available to drive the expansion of the dust cloud. Unfortunately, this is simply not true, as we shall now demonstrate.
The collapsing floors in each tower behaved like giant pistons pushing up to 10,000 m^3 of air out of each floor space. The pressure build-up would have shattered windows almost immediately, expelling the enclosed air. However, the process of collapse would have simultaneously crushed the gypsum wallboard and fiberglass insulation present on every floor and some of this debris would have been expelled also. How fast was this dust cloud expelled? The first collapsing floor fell the 3.7-meter ceiling-to-floor distance in 0.87 seconds and subsequent floors fell much faster. It follows that a volume of dusty air near the center of a collapsing floor traversed a horizontal distance of about 16 meters in 0.87 seconds in exiting the building. This volume therefore had an average expulsion velocity of 66 km per hour. The towers ultimately attained a collapse velocity in excess of 50 m/s in which case the lower floors were crushed in 0.074 seconds and dust expulsion velocities approached 778 km per hour!
Mr. King, you acknowledge that much of “WTC Dust Cloud Energetics and Implications” is based on J. Hoffman’s 2003 study of the expansion of the North Tower dust cloud. In this study Mr. Hoffman estimates that the dust cloud advanced at an average horizontal velocity of about 10 m/s immediately after the tower collapse. I have previously shown that dust was expelled from the upper floors of a collapsing tower at a velocity ~ 20 m/s. The rate of expansion of the dust was subsequently retarded by air resistance, so Mr. Hoffman’s average velocity of 10 m/s over the first minute of expansion appears to be a reasonable estimate. For dust ejected from upper floors, say 300 meters above the ground, a vertical settling velocity of about 5 m/s is also quite reasonable for 60 micron particles. Thus dust formed at 300 m moving at 10 m/s would settle out after about one minute at a distance more than ½ kilometer from the base of the tower, a trajectory which is consistent with the extent of dust dispersion observed.
Let us now consider the energy imparted to the ejected concrete dust. Each tower contained about 69,000,000 kg of concrete flooring. To be favorable to the thesis presented in “WTC Dust Cloud Energetics and Implications” I will assume that all of this was ejected as dust. For an ejection velocity of 20 m/s the kinetic energy imparted to the dust is ½ x 69,000,000 x (20)^2 J = 1.4 x 10^10 J per tower, or 1.3 x 10^8 J per floor. This is much less than the kinetic energy of the 15-storey block of WTC 1 floors at the moment of its impact on the lower floors, (2.1 x 10 ^9 J), or the energy needed to collapse the support structure of one floor, (estimated to be ~ 0.6 x 10^9 J).
For the sake of completeness we may also calculate the energy imparted to the air that was expelled from the collapse of one floor. Ignoring furniture and other office fixtures, the volume of air expelled per floor was about 10,000 m^3. This has an approximate mass of 12,000 kg at normal temperature and pressure. For an ejection velocity of 20 m/s this air carries away ½ x 12,000 x (20)^2 J = 2.4 x 10^6 J of kinetic energy per floor. Once again, this amount of energy is trivial compared to the energy released by the descent of the upper sections of the towers through one floor height or 3.7 meters.
Thus, contrary to “WTC Dust Cloud Energetics and Implications”, I would argue that there was plenty of gravitational potential energy available to expand each WTC dust cloud.
Sincerely.....
I received a response from Mr. King that included the following:
1. While the largest tonnage of particles was in their largest size, Lioy's assay of particle sizes as shown in my table 2 (from Lioy, et al - table now missing from source!) indicates that 123,016 tons - about 42% by weight - of it was smaller than 60 microns. I'd be happy to compute only 42% of the assumed water expansion. Then the observed expansion would have to be more driven thermodynamically, which is energetically more demanding.
2. It seems like you are assuming the pancake theory, but GZ left no floors at all, let alone a stack of them - donuts between the outer wall and the core. There essentially weren't any shattered windows in the WTC tower rubble: see my footnote 29 and the recollection by Lieutenant Allen (Butch) Williams of NYFD.
3. For the whole building collapse to have taken little over 9.1 seconds the floors on average could not have transferred much energy to the air until they hit the ground. Since none remained on the ground, floor-hitting didn't happen. Besides the largest quantity of dust became apparent about 3 seconds into the "collapse" when there was little energy available by that mechanism
4. You are assuming that the particles were moving independently due to gravity and subject to turbulence. But as the videos clearly show, the clouds moved as pyroclastic flows, falling and rolling more like a dense fluid, like volcanic ash
5. You have calculated the energy to give the air and dust a velocity - as if it were pushed out of an elevator shaft, which is far less than that required by the Ideal Gas Law to expand a volume of air - especially where the suspended dust adds so much thermal capacity. Along the way, you seem to want the floors to be both solid to do the pushing, and to be the dust that is pushed.
I responded today with this:
Thanks for your quick response to my e-mail. I will try to address the points you have raised:
(i) You do indeed claim to show the dust cloud was heated by at least 600 deg C. Why therefore do you also claim that 100 deg C of heating is "adequately plausible"?
(ii) I do not believe Lioy or any other researchers who have reported size distributions for WTC dust specify how much dust was produced by the collapse of the Towers. Thus to claim that precisely 123,016 tons of dust was smaller than 60 microns is not supported by actual measurements. Many photos of the rubble pile show large chunks of concrete mixed in with a material that looks like gravel. (See the book Aftermath by Joel Meyerowitz). I believe the average concrete particle size was ~ 1 mm in which case there was sufficient gravitational energy to collapse the Towers and pulverize the concrete to the degree observed.
(iv) Stacks of floors were found at GZ - see Meyerowitz's photo collection. Also the famous "meteorite" located in Hangar 17 at Kennedy International Airport shows a "sandwich" made up of several WTC floor sections.
(v) You claim that little kinetic energy was available in the first 3 seconds of the collapse. This is simply not true! Just 2 seconds into the collapse of WTC 1 the upper block had dropped about 20 meters, taking 5 floors with it, and was moving at about 20 m/s. It therefore had a KE of about 1.5 x 10^10 J. This would have impacted the 627 tonnes of concrete on the 90th floor with a specific impact energy of about 20 J/g; this energy is 200 times larger than the elastic strain energy capacity of concrete and more than sufficient to reduce it to mm sized fragments.
Final Comment
It is interesting that Jim Hoffman and others frequently refer to the expanding dust clouds produced by the collapse of WTC 1 & 2 as pyroclastic surges. This terminology is based on the supposed resemblance of the WTC dust clouds to the hot turbulent flows of gas, dust and debris ejected by many volcanic eruptions. L. M. Schwarzkopf et al. have pointed out that pyroclastic surges are promoted by breaks in the slope of a volcano where the cascades of falling rock and ash are subject to hard impacts. These impacts result in intensified production of fines through fragmentation of the larger rocks entrained in the flow.
In order to study this phenomenon further, Schwarzkopf et al. performed drop experiments with basaltic rock samples pre-heated to temperatures up to 850 deg C. The samples weighed about 50 grams and the drop height was about 3 meters so that the impact kinetic energy was about 30 J/g. The remarkable result of these experiments was that the samples heated above 200 deg C exploded on impact and the degree of fragmentation increased as the temperature was increased.
See www.electronic-earth-discuss.net/1/81/2006/
I believe that the same thing would happen to the concrete slabs in the fire-affected zones of WTC 1 & 2 as the upper sections collapsed. Concrete typically contains at least 5 % water in pores and capillaries. When concrete is heated above 100 deg C, the water is converted to steam under very high pressure, sometimes leading to explosive spalling of the outermost layers of the material. However, a hard impact on hot concrete would most certainly result in total fragmentation of the concrete, accompanied by the explosive release of hot, (superheated!), steam.
I have started an e-mail exchange about this article with Mr. King and I thought JREFers would be interested in reading some of it so here we go:
Dear Mr. King,
I have read with great interest your recent article entitled “WTC Dust Cloud Energetics and Implications”, (published in Volume 1 of the 9/11 Scholars Review). My interest stems from the fact that I have also carried out research on this topic. I have to say that I have many problems with the physics and chemistry of your article, but for now I would like to focus on one issue: the energy needed to vaporize the moisture in the pulverized concrete, as discussed on page 14 of your article. I would like to review this part of your calculation because it constitutes one of your largest energy terms.
On page 15 of your article you calculate the energy needed to vaporize the water in the WTC concrete using two assumptions:
(i) The concrete contained 1 % water by weight.
(ii)The water was liberated “within a second” at 100 deg C, the boiling point of water, so that the heat of vaporization of the water in the concrete is ~ 540 cal/g.
I would question both of these assumptions…
Concrete is made from Portland cement paste, (300 – 400 kg/m^3), and fine or coarse “aggregate” materials such as sand, gravel, crushed rock, gypsum, silica, etc, (1500 – 2000 kg/m^3). Dry, unhydrated, cement is mostly composed of calcium, iron and aluminum silicates, such as tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite.
In the presence of added water, typically 150 – 200 kg/m^3, up to 24 % of the cement is converted to calcium hydroxide which forms a gel that sets and hardens as it dries. Nevertheless, even after years of exposure to dry air, “dried” concrete retains a portion of the initially added water. This water is believed to be present in three forms: capillary water, gel water and chemically combined water. Capillary water is the most loosely bound form of water in hardened cement, existing as a liquid in the pores and capillaries of the material; this water is partially removed by air-drying at ambient temperatures. By comparison, gel water is not removed by drying cement at room temperature but is lost by heating the material to ~ 110 deg C. Chemically combined water, also known as non-evaporable water, is an integral part of the calcium silicate hydrate - calcium hydroxide gel and is only removed by heating the material to temperatures in the range 450 – 500 deg C. Thermo-gravimetric analysis of concrete shows that most samples loose at least 8 % of their weight as evaporated water upon heating to 600 deg C. Additional weight loss usually occurs by heating samples in the range 600 – 1000 deg C due to CO2 loss from the decomposition of calcium carbonate also present in cement.
These properties of cement and concrete show that there is no experimental basis for the assumption that the WTC concrete contained only 1 % water, it would have been at least eight times higher than this! But this is only the beginning of problems with “WTC Dust Cloud Energetics and Implications”. The diffusion coefficient of capillary water in concrete at temperatures below 100 deg C is reported to be about 10^-9 m^2/s. Now, since it is argued on page 14 of “WTC Dust Cloud Energetics and Implications” that the moisture in the pulverized concrete would have been liberated “within a second”, let’s consider how far water molecules would be able to migrate in concrete in one second. The average distance that a species with diffusion coefficient D migrates in time t is ~ Sqrt[Dt]. Using the value of D noted above we conclude that water molecules would migrate in concrete only about 30 microns in one second. Thus, to escape from the WTC concrete in this time interval, the concrete would need to be pulverized to particles that were 60 microns or smaller.
The most detailed analyses of WTC dust appear to be those reported by P. J. Lioy et al. in Vol. 110, page 703, of Environmental Health Perspectives, J. K. McGee et al. in Vol. 111, page 972, of Environmental Health Perspectives , and by G. P. Meeker in the USGS Report No. 2005–1031. These authors collected samples shortly after 9-11 at several locations within 1 km of ground zero and carried out detailed chemical and particle size analyses; their measurements show that, for samples collected closest (< 400 meters) to ground zero, the particle size distribution was: 16 % greater than 300 microns; 46 % in the range 75 – 300 microns; 38 % less than 75 microns. This shows that capillary water could not have escaped from the majority of the pulverized concrete in the short time interval proposed in “WTC Dust Cloud Energetics and Implications”.
The situation for the gel water and chemically combined water in the WTC concrete would have been even less favorable to the loss of water “within a second” because the pulverized concrete would have to have been at a temperature over 450 deg C. This level of pre-heating might have occurred for concrete in the upper, fire-ravaged floors of WTC 1 & 2, but certainly did not occur for floors in the lower two thirds of the Towers. Nevertheless, “WTC Dust Cloud Energetics and Implications” appears to be saying that all the concrete in the Towers must have been flash heated to at least 100 deg C to account for the observed expansion of the dust cloud. This view assumes that shock heating is the only possible source of energy available to drive the expansion of the dust cloud. Unfortunately, this is simply not true, as we shall now demonstrate.
The collapsing floors in each tower behaved like giant pistons pushing up to 10,000 m^3 of air out of each floor space. The pressure build-up would have shattered windows almost immediately, expelling the enclosed air. However, the process of collapse would have simultaneously crushed the gypsum wallboard and fiberglass insulation present on every floor and some of this debris would have been expelled also. How fast was this dust cloud expelled? The first collapsing floor fell the 3.7-meter ceiling-to-floor distance in 0.87 seconds and subsequent floors fell much faster. It follows that a volume of dusty air near the center of a collapsing floor traversed a horizontal distance of about 16 meters in 0.87 seconds in exiting the building. This volume therefore had an average expulsion velocity of 66 km per hour. The towers ultimately attained a collapse velocity in excess of 50 m/s in which case the lower floors were crushed in 0.074 seconds and dust expulsion velocities approached 778 km per hour!
Mr. King, you acknowledge that much of “WTC Dust Cloud Energetics and Implications” is based on J. Hoffman’s 2003 study of the expansion of the North Tower dust cloud. In this study Mr. Hoffman estimates that the dust cloud advanced at an average horizontal velocity of about 10 m/s immediately after the tower collapse. I have previously shown that dust was expelled from the upper floors of a collapsing tower at a velocity ~ 20 m/s. The rate of expansion of the dust was subsequently retarded by air resistance, so Mr. Hoffman’s average velocity of 10 m/s over the first minute of expansion appears to be a reasonable estimate. For dust ejected from upper floors, say 300 meters above the ground, a vertical settling velocity of about 5 m/s is also quite reasonable for 60 micron particles. Thus dust formed at 300 m moving at 10 m/s would settle out after about one minute at a distance more than ½ kilometer from the base of the tower, a trajectory which is consistent with the extent of dust dispersion observed.
Let us now consider the energy imparted to the ejected concrete dust. Each tower contained about 69,000,000 kg of concrete flooring. To be favorable to the thesis presented in “WTC Dust Cloud Energetics and Implications” I will assume that all of this was ejected as dust. For an ejection velocity of 20 m/s the kinetic energy imparted to the dust is ½ x 69,000,000 x (20)^2 J = 1.4 x 10^10 J per tower, or 1.3 x 10^8 J per floor. This is much less than the kinetic energy of the 15-storey block of WTC 1 floors at the moment of its impact on the lower floors, (2.1 x 10 ^9 J), or the energy needed to collapse the support structure of one floor, (estimated to be ~ 0.6 x 10^9 J).
For the sake of completeness we may also calculate the energy imparted to the air that was expelled from the collapse of one floor. Ignoring furniture and other office fixtures, the volume of air expelled per floor was about 10,000 m^3. This has an approximate mass of 12,000 kg at normal temperature and pressure. For an ejection velocity of 20 m/s this air carries away ½ x 12,000 x (20)^2 J = 2.4 x 10^6 J of kinetic energy per floor. Once again, this amount of energy is trivial compared to the energy released by the descent of the upper sections of the towers through one floor height or 3.7 meters.
Thus, contrary to “WTC Dust Cloud Energetics and Implications”, I would argue that there was plenty of gravitational potential energy available to expand each WTC dust cloud.
Sincerely.....
I received a response from Mr. King that included the following:
1. While the largest tonnage of particles was in their largest size, Lioy's assay of particle sizes as shown in my table 2 (from Lioy, et al - table now missing from source!) indicates that 123,016 tons - about 42% by weight - of it was smaller than 60 microns. I'd be happy to compute only 42% of the assumed water expansion. Then the observed expansion would have to be more driven thermodynamically, which is energetically more demanding.
2. It seems like you are assuming the pancake theory, but GZ left no floors at all, let alone a stack of them - donuts between the outer wall and the core. There essentially weren't any shattered windows in the WTC tower rubble: see my footnote 29 and the recollection by Lieutenant Allen (Butch) Williams of NYFD.
3. For the whole building collapse to have taken little over 9.1 seconds the floors on average could not have transferred much energy to the air until they hit the ground. Since none remained on the ground, floor-hitting didn't happen. Besides the largest quantity of dust became apparent about 3 seconds into the "collapse" when there was little energy available by that mechanism
4. You are assuming that the particles were moving independently due to gravity and subject to turbulence. But as the videos clearly show, the clouds moved as pyroclastic flows, falling and rolling more like a dense fluid, like volcanic ash
5. You have calculated the energy to give the air and dust a velocity - as if it were pushed out of an elevator shaft, which is far less than that required by the Ideal Gas Law to expand a volume of air - especially where the suspended dust adds so much thermal capacity. Along the way, you seem to want the floors to be both solid to do the pushing, and to be the dust that is pushed.
I responded today with this:
Thanks for your quick response to my e-mail. I will try to address the points you have raised:
(i) You do indeed claim to show the dust cloud was heated by at least 600 deg C. Why therefore do you also claim that 100 deg C of heating is "adequately plausible"?
(ii) I do not believe Lioy or any other researchers who have reported size distributions for WTC dust specify how much dust was produced by the collapse of the Towers. Thus to claim that precisely 123,016 tons of dust was smaller than 60 microns is not supported by actual measurements. Many photos of the rubble pile show large chunks of concrete mixed in with a material that looks like gravel. (See the book Aftermath by Joel Meyerowitz). I believe the average concrete particle size was ~ 1 mm in which case there was sufficient gravitational energy to collapse the Towers and pulverize the concrete to the degree observed.
(iv) Stacks of floors were found at GZ - see Meyerowitz's photo collection. Also the famous "meteorite" located in Hangar 17 at Kennedy International Airport shows a "sandwich" made up of several WTC floor sections.
(v) You claim that little kinetic energy was available in the first 3 seconds of the collapse. This is simply not true! Just 2 seconds into the collapse of WTC 1 the upper block had dropped about 20 meters, taking 5 floors with it, and was moving at about 20 m/s. It therefore had a KE of about 1.5 x 10^10 J. This would have impacted the 627 tonnes of concrete on the 90th floor with a specific impact energy of about 20 J/g; this energy is 200 times larger than the elastic strain energy capacity of concrete and more than sufficient to reduce it to mm sized fragments.
Final Comment
It is interesting that Jim Hoffman and others frequently refer to the expanding dust clouds produced by the collapse of WTC 1 & 2 as pyroclastic surges. This terminology is based on the supposed resemblance of the WTC dust clouds to the hot turbulent flows of gas, dust and debris ejected by many volcanic eruptions. L. M. Schwarzkopf et al. have pointed out that pyroclastic surges are promoted by breaks in the slope of a volcano where the cascades of falling rock and ash are subject to hard impacts. These impacts result in intensified production of fines through fragmentation of the larger rocks entrained in the flow.
In order to study this phenomenon further, Schwarzkopf et al. performed drop experiments with basaltic rock samples pre-heated to temperatures up to 850 deg C. The samples weighed about 50 grams and the drop height was about 3 meters so that the impact kinetic energy was about 30 J/g. The remarkable result of these experiments was that the samples heated above 200 deg C exploded on impact and the degree of fragmentation increased as the temperature was increased.
See www.electronic-earth-discuss.net/1/81/2006/
I believe that the same thing would happen to the concrete slabs in the fire-affected zones of WTC 1 & 2 as the upper sections collapsed. Concrete typically contains at least 5 % water in pores and capillaries. When concrete is heated above 100 deg C, the water is converted to steam under very high pressure, sometimes leading to explosive spalling of the outermost layers of the material. However, a hard impact on hot concrete would most certainly result in total fragmentation of the concrete, accompanied by the explosive release of hot, (superheated!), steam.
