Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 CARBON LOADED CONCRETE PRODUCTS
2
3 This invention relates to carbon loaded concrete and
4 cementicious products having reduced thermal
conductance.
6
7 Heat transfer through a composite material occurs via a
8 combination of convection, conduction and radiation.
9 In practice, composite thermal conductivity depends, in
part, on the volume of the solids) versus pore volume,
11 and the conductivity of the bulk solid.
12
13 In general terms, for a porous material, the greater
14 the porosity (lower density), the more significant is
convection through pores and radiation from cell walls.
16 The relative importance of convection depends on the
17 degree and type of porosity, for example, the pro-
18 portion of open to closed porosity, pore diameter and
19 shape. Below a certain pore size, in-pore gases are
effectively static and convection is drastically
21 reduced.
22
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1 Conversely, heat transfer by convection increases with
2 moisture content of the concrete.
3
4 An additional effect of pore size is that when there
are many very small pores, as against a few larger
6 ones, there are a greater number of narrow, solid,
7 heat-bridges, thus constricting thermal conduction
8 through the solid.
9
Further, the greater number of solid barriers through a
11 given volume in a system of small pores, results in a
12 higher impedance to thermal transfer by radiation.
13 This is due to the fact that heat energy must be
14 absorbed and re-radiated many times for heat transfer
to occur.
16
17 According to one aspect of the present invention, there
18 is provided a concrete or cementicious product having
19 one or more forms of carbon dispersed therethrough so
as to reduce thermal conductance across the product.
21
22 In another view of the present invention, there is
23 provided a concrete or cementicious product having one
24 or more forms of carbon dispersed therethrough in small
clusters and/or agglomerates that are wholly or
26 substantially isolated from each other.
27
28 Particulate loadings, especially carbons, may be used
29 to reduce heat transfer by any or a combination of the
following, depending on the other components in the
31 matrix and the processing methods:
32
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1 Increase impedance to heat transfer by radiation
2 because certain carbons are good infra-red
3 absorbers..
4
Provide particles with a chosen porosity to
6 influence convection.
7
8 Depending on other components and processing
9 methods,they may influence the size and form of a
proportion of the porosity, other than their own
11 porosity, as has been observed for carbon and/or
12 silica composite systems other than concrete.
13
14 Carbons suitable for use in the present invention will
typically have a BET surface area of < 550 m2/g.
16
17 One typical form of carbon for use with the present
18 invention is carbon black.
19
Carbon blacks are composed of spheroidal primary
21 particles which partially coalesce during manufacture
22 to form interlinked clusters and chains of carbon
23 spheres. The structure of a carbon black is defined in
24 terms of the growth of the clusters and chains. The
carbon black industry defines a "low structure" black
26 as consisting of small clusters of spheroids, whereas a
27 "high structure" black contains extensive chains and
28 clusters, which tend to interlock further to form large
29 agglomerates.
31 The forms) of carbon black suitable for use with the
32 present invention preferably have a medium to low
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4
1 "structure" and a high intrinsic electrical
2 resistivity.
3
4 Also preferred in some cases are forms of carbon with a
low pH in dry dispersion in cement, and/or a small
6 particle size.
7
8 The "structure" of the carbon black can be defined by
9 its DBP Index. This is the amount of di-butyl
phthalate which a carbon can take up to form a paste of
11 a prescribed consistency. A low DBP index indicates a
12 "low structure". DBP Index values for carbons for use
13 with the present invention range typically from 35 to
14 170 ml/100g and more preferably have a DBP index in the
range of 40 - 105 mls/100g.
16
17 An aim of the present invention is to disperse a carbon
18 through the concrete or a cementicious material so that
19 clusters, chains and small agglomerates are largely
isolated and do not form linked pathways through the
21 block. In this way, use is made of the carbon's
22 ability to absorb radiant heat, without creating
23 additional routes for convection and/or conduction.
24
The concrete or cementicious products of the present
26 invention can be of any form, size, shape and design.
27 One typical example is concrete blocks, from which
28 structures can be formed and/or built. Furthermore
29 blocks of the Autoclaved Aerated Concrete (AAC) type
are suitable for the application of this invention.
31
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1 According to another aspect of the present invention,
2 there is provided a method of forming a concrete or
3 cementicious product having one or more forms of carbon
4 dispersed therethrough so as to reduce thermal
5 conductance across the product, wherein cement or other
6 cementicious material, water and the or each form of
7 carbon are admixed, cast and cured.
8
9 The carbon is preferably added as a percentage of the
cementicious material in the range 0.2 to 3.0 wt%,
11 preferably 0.5 to 2.0 wt%.
12
13 Cementicious material can be: Portland Cement; Calcium
14 Aluminate Cement; Pozzolanic materials such as
Pulverised Fuel Ash (PFA), volcanic ash etc; finely
16 ground silica; Latent Hydraulic materials such as
17 Ground Granulated Blastfurnace (GGBS) and other slags
18 etc; Microsilica; Metakaolin; or mixtures thereof.
19 This list is not exhaustive.
21 An embodiment of the present invention will now be
22 described by way of example only and with reference to
23 the accompanying Figures as referred to in the text:
24
Suitable forms of PFA comply with BS3892: Part l: 1993
26 or BS EN 450 . 1995. A suitable source of PFA is from
27 Drax power station (UK). Other forms and sources of
28 PFA may also be used.
29
A suitable Plasticiser for use in this invention is
31 Sikament 10. Other types of plasticiser may also be
32 used.
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1
2 Suitable types of Coated Aluminium Powder are Higas 100
3 and Higas 220. Other types of aluminium
powder may
4 also be used.
6 (I) Formation of Blocks
7
8 The trials were based on the following
dry weight
9 standard formulation:
11 PFA 71.82%
12 Plasticiser 0.54%
13 Ordinary Portland Cement 17.440
14 Calcium Sulfate Anhydrite 1.54%
Hydrated Lime 8.21%
16 Coated Aluminium Powder 0.45%
17
18 Water at ambient temperature was used to make the
19 wet mix at between 40-50% of the dry weight of the
ingredients.
21 The following carbon blacks were used:
22
23 BET Surface Area DBP Index
24 (m2/g) (g/100m1)
26 Carbon 1 40 48
27 Carbon 2 60 64
28 Carbon 3 82 102
29 Carbon 4 525 98
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1 Carbon was added as a percentage of cementicious
2 material (PFA + Ordinary Portland Cement) in the
3 range 0.5 t.o 2.0 wt. o.
4
Components were mixed as follows:
6
7 a. Carbon and approximately l00 of the PFA were
8 dispersed in approximately 150 of the mixing
9 water containing approximately half the
plasticiser in a high shear mixer.
11
12 b. Cement, Calcium Sulfate Anhydrite, the rest
13 of the PFA, Plasticiser and mixing water were
14 vigorously agitated to form a slurry with a).
16 (For mixes without carbon addition step a)
17 was omitted)
18
19 c. Lime and the Aluminium Powder were combined
and were then added to the slurry with
21 further vigorous agitation to obtain an
22 homogenous mixture.
23
24 The mixing regime should be chosen such that
substantially discrete particles of carbon are
26 evenly dispersed throughout the mix. Overmixing
27 of some forms of carbon may lead to agglomeration
28 of the carbon particles and result in poor
29 performace of the blocks.
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1 Moulds were coated with release agent. The slurry
2 was immediately poured into the mould. The mix
3 rises typically between 80 to 100%.
4
(II) Autoclaving
6
7 Blocks were put in the autoclave up to 12 hours
8 after casting.
9
System was ramped to temperature over 4 hours
11 maintained at 180°C for 8 hours, and cooled down
12 over a period of 4-8 hours.
13
14 The following are examples of autoclaved blocks:
16 Carbon Type Addition
17
18 1 1.0%
19 2 0.50
3 1.0%
21 Standard Block 0%
22
23 (III)Non-autoclaved blocks
24
In addition the following block was also produced
26 for comparative purposes without autoclaving:
27
28 Carbon Type Addition
29
4 0.5%
31
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1 (IV) Drying of Samples
2
3 According t_o BS 874 Part 2 . Section 2.2 1988,
4 samples must not lose more that 4% weight during
thermal measurements for the k-valve to be valid.
6 All blocks were oven dried at 100 to 150°C and
7 weighed before and after measurement.
8
9 (V) Thermal Measurement
11 A "plain unguarded hot plate" apparatus was set up
12 according to BS 874 Part 2 . Section 2.2 1988.
13
14 (VI)Results
k-Values
Density/kgm-3 k/Wm-1K-1
Standard - no carbon 505 0.150
Carbon No4, 0.50 579 0.115
Carbon Nol, 1.0o 580 0.136
Carbon No3, 1.0% 483 0.120
Carbon No2, 0.50 698 0.142
Commercially 490 0.140
available aerated
block
16
17
18 As the density of most materials, including
19 aerated concrete, increases so does the k-value.
It can be seen from the above results that where
21 the density has increased compared to the standard
22 block there has been a reduction in the k-value
23 and where the density has reduced the decrease in
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1 k-value is greater than that expected from the
2 density reduction alone.
3
4 (VII)Pore Structure
5
6 Fracture surfaces of autoclaved samples were gold
7 coated and examined in a scanning electron
8 microscope.
9
10 The aerated commercial sample consisted of roughly
11 spherical, blow pores of 0.1 to lmm diameter
12 (Figure 1). Pores are not completely closed.
13 Pore walls are relatively smooth (Figure 2) with
14 further irregular, open porosity (up to 0.05~.m)
between acicular crystals. The matrix between the
16 blown pores consists predominantly of loosely
17 bonded PFA spheres, in the size range 1 to 10 m
18 (Figure 3). with considerable open porosity
19 between.
21 At low magnification (Figure 4), the standard
22 formulation appears slightly irregular compared
23 with aerated commercial sample. The size range of
24 blown pores is again 0.1 to lmm dia., and pore
wall thickness is similar. The pore wall
26 structure (Figure 5) is loosely bonded PFA, with a
27 size range similar to the aerated commercial
28 sample. There is considerable "debris" around the
29 PFA particles. Very few acicular crystals were
seen.
31
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1 In a carbon black (Nol) loaded sample at 0.5o
2 carbon addition, blown pores were less regular in
3 shape (Figure 6) (size range 0.1 to 2mm dia.).
4 Again, pores were not completely closed. The
internal pore surface was much rougher (Figure 7).
6 The matrix was less regular and composed of
7 particles in the range 0.5 to 10~,m. The majority
8 of particles were 0.5 to 1.O~,m, hence the porosity
9 in the matrix of the carbon black loaded sample
contains relatively few larger pores.
11
12 Conclusions
13
14 1. Carbon loaded aerated concretes have been formed
with k-values lower than the standard (no carbon)
16 aerated concrete even where the carbon loaded
17 concretes were of increased density.
18
19 2. Carbon influences pore structure as follows .
blown pores become less regular and pore surfaces
21 appear rougher than either the standard formula or
22 the aerated sample.
23
24 3. Carbons which give the best results are for
example high resistivity carbon blacks, with
26 medium to low structures,(DBP 40 to 105m1s/100g).
27
28 Brief Description of the Figures
29
Figure 1 shows the pore structure of commercial aerated
31 block magnified x 20.
32
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1 Figure 2 shows the pore structure of commercial aerated
2 block magnified x 3000.
3
4 Figure 3 shows the pore structure of commercial aerated
block magnified x 3000.
6
7 Figure 4 shows the pore structure in standard
8 formulation x 20.
9
Figure 5 shows the pore structure in standard
11 formulation x 3000.
12
13 Figure 6 shows the pore structure in a carbon black
14 (Nol) loaded sample at addition
0.5% carbon x
20.
16 Figure 7 shows the pore structure in a carbon black
17 (Nol) loaded sample at addition
0.5o carbon x
1500.
