WO2000069789A1

WO2000069789A1 - Carbon loaded concrete products

Info

Publication number: WO2000069789A1
Application number: PCT/GB2000/001845
Authority: WO
Inventors: Stephen John Bloomer; John Griffiths; Jaqueline Lander
Original assignee: Mantle & Llay Limited
Priority date: 1999-05-14
Filing date: 2000-05-15
Publication date: 2000-11-23
Also published as: NO20015553D0; CA2373436A1; EP1187795A1; AU5082600A; NO20015553L; EE200100602A; NZ515494A; GB9911165D0

Abstract

The invention provides a concrete or cementicious product having one or more forms of carbon dispersed therethrough so as to reduce thermal conductance across the product. The one or more forms of carbon are preferably dispersed therethrough in small clusters and/or agglomerates that are wholly or substantially isolated from each other. The carbon(s) have a BET surface area of less than 550 m2/g and include(s) carbon black. The invention also provides a method of forming such a concrete or cementicious product.

Description

CARBON LOADED CONCRETE PRODUCTS

This invention relates to carbon loaded concrete and cementicious products having reduced thermal conductance .

Heat transfer through a composite material occurs via a combination of convection, conduction and radiation. In practice, composite thermal conductivity depends, in part, on the volume of the solid (s) versus pore volume, and the conductivity of the bulk solid.

In general terms, for a porous material, the greater the porosity (lower density) , the more significant is convection through pores and radiation from cell walls. The relative importance of convection depends on the degree and type of porosity, for example, the pro- portion of open to closed porosity, pore diameter and shape. Below a certain pore size, in-pore gases are effectively static and convection is drastically reduced. Conversely, heat transfer by convection increases with moisture content of the concrete.

An additional effect of pore size is that when there are many very small pores, as against a few larger ones, there are a greater number of narrow, solid, heat-bridges, thus constricting thermal conduction through the solid.

Further, the greater number of solid barriers through a given volume in a system of small pores, results in a higher impedance to thermal transfer by radiation. This is due to the fact that heat energy must be absorbed and re-radiated many times for heat transfer to occur.

According to one aspect of the present invention, there is provided a concrete or cementicious product having one or more forms of carbon dispersed therethrough so as to reduce thermal conductance across the product.

In another view of the present invention, there is provided a concrete or cementicious product having one or more forms of carbon dispersed therethrough in small clusters and/or agglomerates that are wholly or substantially isolated from each other.

Particulate loadings, especially carbons, may be used to reduce heat transfer by any or a combination of the following, depending on the other components in the matrix and the processing methods: Increase impedance to heat transfer by radiation because certain carbons are good infra-red absorbers .

Provide particles with a chosen porosity to influence convection.

Depending on other components and processing methods, they may influence the size and form of a proportion of the porosity, other than their own porosity, as has been observed for carbon and/or silica composite systems other than concrete.

Carbons suitable for use in the present invention will typically have a BET surface area of < 550 m²/g.

One typical form of carbon for use with the present invention is carbon black.

Carbon blacks are composed of spheroidal primary particles which partially coalesce during manufacture to form interlinked clusters and chains of carbon spheres. The structure of a carbon black is defined in terms of the growth of the clusters and chains. The carbon black industry defines a "low structure" black as consisting of small clusters of spheroids, whereas a "high structure" black contains extensive chains and clusters, which tend to interlock further to form large agglomerates.

The form(s) of carbon black suitable for use with the present invention preferably have a medium to low "structure" and a high intrinsic electrical resistivity.

Also preferred in some cases are forms of carbon with a low pH in dry dispersion in cement, and/or a small particle size.

The "structure" of the carbon black can be defined by its DBP Index. This is the amount of di -butyl phthalate which a carbon can take up to form a paste of a prescribed consistency. A low DBP index indicates a "low structure". DBP Index values for carbons for use with the present invention range typically from 35 to 170 ml/lOOg and more preferably have a DBP index in the range of 40 - 105 mls/lOOg.

An aim of the present invention is to disperse a carbon through the concrete or a cementicious material so that clusters, chains and small agglomerates are largely isolated and do not form linked pathways through the block. In this way, use is made of the carbon's ability to absorb radiant heat, without creating additional routes for convection and/or conduction.

The concrete or cementicious products of the present invention can be of any form, size, shape and design. One typical example is concrete blocks, from which structures can be formed and/or built. Furthermore blocks of the Autoclaved Aerated Concrete (AAC) type are suitable for the application of this invention. According to another aspect of the present invention, there is provided a method of forming a concrete or cementicious product having one or more forms of carbon dispersed therethrough so as to reduce thermal conductance across the product, wherein cement or other cementicious material, water and the or each form of carbon are admixed, cast and cured.

The carbon is preferably added as a percentage of the cementicious material in the range 0.2 to 3.0 wt%, preferably 0.5 to 2.0 wt%.

Cementicious material can be: Portland Cement; Calcium Alu inate Cement; Pozzolanic materials such as Pulverised Fuel Ash (PFA) , volcanic ash etc; finely ground silica; Latent Hydraulic materials such as Ground Granulated Blastfurnace (GGBS) and other slags etc; Microsilica; Metakaolin; or mixtures thereof. This list is not exhaustive.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying Figures as referred to in the text:

Suitable forms of PFA comply with BS3892: Part 1: 1993 or BS EN 450 : 1995. A suitable source of PFA is from Drax power station (UK) . Other forms and sources of PFA may also be used.

A suitable Plasticiser for use in this invention is Sikament 10. Other types of plasticiser may also be used. Suitable types of Coated Aluminium Powder are Higas 100 and Higas 220. Other types of aluminium powder may also be used.

(I) Formation of Blocks

The trials were based on the following dry weight standard formulation:

PFA 71.82% Plasticiser 0.54% Ordinary Portland Cement 17.44% Calcium Sulfate Anhydrite 1.54% Hydrated Lime 8.21% Coated Aluminium Powder 0.45%

Water at ambient temperature was used to make the wet mix at between 40-50% of the dry weight of the ingredients. The following carbon blacks were used:

BET Surface Area DBP Index (m²/g) (g/lOOml)

Carbon 1 40 48 Carbon 2 60 64 Carbon 3 82 102 Carbon 4 525 98 Carbon was added as a percentage of cementicious material (PFA + Ordinary Portland Cement) in the range 0.5 to 2.0 wt . % .

Components were mixed as follows:

a. Carbon and approximately 10% of the PFA were dispersed in approximately 15% of the mixing water containing approximately half the plasticiser in a high shear mixer.

b. Cement, Calcium Sulfate Anhydrite, the rest of the PFA, Plasticiser and mixing water were vigorously agitated to form a slurry with a) .

(For mixes without carbon addition step a) was omitted)

c. Lime and the Aluminium Powder were combined and were then added to the slurry with further vigorous agitation to obtain an homogenous mixture.

The mixing regime should be chosen such that substantially discrete particles of carbon are evenly dispersed throughout the mix. Overmixing of some forms of carbon may lead to agglomeration of the carbon particles and result in poor performace of the blocks. Moulds were coated with release agent . The slurry was immediately poured into the mould. The mix rises typically between 80 to 100%.

[II) Autoclaving

Blocks were put in the autoclave up to 12 hours after casting.

System was ramped to temperature over 4 hours maintained at 180°C for 8 hours, and cooled down over a period of 4-8 hours.

The following are examples of autoclaved blocks:

Carbon Type Addition

1 1.0% 2 0.5% 3 1.0% Standard Block 0%

;iII)Non-autoclaved blocks

In addition the following block was also produced for comparative purposes without autoclaving:

Carbon Type Addition

0.5* (IV) Drying of Samples

According to BS 874 Part 2 : Section 2.2 1988, samples must not lose more that 4% weight during thermal measurements for the k-valve to be valid. All blocks were oven dried at 100 to 150°C and weighed before and after measurement .

(V) Thermal Measurement

A "plain unguarded hot plate" apparatus was set up according to BS 874 Part 2 : Section 2.2 1988.

(VI) Results k-Values

As the density of most materials, including aerated concrete, increases so does the k-value . It can be seen from the above results that where the density has increased compared to the standard block there has been a reduction in the k-value and where the density has reduced the decrease in k-value is greater than that expected from the density reduction alone.

(VII) Pore Structure

Fracture surfaces of autoclaved samples were gold coated and examined in a scanning electron microscope.

The aerated commercial sample consisted of roughly spherical, blow pores of 0.1 to 1mm diameter (Figure 1) . Pores are not completely closed. Pore walls are relatively smooth (Figure 2) with further irregular, open porosity (up to 0.05μm) between acicular crystals. The matrix between the blown pores consists predominantly of loosely bonded PFA spheres, in the size range 1 to 10 m (Figure 3) . with considerable open porosity between.

At low magnification (Figure 4), the standard formulation appears slightly irregular compared with aerated commercial sample. The size range of blown pores is again 0.1 to 1mm dia., and pore wall thickness is similar. The pore wall structure (Figure 5) is loosely bonded PFA, with a size range similar to the aerated commercial sample. There is considerable "debris" around the PFA particles. Very few acicular crystals were seen. In a carbon black (Nol) loaded sample at 0.5% carbon addition, blown pores were less regular in shape (Figure 6) (size range 0.1 to 2mm dia.) . Again, pores were not completely closed. The internal pore surface was much rougher (Figure 7) . The matrix was less regular and composed of particles in the range 0.5 to lOμm. The majority of particles were 0.5 to 1. Oμm, hence the porosity in the matrix of the carbon black loaded sample contains relatively few larger pores.

Conclusions

1. Carbon loaded aerated concretes have been formed with k-values lower than the standard (no carbon) aerated concrete even where the carbon loaded concretes were of increased density.

2. Carbon influences pore structure as follows : blown pores become less regular and pore surfaces appear rougher than either the standard formula or the aerated sample.

3. Carbons which give the best results are for example high resistivity carbon blacks, with medium to low structures, (DBP 40 to 105mls/100g) .

Brief Description of the Figures

Figure 1 shows the pore structure of commercial aerated block magnified x 20. Figure 2 shows the pore structure of commercial aerated block magnified x 3000.

Figure 3 shows the pore structure of commercial aerated block magnified x 3000.

Figure 4 shows the pore structure in standard formulation x 20.

Figure 5 shows the pore structure in standard formulation x 3000.

Figure 6 shows the pore structure in a carbon black (Nol) loaded sample at 0.5% carbon addition x 20.

Figure 7 shows the pore structure in a carbon black (Nol) loaded sample at 0.5% carbon addition x 1500.

Claims

1. A concrete or cementicious product having one or more forms of carbon dispersed therethrough so as to reduce thermal conductance across the product .

2. A concrete or cementicious product as claimed in claim 1 having one or more forms of carbon dispersed therethrough in small clusters and/or agglomerates that are wholly or substantially isolated from each other.

3. A concrete or cementicious product as claimed in claim 1 or 2 wherein the carbon (s) have a BET surface area of less than 550 m²/g.

4. A concrete or cementicious product as claimed in any of the preceding claims wherein the one or more form of carbon is or includes carbon black.

5. A concrete or cementicious product as claimed in claim 4 wherein the carbon black has a medium to low structure and a high intrinsic electrical resistivity.

6. A concrete or cementicious product as claimed in any of the preceding claims wherein the carbon has a low pH in dry dispersion in cement, and/or a small particle size.

7. A concrete or cementicious product as claimed in any of the preceding claims wherein the one or more form of carbon has a DBP Index value of from 35 to 170 ml/lOOg.

8. A method of forming a concrete or cementicious product as claimed in any of the preceding claims wherein cement or other cementicious material, water and the or each from of carbon are admixed, cast and cured.

9. A method as claimed in claim 8 where the carbon is added as a percentage of the cementicious material in the range 0.2 to 3.0 wt%.

10. Use of carbon black in the production of a product as claimed in any of claims 1 to 7.

11. Use of carbon black in a method as claimed in claim 8 or 9.