WO1993003234A1

WO1993003234A1 - Element and method of preparing this element

Info

Publication number: WO1993003234A1
Application number: PCT/DK1992/000237
Authority: WO
Inventors: Thorkild Bach; Knud Lund Eriksen
Original assignee: Lund Eriksen Consult Aps
Priority date: 1991-08-02
Filing date: 1992-08-03
Publication date: 1993-02-18
Also published as: AU2447792A; DK142491D0

Abstract

A method of preparing a distance heating pipe insulated with foamed cement, said pipe comprising an inner metal pipe and an outer plastic pipe, wherein a foamable cementitious mixture is introduced into the space between the outside of the inner pipe and the inside of the outer pipe, said mixture containing, based on the weight of cement, 70-80 % by weight of water, 0.15-0.5 % by weight of a foaming agent and 4-6 % by weight of setting controlling agent in the form of one or more calcium aluminates, and wherein the mixture is caused to foam and set.

Description

ELEMENT AND METHOD OF PREPARING THIS ELEMENT.

The present invention relates to a method of preparing elements comprising a foamed cementitious insulating layer wherein a mixture of cement, water and a foaming agent is introduced into a space between at least two boundaries and the mixture is caused to foam and to set.

SE,B,436,506 discloses a method of preparing a pre-manufactured building element consisting of two thin metal plates separated by an insulating layer made of foamed concrete. In this prior art method water, cement and one component of a two-component foaming agent are mixed and subsequently the mixture is vigorously stirred. The other component of the two-component foaming agent is then added during stirring and simultaneous introduction of air, and the mixture thus formed is pumped into the space between two metal plates arranged parallel to each other and suitably spaced.

DE,C,875,402 also discloses a method of preparing a building element comprising two facing sheets and an intervening insulating layer of ligth concrete. In this prior art method, the insulating layer is prepared by pouring the concrete into the space between said sheets.

The prior art methods present the advantage that the insulating layer binds strongly to the sheets which define the moulding cavi¬ ties.

However, the prior art methods of the above type suffer from the drawback that the setting of the cementitious mixture occurs after the termination of the foaming process and after the foam has begun collapsing. Consequently, the insulating layer does not optain optimum density and hence not maximum insulating characteristics, and a desired filling of the space between the boundaries is not obtained.

The object of the present invention is to provide a method of the above mentioned type which ensures that the setting of the foamed cementitious mixture takes place when the foaming is at its maximum and that the space is completely filled.

According to the invention this is obtained by using a foamable cementitious mixture which based on the weight of cement contains 70-80% by weight of water, 0.15-0.5% by weight of a foaming agent and 4-6% by weight of a setting controlling agent in the form of one ^' or more calcium aluminates.

By using the above mixture it is possible to accellerate the setting to such an extent that it begins before any significant collapse of the foam formed by means of the foaming agent has occured.

DE,A1,3041901 discloses a method of preparing micro-porous refrac¬ tory building materials. In this prior art method, air is stirred into a mixture of cement, such as calcium aluminate cement, a particul te aggregate and a foaming agent, such as water-soluble casein, and optionally alkali polyphosphates for changing the setting rate.

The cement component of the above mixture preferably comprises Portland Cement and, in case aluminium powder is used as the foaming agent, Portland Cement to which an al aline material has been added, which alkaline material by reacting with the Al-powder generates of hydrogen. Filter dust is preferably used as the alkaline material and in particular filter dust formed by purification of flue gas from cement kilns. The cement mixture may contain various additives, such as additives of the pozzolan type, e.g. fly-ash, slag, micro- silica, gypsum and chalk. Examples of other additives are foaming agents, such as cellulose derivatives, polyethylenes, polypropy- lenes. Examples of suitable cellulose derivatives are carboxymeth- ylcelluloses, hydroxyethylcelluloses and HPMC.

Examples of further additives are surfactants, such as Berol®09 and/or BeroT^®08, thickening agents, colouring agents, e.g. coloured pigments, corrosion inhibitors, dispersants and flocculants.

Examples of suitable foaming agents comprise hydrogen peroxide and metals which are electro-chemically less noble than silver. A particularly suitable foaming agent of the latter type is aluminium, preferably in the form of a powder having a surface area (Blaine) of from 5000 to 9100 m²/g.

When using a pulverized metallic foaming agent, it may be coated with an agent, such as wax, to retard the gas generating reaction.

A particularly suitable setting controlling agent is aluminate cement which e.g. is commercially available under the names Secar 71 and Alcoa. Both of these aluminate cements have a high content of monocalciu aluminate and calcium bialuminate.

The components used for forming the cement mixture may be mixed at the temperature of the surroundings but the mixing may also be effected at an elevated temperature, e.g. by heating one or more of the components, such as water, or by using a heated mixer.

A further possibility comprises the use of a chemical compound which reacts exothermally with one of the other components of the mixture, such as water.

When a particularly fast setting of the cement is desired, a mixture having a temperature of 40-90^βC is preferably used.

When preparing the cement mixture it is preferred to disperse the setting controlling agent, such as aluminate cement, in water and then to mix the dispersion thus obtained with the other components. The dispersion and the other components are preferably mixed immediately before the finished mixture is poured into the space between the boundaries.

When using a powdered foaming agent, it is preferably dispered in a portion of the water before the foaming agent is mixed with the other components.

The method according to the invention is particularly suitable for preparing heat (cold) insulated pipes for transporting liquid media, said pipes comprising an inner pipe and an outer pipe concentrically located around the inner pipe and an insulating layer disposed in the space between the outside of the inner pipe and the inside of the outer pipe.

Well known pipes of this type are district heating pipes, i.e. pipes for transporting hot water. Such a district heating pipe typically consists of an inner metal pipe, such as an iron pipe, and an outer shell pipe, e.g. a plastic pipe, such as a polyethylene pipe.

When using the method according to the invention for preparing such district heating pipes, the concentrically located pipes are dis- posed in an inclined or vertical position and the finished foamable cement mixture is introduced into the space between the two pipes in such an amount that the entire space between the pipes is filled after the termination of the foaming process.

The foamable mixture is preferably introduced through an inlet having means for discharging gas, including the air present in the said space prior to the filling with the foamable cement mixture.

In the following examples the method according to the invention is explained in further details in connection with the preparation of district heating pipes. However, it should be emphasized that the method is also suitable for prepararing insulating layers on other objects than pipes, including insulating layers on board elements and for elements where one or both boundaries form part of the finished product.

As indicated above, the method according to the invention is par¬ ticularly suitable for forming an insulating layer on hot objects. Thus, the method according to the invention allows the preparation of insulating layers tolerating temperatures of up to lOOO'C.

However, the method can also be used for insulating objects having temperatures of less than 0°C, e.g. for insulating cooling pipes, cold stores and the like having temperatures as low as -40^*C.

In the examples below the following materials were used:

"Blok Cement" Blok Cement is a filler cement produced by Aalborg Portland and having a relatively high green strength and a relatively low final strength. Blok Cement is prepared by grinding Portland cement clinker with an inactive filler (pulverized chalk) to which a chromium neutralizing substance has been added. Blok Cement differs from the other Danish cements in having a high alkali content, i.e. 1% equivalents of Na-O, which ensures a high green strength. Blok Cement, which is grey, is a certified non-sulfate resistant filler cement.

'Standard Cement"

Standard Cement is an ordinary-setting cement containing fly-ash. Standard Cement is prepared by grinding Portland clinker with fly-ash and small amounts of gypsum, and adding a chromium neutralizing substance thereto. The maximum fly-ash content amounts to 35% and is typically in the range of 20-25%. Standard Cement, which is grey, is a certified fly-ash cement produced by Aalborg Portland, which complies with the DS 427 strength requirements for ordinary-setting cement. It is moderately sulfate resistant and has a moderate alkali content.

"White Portland Cement"

White Portland Cement is a fast-setting cement produced by Aalborg Portland. White Portland Cement is prepared by grinding White Portland clinker together with small amounts of gypsum, and adding a chromium neutralizing substance thereto. No filler is added during the preparation. White Portland Cement, which is white, is a certified Portland cement which complies with the DS 427 strength requirement for fast-setting cement having a 24 hour compressive strength of >16 N /m². It is highly sulfate resistant and has an extra low alkali content.

"Secar 71"

Secar 71 is a hydraulic cement having a high content of aluminium oxide which preferably is bound in the form of monocalcium aluminate and calcium bialuminate. The aluminium oxide content is higher than 69%, the calcium oxide content is less than 30% and the contents of other oxide compounds are all less than 1%. The alkali content measured as oxide is less than 0.5 %. Absolute density: about 3.0 g/cm³. Fineness measured as specific surface (Blaine) is typically in the range of 3900-4500 [cm²/g] and the sieve residue on a 90 μm sieve is less than 5%. Secar 71 is produced by Lafarge Fondu Inter¬ national, 157 Avenue Charles de Gaulle, 92521 Neuilly Cedex, France.

"Alcoa CA-14"

Alcoa CA-14 is an aluminate cement produced by Alcoa (Aluminium Company of America, Pittsburg, PA, U.S.A.) and sold in Northern Europe by Alcoa Chemie Verkoop, Bruistensingel 154, 5232 AC's-Her- togenbosch, the Netherlands. The technical specifications of Alcoa CA-14 correspond exactly to those of Secar 71.

"Al-powder"

Al-powder is produced by Carlfors Bruk, Huskvarna, Sweden. The type used is TA 5000 R having a free aluminium metal content of 92.3% and a specific surface (according to Blaine) of 9100 cm²/g.

"Berol 08"

Berol 08 is a powder of a non-ionic tenside based on normal primary alcohol (fatty alcohol). It has a very strong hydrophilic (water- soluble) character (HLB-value of 18.7) and exhibits a surface tension of 48 mN/m according to Du Noϋy, 25^*C, 0.1% content of active material. Berol 08 is chemically stable in weak acids and bases, but it is not stable for long periods in strong acids and bases. Berol 08 is produced by Berol Nobel AB 444 85 Stenungsund, Sweden and is marketed in Denmark by Berol Nobel A/S, Box 106, DK-3000 Helsingør.

"Berol 09"

Berol 09 is a clear liquid of a non-ionic tenside of the alkyl phenol-ethyleneoxide adduct type. It has a hydrophilic (water- soluble) character (HLB-value of 13.3) and exhibits a surface tension of 33 mN/m according to Du Noiiy, 25^βC, 0.1% content of active material. Berol 09 is chemically stable in weak acids and bases, but it is not stable for long periods in strong acids and bases.

Example

Foamable cement mixtures 1-7 were prepared according to the inven¬ tion by mixing the constituents shown in table 1 in the weight ratios stated.

TABLE 1

Mixture No. 1

2.5 g of Al powder was suspended in approx. 25 g of water at approx. 20^βC. The suspension was kept stirred while 1,350 g of Standard Cement was mixed with 600 g of water at 20^βC in a Hobart mixer at a low mixing rate for 3 minutes. After 3 minutes the Hobart mixer was stopped and the suspension of Al-powder in water was added and then the Hobart mixer was immediately started at a low mixing rate. After 5 seconds at low rate the rate was increased to high rate. 20 Seconds after the start of the Hobart mixer, i.e. calculated from the addition of the suspension of Al-powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

After about 1 minute the mixture began to expand. After further 3 minutes the mixture had not stopped expanding, but the surface of the mixture began to crack and disintegrate almost at the same time as the the surface appearance changed f om glossy to dul1. The hydrogen subsequently generated was lost through the cracks formed, which gradually increased in number and size. The final expansion was very small (approx. 50%). In spite of the clearly dried out structure, the surface shrunk slightly after maximum expansion. The example shows that the mixture does not contain sufficient liquid to ensure the cohesion of the "concrete" formed. Presumably this is due to the fact that the Al-powder consumes water during the generation of hydrogen, that the air bubbles are formed as a water film and that the air bubbles act as a kind of aggregate in the cement paste formed.

Mixture No. 2

2.5 g of Al powder was suspended in 45 g of water at approx. 40"C. The suspension was kept stirred while 1,350 g of Standard Cement was mixed with 900 g of water at 40^βC in a Hobart mixer at a low mixing rate for about 3 minutes. After 3 minutes the Hobart mixer was stopped and the suspension of Al-powder in water was added and then the Hobart mixer was immediately started at a low rate. After 5 seconds at low mixing rate the rate was increased to high rate. 20 Seconds after the start of the Hobart mixer, i.e. calculated from the addition of the suspension of Al-powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

After about 1 minute the mixture began to expand. Approximately after further 8 minutes the surface of the mixture was dull and no further expansion took place. However, a certain amount of hydrogen continued to generate, which mainly escaped through the cracks formed. The final expansion was insufficient (only about 100%). After about 10 minutes the surface began to slightly shrink corres¬ ponding to the escape of hydrogen.

When the form was removed the following day, some material stuck to the bottom of the plastic bucket. This indicates either that hy¬ drogen continues to generate after the establishment of the solid (set/dried-out) structure had begun, the diameter of the sides of the plastic bucket being slightly greater during the setting process than originally (after termination of the generation of hydrogen the sides of the plastic bucket have thus squeezed sufficiently around the material to cause the lower part of the material to stick to the bottom when the form was removed), or that cooling has caused the plastic bucket to squeeze around the foamed material during removal of the form.

The example indicates that the water content has been insufficient.

Mixture No. 3

2.5 g of Al powder was suspended in approx. 150 g of water at approx. 45'C. The suspension was kept stirred while 1,350 g of Standard Cement was mixed with 850 g of water at about 45'C in a Hobart mixer at a low mixing rate for about 3 minutes. After 3 minutes the Hobart mixer was stopped and the suspension of Al-powder in water was added and then the Hobart mixer was immediately started at a low rate. After 5 seconds at low mixing rate the rate was increased to high rate. 20 Seconds after the start of the Hobart mixer, i.e. calculated from the addition of the suspension of Al-powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

After about 1 minute the mixture began to expand. After further 11 minutes maximum expansion was reached and then a certain amount of hydrogen escaped and the surface shrunk somewhat. At that time the material was not fully set. Maximum expansion was about 150%. An insufficient dispersion of the Al powder, resulting in too large bubbles, was considered to be one of the reasons for the escape of hydrogen.

Mixture No. 4

Mixture No. 4 was prepared in the same manner as mixture No. 3 except that the mixing time, after the addition of the suspension of Al powder in water, was increased from 20 seconds to 60 seconds.

The foamed structure had a slightly more pleasant appearance, but the surface shrunk as described for mixture No. 3. On the basis of this example it was found necessary to accellerate the setting.

Mixture No. 5

Mixture No. 5 was prepared in the same manner as mixture No. 4 except that White Portland Cement (fast-setting) was used instead of Standard Cement (ordinary-setting).

The alkalinity of White Portland Cement is so low that the foaming is very slowly progressing. A 50% expansion was not even obtained after about 10 minutes. The settting took place so quickly that a further expansion did not occur.

Mixture No. 6

2.5 g of Al powder was suspended in 145 g of water at about 45^*C. During continuous stirring of the suspension of Al powder in water, 4.0 g of Berol 09, known from the paint industry as an excellent dispersant, was mixed with 1,350 g of Blok Cement and 800 g of water at about 45^*C in a Hobart mixer at a low mixing rate for about 3 minutes. After 3 minutes the Hobart mixer was stopped and the suspension of Al-powder in water was added and then the Hobart mixer was immediately started at a low mixing rate. After 5 seconds at low rate the mixing rate was increased to high rate. 60 Seconds after the start of the Hobart mixer, i.e. calculated from the addition of the suspension of Al-powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

When the mixture was filled into the plastic bucket, the volume had increased compared to the previous examples. After about 30 seconds the mixture began to expand. After approximately further 8 minutes . the foaming reached its maximum. Maximum expansion was about 250%. After approximately further 3 minutes a fairly strong collapse took place so that the final foaming was less than about 200%. The setting had started and the surface did not undergo a significant further shrinkage.

The example shows that Blok Cement provides a rapid foaming and a fairly fast setting.

Mixture No. 7

3.0 g of Al powder was suspended in 145 g of water at about 45°C. During continuous stirring of the suspension of Al powder in water, 7.0 g of Berol 09 was mixed with 1,350 g of Blok Cement and 800 g of water at about 48^*C in a Hobart mixer at a low mixing rate for about 3 minutes. After the 3 minutes the Hobart mixer was stopped and the suspension of Al-powder in water was added and then the Hobart mixer was immediately started at a low mixing rate. After 5 seconds at a low rate the rate was increased to high rate. 60 Seconds after the start of the Hobart mixer, i.e. calculated from the addition of the suspension of Al-powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

When the mixture was filled into the plastic bucket, the volume had increased compared to mixture No. 6. After about 30 seconds the mixture began to expand. After approximately further 8 minutes the foaming reached its maximum. Maximum expansion was about 350%. After approximately further 3 minutes a collapse of about 10% took place so that the final foaming was about 300%. The setting had started and the surface did not undergo a significant further shrinkage.

The collapse differs in character from the cases where Berol 09 was not used, as no significant escape of hydrogen from the surface seems to occur. A fairly uniform collapse takes place.

Blok Cement provides a rapid foaming and a fairly fast setting. Berol 09 enhances the formation of foam, but the foam does not seem to be stable.

Mixtures Nos. 8-14 were prepared according to the method of the invention with the weight amounts stated in table 2.

TABLE 2

Mixture No. 8

2.0 g of Al powder was suspended in 90 g of water at about 45*C. During continuous stirring of the suspension of Al powder in water, 1,350 g of Blok Cement was mixed with 855 g of water at about 50^βC in a 5-liter steel container mounted on a DIAF mixer for about 3 minutes. The admixture of cement was effected at about 1,800 r.p.m. within about 30 seconds. 3 minutes after the start of the mixing, 2.7 g of Berol 08 and the suspension of Al powder in water were added simultaneuously during continuous stirring within about 5 seconds. The stirring rate was slowly increased to about 2,500 r.p.m. 60 Seconds after the start of the addition of Berol 08 powder and the suspension of Al powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 5 liters.

When the mixture was filled into the plastic bucket, the mixture was foamy. After about 30 seconds the mixture began to expand. After approximately further 9 minutes the foaming of the mixture reached its maximum. Maximum expansion was about 350%. However, the foam collapsed again substantially as mixture No. 7. The lack of sta¬ bility might be caused by an insufficient amount of material in the mixer and/or by a too slow setting.

Mixture No. 9

In order to reduce the uncertainty as to the influece of the amount of materials, the amounts of materials used was doubled compared to mixture No. 8.

6.0 g of Al powder was suspended in 200 g of water at about 50^*C. During continuous stirring of the suspended Al powder in water, 2,619 g of Blok Cement was mixed with 1690 g of water at about 50^βC in a 5-liter steel container mounted on a DIAF mixer for 3 minutes. The admixture of cement was effected at about 1,800 r.p.m. within about 30 seconds. 3 Minutes after the start of the mixing, 81 g of Secar, 5.4 g of Berol 08 and the suspension of Al powder in water were added during continuous stirring within about 5 seconds. After the addition, the stirring rate was slowly increased to about 2,500 r.p.m. 60 Seconds after the start of the addition of Secar 71, Berol 08 powder and the suspension of Al powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 10 liters.

The mixture was now sufficiently dispersed.

When the mixture was filled into the plastic bucket, the mixture was foamy. After about 30 seconds the mixture began to expand. After approximately further 9 minutes the foaming of the mixture reached its maximum. Maximum expansion was about 350%. However, the foam collapsed substantially as in mixture No. 7. The mixture was clearly set after about 30 minutes but not after 10 minutes.

Mixture No. 10

4.0 g of Al powder was suspended in 200 g of water at about 55^*C. During continuous stirring of the suspension of Al powder in water, 2,600 g of Blok Cement was mixed with 1690 g of water at about 60^*C in a 5-liter container mounted on a DIAF mixer for about 5 minutes. The admixture of cement was effected at about 1,800 r.p.m within about 30 seconds. 5 Minutes after the start of the mixing, 100 g of Secar 71 was added under continuous stirring within about 5 seconds. 60 Seconds after the start of the addition of Secar 71, 5.4 g of Berol 08 powder and the suspension of Al powder in water were added during continuous stirring within 5 seconds. After this addition, the stirring rate was slowly increased to about 2,500 r.p.m. 60 Seconds after the start of the addition of Berol 08 powder and the suspension of Al powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 10 liters.

When the mixture was filled into the plastic bucket, the mixture was foamy. After about 15 seconds the mixture began to expand. After further 9-10 minutes the foaming of the mixture reached its maximum and about this point of time the material began to set. The material did not collapse.

Thus, the composition of materials and the process parameters were adjusted so that the setting commenced at the desired point of time, i.e. at the point of maximum foaming, and the strength of the setting was sufficient to freeze the material in this condition.

Mixture No. 11

6.0 g of Al .powder was suspended in 200 g of water at about 50'C. During continuous stirring of the suspension of Al powder in water, 2,565 g of Blok Cement was mixed with 1690 g of water at about 54^*C in a 5-liter steel container mounted on a DIAF mixer for about 5 minutes. The admixture of cement was effected at about 1,800 r.p.m. within about 30 seconds. 5 Minutes after the start of the mixing, 135 g of Secar 71 was added during continuous stirring within about 5 seconds. 60 Seconds after the start of the addition of Secar 71, 1.4 g of Berol 08 powder and the suspension of Al powder in water were added within about 5 seconds during continuous stirring. After this addition the stirring rate was slowly increased to about 2,500 r.p.m. 60 Seconds after the start of the addition of Berol 08 powder and the suspension of Al powder in water, the mixer was stopped and the mixture was immediately poured into a plastic bucket having a volume of about 10 liters.

When the mixture was filled into the plastic bucket, the mixture was clearly less foamy than mixture No. 10. After about 30 seconds the mixture began to expand. After further 9-10 minutes the foaming of the mixture reached its maximum. The material collapsed slightly before setting.

The somewhat lower temperature delayed the setting in spite of an increased content of Secar 71.

Mixture No. 12

Mixture No. 12 corresponded exactly to mixture No. 11 except that the Berol 08 powder was completely omitted, that the Secar 71 was replaced by Alcoa CA-14, that the temperature of the water was increased to 60^βC and that the water content was increased by 50 g.

Approximately 15 minutes after the mixture was filled into the plastic bucket, the mixture began to expand. After further 9-10 minutes the foaming of the mixture reached its maximum. At about this point of time the material began to set. The material did not collapse.

Thus, the combination of aluminate cement and a sufficient tempera¬ ture ensured the start of the setting at the desired point of time. Mixture No. 13

Mixture No. 13 correspondend exactly to mixture No. 12 except that the amounts used were increased by 60% and that the pouring was not effected in a plastic bucket but in the space between an inner steel pipe and a surrounding shell pipe of polyethylene.

The foaming reached its maximum about 10 minutes after the termination of the mixing and at about this point of time the material began to set. No visible collapse of the material took place.

A subsequent longitudinal cut of the pipes showed a homogeneous filling in the entire pipe length about 1 m. There was no immediate visible difference between top and bottom with regard to pore sizes.

In a compressive strength test of two samples, a compressive strength of 0.477 MPa and 0.495 MPa, respectively, was observed and a density in dry condition of about 350 kg/m³.

Mixture No. 14

Mixture No. 14 corresponded exactly to mixture No. 13 except that the amounts used were increased by 64%, however the Al powder by 75%, and the pipe produced was tested for its heat insulating property which proved fully satisfactory.

The pipes produced were subsequently transported on a lorry and loaded and unloaded different places in Denmark. After about 10 loadings/unloadings the insulation was still intact apart from the outer end portions. No damage of the spacers and the insulation in the pipes were observed.

Claims

P a t e n t c l a i m s

1. A method of preparing elements comprising a foamed cementitious insulating layer, wherein a mixture of cement, water and a foaming agent is introduced into a space between at least two boundaries and the mixture is caused to foam and set, c h a r a c t e r i z e d in using a foamable mixture which based on the weight of cement contains 70-80% by weight of water, 0.15-0.5% by weight of a foaming agent and 4-6% by weight of a setting controlling agent in the form of one or more calcium aluminates.

2. A method according to claim 1, c h a r a c t e r i z e d in that the cement component comprises Portland Cement.

3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that in addition to Portland Cement the mixture contains an alkaline material.

4. A method according to claim 3, c h a r a c t e r i z e d in using filter dust as the alkaline material.

5. A method according to any of the preceding claims, c h a r ¬ a c t e r i z e d in using a metal which is electro-chemically less noble than silver as the foaming agent.

6. A method according to claim 5, c h a r a c t e r i z e d in using aluminium as the foaming agent.

7. A method according to claim 6, c h a r a c t e r i z e d in using aluminium powder having a surface area (Blaine) of from 5,000 to 9,100 m²/g as the foaming agent.

8. A method according to claim 5, c h a r a c t e r i z e d in that the metal is coated with an agent delaying the gas generating reaction.

9. A method according to any of the preceding claims, c h a r ¬ a c t e r i z e d in using aluminate cement as the setting control¬ ling agent.

10. A method according to any of the preceding claims and wherein a metal powder is used as the foaming agent, c h a r a c t e r i z e d in that said metal powder is dispersed in water before being mixed with the other components.

11. A method according to any of the claims 1-9, c h a r a c ¬ t e r i z e d in dispersing the setting controlling agent in water and subsequently mixing the dispersion formed with the other com¬ ponents.

12. A method according to any of the preceding claims, c h a r ¬ a c t e r i z e d in that the cementitious foamable mixture is poured into the space between an inner pipe and an outer pipe which is concentrically arranged around the inner pipe.

13. A method according to claim 12, c h a r a c t e r i z e d in that the inner pipe is a metal pipe and the outer pipe is a plastic pipe.

14. A method according to claim 13, c h a r a c t e r i z e d in that the inner pipe is an iron pipe and the outer pipe is a poly¬ ethylene pipe.

15. An element prepared by the method according to any of the preceding claims.