The present invention relates to a method of acce¬ lerating the hardening of concrete, based on Portland cement or other binders, by carbonation with a gas con¬ sisting wholly or partly of carbon dioxide, the con- crete being dewatered and/or deaerated prior to car¬ bonation in a closed mould in which a sub-pressure has been established, and carbonation being commenced at the sub-pressure maintained in the concrete.

The invention basically is a development of the methods of rapidly hardening concrete disclosed in Swe¬ dish Patent 7800077-5 and Swedish Patent Applications 7907235-1 and 8002613-3.

The said patent and the first-mentioned patent application indicate a method of casting and hardening concrete without the need of using a special curing chamber or autoclave, the concrete being cast or shaped after mixture of the components included, whereupon the mass is subjected to vacuum treatment by being ex¬ posed to a sub-pressure during casting in a mould. After that, and while at least initially maintaining the sub- pressure, carbon dioxide gas is introduced into the mass and then, because of said sub-pressure, diffuses into the capillaries in the concrete mass and accom¬ plishes rapid hardening. The most important factors in connection with con¬ crete hardening by carbonation are the reactivity of the binders to the carbon dioxide gas, suitable water absorption mean values of the -concrete mass and its suitable pore and capillary structure (including the aggregate) , and the partial pressure of the carbon di¬ oxide gas. Carbonation, i.e. the reaction between the binders which contain lime, magnesium etc., and the carbon dioxide, occurs only the presence of water. If there is an excess of water, which always occurs in

castable mixtures more rich in water, the carbonation reaction may be very weak or entirely suppressed.

The vacuum treatment method according to the above- mentioned patents makes it possible to remove the large amount of water from the mixture and, simultaneously, to provide a sub-pressure in the concrete mass, where¬ by the partial pressure of the carbon dioxide gas is increased. These two factors promote the carbonation process, i.e. the hardening of the concrete. The method according to Swedish Patent Application 8002613-1 as referred to above is a development of the method according to the Swedish Patent 7800077-5 and Swedish Patent Application 7907235-1 for the carbonation of concrete elements and structures in a closed mould, and furthermore solves the problem of carbonation har¬ dening of concrete in chambers or tunnels.

Swedish Patent Application 8002613-1 discloses a method of removing the moisture during carbonation, if a closed mould is used, and prior to and during car- bonation if the mould has been stripped from the con¬ crete. This method utilises controlled conditions by combining air streams, heat generation and moisture ab¬ sorption in a continuously closed circulation system. The prior art methods referred to above function saitsfactorily as long as the carbon dioxide gas is pure. However, the carbonation of concrete with carbon dioxide gas containing greater or less quantities of air, or waste gases from, for example, cement or lime industries, is difficult or even impossible because the gas not reacting with the concrete binders and the vapour generated during the carbonation reaction ex¬ tend the hardening time or completely inhibit carbona¬ tion.

The present invention proposes a method of the type referred to by way of introduction and eliminating the above-mentioned problem in that the gas not react¬ ing with the concrete binders, and/or the water vapour.

is removed from the concrete during carbonation.

If, according to a further feature of this inven¬ tion, the concrete is precarbonated by supplying carbon dioxide gas during the mixing or, optionally, the cast- ing of the concrete, the short-term strength of the concrete (directly after carbonation) is increased with¬ out detracting from the final strength.

By repeating the carbonation after the main car¬ bonation, according to a further feature of the pre- sent invention, there is obtained a further increase of the short-term strength to maintain the final strength (as compared with normal hardening) .

The above-mentioned and further features of the invention will appear from the following description, reference being made to the accompanying drawings in which

Fig. 1 is a general flow diagram of the method according to the invention;

Fig. 2 illustrates schematically the different methods of removing the gas not reacting with the con¬ crete binders, and/or the water vapour from the con¬ crete;

Fig. 3 is a pressure/time diagram showing the va¬ cuum carbonation in a closed mould, according to Figs. 2:1.1 and 2:1.2;

Fig. 4 is a corresponding diagram showing the car¬ bonation without mould cover according to Fig. 2:1.3;

Fig. 5 also is a corresponding diagram showing repeated carbonation after the main carbonation in a closed mould, according to Figs. 2:1.1 and 2:1.2;

Fig. 6 is a pressure/time diagram showing repeated carbonation after drying with high-frequency electro¬ magnetic waves (HF) on simultaneous vacuum action after the main carbonation in a closed mould, according to Fig. 2:1.1 or 2:1.1;

Fig. 7 also is a similar diagram showing continued repeated carbonation after drying with high-frequency

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electromagnetic waves (HF) of a concrete from which the mould has been partly stripped, according to Fig. 1:2.2, or a concrete from which the mould has been stripped completely, according to Fig. 1:1.4, suitable for carbonation on a belt;

Fig. 8 illustrates schematically precarbonation during mixture of the concrete components, while

Fig. 9 illustrates schematically precarbonation during the actual casting; Fig. 10 shows an example of carbonation of sand¬ wich elements in a battery, according to Figs. 1:1.1 and 1:1.2; and

Fig. 11 shows an example of repeated carbonation during flow-line production A -according to Figs. 1:1.1 and 1:1.2, and production B according to Fig. 1:2.4. In the general flow diagram shown in Fig. 1, the method according to the invention is begun by dewater- ing and/or deaerating the concrete in a closed mould in which a sub-pressure or vacuum is maintained. If one then follows the full-line arrows, a main carbo¬ nation takes place in step 1 in which the vacuum is maintained at least at the beginning of the supply of carbon dioxide gas to the closed mould, the gas not reacting with the concrete binders and/or the water vapour being removed from the concrete either by suc¬ tion with the aid of sub-pressure 1.1 or by opening a valve in the mould 1.2. Alternatively, the main car¬ bonation according to the dotted line can be carried out with concrete compacted under high pressure and vacuum, a part of the mould, preferably its cover, hav¬ ing been removed; see 1.3.

After removal of the gas and/or the water vapour, the mould can be stripped according to 1.4. Instead of stripping, the concrete may be subjected, after re- moval of the gas and/or the water vapour, to repeated carbonation in a closed mould and under vacuum prior to the supply of carbon dioxide gas. Naturally, it is

also possible that the repeated carbonation takes place after stripping, as will appear from Fig. 1.

After the main carbonation and prior to the re¬ peated carbonation, the concrete can be dried by ra- diation combined with a sub-pressure. For this radiation, use is made of high-frequency electromagnetic waves (HF) which are the most effective for this purpose. After drying, the concrete is further carbonated in a closed mould, the non-reactive gas and/or the water vapour being further removed by suction according to

2.1 or an open valve according 2.2. Alternatively, the repeated carbonation after drying may be effected with¬ out vacuum of concrete partly removed from the mould (according to 2.3) or concrete completely removed from the mould (according to 2.4; see the double-dot arrows). As has been mentioned above. Fig. 2 illustrates schematically the different methods of removing the gas not reacting with the concrete binders and/or the water vapour from the concrete. The concrete 1 is dis- posed in a mould 2, and the non-reacting gas 3 and/or the water vapour are removed by suction (vacuum) at the side opposite the carbon dioxide gas supply 4, as is shown by the arrows 3 in Fig. 2:1.1. In Fig. 2:1.2, the gas 3 is unobstructedly expelled through an open valve, as is also shown by arrows 3. In Figs. 2:1.1 and 2:1.2, the gas and the vapour are expelled in a closed mould. In Fig. 2:1.3, finally, the cover of the mould 2 has been removed to allow the gas to escape. This variant can be used only with concrete that has been compacted under high pressure and vacuum, the car¬ bon dioxide gas having been supplied under a low excess pressure.

Fig. 3 shows a pressure/time diagram of the main carbonation in a closed mould, the carbon dioxide gas being supplied initially (step 1) under vacuum, whereupon the non-reacting gas and/or the water vapour are removed by suction (step 2) according to 1.1 or through an open

valve according to 1.2, in the same manner as has been described above. The times t. , t_ and t_ on the time axis may amount to, for example, ≤ 10 min. , 4 min. and ≤ 10 min, , respectively, The diagram according to Fig. 4 illustrates carbo¬ nation when the mould cover has been removed, according to Fig. 2:1.3 (compacted concrete only). Here t ₁ may be < 1 min., t_ < 0.5 min. and t- < 10 min.

Fig. 5 shows how repeated carbonation can be car¬ ried out after the main carbonation in a closed mould according to Figs. 2:1.1 and 2:1.2 with, respectively, suction and open valve. Here t, may amount to, for exampl < 5 min., t < 2 min. and t. < 10 min.

The repeated carbonation illustrated in Fig. 6 takes place after the HF drying (follows upon the main carbonation) in a closed mould, also under suction or wit open valve, t, being for example _ 2 min., t- < 2 min. and^t- + t < 10 min.

Fig. 7, finally, illustrates continued repeated carbonation after HF drying of the concrete partly re¬ moved from the mould, according to Fig. 1:1.3, or of the concrete completely removed from the mould, accord¬ ing to Fig. 1:2.4.

The precarbonation of the concrete 1, as shown in Figs. 8 and 9, may take place, according to Fig. 8, during mixing in a paddle mixer 7, carbon dioxide gas 4 being supplied from a container 8 via a hose 9 and a reduction valve 10. In Fig. 9, precarbonation takes place during casting of the concrete 1 in the mould 2, the precarbonated concrete being designated 11. The concrete 1 is kept in a container 12 for premixed dry concrete, and inside the container 12 an agitator 13 and outside the container a vibrator 14 are provided. As in Fig. 8, a container 8 with carbon dioxide gas is connected to the container 12 via a hose 9 and a valve 10 for supplying said carbon dioxide gas.

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Fig. 10 illustrates schematically an example of the carbonation of sandwich elements stacked upon one another in batteries, the carbonation and the removal of the non-reacting gas being conducted in the manner illustrated in Figs. 1:1.1 and 1:1.2.

Finally, Fig. 11 shows an example of how repeated carbonation can be carried out during flow-line pro¬ duction of concrete elements according to Fig. 1. Cast¬ ing following upon a possible precarbonation is effect- ed at 1, At 2, the mould is subjected to a vacuum (op¬ tionally compaction) , and at 3 the main carbonation and subsequent removal of the gas not reacting with the concrete binders and/or the water vapour takes place. Finally, the concrete is removed from the mould at 4. HF drying takes place at 5, preferably in a tunnel, wherupon the repeated carbonation takes place at 6, preferably also in a tunnel.

The following Tables show the results of tests conducted to establish the effect of different carbo- nation conditions on the strength after a short period of time and after 28 days (normal hardening after car¬ bonation) .

Table I shows the compressive strength in MPa at different CO- contents in the gas and different condi- tions for removing the non-reacting gas and the water vapour after a short period of time (directly after carbonation) and after 28 days. For these tests use was made of lightweight aggregate concrete having a

2 density of 1400 kg/m . Gas containing 50% CO- reacted very weakly, and low direct strength was obtained after carbonation. Gas containing 25% CO- did not react at all during carbonation. This applies to hardening in a closed mould without removal of the non-reacting gas. Upon carbonation with an open valve in the upper mould part (step 2) upon continued supply of gas, the direct and final strength was satisfactory, both for gas con¬ taining 50% CO- and for gas containing 25% CO . Si i-

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lar results were obtained upon suction with vacuum. It should be pointed out that a lower carbon dioxide content in the gas gave a longer carbonation time, and the amount of absorbed carbon dioxide upon removal of the non-reacting gas was more difficult to control, especially with vacuum.

TABLE I

Table II below shows the results obtained with carbonation tests of cement-bonded reinforced fiberboard (cement board) . Different carbonation methods according to the invention, designated in accordance with Fig. 1 et seσ. have been used: Carbonation and repeated car¬ bonation. The concrete either had been compacted under vacuum or under vacuum and pressure (about 1 MPa) prior to carbonation. The carbonation was conducted with a gas containing 50% CO . The tensile strength in bending was tested immediately after the carbon dioxide treat¬ ment (8-18 min.) , and after three days (normal air har¬ dening) . It was found that, compared with concrete that

had been vacuum-compacted only and concrete that had been carbonated only, the direct strength and the strength after 3 days after compaction was increased further if repeated carbonation and HF drying had been carried out.

TABLE II

The invention naturally is not restricted to the embodiments shown in the drawings and described above, but may be modified in several ways within the scope of the appended claims.