CALCIUM NITRATE FOR REDUCING THE PORE SIZE DISTRIBUTION OF A HARDENED CEMENTITIOUS COMPOSITION AND STEEL REINFORCED CONCRETE HAVING AN ELEVATED RESISTANCE TOWARDS CARBONATION

Abstract

The invention relates to the use of calcium nitrate to reduce the pore size distribution of a hardened cementitious composition, preferably, a hardened concrete composition, wherein the cementitious (concrete) composition comprises between 1 weight % to 4 weight % of calcium nitrate of the cement content of the cementitious composition, depending on the type of cement. This results in a reduced permeability for the set cementitious (concrete) composition for carbon dioxide (CO.sub.2) and thus an elevated resistance towards carbonation. The invention furthermore relates to a method for producing such a hardened cementitious (concrete) composition and a pourable and curable (wet) concrete composition. The invention also relates to a steel reinforced concrete solid having an elevated resistance towards carbonation and a method for producing a steel reinforced concrete solid having an elevated resistance towards carbonation.

Claims

1-12. (canceled)

13. Method for producing a cementitious composition having a modified pore size distribution after being set, CHARACTERISED IN THAT the method comprises the step of including between 1 weight % to 4 weight % of calcium nitrate of the cement content of the cementitious composition, depending on the type of cement, into the cementitious composition, wherein calcium nitrate increases the density of voids with a void diameter below 300 ?m by at least 10%.

14. Method according to claim 13, wherein the cementitious composition is a concrete composition.

15. Method according to claim 14, wherein the method comprises the step of including between 300 and 500 kg Portland cement per m.sup.3 hardened concrete into the concrete composition.

16. Method according to claim 14, wherein the method comprises the step of replacing an amount of cement by a cement replacement material at a concentration of between 0.1 weight % to 50 weight % of the cement content of the cementitious composition.

17. Method according to claim 16, wherein the cement replacement material is chosen out of any one of fly ash, ground granulated slag, lime stone or a combination thereof.

18. Method according to claim 13, wherein the method comprises the step of including between 150 and 300 kg water per m.sup.3 hardened concrete into the concrete composition.

19. Method according to claim 13, wherein the method comprises the step of including between 1.500 and 1.800 kg aggregate per m.sup.3 hardened concrete into the concrete composition.

20. Method according to claim 19, wherein the aggregate comprises sand, gravel and stones.

21. Method according to claim 13, wherein calcium nitrate increases the density of voids with a void diameter below 300 ?m by at least 20%; more preferably at least 30%; most preferably at least 40%.

22. Method according to claim 13, wherein calcium nitrate increases the density of voids with a void diameter ranging from at least 30 ?m to at most 300 ?m by at least 5% to at most 50%; preferably at least 10% to at most 45%; more preferably at least 20% to at most 45%; most preferably at least 30% to at most 45%.

23. Method according to claim 13, wherein calcium nitrate increases the density of voids with a void diameter ranging from at least 50 ?m to at most 150 ?m by at least 5% to at most 50%; preferably at least 10% to at most 45%; more preferably at least 20% to at most 45%; most preferably at least 30% to at most 45%.

24. Method according to claim 13, wherein calcium nitrate increases the density of voids with a void diameter ranging from at least 60 ?m to at most 100 ?m by at least 5% to at most 50%; preferably at least 10% to at most 45%; more preferably at least 20% to at most 45%; most preferably at least 30% to at most 45%.

25. Pourable and curable concrete composition, comprising per m.sup.3 cured concrete between 300 and 500 kg cement; between 150 and 300 kg water; between 1.500 and 1.800 kg aggregate; and between 1 weight % to 4 weight % of the cement content of the concrete composition, depending on the type of cement, of calcium nitrate.

26. Concrete composition according to claim 25, wherein an amount of cement is replaced by a cement replacement material at a concentration of between 0.1 weight % to 50 weight % of the cement content of the concrete composition.

27. Concrete composition according to claim 26, wherein the cement replacement material is chosen out of any one of fly ash, ground granulated slag, lime stone or a combination thereof.

28. Concrete composition according to claim 25, wherein the aggregate comprises sand, gravel and stones.

29. Concrete composition according to claim 27, wherein the cement is Portland cement.

30-35. (canceled)

36. Steel reinforced concrete solid having an elevated resistance towards carbonation obtained from hardening the concrete composition according to claim 25.

37. Method for producing a steel reinforced concrete solid having an elevated resistance towards carbonation, comprising the steps of: (I) preparing a concrete composition according to claim 25 comprising mixing the water, the cement, the aggregate and the calcium nitrate; (II) casting the concrete composition into a form provided with a steel reinforcement; and (III) having the concrete composition hardened into the steel reinforced concrete solid with the elevated resistance towards carbonation.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0067] FIG. 1 shows a diagram wherein the carbonation depth in cm of two different cementitious compositions A and B under accelerated conditions is shown;

[0068] FIG. 2 shows a diagram wherein the carbonation depth in cm of two different cementitious composition A and B under normal atmospheric conditions is shown.

[0069] FIG. 3 shows a graph wherein the cumulative frequency in % is displayed in function of the void size in ?m for three different cementitious compositions.

DETAILED DESCRIPTION OF THE INVENTION

[0070] The present invention relates to the use of calcium nitrate to modify the pore size distribution of a hardened cementitious composition, more preferably a hardened concrete composition. A pore is the void space embedded within the solid matrix of a porous media, here the hardened cementitious composition, more specifically the hardened concrete (also called set or cured concrete). The porosity of a hardened concrete is the total pore volume. When the pore size distribution of the cementitious composition is modified versus known state of the art set cementitious compositions, i.e. more smaller and less bigger voids, the permeability of the hardened cementitious composition for carbon dioxide (CO.sub.2) is reduced.

[0071] Depending on the type of cement, 1 weight % (also called mass percent) to 4 weight % of calcium nitrate by weight of cement (cement content of the cementitious composition) is included in the dry cementitious composition (before water is added). All types of calcium nitrate can be used such as calcium nitrate solution or calcium nitrate containing granules.

[0072] The pourable and curable cementitious composition, also called wet cementitious composition, preferably the wet, pourable and curable concrete composition, preferably comprises per m.sup.3 cured concrete: [0073] 300-500 kg cement, preferably Portland cement, which serves as the binder of the cementitious composition (concrete); [0074] 150-300 kg water; [0075] 1.500-1.800 kg aggregate, preferably sand (fine aggregate), gravel and stones (coarse aggregate); [0076] 1 weight % to 4 weight % of calcium nitrate by weight of cement.

[0077] It is remarked that, after water is added to the dry, cementitious (concrete) composition consisting of the binder, the aggregate and the calcium nitrate as an admixture, and mixing the water with this dry, cementitious (concrete) composition, a pourable and curable, wet cementitious (concrete) composition is obtained that sets after a certain period of time.

[0078] The invention also relates to a method for producing a cementitious composition resulting in a set cementitious composition having a reduced pore size distribution. This method comprises the step of including between 1 weight % to 4 weight % of calcium nitrate by weight of cement, depending on the type of cement used.

[0079] The method preferably furthermore comprises the steps of including in the cementitious (concrete) composition: [0080] 300-500 kg cement, preferably Portland cement, which serves as the binder of the cementitious composition (concrete); [0081] 150-300 kg water; and [0082] 1.500-1.800 kg aggregate, preferably sand (fine aggregate), gravel and stones (coarse aggregate).

[0083] Depending on the application and thus the required properties of the concrete, stones such as amongst others crushed rocks such as limestone or granite can be used.

[0084] It is possible to replace 0.1 weight % to 50 weight % of the cement by a cement replacement material such as fly ash, ground granulated slag, lime stone or any combination thereof.

[0085] The invention furthermore relates to the use of calcium nitrate as an admixture for a steel reinforced concrete composition to increase the resistance towards carbonation of the steel reinforced hardened concrete, wherein the concrete composition comprises cement and the calcium nitrate at a dosage of 1 to 4 weight % of the cement weight.

[0086] It has been observed that, when the pore size distribution of the concrete solid is altered by adding a certain amount of calcium nitrate, an increase of the resistance towards the carbonation frontier of the steel reinforced concrete solid is obtained. In other words, the migration of carbon dioxide and the dissolution in water to form hydrocarbon acid, in the steel reinforced concrete, is increased.

[0087] The invention also relates to a method for producing a steel reinforced concrete solid having an elevated resistance towards carbonation, comprising the steps of: [0088] (I) preparing a concrete composition according to the invention as disclosed above comprising mixing the water, the cement, the aggregate and the calcium nitrate; [0089] (II) casting the concrete composition into a form provided with a steel reinforcement; and [0090] (III) having the concrete composition hardened into the steel reinforced concrete solid with the elevated resistance towards carbonation.

[0091] The invention finally relates to a steel reinforced concrete solid obtained from hardening the concrete composition according to the invention as disclosed above.

EXAMPLES

Example 1: Accelerated Tests

[0092] In a first case study, the carbonation in a number of concrete samples under accelerated conditions was studied. The samples were prepared with a water to cement ratio of 0.5. Two types of cement were used, i.e. CEM I 42.5 R (A) and CEM II/A-V 42.5 R (B). The dosage levels of calcium nitrate were 0 weight % (1), 1 weight % (2) and 2 weight % (3). The samples were cured for a period of 28 days and afterwards exposed to an atmosphere with 2% CO.sub.2 until 56 days. After 56 days, analysis of the carbonation depth was performed. In FIG. 1, a diagram is shown that shows the carbonation depth in average goes down. For CEM I, the carbonation depth is already minimized when using a 1 weight % calcium nitrate dosage, while for CEM II/V-A the carbonation depth is minimized when using 2 weight % calcium nitrate dosage.

Example 2: Non-Accelerated Tests

[0093] In a second case study, the carbonation in a number of concrete samples under normal conditions was studied. The samples were prepared with a water to cement ratio of 0.5. Two types of cement were used, i.e. CEM I 42.5 R (A) and CEM II/A-V 42.5 R (B). The dosage levels of calcium nitrate were 0 weight % (1), 1 weight % (2) and 2 weight % (3). The samples were cured for a period of 28 days and afterwards exposed to ordinary atmosphere until 182 days. After 182 days, analysis of the carbonation depth was performed. In FIG. 2, a diagram is shown that shows that the carbonation depth in average goes down. For CEM I, the carbonation depth is already minimized when using a 2 weight % calcium nitrate dosage, while for CEM II/V-A the carbonation depth is minimized age.

Example 3: Void Size Distribution Tests

[0094] In a third case study, the effect of calcium nitrate on the void size distribution in a number of concrete samples was studied. The samples were prepared with a water to cement ratio of 0.5. The concrete compositions contained Ordinary Portland cement (OPC). Three reinforced concrete samples were prepared in cubes with an edge length of 150 mm.

[0095] The first and the second sample served as comparative reference values; in particular reference A contained a cementitious composition without any additives, while reference B contained a cementitious composition with a porosity increasing additive (e.g. admixture of surfactants) commonly used in the art. The third sample contained the composition of reference B together with 4 weight % of calcium nitrate by weight of cement.

[0096] Measurement of the total air void content indicated that the sample with 4% calcium nitrate lead to the highest porosity value of 6.0%; however, the 0.2% increment over the reference B porosity value of 5.8% was regarded as negligible. It is therefore concluded that the addition of calcium nitrate has almost no discernible effect on the porosity of reinforced concrete.

[0097] In contrast, measurements of the air void size distribution presented in FIG. 3 did display significant differences between the calcium nitrate sample and the two reference samples.

[0098] In general the addition of calcium nitrate caused the total number of voids with a void diameter below 300 ?m to increase, while simultaneously causing the total number of voids with a void diameter above 300 ?m to decrease; thus obtaining an effective void size shift to lower void sizes (i.e. to the left on FIG. 3) after addition of calcium nitrate. The formerly described shift becomes even more pronounced below 150 ?m, and is especially prominent below 100 ?m.

[0099] When focusing on the cut-off value of 100 ?m in the sample containing 4% calcium nitrate, approximately 71% of voids now have a void diameter of 100 ?m or less, while approximately 29% of voids to have a diameter above 100 ?m. In comparison, at the same cut-off value for reference B, only 51% of voids have a void diameter below 100 ?m, and for reference A this value even decreases to 45%. It is therefore concluded that the addition of calcium nitrate causes the void size distribution to shift by approximately 39% towards the sub-100 ?m range; that is, the total of amount of voids with a void size below 100 ?m increases by 39%, while the total of amount of voids with a void size above 100 ?m decreases by the same amount.

[0100] Similar observations are noticeable at different cut-off value of the void diameter, and are presented below in Table 1, which shows an overview of the void size distribution ranges at selected cut-off values of 30, 50, 60, 70, 80, 100, 300, 1000 and 2000 ?m.

[0101] A similar comparison can then be made between samples with the similar porosity values; namely reference B and the sample with 4% wt. calcium nitrate. The results are presented in Table 2.

[0102] In general it is found that the effect of calcium nitrate is already noticeable from 30 ?m onwards, wherein the total number of voids increases by ?40% (i.e. approximation due to the low number of voids with a diameter of 30 ?m or below) up to 300 ?m, wherein the total number of voids increases by 6%.

TABLE-US-00001 TABLE 1 Void size distribution ranges Cut-off value Reference A Reference B Sample 4% CN x (in ?m) (porosity 2.9%) (porosity 5.8%) (porosity 6.0%) 30 1% ? x < 99% 3% ? x < 97% 5% ? x < 95% 50 10% ? x < 90% 23% ? x < 90% 26% ? x < 74% 60 18% ? x < 82% 29% ? x < 71% 40% ? x < 60% 70 27% ? x < 73% 35% ? x < 90% 49% ? x < 51% 80 36% ? x < 64% 40% ? x < 60% 58% ? x < 42% 100 45% ? x < 55% 51% ? x < 49% 71% ? x < 29% 150 50% ? x < 50% 69% ? x < 31% 81% ? x < 19% 300 60% ? x < 40% 85% ? x < 15% 90% ? x < 10% 1000 87% ? x < 13% 96% ? x < 4% 98% ? x < 2% 2000 97% ? x < 3% 100% ? x 100% ? x

TABLE-US-00002 TABLE 2 Comparative increment in void size amount Cut-off value Reference B Sample 4% CN Relative x (in ?m) (porosity 5.8%) (porosity 6.0%) increment 30 3% < x 5% < x ~40% 50 23% < x 26% < x 13% 60 29% < x 40% < x 37% 70 35% ? x 49% ? x 40% 80 40% ? x 58% ? x 45% 100 51% ? x 71% ? x 39% 150 69% ? x 81% ? x 17% 300 85% ? x 90% ? x 6% 1000 96% ? x 98% ? x 2% 2000 100% ? x 100% ? x 0%

[0103] However, the effect becomes clearly defined in the range of 50 to 150 ?m, wherein the calcium nitrate shows an effective increase in void count by at least 13%; especially in the range of 60 to 100 ?m the void count is observed to increase drastically by at least 37%. The highest void increment of 45% is observed for a void diameter of 80 ?m.

[0104] After 300 ?m the three samples are observed to equalize in the total number of voids, thus indicating that the total number of voids with a void diameter above 300 ?m were reduced by the same amounts as previously reported and the total porosity is confirmed to remain unaffected by calcium nitrate.

[0105] In conclusion, the addition of 4% wt. calcium nitrate to the third sample caused the total number of voids with a void diameter below 300 ?m to increase, in particular below 150 ?m, more in particular below 150 ?m; while simultaneously causing the total number of voids with a void diameter above respectively 300, 150 and/or 100 ?m to decrease. As a result, the overall porosity of the hardened cementitious composition remained approximately similar, that is bringing about only discernible changes to the overall porosity, yet the pore size distribution was modified significantly.