Method For Producing A Cementitious Composite, And Long-Life Micro/Nanostructured Concrete And Mortars Comprising Said Composite

Abstract

The invention relates to a method for producing a cementitious composite, comprising: 1) a first step of conditioning silica nanoparticles, in which the nanoparticles are heated to a temperature between 85-235 C. for a sufficiently long time interval so as to obtain a maximum humidity content of 0.3% relative to the total weight of the material resulting from the first step; 2) a dry dispersion step, in which the conditioned nanoparticles in step 1) are dispersed over cement and in which inert grinding balls are used; 3) a step of conditioning the cementitious composite obtained in step 2), in which the grinding balls are separated from the cementitious composite produced. The invention also relates to the resulting composite, to cement derivatives comprising said composite, preferably mortars and concrete, to the production method thereof and to the use of these materials in industry.

Claims

1. A method for producing a cementitious composite comprising the steps of: 1) a first step of conditioning silica nanoparticles, wherein they are heated to a temperature between 85-235 C., for a sufficiently long time period to achieve a maximum humidity content of 0.3% with regard to the total weight of the material resulting from this first step, 2) a dry dispersion step, in which the nanoparticles conditioned in step 1) are dispersed over cement particles and wherein inert grinding balls are used, 3) a conditioning step of the cementitious composite obtained in step 2), wherein the grinding balls are separated from the cementitious composite obtained.

2. The method according to claim 1, wherein, in the first step, the silica nanoparticles are heated between 100 and 140 C.

3. The method according to claim 1, wherein, in the first step silica nanoparticles are heated following ramps between 1 C. and 100 C./min.

4. The method according to claim 1, wherein, in the first step, a drying equipment is used, selected from: a drying oven, an equipment for continuous drying, and an equipment for drying in infrared oven.

5. The method according to claim 1, wherein, in the first step, silica nanoparticles are obtained with a residual percentage of water of less than 0.2% by weight with regard to the total weight, on cement particles.

6. The method according to claim 1, wherein, in the second dispersion step, the silica nanoparticles and cement are present in a weight ratio between 85 and 99.5% of cement and 15 to 0.5% of silica nanoparticles.

7. The method according to claim 1, wherein, in the second dispersing step, a mixer selected from a kneader, a mixing concrete and biconic mixer is used.

8. The method according to claim 1, wherein f the grinding balls used during the second dispersion step have a size of between 1 mm and 100 mm.

9. The method according to claim 1, wherein, in the second dispersion step, the grinding balls are chosen from microballs of 2 mm diameter, of YTZ, ZrSiO.sub.4 microballs, and steel microballs, and mixtures of the same.

10. The method according to claim 1, wherein, in the second dispersion step, a stirring time between 0.2 and 4 hours is used.

11. The method according to claim 1, wherein among the silica nanoparticles, at least 50% of the silica particles have a size of less than 100 nm.

12. A cementitious composite obtained by the method defined in claim 1, comprising: cement particles and silica nanoparticles in a total proportion of silica nanoparticles from 0.5% to 15% by weight with regard to cement.

13. The cementitious composite according to claim 12, selected from: a composite with 8% of microsilica and 2% of nanosilica, and a composite with 10% of microsilica.

14. The cementitious composite according to claim 12, wherein the cement particles are Portland's cement particles.

15. The cementitious composite according to claim 12, wherein among the silica nanoparticles at least 50% of the silica particles have a size of less than 100 nm.

16. A cement-based material prepared with the defined cementitious composite of claim 12 as a cement phase, and which at 28 days of curing comprises ettringite and portlandite crystals of submicron dimensions.

17. The cement-based material according to claim 16, wherein the submicron dimensions of the primary ettringite phase comprise sizes of less than 300 nm, in at least one dimension.

18. The cement-based material according to claim 16, which is selected from one of mortar and concrete.

19. The cement-based material according to claim 18, which is mortar having a resistance to compression at 7 days of at least 77 MPa and a resistance to compression at 28 days of at least 90 MPa, an electrical resistivity at 7 days curing of at least 6.1 kQ.Math.cm and at 28 days of at least 32.2 kQ.Math.cm, and chlorides migration coefficient at 28 days of 2.47 10-12 m.sup.2/s.

20. The cement-based material according to claim 18, which is a concrete having a resistance to compression at 7 days of at least 52 MPa and a resistance to compression at 28 days of at least 67 MPa, an electrical resistivity at 7 days of curing of at least 17.17 kQ.Math.cm and at 28 days of at least 81.82 kQ.Math.cm, and chlorides migration coefficient at 28 days of 0.710.sup.12.Math.m.sup.2/s.

21. A process for the preparation of cement-based material as defined in claim 16, the process comprising the steps of: a) obtaining a cementitious composite comprising: cement particles and silica nanoparticles in a total proportion of 0.5% to 15% by weight with regard to the cement, preferably 1% to 12% by weight with regard to the cement, and a percentage of residual humidity lower than 1% by weight with regard total weight overall, preferably less than 0.5% by weight with regard to the total weight, and b) mixing the obtained cementious composite with at least one aggregate, water and additional components required to obtain a cement-based material.

22. The process of claim 21, wherein the cement-based material is concrete and comprises: a) obtaining a cementitious composite comprising: cement particles and silica nanoparticles in a total proportion of 0.5% to 15% by weight with regard to the cement, preferably 1% to 12% by weight with regard to the cement, and a percentage of residual humidity lower than 1% by weight with regard to the total weight, preferably less than 0.5% by weight with regard to the total weight, and b) mixing the cementitious composite obtained with at least one aggregate, water, and required additional components required to obtain concrete, c) performing operations according to the standard procedure to obtain concrete.

23. The process of claim 21, wherein the cement-based material is a mortar, and comprises: a) mixing the cementitious composite obtained with at least one aggregate, water, and required additional components to obtain a mortar b) performing operations according to the standard procedure to obtain a mortar, with the provision of using 90 strokes in the compaction of the samples.

24. The process of claim 21, wherein the cementitious composite is selected from: a composite with 8% of microsilica and 2% of nanosilica, and a composite with 10% of microsilica.

25. The process of claim 21, wherein the cement particles are Portland's cement particles.

26. The process of claim 21 wherein mortar or concrete are obtained.

27. The process of claim 21, wherein among the silica nanoparticles at least 50% of the silica particles have a size of less than 100 nm.

28. (canceled)

29. The cement based material according to claim 17 which is selected from one of mortar and concrete.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0144] FIG. 1 shows Scanning Electron Microscopy, SEM, micrographs of cement 52.5R.

[0145] FIG. 2 shows SEM micrographs of the cementitious composite of the invention with 10% nanosilica.

[0146] FIG. 3 shows SEM micrographs of cement with 10% FE, this is 10% of microsilica of the company Ferroatlntica S.L.

[0147] FIG. 4 shows a SEM micrograph of the M-3.2 mortar sample after 7 days of curing, where it can be observed the interior of a pore covered with nanometric ettringite.

[0148] In FIGS. 5a), 5b) and 5c) MEB micrographs of M-3.2 mortars at 28 days of age of curing, with different scales are presented, where it can be observed the Interior of a pore clearly covered by nanometric ettringite needles which remain stable.

[0149] FIGS. 6a) and 6b) show SEM micrographs for the dosing of concrete sample H-3.1 after 28 days of curing, in which it is observed that the reduction does not occur when the addition is of micrometric size.

[0150] FIG. 7 shows the SEM micrograph of H-3.3 concrete after 28 days of curing, where nanometric ettringite needles can be seen.

[0151] In FIGS. 8 a) and 8b) the ettringite crystals are observed next to the C3A formations, in SEM micrograph of the H-3.2 concrete after 28 days of curing.

[0152] FIG. 9 shows a X-ray diagram of H-1 at 90 days with a percent ettringite of <0.5% with respect to the total mass.

[0153] FIG. 10 shows a X-ray diagram of H-3.1 at 90 days with a percent ettringite of 1.6% relative to the total mass.

[0154] FIG. 11 shows a X-ray diagram of H-3.2 at 90 days with a percentage of ettringite of 2.4% with respect to the total mass.

[0155] FIG. 12 shows a XRD diagram of H-3.3 at 90 days with a ettringite percentage of 1.5% relative to the total mass.

[0156] FIG. 13 shows Raman spectra of the starting materials used C1 and microsilica and of the cementitious composite systems CC3.1 and CC3.0.8.

[0157] FIG. 14 shows Raman spectra of a selected area between 830 and 870 cm.sup.1 for cement C1 and cementitious composites CC3.1 and CC3.0.8. The discontinuous vertical lines have been included as a visual guide to highlight the shift of the Raman bands.

Examples

Example 1. Preparation a Cementitious Composite

[0158] Table 1 shows the physical and chemical characteristics of the cement used, provided by the manufacturer. Table 2 shows the granulometry of said cement.

TABLE-US-00001 TABLE 1 Physical and chemical characteristics of the cement used Standard Chemical characteristics (%) Results EN/UNE Lost by calcination/Lost by fire 1.60 <5 Insoluble Residue 0.3 <5 Sulfates (SO.sub.3) 3.10 <4 Chlorides 0.01 <0.10 Physical and chemical characteristics Normal consistency water % 35.3 Start setting min 90 >45 Final setting min 127 <720 Le Chatelier expansion mm 0.8 <10 Specific surface (Blaine) cm.sup.2/g 7470

TABLE-US-00002 TABLE 2 Granulometry of the cement used Granolometry (% that passes trough) Sieve 1 Micron 14.0 Sieve 8 Micron 61.0 Sieve 16 Micron 88.0 Sieve 32 Micron 99.8 Sieve 64 Micron 100 Sieve 96 Micron 100 Average Dimeter (Micron) 5.7

[0159] Table 3 shows the specific surface and the average particle size.

TABLE-US-00003 TABLE 3 Specific surface and average particle size of the additions used Nanosilica Microsilica BET Specific Sufarce (m.sup.2/g) 200 23 Average size (m) 0.2-0.3 15.0

1Drying of Silica Nanoparticles

[0160] In a specific example, in the conditioning step of raw materials, 200 grams of nanosilica or microsilica are heated, or a mixture of both at a temperature between 100-200 C., preferably 120 C., for 24 hours, in order to eliminate humidity adsorbed on the silica nanoparticles. This step is critical for the proper dispersion and anchorage of the smaller particles. In another test of the conditioning step, it was found that I gram of nanosilica, or 1 gram of microsilica, or a mixture of both, dried effectively in a heating at 120 C. for 5 minutes with ramps of 20 C./min on an infrared balance.

[0161] Similar treatments to 140, 160 and 180 C. for a similar time have given the same result but require a greater energy consumption to heat the material.

[0162] Preferred conditions for some embodiments were 100 C.24 hours.

[0163] In other examples, the cement microparticles were also dried. However, this process is not necessary and it was found that the same results were obtained without the drying process of the cement particles, since the water absorbed in the cement is not removed by drying as it reacts forming hydrated compounds.

2. Dry Dispersion Process

[0164] In a particular example, weight proportions of 90% of cement particles CEM I 52, 0.5R and 10% of nanosilica or microsilica are used, or 10% of a mixture of both; for example 8% microsilica and 2% nanosilica.

[0165] The appropriate amount of raw materials necessary to form the composite, the silica nanoparticles being previously conditioned, is introduced in a biconical agitation mixer where some particles impact with others. This agitation process is assisted by inert grinding balls of stabilized zirconia with yttria of 2 mm in diameter that helped to generate a greater energy transfer between the particles. The weight ratio between grinding balls and the cement particles used was 1 to 2.

[0166] A biconical mixer of 10 L of useful capacity has been used, constructed in stainless steel AISI-316-L for all the parts in contact with the product. The mixer was mounted on a carbon steel bedplate, dimensioned to allow a useful distance of the 800 mm ground discharge valve.

3. Conditioning of the Cementitious Composite

[0167] In this step, the grinding balls of the product were separated by means of a 500 m vibrosieve of stainless steel light mesh, which ensures that the finished product does not contain grinding balls and also allowed to reduce the possible agglomerates formed due to the agitation of the materials in the mill when releasing said agglomerates.

[0168] The conditioning step of the final product or product obtained in step 2) of dispersion was carried out by means of a circular sieve screen for classification of solid products of Maincer SL, suitable for sifting from 36 m to 25 mm. The sifter has a product inlet in the central part and outlet through the side mouth and is made entirely of stainless steel. It has a vibrating motor with eccentric masses.

[0169] The product was sieved until the grinding balls used are clean and all the agglomerates have been discarded.

[0170] Optionally, the balls can be Inside the mixing system if there is a suitable separating element that allows the exit of the composite microparticles and retain the microballs.

Example 2. Preparation of Mortar Using Cementitious Composite

[0171] For the preparation of the mortar specimens, CEM I 52, 0.5R cement particles were used, supplied by the Cementos Portland Valderrivas Group and manufactured according to the standard (UNE-EN-197-1: 201 1). The characteristics of the cement used are shown in table 1 and 2 above.

[0172] Two different additions were used for the mortars: Microsilica supplied by Ferroatlntica S.L and nanosilica powder CAB-O-SIL M-5 supplied by CABOT.

[0173] The aggregate used for the manufacture of the mortar specimens was a standardized CEN sand meeting the specifications of the standard (UNE-EN 196-1 2005).

[0174] For the tests of mortars, standardized prismatic samples of 4040160 mm were manufactured. The manufacture of these mortar specimens was done according to the procedure described in the standard (UNE-EN 196-1, 2005) with the exception of the compaction of the samples for which 90 strokes were used. The amount of cement particles and the water/cementitious material (w/c) ratio is 0.5, the one specified in the same standard. In the cases in which additions of silica nanoparticles were introduced to obtain the cementitious composite, the amount of cement as a cementitious composite was considered, that is, the silica nanoparticles replaced the cement. In this way, the water/cementitious composite ratio was maintained at a value of 0.5. After 24 hours in the mold in a laboratory environment covered by a damp cloth to prevent drying, the test pieces were demolded and cured submerged in water, maintaining it at (201) C.

[0175] Two methods of including the silica nanoparticles into the mixture were compared. The first one was to add the silica nanoparticles during the kneading process; that is, the conventional method called as manual method of including silica nanoparticles. In the second method the silica nanoparticles were added using the method object of the present invention described above in the section description of the invention and the examples of preparation of cementitious composite, which achieves a dry dispersion of the silica nanoparticles on the cement particles. This mixture is used as conventional cement with good workability in the preparation of mortars and concretes.

[0176] Dosages with different content of silica nanoparticles were tested. In the dosages prepared in a conventional manner for comparative purposes, it was necessary to add a superplasticizer additive to improve the workability of the mortars.

[0177] The best results in mechanical and durable properties were obtained for the dosages with 10% of silica nanoparticles, being the optimum in the durability properties in the combined addition of microsilica and nanosilica, in proportions of 8% of micro and 2% of nanosilice. This mixed addition dosage was only possible with the material obtained using the method of the present invention, since manual mixing was impossible given the enormous demand for water that it required. In the manual mixture it was not possible to avoid the use of the superplasticizer additive in proportions lower than 5% with respect to the weight of cement that allows, at most, the standard. The mixture made by the manual method of including silica nanoparticles, was impossible to knead, even with the maximum content of superfluidizer additive. Following the conventional method of addition of silica nanoparticles, it was only possible to perform the mixture with 10% addition of microsilica. In the following, the results of the different tests of mechanical and durable properties that have been carried out will be presented for the following dosages: [0178] M1, reference dosage made with CEM I 52.5R cement particles without any addition. [0179] M2, conventional dosage with the same cement and manual addition of 10% of microsilica. [0180] M-3.1, dosage with the same cement and addition of 10% of dispersed micro silica with the method of the invention. [0181] M-3.2, dosage with the same cement and addition of 8% of micro silica and 2% of nano silica dispersed with the method of the invention

[0182] The resistance to compression is used as the main mechanical characteristic of cementitious materials. The compression resistance test was performed according to the standard (UNE-EN 196-1, 2005). At the ages of 7 and 28 days, six semiprisms of 3 test tubes of 4416 cm obtained previously to the bending break of each prepared dosages, were broken. The testing machine used was an Ibertest 150 T hydraulic press with Servosis automation. The results found for this test carried out in the mortar are shown in table 4:

TABLE-US-00004 TABLE 4 Resistance to compression at 7 and 28 days of the dosages used Resistance to Resistance to compression at compression at Sample 7 days (MPa) 28 days (MPa) M-1 59 2 67 1 M-2 62 3 80 1 M-3.1 81 3 97 4 M-3.2 77 3 89 2

[0183] As can be seen in table 4, the additions of microsilica and nanosilica improve the mechanical properties with respect to the mortar without addition used as a reference. The improvement is superior in the case of the use of the materials object of invention. Regarding this property the mortar made with 10% of microsilica provides better results, reaching 100 MPa in some samples made with the cement prepared with the particle dispersion method of the present invention. This method represents an improvement of more than 20% on samples made with the same addition amount included manually. In the case of the dosage made with mixed addition of microsilica and nanosilica with the method of the invention, lower values were obtained than for the 10% of microsilica added also with the method of invention, but higher than the mixture in which it was added in a manual way. On the other hand, in the measurements carried out of durable properties, better results were obtained in the M-3.2 mortar.

[0184] The fundamental parameters measured to assess the durability of the samples were electrical resistivity and migration of chlorides.

[0185] Table 5 shows the average values of the cell constant (K), electrical resistance (Re) and electrical resistivity (pe) for the mortar specimens selected at the curing age of 7 and 28 days of curing. Also included is the risk of chloride penetration for the calculated average value of electrical resistivity because both parameters can be related. This correlation can be obtained from the chloride penetration risk data dictated by the ASTM C12012 standard.

TABLE-US-00005 TABLE 5 Average values of the cell constant (K), electrical resistance (Re), electrical resistivity (pe) and risk of chloride penetration for the selected mortar specimens at 7 and 28 days of curing Age Electric Electric Risk of K = S/L curing Resistance Resistivity penetration Sample (cm) (days) (k) (k .Math. cm) Cl.sup. M-1 5.10 7 0.728 3.71 High 28 0.817 4.17 High M-2 5.61 7 1.135 6.40 Moderate 28 2.075 11.6 Low M-3.1 5.99 7 0.823 4.93 High 28 3.300 22.02 Low M-3.2 5.90 7 3.915 23.1 Very low 28 5.460 32.2 Very low

[0186] Table 6 shows the coefficient of migration of chlorides (Dnssm) at the age of curing of 28 days for the selected mortars.

TABLE-US-00006 TABLE 6 Chloride migration coefficient (Dnssm) after 28 days of curing for the selected mortars Sample Dnssm (10.sup.12 .Math. m.sup.2/s) M-1 13.687 M-2 4.862 M-3.1 2.879 M-3.2 2.476

[0187] By means of the scanning electron microscopy technique, SEM, the different mortars prepared at the age of 7 and 28 days of curing were analyzed and characterized. In these samples, the different hydration products of the mortars were also identified. The morphology of the originating CSH gels, the phases inside the pores, as well as the morphology and phase sizes such as portlandite and ettringite were studied. In addition, the changes originated by the inclusion of the additions to the matrix of the mortar samples and the Interface or transition zone (ITZ) between the aggregate and the paste of the samples have been studied.

[0188] In the cementitious materials of the mortar type proposed by the present invention, in the case of the addition of nanosilica, ettringite and portlandite nanocrystals originated during the hydration of the material are formed. The permanence of nanometric ettringite crystals covering the pores of the hardened material represents a significant advantage, both in terms of stability against sulphate attacks and against the entry of aggressive agents through the porous network. In this way, we obtain a mortar with exceptional durability characteristics and therefore with a very long expected life.

[0189] FIG. 4 shows a SEM micrograph of M-3.2 sample at 7 days of age of curing, where it can be observed the Interior of a pore covered by primary nanometric ettringite.

[0190] FIG. 5a) b) and c) show SEM micrographs (of the sample M-3.2) at 28 days of age of curing with different scales, where the interior of a pore can be observed, clearly upholstered with nanometric ettringite needles which remain stable.

[0191] For the mortars made from cementitious composites of the present invention, prepared with additions of silica nanoparticles on CEM I 52.5R anhydrous cement, it is observed that: [0192] All of them increase their values of resistance to compression with respect to the sample without additions used as a reference, as well as on the samples in which the addition of nanosilice and microsilica was carried out in a conventional manner, the best being 10% micro-nanosilica, and 8% microsilica+2% of nanosilica at the age of 28 days of curing. [0193] All of them lead to higher percentages of hydration degree and CSH gel, the general trend being the decrease of the dehydroxylation percentages. [0194] A refinement of the porous structure is obtained in all cases with lower values of the chloride migration coefficient and higher electrical resistivities. [0195] Scanning electron microscopy (SEM) images show more compact and dense gels than in the CEM I 525R cement reference mortar without additions, as well as a better adhesion between the paste and the aggregate. In the samples with nanosilica, an upholstery of micrometric primary ettringite is observed in the internal walls of the pores, that does not appear for the microsilica or in the reference mortar.

[0196] It stands out that for 28 days of curing the micrometric primary ettringite phase remains unchanged. This effect is particularly remarkable, since it shows that this phase does not degrade, which means an improvement in durability against attack by sulfates. Usually the primary ettringite phase formed during the hydration of the cements is not stable and goes into a monosulfate state, with less sulphate content, thus being susceptible of being attacked by the entrance of sulfates from the outside, reacting with it to give again hydrated calcium trisulfoaluminate in hardened state, which is called secondary ettringite. The formation of secondary ettringite produces a large increase in volume inside the hardened material, an effect that causes great internal stresses, and as a consequence causes an important cracking and degradation of the material.

Example 3. Preparation of Concrete Using Cementitious Composite

[0197] For the manufacture of the concrete specimens, three dosages were selected among those studied that gave better results in paste and mortar. These were prepared with the same cement particles (CEM I 52.5R). In addition, concrete was prepared only with cement, to be used as a reference (H-I) against the mixtures under study. The compositions selected were the following, in all those that had addition, this was included by the method of the present invention: [0198] H1, reference dosage made with CEM I 52.5R cement particles without any addition. [0199] H3.1, dosage with the same cement and addition of 10% of microsilica.H3.2, dosage with the same cement and addition of 8% of microsilica and 2% of nanosilice [0200] H3.3, dosage with the same cement and addition of 10% nanosilicate.

[0201] Table 7 shows the dosages used for the manufacture of concrete specimens.

TABLE-US-00007 TABLE 7 Dosing for one cubic meter of concrete of the concretes object of study Materials (kg/m3) H-1 H-3.1 H-3.2 H-3.3 CEM I 52.5R CEM U 400 360 360 360 Microsilica (g) 40 32 Nanosilica (g) 8 40 Water (L) 180 180 180 180 Sand (kg) 825 825 825 825 Grit (kg) 419 419 419 419 Gravel (kg) 524 524 524 524 Superplastisizer(% with respect 0.90 1.00 1.80 5.00 to the weight of cement) w/c 0.45 0.45 0.45 0.55 w/c: water/cement

[0202] The elaboration of the same was carried out under laboratory conditions with temperatures of 20-25 C. and average relative humidity of 35%. The procedure used is that described in the standard (UNE-EN 12390-2, 2009). Before weighing the quantities of material indicated for the different dosages obtained, it was necessary to make the relevant corrections in the aggregates, calculating the humidity at the time of use. Once these values were obtained, the final weights of both the aggregates and the mixing water were corrected. To mix the materials, a 100-liter vertical shaft kneader with a mobile container was used to receive the concrete discharge.

[0203] Once the mixture was homogenized, the anhydrous cement particles were included with the additions previously deposited. Once the anhydrous cement was included, it was kneaded for 60 seconds with the aggregates to homogenize the material. Then, the new generation superfluidizer additive previously dissolved in a small amount of the mixing water was added to the mixture. The remaining water was included slowly. Once the batch was completed, two types of cylindrical molds were filled in 3 tons with the concretes prepared to obtain cylindrical specimens with a diameter of 150 mm and 300 mm in height and specimens of 100 mm in diameter and 200 mm in height. For the compaction of the concrete samples a vibrating table was used. After 24 hours in a laboratory environment, covered by a damp cloth to prevent drying, the specimens were demolded and cured under water until the ages of 7 and 28 days.

[0204] Prior to the filling of the molds, the Abrams cone test was carried out, which is a measure of the docility (workability) of the concrete. The results obtained are presented in table 8.

TABLE-US-00008 TABLE 8 Abrams Cone Seat for the dosages used Concrete Samples Designation H-1 H-3.1 H-3.2 H-3.3 Seat (cm) 10 11 6 0

[0205] These results show the Impossibility of putting H-3.3 concrete into operation, due to its zero-value seat.

[0206] In table 9 the results of the compression test are shown after 7 and 28 days of curing the manufactured dosages.

TABLE-US-00009 TABLE 9 Average compression resistance and its corresponding standard deviation for the concrete samples under study Resistance to compression (MPa) Curing time (days) Sample 7 28 H-1 44.8 3.1 50.4 1.5 H-3.1 46.5 0.2 56.3 0.4 H-3.2 51.5 5.3 66.9 0.1 H-3.3 49.5 6.1 52.9 1.1

[0207] The test of resistance to compression at the ages of 7 and 28 days of curing on the concrete specimens was carried out following the standard (UNE-EN 12390-3, 2009). To carry out this test, concrete specimens of 150 mm in diameter and 300 mm in height were used.

[0208] Table 10 shows the average values of the cell constant (K), electrical resistance (Re) and electrical resistivity (pe) for the concretes under study at the curing age of 7 and 28 days. In addition, the risk of chloride penetration is included for the calculated average value of electrical resistivity in each case.

TABLE-US-00010 TABLE 10 Average values of the cell constant (K), electrical resistance (Re), electrical resistivity (pe) and risk of chloride penetration for the selected mortar specimens at 7 and 28 days of curing Age Electric Electric Risk of K = S/L curing Resistance Resistivity penetration Sample (cm) (days) (k) (k .Math. cm) Cl.sup. H-1 3.95 7 1.272 5.02 High/Moderate 28 2.090 8.25 Moderate H-3.1 3.93 7 2.202 8.65 Moderate 28 10.581 41.58 Very low H-3.2 3.93 7 4.370 17.17 Low 28 20.820 81.82 Very low H-3.3 3.97 7 5.930 23.54 Very low 28 7.075 28.09 Very low

[0209] Another test that characterizes the durability of concrete versus the penetration of chlorides is the determination of the migration coefficient. The concrete samples under study underwent the corresponding test according to the NT-BUILT 3040 standard. The results are shown in table 11. They are observed to show the same trends found in the resistivity test. According to these results and applying the models of proposed useful life, the EHE (Spanish Instruction for Structural Concrete), and the equivalences between the coefficients of migration and diffusion of chlorides, a useful life value is obtained that is also included in the same table.

TABLE-US-00011 TABLE 11 Average value of the chloride migration coefficient of the concrete studied Migration Diffusion Service life (years) coefficient 10.sup.12 coefficient 10.sup.12 (from commissioning Dosage (m.sup.2/seg) (m.sup.2/seg) to the start of corrosion) H-1 10.089 2.775 72 H-3.1 1.91 0.554 336 H-3.2 0.761 0.271 801 H-3.3 2.017 0.583 319

[0210] The results by SEM micrographs show that the addition of silica nanoparticles significantly reduces the size of the crystals. The SEM micrographs presented in FIGS. 6a) and 6b) for the H-3.1 dosing after 28 days of curing, and show that the reduction of the size of the crystals does not occur when the addition is of micrometric size.

[0211] In FIGS. 6a) and 6b) SEM micrographs of the H-3.1 concrete are shown.

[0212] FIG. 7 shows the micrograph of H-3.3 concrete after 28 days of curing, where nanometric ettringite needles can be seen.

[0213] In FIGS. 8 a) and 8b) the ettringite crystals are observed next to the C3A formations of the H-3.2 concrete after 28 days of curing.

[0214] The micrographs show that the properties of the crystals obtained with the use of nano additions are maintained, improving the microstructure of the material and doubling its life in service.

[0215] The concrete samples obtained with similar addition of microsilica and nanosilica but following a conventional process for comparative purposes, necessarily had to be limited to the possibility of working the material. It was impossible to work with nanosilica additions greater than 7.5% by weight of the cement. Even so, in this dosage, the amounts of superplasticizing additive necessary to be able to obtain adequate workability, exceed the limit allowed by the EHE (Spanish Instruction for Structural Concrete).

[0216] The studies carried out on concrete samples with additions of micro, nano, and micro and nanosilica mixture gave better results, indicating that all cases give rise to samples with better mechanical and durable properties than the corresponding conventional concrete used as reference. The improvement of mechanical properties can be related to higher contents of CSH gel and higher degree of hydration than the concrete used as reference. On the other hand, the improvement of durable properties can be related to the formation of a more refined and consolidated porous structure, noticeably greater electrical resistivities, and rather lower chlorides migration coefficients. Lower percentages of portlandite also appear as significant improvements, which Is the hydrated compound more susceptible to be leached, together with a better adhesion between the aggregate and the pulp.

[0217] In summary, in all of them a notable quantitative leap in the relevant parameters of their potential mechanical properties and especially in the durable ones was observed.

[0218] With the method of the present invention, concretes having percentages of ettringite of at least 1.5% at 90 days have been obtained.

Example 4. Characterization of the Cementitious Composite of Example 1

[0219] The materials obtained following the method described in Example 1 using both, the same starting cement and the microsilica and nanosilica, were characterized in terms of specific surface area and Raman spectroscopy.

[0220] In all cases, the drying materials were dried in an oven at 90 C. for 12 hours until they reached a humidity of less than 0.05%.

[0221] Cements, C, and cementitious composites, CC, prepared were: [0222] C1, cement CEM I 52.5R without any addition. [0223] C2, cement CEM I 52.5R and manual addition of 10% by weight of microsilica. [0224] CC3.1, CEM cement 1 52.5R and addition of 10% of dispersed microsilica with the method of the invention. [0225] CC3.2, CEM cement I 52.5R and addition of 8% of microsilica and 2% of nanosilice dispersed with the method of the invention

[0226] Additionally, and following the same procedure described in Example 1, the C2b and CC-3.1 cementitious composite were prepared from the same cement as in Example 1 and a microsilica from Elkem Microsilica Grade 940 with a specific surface area of 20.4 m.sup.2/g: [0227] C2b, cement CEM I 52.5R and manual addition of 10% by weight of microsilica. [0228] CC3.1 b, CEM cement I 52.5R and 10% addition of dispersed micro silica, with the method of the invention.

[0229] In the preparation, drying of the starting materials was carried out, consisting of drying in an oven at 90 C. for 12 hours until it reached a humidity of less than 0.05%.

[0230] Table 12 shows the values of the specific surface area determined by the BET method (Brunauer, Emmett and Teller) multipoint for these materials and the % variation corresponding to the percentage of variation of the experimental area compared to the theoretical value obtained by the rule of mixtures with respect to the specific surfaces of the components of the mixture weighted by the composition of the mixture.

TABLE-US-00012 TABLE 12 BET specific surface of cementitious composites % decrease of Mortar of specific surface Cements and example value in relation to cementitious 2 where it BET Specific the calculated value composites is used Surface (m.sup.2/g) using the mix rule C1 M-1.sup. 1.34 C2 M-2.sup. 3.48 0.75 CC3.1 M-3.1 3.41 2.74 CC3.2 M-3.2 6.63 8.23 CC3.1b 3.18 2.00 CC3.2b 2.82 13.23

[0231] The cementitious composites of the present invention are characterized by a decrease in the specific surface area of the composite that is >2% higher than the value of the specific surface calculated by the mixing rule. The decrease in the value of the specific surface area with respect to the value calculated by the mixing rule for the cementitious composites of the present invention is at least three times the value of the decrease of the specific surface area with respect to the value calculated by the mixing rule for a material of similar composition prepared by a manual mixing procedure. The greater decrease of the values of the specific surface area with respect to the value calculated by means of the rule of mixtures for the cementitious composites correlates with an effective dispersion of the microsilica particles and also implies a variation of the hydration capacity of the surface. The addition of nanosilica to the cementitious composite also results in a greater decrease in the value of the specific surface area compared to the value calculated by the mixing rule.

[0232] The effective dispersion of the microsilica particles or of the silica nanoparticles or of the combination of microsilica particles plus nanosilica nanoparticles is associated with a modification of the structure of the cementitious composite. This modification of the structure in the cementitious composites of the present invention is characterized by changes in the bands obtained by spectroscopy and/or shift of said Raman bands with respect to the Raman bands of the anhydrous Portland cement. The starting materials were characterized by Raman spectroscopy: CEM 52.5R (C1) and Microsilica; as well as the cementitious composite CC3.1. Additionally, a cementitious composite was characterized following example 1 of the present invention for the sample CC3.1 wherein the percentage of addition of microsilica was modified to obtain 8% by weight and which we shall denominate CC3.0.8. In FIG. 13 it can be seen the different Raman spectra for all the mentioned systems.

[0233] To carry out the study of the effect of the addition of the microsilica on cement C1, anhydrous Portland cement, we proceeded, first, to the characterization of the starting materials separately to identify their major mineralogical phases. In the case of anhydrous Portland cement, there are numerous phases, such as C2S (dicalcium silicate or belite), C3S (tricalcium silicate or alite), C3A (tricalclum aluminate), C4AF (ferritic phase), etc. However, to try to characterize the behavior of the additions of microsilica (whose chemical composition is >85% by weight of SiO.sub.2) to the cement, the Raman modes that appear around 840 cm.sup.1, FIG. 14, are used, which allow to determine the presence of C.sub.2S and C.sub.3S phases of the cement.

[0234] The C1 cement has a Raman spectrum where a Raman band located around 840 cm.sup.1, assigned to the presence of the C3S or alite phase, can be appreciated. This Raman band presents a shoulder towards higher values of Raman shift, greater value of cm.sup.1. A second intense and narrow band also appears around 1022 cm.sup.1. Both bands with respective characteristics of the presence of the majority phases of the cement: the tricalcium silicate or alite (C.sub.3S) and the dicalcium silicate or belite (C.sub.2S).

[0235] The Raman spectrum of the microsilica shows the existence of very widened Raman bands because the angles of the SiOSi bonds are widely distributed throughout the structure. The defect bands D1 and D2 located at 484 and 596 cm.sup.1, respectively, as well as the bands located at 460, 800 and 1 100 cm.sup.1 assigned to the SiOSI bonds are clearly visible. The position of the maximum and Raman bands varies within the microsilica, in particular for the characteristic Raman band located at 500 cm.sup.1, being a signal of the differences in crystallization and stress that can be found within the microsilica.

[0236] The cementitious composites of the present invention showed a significant modification in the position and intensity of the characteristic Raman bands related to the phases of anhydrous Portland cement. The Raman shift towards the blue of the Raman bands that appears around 840 cm.sup.1 and 857 cm.sup.1, has been found for the cementitious composites of the present invention. The Raman shift towards the blue (higher values of Raman displacement in terms of cm.sup.1) implies that the bond strength constant corresponding to the Raman mode is stronger, that is, the bond is shorter and therefore of higher energy. This Raman shift towards blue means that in the cementitious composites of the present invention the presence of silica particles dispersed on the surface of the same particles modify the crystalline structure of the cement, making its bonds stronger. This effect is evidence of the effective anchoring of the silica particles in the cementitious composite according to the method described in the present invention. In addition, the increase in intensity corresponding to the Raman band at 840 cm.sup.1 with respect to the Raman band at 847 cm.sup.1 evidences a greater presence on the surface of the first phase corresponding to said Raman mode. The aforementioned effects correlate with the modification of the reactivity of the cementitious composites of the present invention and allow modifying the cement microparticles to obtain mortars and long-lasting concrete from the cementitious composites as described in the present invention.

[0237] The Raman band corresponding to the microsilica that appeared around 800 cm.sup.1 has an intensity much lower than that expected for the percentage of addition used. This aspect, together with the differences in Raman displacement of the microsilica, makes it impossible to evaluate whether there are modifications in the bonds corresponding to the microsilica. However, the low intensity represents a sign of adequate dispersion since it is not possible to find areas with the exclusive presence of microsilica. This aspect is important to produce a greater degree of reaction during the subsequent hydration process. The adequate dispersion of the particles observed by scanning electron microscopy is confirmed in this way. Therefore, the different additions cause a better homogeneity and distribution of both major phases of the cement (C.sub.2S and C.sub.3S).

[0238] In the cementitious composites of the present invention that include silica nanoparticles, these effects have shown to be analogous to those described for microsilica.

[0239] In this way, the products of cementitious composites of the present invention are characterized by showing a Raman shift towards the blue of the phases corresponding to the cement with respect to the starting cement. This Raman shift towards higher cm.sup.1 values characterizes the cementitious composite as a material with a structural modification that is produced by the presence of silica particles or silica nanoparticles or by the combination of microsilica and nanosilica. Said silica particles are preferably anchored to the surface of the cement particles. The structural modification of the cement phases is correlated with the modified response of the cementitious composites with respect to conventional cement, since there is a considerable increase in the mechanical resistance at short ages, as well as the values of the electrical resistivity, together with a strong decrease in chlorides migration coefficients compared to mortars and conventional concrete or with mortars and concrete with conventional addition of microsilica and nanosilica. The modification of the cement structure in the cementitious composites of the present invention demonstrates the dispersion of the microsilica or nanosilica particles which thus present an improvement in the appearance of the main cement hydration product (primary CSH gel), and gives rise to the appearance of secondary gels due to the pozzolanic activity of the silica. This effect has been found for mortars prepared in the present invention following example 2. By means of Differential Thermal Analysis, the percentage of the gel phase, the percentage of the portlandite phase, which is a hydrated phase of the cement, and the relationship between these phases, were determined for the mortars, Table 13. A significant increase in gel formation was determined for the mortar prepared from the cementitious composite of the present invention.

TABLE-US-00013 TABLE 13 BET specific surface of cementitious composites 7 days 28 days M1 M-3.2 M1 M-3.2 % gel 2.602 2-963 3.181 3.381 % free portlandite 1.157 0.968 1.263 0.981 phase gel/portlandita 2.249 3.060 2.520 3.448