NANOTUBES, PROCESS FOR OBTAINING THEM AND CEMENTITOUS COMPOSITIONS COMPRISING THEM

Abstract

The present invention describes the preparation of nanotubes made from portlandite, the naturally occurring form of calcium hydroxide, Ca(OH).sub.2. Portlandite nanotubes are obtained by a process comprising the following steps: a) reacting calcium chloride with calcium oxide in aqueous solution, thus obtaining an aqueous dispersion; b) feeding as such the aqueous dispersion obtained in step a) to a hydrothermal reaction, thus obtaining portlandite nanotubes. The invention also concerns the use of the portlandite nanotubes as a component for cementitious compositions to provide reinforced mortar or concrete.

Claims

1. Portlandite nanotubes obtained by a process comprising the following steps: a) reacting calcium chloride with calcium oxide in aqueous solution, thus obtaining an aqueous dispersion; b) feeding as such the aqueous dispersion obtained in step a) to a hydrothermal reaction, thus obtaining portlandite nanotubes.

2. Portlandite nanotubes according to claim 1 obtained by a process comprising the following steps: a) heating a CaCl.sub.2.2H.sub.2O aqueous solution at a temperature between 40 C. and 100 C., adding solid CaO to the heated solution, letting the reaction run at room temperature; b) feeding as such the aqueous dispersion obtained in step a) to the hydrothermal reaction, heating the dispersion at a temperature in a range between 160 C. and 270 C., for a time of a least 4 hours, thus obtaining portlandite nanotubes.

3. Portlandite nanotubes according to claim 2 wherein in step a) solid CaO is added to the heated solution and stirred for a time of at least 5 minutes and the reaction is run at room temperature for a time of at least 24 hours.

4. Portlandite nanotubes according to claim 1 defined by characterisation based on X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM).

5. Process for producing portlandite nanotubes, comprising the following steps: a) reacting calcium chloride with calcium oxide in aqueous solution, thus obtaining an aqueous dispersion; b) feeding as such the aqueous dispersion obtained in step a) to a hydrothermal reaction, thus obtaining portlandite nanotubes.

6. Process according to claim 5 comprising the following steps: a) heating a CaCl.sub.2.2H.sub.2O aqueous solution at a temperature between 40 C. and 100 C., adding solid CaO to the heated solution, letting the reaction run at room temperature; b) feeding as such the aqueous dispersion obtained in step a) to the hydrothermal reaction, heating the dispersion at a temperature in a range between 160 C. and 270 C. for a time of a least 4 hours, thus obtaining portlandite nanotubes.

7. Process according to claim 6 wherein in step a) solid CaO is added to the heated solution, stirred for a time of at least 5 minutes and the reaction is run at room temperature for a time of at least 24 hours.

8. Process according to claim 6 wherein in step a) solid CaO is added to the CaCl.sub.2.2H.sub.2O aqueous solution in an amount according to a molar ratio CaCl.sub.2.2H.sub.2O/CaO between 1:1 to 10:1, respectively.

9. Process according to claim 8 wherein in step a) solid CaO is added to the CaCl.sub.2.2H.sub.2O aqueous solution in an amount according to a molar ratio CaCl.sub.2.2H.sub.2O/CaO between 1:1 to 5:1, respectively.

10. Process according to claim 5 wherein the portlandite nanotubes are defined by characterisation based on X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM).

11. Use of the portlandite nanotubes according to claim 1 as a component in cementitious compositions to provide mortars or concrete.

12. Use according to claim 11 as a reinforcing material in cementitious compositions.

13. A cementitious composition comprising an hydraulic binder and aggregates for the production of mortars or concrete, wherein it comprises portlandite nanotubes according to claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0037] As described above,

[0038] FIGS. 1 to 3 of the enclosed drawings show X-ray diffractogram, XRD (FIG. 1) and transmission electron microscopy (TEM) images (FIGS. 2 and 3) of reference products made for comparative purposes and falling outside the scope of the present invention.

[0039] FIG. 4 and FIG. 11 show X-ray diffractograms, XRD, of samples as obtained in the following Examples 1 and 2, respectively, according to the invention.

[0040] FIGS. 5 and 6 show transmission electron microscopy (TEM) images of the sample as obtained in the following Example 1 according to the invention.

[0041] FIG. 7 shows an electron density profile of the sample of FIG. 6.

[0042] FIGS. 8 and 9 show a high-resolution transmission electron microscopy (HRTEM) image of the sample as obtained in the following Example 1 according to the invention.

[0043] FIG. 10 shows an electron density profile of the sample of FIG. 9.

[0044] FIG. 12 shows a transmission electron microscopy (TEM) image of the sample as obtained in the following Example 2 according to the invention.

[0045] FIG. 13 shows an enlargement of FIG. 12.

[0046] FIG. 14 shows an electron density profile of the sample obtained in the same Example 2.

[0047] FIG. 15 and FIG. 19 show X-ray diffractograms, XRD, of samples as obtained in the following Examples 3 and 4, respectively, according to the invention.

[0048] FIGS. 16 and 17 show a transmission electron microscopy (TEM) image of the sample as obtained in the following Example 3 according to the invention.

[0049] FIGS. 20 and 21 show a transmission electron microscopy (TEM) image of the sample as obtained in the following Example 4 according to the invention.

[0050] FIG. 18 shows an electron density profile of the sample of FIG. 17.

[0051] FIG. 22 shows an electron density profile of the sample of FIG. 21.

EXAMPLES

[0052] The following examples illustrate the invention without in any way limiting the scope thereof. The characterisation of the final product obtained in the examples was carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) as follows. The X-ray diffraction measurements were made by a Philips XPert Pro MPD pw3040/60, equipped with a copper ceramic tube and employing a continuous scanning in the 2 range from 2 to 75, and a generator power of 40 kV and 40 mA.

[0053] The final products were then characterised by a JEOL JEM-1230 thermionic emission transmission electron microscopy (120 kV) with a digital camera and some of the samples were also observed on a JEM-2200 FS/CR (JEOL, Ltd.) field emission gun (FEG) transmission electron microscopy operated at 200 kV. The samples were dispersed in acetone and a drop of the dispersion was applied to a carbon grid.

Example 1

[0054] 3 M aqueous solution of CaCl.sub.2.2H.sub.2O is prepared. While heating the solution at 70 C., solid CaO is added and stirred for 10 minutes. The mixture is left reacting for 68 hours, at room temperature. Then the so obtained dispersion is directly fed as such, without filtering or washing, to a reactor and heated to 200 C. The hydrothermal reaction is carried out for 15 hours. The solid obtained is filtered and rinsed with water.

[0055] The final solid was characterized by X-ray diffraction as shown in the enclosed FIG. 4. The XRD pattern of the solid is ascribed to the hexagonal phase of Ca(OH).sub.2 (JCPDS 01-084-1271).

[0056] The solid was also analysed by transmission electron microscopy (JEOL JEM-1230 at 120 kV), scale bar 0.2 m, see FIG. 5. The product obtained shows fibre-like/rod-like morphologies of micrometres in length and diameters between 23 nm and 44 nm. At higher magnification (scale bar 50 nm), see FIG. 6, a nanotube morphology can clearly be defined, as indicated by the arrows.

[0057] The nanotube nature of the fibre can be further confirmed by extracting electron density profiles from the TEM images, see FIG. 7, which shows the electron density profile extracted from the area delimited by the box traced on the image of FIG. 6. The ordinate refers to counts and the abscissa to d, measured as nm.

[0058] As mentioned above, the electron density profile shows the intensity of the electrons that pass through the analysed material. When the intensity is high or relatively high, it means that the electrons do not go through any material or they go through a very thin layer of material. When the intensity is very low or almost zero, it means that the electrons are not able to go through the material which indicates that the material is quite compact. The electron density profile of the nanotube shows a high intensity outside the tube, which lowers to nearly zero where the electrons do not go through the walls of the nanotube formed by layers of Ca(OH).sub.2, and then increases in the middle of the tube to a certain extent, which indicates that the electrons there pass through a thinner layer of material, as in the case of a hollow tube. Furthermore, from the electron density profile, the inner diameter of the nanotube can also be measured, which turns out to be around 42 nm in the case of the example at issue.

[0059] The product obtained in Example 1 was then analysed by high resolution transmission electron microscopy, HRTEM (JEOL JEM-2200 FS/CR operated at 200 kV). In FIG. 8 a Ca(OH).sub.2 nanotube is shown at high resolution (indicated by the arrows). In the image of FIG. 8 the different layers that form the wall of the nanotube can be seen, clearly showing its multiwalled nature. The inner diameter of the nanotube is 8.8 nm.

[0060] FIG. 9 is an enlargement of FIG. 8, where a specific area is delimited by a box, again for electron density profile purpose. The corresponding electron density profile as obtained from that traced box is shown in FIG. 10. The ordinate refers to counts and the abscissa to d, measured as nm.

Example 2

[0061] To a 3 M CaCl.sub.2.2H.sub.2O aqueous solution kept at 70 C., solid CaO is added and stirred for 30 minutes and the dispersion is left reacting for 44 hours at room temperature. Then the so obtained dispersion, without previous filtering or washing, is fed as such to a reactor and heated to a temperature of 250 C. The reaction is carried out for 15 hours. The solid thus obtained is filtered and rinsed with water.

[0062] The solid was characterized by X-ray diffraction, see FIG. 11. Apart from a set of peaks of low intensity which can be assigned to the rhombohedral phase of calcium carbonate (JCPDS 01-081-2027), possibly formed due to a small quantity of CO.sub.2 unremoved from the water, the main peaks obtained can be ascribed to the hexagonal phase of Ca(OH).sub.2 (JCPDS 01-084-1265).

[0063] The product obtained from Example 2 was also characterized by transmission electron microscopy (JEOL JEM-1230 at 120 kV), see FIG. 12, scale bar 0.2 m, and the enlargement of FIG. 13, scale bar 100 nm. TEM images show that fibre-like morphologies of portlandite where formed in the synthesis, as well as nanotube particles shown in FIG. 13 and marked with arrows, scale bar 100 nm. The diameter of the fibres or nanotubes of Example 2 ranges between 17 nm and 44 nm.

[0064] As shown in FIG. 14, the electron density profile is reported with reference to the area marked by a box on the nanotube of FIG. 13. The ordinate refers to counts and the abscissa to d, measured as nm. The electron density profile of the marked area confirms the nanotube structure at issue. The inner diameter of the nanotube is 29.84 nm.

Example 3

[0065] Whilst in example 1 and example 2 above CaO is added to said solution according to a molar ratio CaCl.sub.2.2H.sub.2O to CaO=10/1 respectively, a lower molar ratio CaCl.sub.2.2H.sub.2O/CaO=5/1 is used in the present example as follows.

[0066] To a 3M CaCl.sub.2.2H.sub.2O solution at 70 C., solid CaO is added according to a molar ratio CaCl.sub.2.2H.sub.2O to CaO=5/1 in 30 minutes and left reacting for 72 hours at room temperature.

[0067] Then the so obtained dispersion is directly fed as such, without filtering or washing, to a reactor and heated to 200 C. The hydrothermal reaction is carried out for 15 io hours. The solid obtained is filtered, rinsed with water and washed with absolute ethanol.

[0068] The final solid is characterized by X-ray diffraction, see FIG. 15. The obtained sample is mainly made of calcium hydroxide and some calcium carbonate. A set of peaks is visible to be assigned to the rhombohedral phase of calcium carbonate (JCPDS 00-001-0837), possibly formed due to a small quantity of unremoved CO.sub.2. The remaining peaks are to be ascribed to the hexagonal phase of Ca(OH).sub.2 (JCPDS 01-084-1263).

[0069] The product obtained was also characterized by transmission electron microscopy (TEM) (JEOL JEM-1230 at 120 kV), see FIG. 16, scale bar 200 nm. Fibre-like morphologies were obtained as shown in FIG. 16, as well as nanotube particles as shown in FIG. 17 in which one of the fibres, indicated by the arrows, is shown to be shaped like a nanotube.

[0070] The nanotube structure is also confirmed by the corresponding electron density profile which is shown in FIG. 18. The electron density profile indicates that the diameter of the nanotube of FIG. 17 is 134 nm.

Example 4

[0071] An even lower molar ratio CaCl.sub.2.2H.sub.2O to CaO=1/1 is used in the present example.

[0072] To a 3M CaCl.sub.2.2H.sub.2O solution kept at 70 C., solid CaO according to a CaCl.sub.2.2H.sub.2O/CaO ratio of 1/1 is added in 30 minutes and left reacting for 72 hours at room temperature.

[0073] Then the so obtained dispersion is directly fed as such, without filtering or washing, to a reactor and heated to 200 C. The hydrothermal reaction is carried out for 15 hours. The solid obtained is filtered, rinsed with water and washed with absolute ethanol.

[0074] The final solid was characterized by X-ray diffraction, see FIG. 19. The sample was mostly made of portlandite and calcium carbonate, probably formed due to the CO.sub.2 that was redissolved in water during the hydrothermal reaction. Some CaClOH was also present in the sample.

[0075] The X-ray spectrum shows that portlandite is present (hexagona crystal system, JCPDS 00-044-1481, the peaks are marked with blue circles in FIG. 19), rhombohedral calcium carbonate (JCPDS 01-085-0849) and an impurity made of CaCIOH (JCPDS 00-036-0983) are also present.

[0076] The sample was analysed by transmission electron microscopy (JEOL JEM-1230 at 120 kV), see FIG. 20 showing a mixture of morphologies, that is, particle-like morphologies mixed with fibre-like morphologies.

[0077] When analysing further the fibre-like morphologies by TEM, the nanotube structure of the fibres was confirmed according to FIG. 21, showing a TEM image of a nanotube-like morphology.

[0078] FIG. 22 shows the corresponding electron density profile which confirms the nanotube structure of the fibre. The inner diameter of that nanotube is 48.5 nm.

[0079] In conclusion, the present invention can finally accomplish the high-felt need of providing portlandite with a nanotube structure, portlandite being a material highly compatible with a cementitious matrix. In particular, the portlandite nanotubes according to the present invention appear to be an ideal reinforcing component in cementitious compositions so as to provide final cementitious products, such as mortars or concrete, with improved mechanical properties, such as improved resistance under tensile stress.

[0080] Moreover, portlandite nanotubes according to the present invention are a product-by-process where such process is substantially characterized by simple and straightforward steps, under mild operating conditions. In particular, no process intermediates need to be separated or isolated, and the whole process is carried out in aqueous solution, these being highly advantageous conditions for industrial scale-up.