CN111574091A

CN111574091A - Self-repairing material and preparation method and application thereof

Info

Publication number: CN111574091A
Application number: CN202010424723.XA
Authority: CN
Inventors: 刘汉东; 李志明; 底杰; 赵晓栋; 张磊; 许光; 李丽莎; 康屾; 陈红远
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-05-19
Filing date: 2020-05-19
Publication date: 2020-08-25
Anticipated expiration: 2040-05-19
Also published as: CN111574091B

Abstract

The invention relates to the technical field of concrete materials, and particularly discloses a self-repairing material and a preparation method and application thereof. The self-repairing material comprises an inner reinforcing component, an inner expanding component and an outer double-layer coating layer, wherein the first layer of the double-layer coating layer is a mineral adsorption component layer, and the second layer of the double-layer coating layer is a slow-release component layer coated outside the mineral adsorption component layer; wherein the enhancing component is a carbonate or bicarbonate; the expansion component is a calcium salt or an oxide of calcium; the mineral adsorption component layer is made of pumice powder; the material of the slow release component layer is polyvinyl butyral. The self-repairing material provided by the invention can be used for quickly filling and repairing micro cracks or micro damages of cement-based materials such as mortar, mortar and concrete caused by various factors, has a good reinforcing effect and has good repeated repairability.

Description

Self-repairing material and preparation method and application thereof

Technical Field

The invention relates to the technical field of concrete materials, in particular to a self-repairing material and a preparation method and application thereof.

Background

After the development of concrete for over a century, the initial cement concrete consisting of simple sand, stone and cement has been developed into high-performance concrete consisting of a fifth component, a sixth component and the like, but the problems of low tension-compression ratio, poor crack resistance and the like of the concrete due to the inherent brittleness of the concrete still remain as research difficulties in the field of concrete engineering. If the micro cracks formed in the early stage of the concrete are not repaired in time, the micro cracks (or micro damages) can be continuously expanded to form macro cracks under the action of external load, so that the mechanical property of the building is reduced, the overall performance of the building is deteriorated, and the service life of the building is shortened.

At present, concrete cracks are mainly repaired by two methods: one is a repair method, i.e., a passive repair method such as grouting, caulking, and concrete replacement is used after macroscopic cracks are generated, which can be observed by naked eyes. The repair mode has more repeated construction times, improves the repair cost, and also has the problems of untimely crack discovery and repair, poor repair effect and the like. The other is a self-repairing mode, namely a timely and effective active repairing mode is carried out before the microcracks (or micro-damage) develop into the macrocracks, such as microbial self-repairing, infiltration crystallization self-repairing, mineral self-repairing and the like. The inorganic mineral self-repairing method becomes a research hotspot in the field due to the advantages of simple construction, low cost and the like. However, most of the currently marketed concrete mineral repair materials fill cracks through self-hardening, the activity of unhydrated cement cannot be effectively excited, the effect of reinforcing concrete cannot be achieved, and part of the repair materials are consumed due to reaction in the early stage of concrete hardening, so that the ideal crack effect cannot be achieved. Therefore, how to effectively excite the activity of unhydrated cement in concrete and delay the release speed of repair components, so that the self-repair product and a concrete matrix are well integrated into a whole, and a better repair and reinforcement effect is achieved, which is a problem to be solved in the field of concrete repair materials.

Disclosure of Invention

The invention provides a self-repairing material and a preparation method and application thereof, aiming at the problem that the self-repairing material in the prior art cannot achieve an ideal crack repairing effect.

In order to solve the technical problems, the technical scheme provided by the invention is as follows:

the self-repairing material comprises an inner reinforcing component, an inner expanding component and an outer double-layer coating layer, wherein the first layer of the double-layer coating layer is a mineral adsorption component layer, and the second layer of the double-layer coating layer is a slow-release component layer coated outside the mineral adsorption component layer;

wherein the enhancing component is a carbonate or bicarbonate; the expansion component is a calcium salt or an oxide of calcium; the mineral adsorption component layer is made of pumice powder; the material of the slow release component layer is polyvinyl butyral.

Compared with the prior art, the self-repairing material provided by the invention utilizes the strong adsorbability of the pumice powder to enable the pumice powder to be uniformly adsorbed on the surfaces of the expansion component and the reinforcing component to form the repairing component powder coated once, and adopts polyvinyl butyral (PVB) as a slow release layer to carry out secondary film-forming coating, so as to obtain the double-layer slow-release blocking repairing material. Wherein, the primary coating adopts the strong adsorption of the pumice powder to carry out coating, the incomplete coating effect can be generated, and the secondary coating adopts PVB film-forming coating to form complete coatingThe repairing component is prevented from being consumed in advance due to hydration in the concrete mixing process. When cracks are generated, water enters to form alkaline substances, a PVB coating film is dissolved, and pumice powder is rapidly mixed with Ca in concrete²⁺C-S-H gel with strong cementing power is generated by reaction, so that the cohesiveness with a concrete matrix is enhanced; meanwhile, the expansion component and the reinforcing component which are not coated by the pumice powder also quickly react to generate high-strength or expanded crystals, finally, a network structure of gel coated crystals and tightly interwoven crystals is formed in cracks, and the network structure is well cemented with the surrounding hardened slurry, so that the excellent repairing effect and the good reinforcing effect are achieved; the expansion component and the reinforcing component coated by the pumice powder have the advantages that C-S-H gel generated by the reaction of the pumice powder is compact and has low hydration speed, so that the repairing component can be continuously provided when the hydration reaction part is not reached in the early stage and cracks are generated again in the later stage, and secondary expansion and secondary hydration are generated, so that the system has repeated repairability.

Preferably, the mass ratio of the reinforcing component to the expansion component to the pumice powder to the polyvinyl butyral is 20-30:40-60:10-15: 10-20.

PVB is selected as a slow release layer component for secondary coating, because PVB has excellent film-forming property, the coating rate can be improved, and further the slow release effect of the material is effectively improved, meanwhile, PVB has unique wetting and dispersing effects on primary coating particles in the invention, the dispersibility among primary coating powder can be improved, and further, the good dispersing effect can be achieved without grinding and dispersing in the preparation process.

In the repairing process, the pumice powder is rapidly mixed with Ca in the concrete²⁺The reaction produces a C-S-H gel with very strong binding capacity, while the calcium oxide and calcium salt provided by the swelling component react rapidly with water to form Ca (OH)₂And expansive crystal substances such as ettringite (Aft), etc., increase solid phase volume in cracks, improve filling rate of cracks, provide a large amount of carbonate ions or bicarbonate ions by the reinforcing component, and can be used as Ca in concrete and expansive components²⁺Reaction to form high strength CaCO₃Simultaneously excite the membraneThe hydration of the surrounding unhydrated cement further improves the filling rate of the crack and the bonding fusion property of the product and the matrix; the C-S-H gel and the high-strength or expanded crystal are rapidly generated at the same time, finally, a C-S-H gel coated crystal and a tightly interwoven network structure among the crystals are formed in the crack and are well fused with the surrounding concrete, so that the crack can be well repaired, the repair speed is improved, and a higher reinforcing effect can be achieved.

The optimized proportional relation among the components can not only improve the repairing effect, but also achieve higher reinforcing effect.

Preferably, the reinforcing component is at least one of lithium carbonate, potassium carbonate or calcium bicarbonate.

The carbonate or bicarbonate in the reinforcing component may be combined with Ca in the concrete and expansive components²⁺Formation of CaCO₃Compact filling and repairing of crack, and exciting hydration of un-hydrated cement, raising crack filling rate and adhesion and fusion between the product and the matrix, and high strength CaCO₃The production can also improve the overall strength of the repaired matrix.

Preferably, the expansion component is a mixture of light-burned calcium oxide, calcium sulphoaluminate clinker and anhydrite in a mass ratio of 45-55:10-15: 10-20.

Preferably, the specific surface area of the expansion component is 150-250m²/Kg。

The optimized expansion component and the reinforcing component are cooperated, so that the crack resistance of the matrix can be improved, micro cracks or micro damages in the matrix can be repaired continuously in time, and higher repairing and reinforcing effects are achieved.

Preferably, the fineness of the pumice powder is 1250-.

The preferred fineness of the pumice powder is beneficial to coating the reinforcing component and the expanding component with the pumice powder components and improving the reactivity of the pumice powder, so that the pumice powder can be quickly mixed with Ca in concrete²⁺The reaction produces C-S-H gel.

The invention also provides a preparation method of the self-repairing material, which at least comprises the following steps:

step one, weighing all components according to a design ratio, and uniformly mixing the weighed reinforcing component and expansion component to obtain composite powder;

step two, stirring the composite powder and the pumice powder for 2-3h under the condition of 700-900r/min to obtain primary coating repair powder;

dissolving the weighed polyvinyl butyral in ethanol to obtain a slow-release solution;

and step four, uniformly mixing the primary coating repair powder and the slow release solution, and drying to obtain the self-repairing material.

The preparation method of the self-repairing material provided by the invention is simple, can achieve a higher dispersing effect without grinding, is low in cost, and is suitable for industrial production.

Preferably, in the third step, the mass concentration of the polyvinyl butyral in the sustained-release solution is 5-10 wt%.

The concentration of the optimized slow-release solution is beneficial to coating and film forming, and the wetting dispersibility of the once-coated repair powder is improved.

The invention also provides application of the self-repairing material in concrete.

Preferably, the dosage of the self-repairing material is 6-10% of the mass of the cementing material in the concrete.

More preferably, the dosage of the self-repairing material is 8% of the mass of the cementing material in the concrete.

When the self-repairing material provided by the invention is applied to concrete, the doping amount is only 6-10% of the mass of the cementing material, and under the condition of smaller doping amount, the crack can be effectively filled and repaired, the repairing speed is high, the repairing effect is good, the integral strength of the matrix can be improved, and the practical value and the popularization value are higher.

Drawings

FIG. 1 is a diagram of an initial crack with a width of 0.8mm prepared by a three-point bending method after standard curing of a mortar test piece doped with the repairing agent prepared in example 1 of the invention to 28 d;

FIG. 2 is a diagram of a second standard curing of a mortar specimen doped with the repairing agent prepared in example 1 of the present invention to a width crack repair of 7 d;

FIG. 3 is a diagram of a second standard curing of a mortar specimen doped with the repairing agent prepared in example 1 of the present invention to a width crack repair of 14 d;

FIG. 4 is a diagram of a second standard curing of a mortar specimen doped with the repairing agent prepared in example 1 of the present invention to a crack repair width of 28 d;

FIG. 5 is an X-ray diffraction (XRD) pattern of the crack formation from standard secondary cure to 28d for a mortar coupon incorporating the repair agent prepared in example 1 of the present invention;

FIG. 6 is an electron Scanning Electron Microscope (SEM) spectrum of the crack formation of a secondary standard cure to 28d mortar test piece incorporating the repair agent prepared in example 1 of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Example 1

The embodiment of the invention provides a cement-based material microcrack repairing agent for buildings, which comprises the following raw materials in parts by weight:

15 parts of pumice powder, 55 parts of an expansion component, 5 parts of lithium carbonate, 15 parts of potassium carbonate and 10 parts of PVB.

The preparation method of the cement-based material microcrack repairing agent for the building comprises the following steps:

secondly, stirring the composite powder and the pumice powder for 3 hours under the condition of 700r/min to obtain primary coating repair powder;

step three, adding the weighed polyvinyl butyral into ethanol, heating to 60 ℃ for 30min to obtain a slow-release solution;

and fourthly, stirring the primary coating repair powder and the slow release solution for 2.5 hours under the condition of 800r/min, and then drying for 12 hours at 55 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Example 2

10 parts of pumice powder, 40 parts of an expansion component, 6 parts of lithium carbonate, 24 parts of potassium bicarbonate and 20 parts of PVB.

secondly, stirring the composite powder and the pumice powder for 2 hours under the condition of 900r/min to obtain primary coating repair powder;

step three, adding the weighed polyvinyl butyral into ethanol, heating to 65 ℃ for 30min to obtain a slow-release solution;

and fourthly, stirring the primary coating repair powder and the slow release solution for 2 hours under the condition of 850r/min, and then drying for 9 hours at 56 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Example 3

10 parts of pumice powder, 50 parts of an expansion component, 20 parts of potassium carbonate, 10 parts of potassium bicarbonate and 10 parts of PVB.

secondly, stirring the composite powder and the pumice powder for 2 hours under the condition of 800r/min to obtain primary coating repair powder;

step three, adding the weighed polyvinyl butyral into ethanol, heating to 62 ℃, and heating for 30min to obtain a slow-release solution;

and step four, stirring the primary coating repair powder and the slow release solution for 3 hours at the speed of 750r/min, and then drying the mixture for 11 hours at the temperature of 58 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Example 4

13 parts of pumice powder, 60 parts of an expansion component, 5 parts of lithium carbonate, 10 parts of potassium carbonate, 5 parts of calcium bicarbonate and 10 parts of PVB.

secondly, stirring the composite powder and the pumice powder for 2.5 hours under the condition of 750r/min to obtain primary coating repair powder;

step three, adding the weighed polyvinyl butyral into ethanol, heating to 64 ℃ for 30min to obtain a slow-release solution;

and fourthly, stirring the primary coating repair powder and the slow release solution for 2 hours under the condition of 800r/min, and then drying for 10 hours at 60 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Example 5

12 parts of pumice powder, 48 parts of an expansion component, 25 parts of potassium carbonate and 15 parts of PVB.

secondly, stirring the composite powder and the pumice powder for 2 hours under the condition of 750r/min to obtain primary coating repair powder;

and fourthly, stirring the primary coating repair powder and the slow release solution for 3 hours at the speed of 700r/min, and then drying the mixture for 10 hours at the temperature of 60 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Example 6

15 parts of pumice powder, 45 parts of an expansion component, 10 parts of lithium carbonate, 10 parts of calcium bicarbonate and 20 parts of PVB.

secondly, stirring the composite powder and the pumice powder for 3 hours under the condition of 850r/min to obtain primary coating repair powder;

step three, adding the weighed polyvinyl butyral into ethanol, heating to 63 ℃ for 30min to obtain a slow-release solution;

and fourthly, stirring the primary coating repair powder and the slow release solution for 2.5 hours under the condition of 900r/min, and then drying for 9 hours at 55 ℃ to obtain the cement-based material microcrack repairing agent for the building.

In the examples 1-6, the expansion component is an additional composite expanding agent with the content of light-burned calcium oxide of 45-55%, the content of calcium sulfoaluminate clinker of 10-15%, the content of anhydrite of 10-20% (the balance being impurities), and the specific surface area is 150-250m²The specific external expanding agent selected within the above parameters has no obvious influence on the performance of the prepared crack repairing agent.

Comparative example 1

The comparative example provides a cement-based material microcrack repairing agent for buildings, the composition of the raw materials and the preparation method are the same as those in example 6, and the difference is only that the pumice powder is replaced by sepiolite powder.

Comparative example 2

The comparative example provides a cement-based material microcrack repairing agent for construction, which has the same raw material composition and preparation method as example 6, except that PVB is replaced by polyvinylpyrrolidone.

Comparative example 3

The comparative example provides a cement-based material microcrack repairing agent for buildings, which comprises the following raw materials: 45 parts of expansion component, 10 parts of lithium carbonate, 10 parts of calcium bicarbonate and 20 parts of PVB.

step two, adding the weighed polyvinyl butyral into ethanol, heating to 63 ℃ for 30min to obtain a slow-release solution;

and step three, stirring the composite powder and the slow-release solution for 2.5 hours under the condition of 900r/min, and then drying for 9 hours at 55 ℃ to obtain the cement-based material microcrack repairing agent for the building.

Comparative example 4

The comparative example provides a cement-based material microcrack repairing agent for buildings, which comprises the following raw materials: 15 parts of pumice powder, 45 parts of an expansion component, 10 parts of lithium carbonate and 10 parts of calcium bicarbonate.

and secondly, stirring the composite powder and the pumice powder for 3 hours under the condition of 700r/min to obtain the cement-based material microcrack repairing agent for the building.

The fineness of each substance in the reinforcing component and the expanding component in the above examples and comparative examples of the present invention is 100-200 mesh, and the fineness of the pumice powder and the sepiolite powder is 1250-2500 mesh.

In order to demonstrate the effect of the cement-based material microcrack repairing agents for construction prepared in the examples of the present invention and the comparative examples, the repairing agents in examples 1 to 6 and comparative examples 1 to 4 were subjected to performance tests.

The crack width repairing capability is tested by adopting the mortar, and the mixture ratio of the cement, the sand, the water and the self-repairing material in the examples 1-6, the comparative examples 1-2 and the blank control group is kg/m³The self-repairing materials prepared in examples 1-6 and comparative examples 1-2 were added to the mortar, respectively, at an amount of 8% of the cement mass, as shown in table 1.

TABLE 1

Group of	Water (W)	Cement	Sand	Repairing agent
					Blank control group	225	450	1350	0
Example 1	225	450	1350	36
					Example 2	225	450	1350	36
Example 3	225	450	1350	36
					Example 4	225	450	1350	36
Example 5	225	450	1350	36
					Example 6	225	450	1350	36
Comparative example 1	225	450	1350	36
					Comparative example 2	225	450	1350	36
Comparative example 3	225	450	1350	36
					Comparative example 4	225	450	1350	36

After standard curing of the mortar test pieces prepared in examples 1 to 6, comparative examples 1 to 4 and the blank control group is carried out for 28d, cracks are prepared by a three-point bending method, the initial width of the cracks is controlled within the range of 0.2-0.4 mm, when the test pieces are cured again for 7d, 14d and 28d, the repair situation of the cracks is observed and compared with the blank control group, and the repair effect graph of the mortar test piece in example 1 is shown in fig. 1-4.

To facilitate evaluation of fracture repair, fracture width repair capability is now divided into four grades: a-crack filling 0% -25%, B-crack filling 25% -50%, C-crack filling 50% -75%, D-crack filling 75% -100%, and the test results are shown in Table 2.

TABLE 2

As can be seen from the table, the secondary standard maintenance 28d of the prefabricated crack mortar test piece in the examples 1 to 6 can completely fill and repair the crack of the cement-based material, and the filling rate reaches 100%. Comparative examples 1 to 4 have a remarkable repairing effect as compared with examples.

After the mortar sample prepared in example 1 was subjected to secondary standard curing for 28d, the products in the cracks were scraped, collected, oven-dried, and ground, and then subjected to XRD (X-ray diffraction) and SEM (scanning electron microscope) tests, as shown in fig. 1 and 2, respectively. CaCO appears in XRD pattern₃、Ca(OH)₂Characteristic diffraction peaks for Aft and C-S-H (bulge) indicating the formation of a large amount of CaCO in the crack₃、Ca(OH)₂Aft, C-S-H, and the like. As seen in the SEM image, approximately regular crystal substances and flocculent gel bodies are generated at the cracks, and the crystals and the gel bodies are in an interwoven structure and are very dense. The above results demonstrate that the repair process reacts to form high strength CaCO₃、Aft、Ca(OH)₂The components are organically combined with each other, so that the crack is filled and repaired, and the C-S-H gel is compatible with and cemented with substances at two ends of the crack, thereby improving the overall strength of the matrix.

The mechanical properties and the mechanical properties after crack repair are tested by adopting concrete, and the mixture ratio of the cement, the sand, the pebble, the water and the self-repairing material in the examples 1-6, the comparative examples 1-2 and the blank control group is kg/m³The self-repairing materials prepared in examples 1-6 and comparative examples 1-2 were added to the mortar, respectively, at an amount of 8% of the cement mass, as shown in table 3.

TABLE 3

Group of	Water (W)	Cement	Sand	Stone	Repairing agent
						Blank control group	178	445	668	1063	0
Example 1	178	445	668	1063	35.6
						Example 2	178	445	668	1063	35.6
Example 3	178	445	668	1063	35.6
						Example 4	178	445	668	1063	35.6
Example 5	178	445	668	1063	35.6
						Example 6	178	445	668	1063	35.6
Comparative example 1	178	445	668	1063	35.6
						Comparative example 2	178	445	668	1063	35.6
Comparative example 3	178	445	668	1063	35.6
						Comparative example 4	178	445	668	1063	35.6

The prepared concrete test piece is subjected to standard tests on mechanical properties and mechanical properties after crack repair according to the GB/T50081-2002 concrete mechanical property test method, and the test results are shown in Table 4. In table 2, 28+28d indicates that the concrete sample of standard curing 28d is cured again to the secondary compressive strength of 28d after being prefabricated with cracks. The strength recovery rate is [ secondary compressive strength after fracture repair/primary compressive strength before fracture prefabrication ] × 100%.

TABLE 4

Group of	Primary compression strength/MPa	Secondary compression strength/MPa	Percent strength recovery/%)
				Blank control group	47.7	35.8	75
Example 1	45.0	49.1	109
				Practice ofExample 2	44.5	49.8	112
Example 3	43.5	50.5	116
				Example 4	44.1	49.0	111
Example 5	43.8	50.4	115
				Example 6	43.9	50.0	114
Comparative example 1	44.0	42.2	96
				Comparative example 2	43.3	42.0	97
Comparative example 3	43.8	37.2	85
				Comparative example 4	46.6	40.5	87

As can be seen from the above table, the compressive strength recovery rates of the concrete samples of examples 1-6 are both greater than those of the blank group test piece and the comparative group test piece, and it is fully demonstrated that the crack repair speed of the experimental group test piece is faster and the strength growth speed is faster, and further, the incorporation of the cement-based material microcrack repairing agent for buildings provided by the invention can rapidly fill and repair microcracks or microcracks of cement-based materials, and has a better reinforcing effect.

It can be seen from comparative examples 1-2 that significant reductions in strength recovery occurred after replacing pumice powder with sepiolite powder and replacing PVB with polyvinylpyrrolidone. It can be seen from comparative examples 3-4 that the effect of using single-layer coating is inferior to the repair effect of double-layer coating on cracks. The reason is that when only PVB is coated (comparative example 3), the repair composition after later release generates more crystals, but the absence of the gel binds these crystals together, resulting in poor adhesion to the substrate; when only pumice powder is coated in a single layer (comparative example 4), the repairing component is prematurely consumed due to the fact that the repairing component is dissolved in water in the early stage of mixing and hydrating the cement-based material, and effective components playing a repairing role are greatly reduced, so that the repairing effect on later-stage cracks is reduced.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents or improvements made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. The self-repairing material is characterized by comprising an inner layer of reinforcing component and expansion component, and an outer layer of double-layer coating layer, wherein the first layer of the double-layer coating layer is a mineral adsorption component layer, and the second layer of the double-layer coating layer is a slow-release component layer coated outside the mineral adsorption component layer;

wherein the enhancing component is a carbonate or bicarbonate; the expansion component is calcium salt and calcium oxide; the mineral adsorption component layer is made of pumice powder; the material of the slow release component layer is polyvinyl butyral.

2. The self-healing material of claim 1, wherein the reinforcing component, the intumescent component, the pumice powder, and the polyvinyl butyral are present in a mass ratio of 20-30:40-60:10-15: 10-20.

3. The self-healing material of claim 1 or claim 2, wherein the reinforcing component is at least one of lithium carbonate, potassium carbonate, or calcium bicarbonate.

4. The self-healing material of claim 1 or claim 2, wherein the expansion component is a mixture of light-burned calcium oxide, calcium sulfoaluminate clinker and anhydrite in a mass ratio of 45-55:10-15: 10-20.

5. The self-healing material of claim 4, wherein the specific surface area of the intumescent component is 150-²/Kg。

6. The self-repairing material of claim 1 or 2, wherein the fineness of the pumice powder is 1250-2500 meshes.

7. The preparation method of the self-repairing material of any one of claims 1 to 6, characterized by at least comprising the following steps:

8. The method for preparing the self-repairing material of claim 7, wherein in the third step, the mass concentration of the polyvinyl butyral in the sustained-release solution is 5-10 wt%.

9. Use of the self-healing material of any one of claims 1 to 6 in concrete.

10. The use of the self-healing material of claim 9 in concrete, wherein the amount of the self-healing material is 6-10% of the mass of cementitious material in the concrete.