NOX-cleaning paving block

Abstract: An NO x -cleaning paving block comprising a surface layer which contains 100 parts by weight of cement, 10-50 parts by weight of titanium dioxide powder and 100-700 parts by weight of sand and is layered on a concrete-made base layer, preferably with a surface layer thickness of 2-10 mm, wherein the surface layer has a void volume of 10-40% and a water permeability of 0.01 cm/sec. or higher. Further, the surface layer of the block preferably is made uneven, for example, is trapezoid in cross section, corrugated or zigzagged. The NO x -cleaning paving block can be finished so as to have a rough surface by tapping the surface layer containing titanium dioxide with many projections from a surface-roughening tool, for example. The surface layer can further incorporate an adsorbing material mixed at a weight ratio of 5-30 to 100 parts by weight of a total composition of the surface layer except water. One or more of zeolite, Magadiite, petalite and clay is used as the adsorbing material. Ten to fifty percent of the sand can be replaced by glass beads and/or glass powder having a particle size of 1-6 mm.

Description: FIELD OF THE INVENTION The present invention relates to an NO x -cleaning paving block, and more particularly to an NO x -cleaning paving block which has an enhanced NO x -cleaning capability due to an increased efficiency of fixing NO x from the air and an increased pluvial NO x -cleaning efficiency, thus helping to protect the environment, and is provided with a non-slip property, wear resistance and further a very decorative property. DESCRIPTION OF THE RELATED ART Recently, air pollution due to NO x which is contained in exhausting gas from automobiles, particularly diesel motorcars, is going from bad to worse because of the ever increasing number of automobiles and resulting increased traffic congestion, etc. Known NO x concentration-lowering materials include metal oxides, of which titanium dioxide is known to have a strong photocatalytic action. Research into removing NO x through the use of the strong photocatalytic action of titanium dioxide has attracted increasing interest in recent years, and removal of NO x by the external walls of buildings constructed of sheets or panels incorporating titanium oxide is nearing the commercial stage. In JP-A-07331120 is disclosed a coating for removing nitrogen oxide which contains titania fine powder and can be applied to various surfaces including vehicles, road surfaces of parking lots, guard rails, pedestrian crossings, outer walls of buildings and bridges. EP-A-0614682 discloses a method for removing low-concentration pollutants such as nitrogen oxides in the environmental atmosphere and pollutants such as volatile organic chlorine compounds in water. The purifying material used in this method is based mainly on titanium dioxide or a mixture of titanium dioxide and activated charcoal and is molded by means of a synthetic resin to a sheet or panel, which are disposed in the outdoors like on exterior walls of buildings. Therefore, the present inventors have conducted research on sheets and panels which can be used for constructing the external walls of buildings, particularly on the conditions under which they can be used for paving purposes, and have found that a paving block formed of a combination of a dough comprising cement, titanium dioxide powder and sand, and a concrete base is a NO x -cleaning paving block (interlocking block) which efficiently removes NO x without suffering from impairment of the catalytic property of titanium dioxide, and has an excellent non-slip property and wear resistance suitable for paving purposes, and further a very decorative property. The NO x -cleaning paving block has been studied from various aspects in order to increase its performance, with the findings that: 1) the surface layer material must be exposed to a maximum dose of ultraviolet light; and 2) the NO x gas which is evolved during the night must be temporarily adsorbed on an adsorbing material from which the gas must be removed during the day. The present invention has been accomplished on the basis of these findings. OBJECT AND SUMMARY OF THE INVENTION It is therefore a first object of the present invention to provide an NO x -cleaning paving block which efficiently removes NO x , has an excellent non-slip property and durability, and thus helps to protect the environment. It is a second object of the present invention to provide an NO x -cleaning paving block which has an increased NO x -removing efficiency and an enhanced pluvial NO x -cleaning effect, and thus helps to protect the environment. It is a third object of the present invention to provide an NO x -cleaning paving block which has an increased NO x -cleaning capability and non-slip property, is capable of cleaning NO x even during the night, and is very decorative. The NO x -cleaning paving block according to the present invention is characterized by comprising a surface layer which contains titanium dioxide and is layered on a concrete-made base layer, and is further characterized by comprising a surface layer which contains 100 parts by weight of cement, 5-50 parts by weight of titanium oxide powder and 100-700 parts by weight of sand and is layered on a concrete-made base layer according to a preferred embodiment thereof, wherein the concrete-made base layer provides the NO x -cleaning paving block according to the present invention with durability. The NO x -cleaning paving block according to the present invention changes NO x to nitrate ions by the action of titanium oxide. Since the nitrate ions are stabilized by being neutralized with the alkaline components of the concrete, the environment is satisfactorily protected. Since the slippery property of the titanium dioxide powder is suppressed by the addition of sand to the surface layer, the surface layer has an excellent anti-slip property. The surface layer has an excellent anti-slip property and durability due to the incorporation of cement and sand. As a result, the NO x -cleaning paving block according to the present invention can be satisfactorily used for paving purposes. The NO x -cleaning paving block according to a preferred embodiment of the present invention is characterized by the asperities on the surface layer containing a titanium oxide, specifically titanium dioxide, which are formed with taps. This characteristic aspect of the preferred embodiment of the present invention enhances the NO x -cleaning capability, anti-slip property and decorative property. In addition, since the NO x -cleaning paving block according to another preferred embodiment of the present invention contains an adsorbing material in the surface layer, not only the NO x adsorbed thereon during the day is removed by decomposition by sunlight (specifically ultraviolet light) during the day, but also NO x is adsorbed on the adsorbing material even during the night and decomposed by ultraviolet light during the day, thus removing the NO x . The photocatalytic efficiency can be increased by increasing the available surface area by setting the void volume of the surface layer of the NO x -cleaning paving block to 10-40% or higher and the water permeability thereof to 0.01 cm/sec. or higher. Further, when the NO x -cleaning paving block according to the present invention has an increased water permeability, the pluvial cleaning effect is also increased. Accordingly, there is provided a highly economical, NO x -cleaning paving block. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view illustrative of an NO x -cleaning paving block according to the present invention; Fig. 2 is a cross sectional view illustrative of a die plate for forming an NO x -cleaning block with trapezoid cross-section asperities formed thereon; Fig. 3 is a cross sectional view illustrative of a die plate for forming an NO x -cleaning block with corrugated asperities formed thereon; Fig. 4 is a cross sectional view illustrative of a die plate for forming an NO x -cleaning block with zigzag asperities formed thereon; Fig. 5 is a cross sectional view illustrative of an NO x -cleaning paving block with trapezoid cross-section formed asperities thereon; Fig. 6 is a cross sectional view illustrative of an NO x -cleaning paving block with corrugated asperities formed thereon; Fig. 7 is a cross sectional view illustrative of an NO x -cleaning paving block with zigzag asperities formed thereon; Fig. 8 is a perspective view illustrative of an NO x -cleaning paving block according to a preferred embodiment of the present invention; Fig. 9 is a perspective view illustrative of an NO x -cleaning paving block according to the present invention, but prior to roughening of the surface; Fig. 10 includes views illustrative of a surface-roughening tool which can be used according to the present invention, wherein Fig. 10a is a perspective view illustrative of projections from the surface of the surface-roughening tool, Fig. 10b is a perspective view illustrative of the shape of one of the projections, and Fig. 10c is a plan view thereof; Fig. 11 includes views illustrative of another surface-roughening tool which can be used according to the present invention, wherein Fig. 11a is a perspective view illustrative of other projections from the surface of the surface-roughening tool, Fig. 11b is a perspective view illustrative of the shape of one of the projections, and Fig. 11c is a plan view thereof; and Fig. 12 includes views illustrative of yet another surface-roughening tool which can be used according to the present invention, wherein Fig. 12a is a perspective view illustrative of yet other projections from the surface of the surface-roughening tool, and Fig. 10b is a perspective view illustrative of the shape of one of the projections. DETAILED DESCRIPTION OF THE INVENTION Throughout the specification and the claims, "for paving purposes" includes "for paving sideways or roadways", and "paving block" includes "block which can be laid on sideways or roadways". On the other hand, "NO x -cleaning" means "removing NO x by decomposition by the catalytic action of titanium dioxide under irradiation with light such as sunlight". Embodiments of the NO x -cleaning paving block according to the present invention include the preferred embodiments as defined in the claims, without being limited thereto. The present invention will now be described in more detail. The NO x -cleaning paving block according to the present invention is characterized by comprising a surface layer which contains titanium dioxide, and is further characterized by comprising a surface layer which contains 100 parts by weight of cement, 5-50 parts by weight of titanium oxide powder and 100-700 parts by weight of sand according to a preferred embodiment thereof, wherein the titanium oxide changes NO x to nitrate ions. Since the nitrate ions are stabilized by being neutralized with the alkaline components of the concrete, the environment is satisfactorily protected. Since the slippery property of the titanium dioxide powder is suppressed by the addition of sand to the surface layer, the surface layer has an excellent anti-slip property. The surface layer has an excellent anti-slip property and durability due to the incorporation of cement and sand. As a result, the NO x -cleaning paving block according to the present invention can be satisfactorily used for paving purposes. The NO x -cleaning paving block according to a preferred embodiment of the present invention is characterized by the asperities on the surface layer containing a titanium oxide, specifically titanium dioxide, which are formed with taps. This characteristic aspect of the preferred embodiment of the present invention enhances the NO x -cleaning capability, anti-slip property and decorative property. In addition, since the NO x -cleaning paving block according to another preferred embodiment of the present invention contains an adsorbing material in the surface layer, not only the NO x adsorbed thereon during the day is removed by decomposition by sunlight (specifically ultraviolet light) during the day, but also NO x is adsorbed on the adsorbing material even during the night and decomposed by ultraviolet light during the day, thus removing the NO x . The photocatalytic efficiency can be increased by increasing the available surface area by setting the void volume of the surface layer of the NO x -cleaning paving block to 10-40% and the water permeability thereof to 0.01 cm/sec. or higher. Further, when the NO x -cleaning paving block according to the present invention has a high water permeability, the pluvial cleaning effect is also increased. Accordingly, there is provided a highly economical, NO x -cleaning paving block. According to the present invention, the thickness of the surface layer is 2-15 mm, and preferably 2-10 mm. When the present invention is attempted to be carried out, if the thickness of the surface layer exceeds 15 mm, then sunlight is hard to penetrate into the interior of the surface layer with the result of decreasing the action on the titanium dioxide, and thus it is useless to provide the titanium dioxide layer with a larger thickness. On the other hand, the NO x -cleaning efficiency is reduced, and further the durability is impaired when the thickness is smaller than 2 mm. When glass beads or silica sand having a high light transmission property is used as the sand which composes the surface layer according to the present invention, a sufficient amount of light penetrates to the depths to increase the NO x -cleaning efficiency. When it is desired to form a surface layer having a large void volume, sand of extremely uniform particle size is used to form a film of a mixture of cement and titanium oxide on the surface of the sand. Particles of the sand which are coated with the mixture of cement and titanium oxide are preferably combined with each other through the function of the mixture. The configuration results in an increase in the available surface area which contributes to a higher photocatalytic efficiency and thus increases the NO x -cleaning efficiency, and also results in an increase in the water permeability which enhances the cleaning effect. The particle size of the sand which can be used according to the present invention preferably ranges from 1.2 to 5 mm. The presence of a mixture of cement and titanium oxide powder as the binding material in an excess proportion relative to the sand results in a lower void volume. Accordingly, the effective void volume is 10-40%, though preferably is 15-30%. When the void volume is smaller than 10%, a sufficient available surface area cannot be provided, and thus the NO x -cleaning efficiency is reduced. On the other hand, when the void volume exceeds 40%, though the NO x -cleaning efficiency is increased, the strength is reduced, and thus durability required as a paving block is not ensured. In order to set the void volume within a range of 10-40%, the ratio between the sand and the binding material is preferably such that the binding material is present in a proportion of 18-100% by weight of the sand. Furthermore, the NO x -cleaning efficiency can be increased by providing a photoreflective layer beneath the surface layer. It is desirable to compose the photoreflective layer with a mixture of cement and white particles or a white pigment, such as titanium dioxide. Furthermore, the surface of the surface layer can be made uneven to increase the light absorbability and the area of contact with the air, and further the anti-slip property. The unevenness can be established by providing a form for forming the blocks with asperities or by grinding. The grinding can be performed with a diamond blade or another tool, or by any appropriate processing such as sand blasting. The asperities can be zigzagged, corrugated or trapezoid in cross section. The asperities can be formed by tapping the surface layer to roughen the surface. As a result, the surface layer has a very rough surface with large asperities. The surface layer can be finished so as to have a rough surface by tapping it with many projections from a surface-roughening tool, which is described later, on the mortar-made surface layer, for example; the surface of the surface layer can be made uneven by other methods as well, thereby increasing the light absorbability and the area of contact with the air, and further improving the anti-slip property. In addition, the surface of the surface layer can be decorated as desired provided that the effects described above are not impaired. In cases where the surface-roughening processing is successively carried out, the blocks are placed on carriages and tapped from above with a vertically reciprocating (over a span of 3 cm for example), surface-roughening tool having downward-pointing projections a required number of times (1-10) while horizontally reciprocating the carriages. The surface layer which has an uneven surface provided by surface-roughening processing which includes tapping on the surface layer in this way, is highly aesthetically pleasing and decorative. According to the present invention, the unevenness of the surface of the surface layer preferably measures 2-7 mm in depth, with a preferable crest spacing of 4-10 mm. The mortar which can be used according to the present invention to compose the surface layer preferably consists of 100 parts by weight of cement, 5-50 parts by weight of titanium dioxide powder and 100-700 parts by weight of sand, and the NO x removed by the action of titanium dioxide is converted into nitrate ions which are then stabilized by neutralization by the alkaline components of the concrete, thus helping to protect the environment. In addition, the incorporation of sand minimizes the slipperiness of the titanium dioxide powder and thus provides the block with anti-slip property. Furthermore, since cement and aggregate are contained, the anti-slip property and the durability are at adequate levels for paving purposes. The proportion of titanium dioxide which can be used according to the present invention to compose the surface layer, though depending on the types, particle sizes, etc. of the used titanium dioxide, is 5-50 parts by weight and preferably 10-50 parts by weight in terms of titanium dioxide powder, per 100 parts by weight of cement. More preferably, the titanium dioxide powder comprises 20-50 parts by weight. In cases where the proportion of titanium dioxide powder in the surface layer according to the present invention is lower than 5 parts by weight per 100 parts by weight of cement, the NO x -cleaning efficiency of the surface layer is impaired, whereas not only the anti-slip property is reduced, but also the wear resistance is impaired when the proportion exceeds 50 parts by weight. On the other hand, the proportion of the sand is 100-700 parts by weight, and preferably 100-400 parts by weight per 100 parts of cement. If the proportion of the sand is lower than 100 parts by weight per 100 parts by weight of cement, the surface layer has a lower anti-slip property and wear resistance, whereas when the proportion of the sand is higher than 700 parts by weight, the NO x -cleaning efficiency is impaired, since the amount of titanium dioxide powder relatively reduces. The surface layer according to the present invention preferably has the composition consisting of 10-50 parts by weight, more preferably 20-50 parts by weight, of titanium dioxide powder and 100-400 parts by weight of sand per 100 parts by weight of cement. The NO x -cleaning paving block which is used according to the present invention can be prepared by any appropriate method, including a method by which a concrete dough is charged into a form and levelled, and a surface layer-forming dough is then placed on the concrete dough for forming a laminate ( cf . Japanese Unexamined Patent Application Disclosure HEI 3-169901, lines 14-19 in the left column on page 1). In addition to the method described above, the concrete-made base layer and the surface layer can be separately formed and set, and then combined, though the former method is preferable. The block base layer which is used according to the present invention is a concrete-made base layer. The concrete-made base layer is prepared with a dough of cement and aggregate. The base layer-forming aggregate can be any of the materials which are conventionally used in the technical field concerned, including sand, natural stones, gravel, crushed stones, etc. The cement does not need to be restricted to any particular type, and includes normal Portland cement, high early strength Portland cement, moderate heat Portland cement, super high early strength portland cement, sulfate-resistant Portland cement, white Portland cement, Portland blast furnace cement, fly ash cement, silica cement, alumina cement, superfast-setting cement, etc. The surface layer which is layered on the block base layer is preferably made of mortar comprising cement, titanium dioxide powder and aggregate, wherein the cement which is used in the mortar can be any of Portland cement, high early strength cement, white cement, alumina cement and the like. White cement is preferable for its enhanced photoreflectivity. The titanium dioxide can be of either rutile or anatase crystal structure, and preferably is anatase-type titanium dioxide for its higher activity. In addition, the sand used in the surface layer can be either natural sand or artificial sand. Further, the sand can be replaced by one or more selected from a group of granite, quartzite, lightweight aggregate, glass grains and silica sand. The particle size of these materials is desired to be from 1.2 to 5 mm. However, in order to guide beams of ultraviolet light to the depths of the surface layer of the block to increase the NO x -cleaning and adsorbing efficiency according to the present invention, it is desirable to replace part of the sand by highly light-transmittable aggregate (hereunder referred to as "light-transmittable aggregate") which can be glass beads, glass powder, silica sand or the like, preferably with a particle size of 1-6 mm in diameter; when the light-transmittable aggregate used has a larger particle size, the strength decreases due to the increased proportion of the cement-free portions, whereas the light transmittability is impaired when the light-transmittable aggregate used has a smaller particle size. In cases where part of the sand is replaced by light-transmittable aggregate such as glass beads, glass powder, silica sand or the like, the proportion relative to the total amount of the aggregate preferably is 10-50% by weight, more preferably 15-50% by weight, and even more preferably 20-50% by weight. When the proportion of the light-transmittable aggregate is lower than 10% by weight, the light-transmitting effect is not produced, whereas the proportion of the sand is reduced and eventually the block has a lower strength if the proportions is higher than 50% by weight. The light-transmittable aggregate available for use has a wide range of particle size or grain size, which is appropriately selected depending on the use. Also, according to the present invention, the surface layer can contain an adsorbing material which can be any conventional catalytic carrier. A particularly preferable material is zeolite, a conventional carrier which has a number of uses and has been confirmed to have a high adsorbing effect. The zeolite can be either naturally occurring zeolite or artificial zeolite. Arduinite, a zeolite mineral which is represented by the chemical formula: (Ca, K 2 , Na 2 ) (AlSi 5 O 12 )·7H 2 O, contains water of crystallization. The water of crystallization is present as molecules of water; the structure is not destroyed even after having been dehydrated, and the spaces from which the molecules of water have been removed become voids. It has been reported that the voids adsorb gas, for which the mineral has an adsorbing action. Clay minerals, which have water of crystallization as well, are effective as the adsorbing material. The clay available for use can be any, and includes Nenme clay and Kibushi clay, for example. Magadiite (Na 2 Si 14 O 29 ·nH 2 O) is also effective as the adsorbing material. Magadiite is a clay mineral-resembling, hydrated sodium silicate of a laminar crystal structure which is, however, free from aluminum. In addition to the above-mentioned concrete examples of the adsorbing material, petalite can used as the adsorbing material according to the present invention as well. Petalite is a white or gray mineral which resembles quartz in appearance, and mineralogically is a silicate mineral (Li 2 OAl 2 O 3 ·8SiO 2 ) which belongs to the feldspar family. Petalite contains a smaller proportion of impurities and is found in a larger quantity than other lithium minerals. Petalite is a conventional ceramic raw material used for many uses, and is further capable of adsorbing NO and HO 2 gas. When compared with other adsorbing materials, petalite is characterized by contributing to the increased strength, particularly bending strength of products containing it. Although the details of the mechanism have not yet known, it is sure that petalite not only serves only as an adsorbing material, but also as aggregate, and thus is suitable as a paving material. The adsorbing material, which can be selected from a variety of candidates, can be incorporated in the surface layer in an appropriate proportion which allows NO and NO 2 gas to be adsorbed; the proportion can be determined as desired, by a simple test, with consideration given to the cost, the quantitative correlation between the cement and titanium dioxide, the strength, the N x -cleaning efficiency, etc. The adsorbing material is mixed into the surface layer in a proportion of 5-30 parts by weight, preferably approximately 7-25 parts by weight per 100 parts by weight of the surface layer except water. When the proportion of the adsorbing material is lower than 5 parts by weight, the adsorbing effect is insufficient, and the adsorbing performance is imperfect during the night free of sunlight. On the other hand, in proportions which exceed 30 parts by weight, the cost increases, and the quantities of cement and titanium dioxide relatively decrease, thus lowering the strength and the NO x -cleaning efficiency. The present invention also allows a photoreflective layer to be provided beneath the surface layer to increase the NO x -cleaning efficiency. The photoreflective layer is desirable to be composed of a mixture of cement and white particles or a white pigment, such as titanium dioxide. A surface-roughening tool which can be used according to the present invention will now be described in detail with reference to Fig. 10 through Fig. 12, without being restricted thereto; all the surface-roughening tools which are similar to and have the same effects of the illustrated one are of course within the scope of the present invention. In the drawings, like numerals indicate like structural members. In Fig. 10(a), projections 13 are placed on the surface 12 of a surface-roughening plate 11 in order or in a random fashion. As illustrated in Fig. 10(b) and Fig. 10(c), the tip of each projection 13 is shaped as a pointed end 14 with which the surface of each NO x -cleaning block is tapped to cut into the surface in order to form predetermined hollows, thereby roughening the surface. The hollows are desirable to be 2 mm or more in depth when the anti-slip property and the NO x -cleaning capability are considered. In Fig. 11(a), projections 15 are placed on the surface 12 of the surface-roughening plate 11 in order or in a random fashion. As illustrated in Fig. 11(b) and Fig. 11(c), the tip of each projection 15 is shaped as a pointed end 16 such as tip of a Phillips screwdriver, with which the surface of each NO x -cleaning block is tapped to cut into the surface in order to form predetermined hollows, thereby roughening the surface. In Fig. 12(a), projections 17 are placed on the surface 12 of a surface-roughening plate 15 in order in the direction of width. As illustrated in Fig. 12(b), the tip of each projection 17 is shaped as a pointed end 18 with which the surface of each NO x -cleaning block is tapped to cut into the surface in order to form predetermined hollows, thereby roughening the surface. When the NO x -cleaning block according to the present invention is prepared, the surface roughening treatment is preferably performed after a curing period of 2-7 days, more preferably 3-5 days, and even more preferably 3 days following the formation of the block. In carrying out the present invention, if the surface-roughening step is conducted too early after the curing period, the degree of surface roughness is insufficient, and this results in not only failure to increase the anti-slip property and the surface area, but also in a larger number of times the surface-roughening tool must be cleaned to remove the visible grime thereon, thus impairing the operational efficiency. On the other hand, when the surface-roughening step is conducted too late after the curing period, the cement in the surface layer excessively sets, and this makes it difficult to form satisfactory hollows in the surface-roughening step. According to the present invention, the number of times the tapping is conducted with a surface-roughening tool is not particularly restricted provided that the hollows are properly formed, and is appropriately determined depending on the degree of cure of the surface of the NO x -cleaning block. When the block is prepared using a concrete-made base layer according to the present invention, the block is provided with durability, and the nitrate ions resulting from the NO x are neutralized by the alkaline components of the concrete. The use of glass or silica glass as the sand provides the block with a highly light-transmittable property in addition to an anti-slip property and wear resistance. It is also possible according to the present invention to allow ultraviolet light to penetrate into the depths of the surface layer by replacing part of the sand in the surface layer by light-transmittable aggregate. In addition, the unevenness of the surface allows light to strike thereon from all directions. Furthermore, the surface roughness provided by tapping the surface results in an increased anti-slip property, and further allows light to strike on the surface from all directions, thereby enhancing the NO x -cleaning capability and improving the aesthetically pleasing and decorative property. Further, the surface roughness of the NO x -cleaning block is efficiently accomplished only by tapping the surface with simple means such a surface-roughening tool with many projections, as described above. The adsorbing material which is contained in the surface layer of the NO x -cleaning block adsorbs and removes NO x produced during the night. The NO x produced during the night is decomposed and removed by exposure to ultraviolet light of sunlight during the day. In addition, the void volume of the surface layer set to 10-40% and the water permeability coefficient set to 0.01 cm/sec. or higher increase the available surface area and the photocatalytic efficiency; the increase in the water permeability coefficient results in a higher degree of cleaning effectiveness. EXAMPLES The present invention will now be described in more detail with reference to the Examples, without being restricted thereto. Example 1 In order to prepare a dough for forming a concrete-made base layer, 100 parts by weight of Portland cement, 31 parts by weight of water, 190 parts by weight of crushed stones and 240 parts by weight of sand were combined and kneaded. Separately, in order to prepare a dough for forming a surface layer, sand, 20 parts by weight of titanium dioxide and 80 parts by weight of Portland cement were combined and kneaded. The sand was used in proportions relative to the cement, listed in Table 1, to prepare Samples 1-4. Samples 1-4 were used to prepare four types of paving blocks as follows. The dough for forming a concrete-made base layer was charged into a 10 x 20-cm form and formed into figures while vibrating, after which the dough for forming a surface layer was placed on the base layer and covered with a die plate, and then subjected to vibration under pressure (pressure: 0.25 kg/cm 2 , number of vibrations: 3,140 r.p.m., amplitude of vibration: 1.4 mm; and vibration time under pressure: 3 sec.), followed by curing, to prepare Test specimens 1-4 which were 20 cm long, 10 cm wide and 8 cm thick, corresponding to Samples 1-4, respectively. Fig. 1 is a perspective view illustrative of one of the blocks prepared as described above. The block 1 has a base layer 2 on which a 7-mm-thick surface layer 3 is layered. The results obtained are listed in Table 1. The anti-slip properties of the paving blocks were measured according to ASTM E303. The method defined there is designed so as to rub a rubber slider mounted on one end of a pendulum arm against the surface to be tested and to measure the energy lost at that time; the measurement is required to be equal to or larger than 65 BPM. The wear resistance test (labeling test) conducted was designed so that falling, rotating (200 r.p.m.) wheels (outer diameter: 205 mm, and width: 100 mm) half of which each had twelve chains with ten links came into contact with a horizontally reciprocating (66 times per min.) test specimen to cut into and wear the surface, and the amount of wear (unit: g per cm 2 ) was measured. Test specimen Cement : sand ratio (wt. ratio) Slip resistance (BPM) Labelling test (g/cm 2 ) Remarks 1 1 : 3 85 1.5 Wet state (slip occurs unless BPM is 65 or more) 2 1 : 2 78 1.5 3 1 : 1 72 2.0 4 1 : 0 56 4.0 Example 2 Test specimen 5 having a 1-to-3 cement-to-sand ratio was prepared in the same manner as in Example 1, except for using, as the die plate with an uneven surface layer, a die plate 4 having a surface 5 trapezoid in cross section which was illustrated in the perspective view of Fig. 2, instead of plate used in Example 1. The block represented by Test specimen 5 is illustrated in the cross sectional view of Fig. 5. Test specimen 5 obtained had both a high anti-slip property and light transmittability, and efficiently removed NO x . In addition, Test specimens 6 and 7 both having the same 1-to-3 cement-to-sand ratio as Test specimen 1 were prepared by using a die plate 6 and a die plate 8 which had a corrugated surface 7 illustrated in the perspective view of Fig. 3 and a zigzagged surface 9 illustrated in the perspective view of Fig. 4, respectively, instead of the plate 4 having the surface 5 trapezoid in cross section which was illustrated in Fig. 2. The obtained blocks are illustrated in the cross sectional views of Fig. 6 and Fig. 7. All the blocks prepared had the excellent effects of the present invention. Example 3 Test specimen 8 having a 1-to-3 cement-to-sand ratio was prepared in the same manner as in Example 1, except for using 100 parts by weight of glass grains instead of the sand to prepare the dough for forming a surface layer. Test specimen 8 obtained had both a high anti-slip property and light transmittability, and efficiently removed NO x . Example 4 Test specimen 9 having a 1-to-1 cement-to-sand ratio was prepared in the same manner as in Example 1, except for using 100 parts by weight of silica sand instead of the sand to prepare the dough for forming a surface layer. Test specimen 9 obtained had both a high anti-slip property and light transmittability, and efficiently removed NO x . Example 5 Test specimen 10 was prepared in the same manner as in Example 4, except for roughening the surface of the surface layer by sand blasting. Test specimen 10 obtained had both a high anti-slip property and light transmittability, and efficiently removed NO x . Example 6 In order to prepare a dough for forming a concrete-made base layer, 100 parts by weight of Portland cement, 25 parts by weight of water and 300 parts by weight of crushed stones were combined and kneaded. Separately, in order to prepare a dough for forming a surface layer, 600 parts by weight of crushed sand, an artificial sand, 30 parts by weight of titanium dioxide, 100 parts by weight of Portland cement and 25 parts by weight of water were combined and kneaded. The dough for forming a concrete-made base layer was charged into a 10 x 20-cm form and formed into a figure while vibrating, after which the dough for forming a surface layer was placed on the base layer and covered with a die plate, and then subjected to vibration under pressure in the same manner as in Example 1. A paving block was obtained after curing. The resulting block, comprising the 7-cm-thick base layer and the 1-cm-thick surface layer, had a base-layer void volume of 26%, a surface-layer void volume of 20%, and a water permeability of 0.10 cm/sec.; the block further had both a high anti-slip property exemplified by 87 BPM and a high cleaning effect, and efficiently removed NO x . Examples 7-10 In order to prepare a dough for forming a concrete-made base layer, 500 parts by weight of Portland cement, 130 parts by weight of water and 180 parts by weight of aggregate (#4 crushed stones) were combined and kneaded. Separately, in order to prepare a dough for forming a surface layer, four formulations having the compositions listed in Table 2 were used to prepare the test specimens designated as Examples 7-10. Samples 7-10 were used to prepare four types of paving blocks as described below. The dough for forming a concrete-made base layer was charged into 10 x 20-cm forms and formed into figures while vibrating, after which the four types of dough for forming surface layers were placed on the respective base layers and covered with die plates, and then subjected to vibration under pressure (pressure: 0.25 kg/cm 2 , number of vibrations: 3,140 r.p.m., amplitude of vibration: 1.4 mm; and vibration time under pressure: 3 sec.) in the same manner as in Example 1, followed by curing and surface-roughening processing. The surface-roughening processing was conducted with a surface-roughening tool (shown in Table 3) with many nail-like projections (made of steel,  4 mm) spaced every 2.5 cm which was moved vertically over a span of 3 cm (6 times/min.) and horizontally over a span of 5 cm (twice/min.) while horizontally moving the blocks at a speed of up to 3 m/min. The resulting blocks 20 cm long, 10 cm wide and 7 cm thick were used as the test specimens. Comparative Examples were prepared in the same manner as the above, but without conducting any surface-roughening processing after curing. One of the resulting blocks is illustrated in the perspective view of Fig. 9. The illustrated block comprises a base layer 2 on which a surface layer 3 having a thickness of 7 mm is layered. The results obtained are listed in Table 3. Here, the anti-slip properties of the paving blocks were measured according to ASTM E303. The method defined there is designed so as to rub a rubber slider mounted on one end of a pendulum arm against the surface to be tested and to measure the energy lost at that time; the measurement is required to be equal to or larger than 65 BPM. On the other hand, the wear resistance test (labeling test) conducted was designed so that falling, rotating (200 r.p.m.) wheels (outer diameter: 205 mm, and width: 100 mm) half of which each had twelve chains with ten links came into contact with a horizontally reciprocating (66 times per min.) test specimen to cut into and wear the surface, and the amount of wear (unit: g per cm 2 ) was measured. The NO x -cleaning capability was expressed as NO x -removal coefficient, a ratio of the NO x concentration at the entrance and that at the exit of a vessel which held 20 cm 2 of a sample under ultraviolet irradiation, which was measured while streaming air having a NO x content of 1 ppm therethrough. Type Formulation Surface-roughing tool Slip resistance (BPM) NO x removal coefficient (%) Wet state Dry state Comp. Ex. 1 1 Not used 66 74 65 Example 7 1 Fig. 10 71 80 68 Comp. Ex. 2 2 Not used 68 77 79 Example 8 2 Fig. 10 72 87 84 Comp. Ex. 3 3 Not used 70 84 76 Example 9-1 3 Fig. 10 72 91 79 Example 9-2 3 Fig. 11 73 91 80 Comp. Ex. 4 4 Not used 68 80 84 Example 10-1 4 Fig. 10 70 88 88 Example 10-2 4 Fig. 12 73 86 88 As is apparent from Table 3, Example 7 had a high anti-slip property due to the incorporation of Portland cement. In addition, the NO x -cleaning capability was enhanced. Example 8 also had a high anti-slip property and further an even more excellent NO x -cleaning capability due to the incorporation of white cement. This is believed as the result of the incorporation of white cement and the uneven pattern resulting from tapping. It is apparent that both Example 9 and Example 10, prepared by using the surface-roughening tools illustrated in Fig. 10, Fig. 11 and Fig. 12, had satisfactory NO x -cleaning capabilities. Example 11 The procedures described in Example 9 were followed, except that the surface-roughening processing was conducted after a curing period of 2 days or 7 days. When the surface-roughening processing was conducted after a curing period of 2 days, the required number of taps was decreased, but the product must be allowed to stand for a while after the surface-roughening processing. The surface-roughening tool used had visible grime. In contrast, when the surface-roughening processing was conducted after a curing period of 7 days, the surface set somewhat excessively, and this caused the required number of taps to increase, with the result that the surface-roughening processing was conducted to a satisfactory degree. The working life of the surface-roughening tool used was, however, slightly shortened. In either case, the effects of the present invention were produced. Example 12 The procedures described in Example 7 were followed, except that the surface layer was prepared using white cement instead of Portland cement, and additionally contained 160 parts by weight of zeolite, and the surface-roughening processing was carried out as described in Example 7. The addition of the adsorbing material allowed the NO x adsorbed during the night to be cleaned off during the day, thus enhancing the NO x -cleaning capability. In addition, the surface-roughening processing contributed to a higher anti-slip property. Examples 13-22 In order to prepare a block base layer, 100 parts by weight of Portland cement, 195 parts by weight of crushed stones, 240 parts by weight of sand and 31 parts by weight of water were combined and kneaded to prepare dough, and the dough was charged into a 10 x 20-cm form which was then vibrated to form a concrete-made base layer on which one of 12 types of dough formulations for forming surface layers was placed to form each of the test specimens designated as Examples 13-22 and Comparative Examples 5-6 in Table 4. The NO removal coefficients of the test specimens were measured after curing. The test was conducted by placing a test specimen in a gastight vessel through the inlet of which a stream of a simulation gas containing 1 ppm of NO gas supplied. The concentration of NO in the gas leaving through the exit was measured to determine the NO removal coefficient. The test was conduced over 24 hours, and the average of the removal coefficients measured every ten minutes was recorded as the test result. Test 1 was conducted with three 10-watt ultraviolet lamp remaining on continuously over the 24 hours. Test 2 was conducted while switching on or off the ultraviolet lamps every two hours. Accordingly, the lamps were glowing for 12 hours throughout the 24-hour test period. Here, of the Examples and Comparative Examples, the test specimens designated as Examples 13, 14, 16 and 17 and Comparative Example 5 were subjected to a strength test called a three-point bending test. The test results are reported in Table 5. NO removal coefficient (%) Bending strength Test 1 Test 2 (Kgf / cm 2 ) Example 13 81 53 65 Example 14 81 54 73 Example 15 84 58 - Example 16 82 55 63 Example 17 83 56 64 Example 18 82 55 - Example 19 81 56 - Example 20 84 55 - Example 21 77 43 - Example 22 78 43 - Comp. Ex. 5 77 42 63 Comp. Ex. 6 72 38 - As is apparent from Table 5, Examples 13-22 incorporating the adsorbing materials are capable of cleaning off more NO x than the Comparative Examples. Particularly, white cement contributed to a higher NO x removal coefficient than Portland cement. Even in Test 2 in which the lamps were switched on and off, more NO x was removed; this effect was particularly significant when glass powder was added. The high-performance NO x -cleaning blocks described above had substantially the same bending strength as well.

Claims: A paving block comprising a surface layer on a concrete-made base layer, characterized in that the surface layer contains 100 parts by weight of cement, 5-50 parts by weight of NO x -cleaning titanium dioxide powder and 100-700 parts by weight of sand. An NO x -cleaning paving block as claimed in Claim 1, wherein the surface layer contains an adsorbing material. An NO x -cleaning paving block as claimed in Claim 2, wherein the adsorbing material is contained in the surface layer at a weight ratio of 5-30 to 100 parts by weight of a total composition of the surface layer except water. An NO x -cleaning paving block as claimed in Claims 2 or 3, wherein one or more of zeolite, Magadiite and clay is used as the adsorbing material. An NO x -cleaning paving block as claimed in Claims 2 or 3, wherein petalite is used as the adsorbing material. An NO x -cleaning paving block as claimed in any of Claims 1 to 5, wherein the thickness of the surface layer is 2-15 mm. An NO x -cleaning paving block as claimed in any of Claims 1 to 6, wherein at least part of the sand comprises glass grains or silica sand. An NO x -cleaning paving block as claimed in any of Claims 1 to 7, wherein 10-50% of the sand has been replaced by glass beads and/or glass powder having a particle size of 1-6 mm. An NO x -cleaning paving block as claimed in any of Claims 1 to 8, wherein the surface layer has a void volume of 10-40% and a water permeability of 0.01 cm/sec. or higher. An NO x -cleaning paving block as claimed in Claim 1, wherein a titanium dioxide-containing mortar-made surface layer is layered on the block base layer and then cured, and the surface of the mortar-made surface layer is then roughened with taps of many projections from a surface-roughening tool. An NO x -cleaning paving block as claimed in Claim 10, wherein the surface layer additionally contains an adsorbing material. An NO x -cleaning paving block as claimed in Claim 11, wherein the adsorbing material is contained in the surface layer at a weight ratio of 5-30 to 100 parts by weight of a total composition of the surface layer except water. An NO x -cleaning paving block as claimed in Claims 11 or 12, wherein one or more of zeolite, Magadiite and clay is used as the adsorbing material. An NO x -cleaning paving block as claimed in Claims 11 or 12, wherein petalite is used as the adsorbing material. An NO x -cleaning paving block as claimed in any of Claims 10 to 14, wherein the thickness of the surface layer is 2-15 mm. An NO x -cleaning paving block as claimed in any of Claims 10 to 15, wherein the surface layer has a void volume of 10-40% and a water permeability of 0.01 cm/sec. or higher.

Inventors: MURATA KOZO, MURATA YOSHIHIKO, OBATA HIROSHI, TAWARA HIDEO, MURATA, KOZO, MURATA, YOSHIHIKO, OBATA, HIROSHI, TAWARA, HIDEO, Murata, Kozo, c/o Cement Research Institute, Murata, Yoshihiko, c/o Cement Research Institute, Obata, Hiroshi, c/o Cement Research Institute, Tawara, Hideo, c/o Cement Research Institute
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1997
Main classification: B01D53
IPC codes: B01D53, C04B41, E01C1, B01J35, C04B28, B01J21