A METHOD OF MAKING CEMENT PRODUCTS HAVING HIGH MECHANICAL STRENGTH

Abstract: A method of making a cement product comprises adding water, and if required, reinforcing material, into a cement and aggregate mixture containing 0 to 5% by weight of a calcium compound based on the amount of CaO relative to the weight of the cement, and kneading the whole carefully, moulding the keaded mixture under pressure into a desired shape, hydrating the moulded product to form a preliminarily hardened product, firing the preliminarily hardened product at a temperature of 650 to 950°C, and hydrating the fired product. The high temperature firing process advantageously permits the smoking treatment of the product surface, and the application thereto of a glaze having a high melting point.

Description: This invention relates to a method of making cement products, and more particularly, an improvement in the method for making a hardened product of cement by moulding a kneaded mixture of cement, an aggregate and water into a desired shape, and hydrating cement particles in water, air or the like. A hardened mass of cement undergoes heavy deterioration in mechanical strength if heated to a high temperature of 4500C or above. For example, it is known that if a hardened mass of Portland cement is heated to 4500C, the calcium hydroxide, Ca(OH)2, formed by hydration of calcium silicate is decomposed into CaO and H20, leading to heavy shrinkage in the mass, and accordingly, a serious reduction in its mechanical strength, particularly compressive strength (Yu Harada and Toru Sakai: "Heat Resistant Concrete", Industry & Products, No. 50, page 119, right column). It is also known that a hardened mass of aluminous cement likewise has its mechanical strength seriously reduced if heated to a high temperature, though for a different chemical reason from that which has been pointed out with reference to Portland cement [Akira Wakabayashi: "Properties of Aluminous Cement at High Temperatures", Journal of the Society of Ceramics, 69, 2 (1961), page 283. Therefore, nobody has heretofore thought of firing a hardened mass of cement at a high temperature with a view to improving its strength. Under these circumstances, it has hitherto been considered that the smoking treatment which is widely utilised for providing a moulded product of clay with a beautifully coloured and lustrous surface is not applicable to a moulded product of cement, because a high smoking temperature reduces its strength. For the same reason, there have hitherto been a lot of unavoid 1 restrictions to the glazing of a moulded product of cement. Unless use is made of a glaze which melts satisfactorily at a temperature in a range below 4500C in which no reduction in the strength of a hardened mass of cement occurs, the heat required for melting the glaze necessarily causes a serious reduction in the mechanical strength of a glazed mass of cement. There has, therefore, been developed a phosphoric acid frit glaze having a low melting point as disclosed in Japanese Patent Publication No. 10530/1974. This glaze is, however, not only inferior in weather and chemical resistance to any ordinary glaze having a higher melting point, but also very expensive. Moreover, it fails to provide a very beautiful finish on the surface of a glazed product of cement. It is, therefore an object of this invention to provide a method of making a cement product having a higher mechanical strength than any such product known in the art. It is another object of this invention to provide a method of making a cement product having a smoked surface formed with a thin film of carbon which is beautiful and lustrous. It is a further object of this invention to provide a method of making a cement product having a beautifully glazed surface formed by a glaze having a high melting point. Other objects of this invention will become apparent from the following description. As a result of extensive research efforts by the inventors of this invention, it-has been discovered that a decrease in the mechanical strength of a hardened mass of cement fired at a high temperature after it has been subjected to a preliminary hydration process is smaller than an increase in its mechanical strength obtained by a principal hydration process following the firing process. In other words, if a process for firing a moulded product of cement is provided between a preliminary hydration process and a principal hydration process, it is possible to obtain a cement product having a higher mechanical strength than the product of any known method obtained without firing a hardened mass of cement at a high temperature. They have also discovered that an increase in the mechanical strength of a cement product is further enhanced if a small amount of a calcium compound is added into a mixture of cement and an aggregate. According to this invention, there is, thus, provided a method of making a cement product, which comprises, in ccr.bin.tOn the steps of adding water, and if required, reLinforcing material, into a mixture of clement and an aggregate containing 0 to 5% by weight of a calcium compound based on the amount of CaO relative to the weight of the cement, and kneading the whole carefully, moulding the kneaded mixture under pressure into a desired shape1 hydrating the resulting moulded product to form a preliminarily hardened product, firing the preliminarily hardened product at a high temperature, and hydrating the fired product. Preferred reinforcing materials include glass fibres, ceramic fibres metal fibres and metal wire. According to-another aspect of this invention, it is possible to burn a smoking agent in a reducing atmosphere during the step of firing the preliminarily hardened product, so that fine grains of carbon may be formed by the thermal decomposition of a hydrocarbon contained in the smoking agent, and deposited on the preliminarily hardened product to provide it with a beautiful, lustrous surface covered with a thin film of carbon. According to a further aspect of this invention, it is possible to apply a glaze having a high melting point on to the surface of the preliminarily hardened product, and fire it at a high temperature, so that the glaze may be melted on the preliminarily hardened product to provide it with a glazed surface which is beautiful and lustrous. The glaze employed preferably comprises a crude glaze such as feldspar and clay, a frit glaze, a volatile glaze, or a mixture of a crude glaze and a frit glaze. The inventors of this invention have not yet fully clarified the mechanism which enables the cement product of this invention to have an increased mechanical strength, but will hereinafter attempt to analyse a mechanism which will most probably work. As is well known, the hardening of cement by hydration proceeds over a very long period of time. There is even a report to the effect that coarse particles of clinker having a particle size of at least 40 ,u retain their original internal structure even up to after thirty or more years. Anderegg and Hubbel report that clinker having a particle size of 15 to 30 u obtained a hydrated depth of only 1.5 P below the particle surfaces after seven days of hydration, only 3.5 P after 28 days, and only 5.0 p after 90 days [Anderegg and Hubbel: Proc. A.S.T.M, II (1929); and Concrete t930)]. In view of these facts, it is presumed that cement particles require a long time of, say, 30 to 100 days to obtain a hydration degree of about 50%, and several more years to permit further hydration to take place very slowly, while their internal portions occupying at leas 302 of their total volume remain unhydrated after such a long period of time. According to the method of this invention, however, an hydrated and hardened layer of each cement particle obtained by preliminary hydration and having a depth of several microns is broken when it is fired at high temperature, and the cracks thus formed in the hydrated layer define a passage for water and CaO formed during the high temperature firing process by Ca(OH)2 liberated during the preliminary hardening process and the calcium compound in the raw material, whereby water and CaO reach the unhydrated portions in the interior of the cement particles. It is, thus, possible to obtain during the final hydration process a hydrated layer having a greater depth than that which could be formed if no firing processes were present between the preliminary and final hydration processes. çonsequently, hydration is caused to proceed faster into the interior of the cement particles. The calcium oxide entering the interior of the particles forms Ca(OH)2 by absorbing water, and expands to substantially fully fill the interstices in the particles. Carbonation of Ca(OH)2 takes place to form a thick hardened surface layer and a dense structure in the cement particles within a relatively short period of time. While trying to verify their assumption which has hereinabove been described, the inventors of this invention have found that the longer the period of the preliminary hydration process, the higher mechanical strength the final product shows, if all the other conditions remain the same. This fact is believed to testify the propriety of the inventors' assumption. The invention will now be described in further detail by way of example. EXAN1PLE A mixture containing 100 parts by weight of ordinary Portland cement, 200 parts by weight of river sark from rWne Shinano River, 2 parts by weight of Adachi lime and 60 parts by weight of water was kneaded for 10 minutes to form a kneaded cement mixture. It was moulded under pressure of 100 kgf/cm2 ( > -9.8 x 10 2) for four minutes in a 500-ton hydraulic press, whereby a moulded product of cement having a water-solid ratio of 0.1 was obtained. After the moulded product was cured in the air for one day for preliminary hardening, it was fired in such a manner that it was heated to 8500C over a period of two hours in an electric furnace, maintained at 8500C for 15 minutes, and cooled to 200C over a period of 10 hours. After it was cooled to 200C, it was cured at 60"C on a wet condition for seven days, whereby a cement product was obtained. The bending strength of the cement product thus obtained was tested by a Tensilon tester having a span of 45 mm and a loading rate of 10 mm/min., and found to be 112.7 kgf/cm2 (¯ 11.05 x 106 Nm-2). For the sake of comparison, the foregoing procedures were repeated, except- that no Adachi lime was used, and the resulting product showed a bending strength of only 81.8 kgf/cm2 (8.02 x 106 Nm2). The term "cement" as herein used does not only mean Portland or aluminous cement, but also includes a mixed cement, such as fly ash cement, blast furnace cement and lime-slag cement. In other words, it should be understood to cover all the substances that are hardened by hydration. For the purpose of this invention, it is necessary to use an "aggregate" which will not undergo any rapid expansion or contraction during the high temperature firing process. Examples of the aggregate material which satisfies such a requirement include a heavy aggregate such as barite, ordinary aggregates such as sand, gravel, rubble, slag and ceramic or porcelain chamotte, and an artificial or other lightweight aggregate. Ceramic or porcelain chamotte is particularly preferable for obtaining an increased mechanical strength according to this invention. While the addition of a calcium compound constitutes an important aspect of this invention, it is possible to employ any calcium compound if it can form CaO during the firing process, and fill the interstices in the cement particles. For example, it is possible to use CaC03, CaO, Ca(OH)2 or CaC03MgC03. The amount of the calcium compound and the bending strength of the final product have a relationship therebetween which depends to some extent on the kind of the cement involved, and the method of preliminary hydration and the length of time spent therefor, i.e., the amount of Ca(OH)2 liberated from the cement during its preliminary hydration, but generally, it is necessary to maintain the amount of the calcium compound in the range of O to 5% by weight based on the amount of CaO relative to the weight of the cement, since any excess over 5% by weight is likely to result in a lower bending strength of the final product than can be obtained when no calcium compound is added, as will hereinafter be shown in Table 1. According to an embodiment of this invention, it is necessary to apply a moulding pressure of at least 30 kgf/cm2 (¯2.94 x 106 Nm-2), and preferably at least 50 kgf/cm2 (¯ 4.9 x 106 Nm-2). It is sufficient to maintain such a moulding pressure for a length of time not longer than five minutes. It is desirable to employ a temperature of 650 C to 950 C for the firing process. A number of mixtures containing 100 parts by weight of Portland cement, 200 parts by weight of porcelain chamotte and different quantities of CaCO3 (Adachi lime) in the range cf O to 30 parts by weight as shown in Table 1 belowwere each kneaded with 60 parts by weight of water for 10 minutes. Each of the kneaded cement mixture thus obtained was moulded under pressure oflOO.kgf/cm2 (H9.8 x 106 Nm ; for four minutes in a 500-ton hydraulic press, whereby a moulded product of cement having a water-solid ratio of 0.1 was obtained. After it was cured in the air for one day, each moulded product was fired and finally hydrated under the same conditions as set forth in the aforesaid Example, whereby a number of cement products were obtained. Table 1 shows the relation between the amount of CaC03 and the bending strength or each product as actually cured. TABLE 1 Amount of CaCO3 Bending strength of (parts by weight the product (kgf/cm2) O 98.5 1 102.3 2 109.3 4 119.8 6 122.4 8 109.3 10 94.0 15 92.5 20 12.5 30 Unmeasurable due to breakage. As is noted fro Table 1, the final product shows a maximum bending strength when it is prepared from raw material containing 6 parts by weight of CaC03 for 100 parts by weight of Portland cement, and the use of 10 ormore parts by weight of CaCOn results in a lower bending strength than can be obtained when ..3 CaCO3 is added. The addition of 8 parts by weight of CaC03 to 100 parts by weight of Portland cement is equivalent to the addition of 4.488 by weight of CaO based on the weight of the cement. Accordingly, Table 1 teaches that it is possible to obtain a product having an increased bending strength of O to 5 by weight of a calcium compound based on the amount of CaO relative to the weight of the cement is employed. Tests were conducted for comparing the mechanical strength of cement products made according to this invention, with those manufactured by a known method including no firing process. Six small pieces 1 to 6 were moulded from a mixture having a cement to aggregate ratio of 1:2, containing 2.0% by weight of CaC03 relative to the weight of the cement, and kneaded with water. They were cured in wet air for one week, fired at 8000C for 10 minutes, and cured in water for three weeks, whereby there were obtained products according to this invention (Group A products). Another set of six small pieces 1' to 6 were moulded from a mixture having a cement to aggregate ratio of 1:2, and only kneaded with water. They were cured in water for four weeks in accordance with a known method, whereby products as identified at Group B were obtained. Each of the Grdup A and B products was tested for bending strength, and the results shown in Table 2 below were obtained. The moulded pieces of cement 1 to 6 and 1' to 6' were all of equal size measuring 210 mm by 30 mm by 30 mm but prepared by using different types of aggregates, i.e., porcelain chamotte having a mesh size not larger than 8 mesh and a bulk density of 2.39 for the test pieces 1 and 1', porcelain chamotte having a mesh size not larger than 8 mesh and a bulk density of 2.36 for the test pieces 2 and 2', procelain chamotte having a mesh size not larger than 35 mesh for the test pieces 3 and 3', ceramic chamotte having a mesh size not larger than 8 mesh for the test pieces 4 and 4', kouka-stone having a mesh size not larger than 35 mesh for the test pieces 5 and 5', and water slam for the test pieces 6 and 6'. The tests were conducted by using a small strength tester FSP-500 made by Tokyo Koki Kabushiki Kaisha, Japan, and having a span of 90 mm and a loading rate of 12 mm/min. TABLE 2 Moulded Water-solid Bending strength kgf/cm2 (x105Nm-2) piece ratio Group A product Group 3 product 1 (1') 0.10 130.5 (128.0) 103.4 (101.4) 2 (2') 0.10 110.2 (108.1) 99.6 (97.7) 3 (3') 0.133 89.8 (88.1) 84.6 (83.0) 4 (4') 0.10 83.7 (82.1) 81.3 (79.7) 5 (5') 0.133 95.0 (93.2) 82.2 (80.6) 6 (6') 0.133 78.9 (77.4) 76.1 (74.6) As is obvious from Table 2, the products (group .R) according to the method of this invention employing a calcium compound in the raw material, and including the firing process show a higher bending strength than the correspcnding products (Group 3) by the known method including no firing process, irrespective of the aggregate employed. The method of this invention is not merely advantageous in its capability of producing a cement product having high mechanical strength. According to a further aspect of this invention, it is advantageously possible to provide smoking treatment on a cement product, and apply a glaze having a high melting point thereto, since the high temperature firing process improves its mechanical strength. Particularly, the method of this invention makes it possible to use a frit glaze which is inexpensive, but which has a high melting point, as opposed to any known method, and moreover, to mix an appropriate amount of feldspar, clay or the like into the frit glaze as shown, by way of example, in Tables 3 and 4, whereby a glaze which is very suitable for a cement product is prepared to provide it with a glazed surface which is very satisfactory in colour and lustre, and which has superior weather, wear and chemical resistance. Thus, the method of this invention can produce cement products which are available for a wide range of new applications, and has a high degree of utility in building and associated industries. TABLE 3 M25 frit 10 parts 1121 frit 20 parts Glass powder 65 parts ZnO 5 parts TABLE 4 3909 frit 20 parts Glass powder 60 parts Feldspar 10 parts Clay 5 parts Zr02 5 parts

Claims: CLAIMS 1. A method of making a cement product, which comprises in combination the steps of: adding water to a mixture of cement and an aggregate containing 0 to 5% by weight of a calcium compound based on the amount of CaO relative to the weight of the cement, and kneading the whole carefully; moulding the resulting kneaded mixture under pressure into a desired shape; hydrating the resulting moulded product to form a preliminarily hardened product; firing said preliminarily hardened product at a temperature 0± 6500C to 9500C; and hydrating the f.i red product for final hard- ening it. 2. A method according to claim 1 further comprising the step of addng. a reinforcing material to the mixture of cement and the aggregate. 3. A method accruing to claim w erein said reinforcing material comprises glass fires, ceramic fibres, metal fibres, or metal wire. 4. A method according to any preceding claim, wherein said step or firing includes burning In a reducing atmosphere a smoking agent containing a hydrocarbon, whereby said hydrocarbon is thermally decomposed to form fine grains of carbon on t surface of said fIred product. 5. A method cording to claim 1, 2 or 3, wherein said step of firing includes appling a glaze having a high melting point an to the surface of said preliminarily hardened product. 6. A method according to claim 5, wherein said glaze is selected from among a crude Glaze, a frit t glaze, a volatile glaze, and a mixture of a crude glaze and a frit glaze. 7. A method according to claim 6, wherein said crude glaze comprises feldspar and/or clay.

Inventors: HARADA, SHOZO, KOIDE, TETSUYA, MIWA, MASAKI, NAGATA, TATSUYA
Applicants: INA SEITO CO., LTD
Application year: 1980
Main classification: C04B15
IPC codes: C04B41, C04B40