GB1590555A - Ettringite composition - Google Patents

Description

(54) ETTRINGITE COMPOSITION (71) We, MINORU TANAKA, of 1-1-9 Matsugaecho, Kitaku, Kobe, Japan, GENZO HASHIZUME of 2-13-4 Tanakacho, Higashinadaku, Kobe, Japan, HIROSHI MATSUI, of 2-96 Edayoshi, Tarumiku, Kobe, Japan and SATORU NAKAGAWA of 2-37-3 Shirakawadai, Sumaku, Kobe, Japan, all Japanese citizens, do hereby declare this invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to an ettringite-based composite material.

Ettringite is a sulfate double salt with calcium aluminate and has the formula C3A.3CaSO4.32H2O (according to cement chemistry notation: C=CaO, A=Al203,). It forms in the initial stage of hydration to Portland cement, and once was thought to be a harmful substance to concrete. Later, however, the study by G.L. Kalousek et al., (G.L.

Kalousek: Sulfoaluminates of calcium as stable and metastable phases, Ph. D. Thesis, University of Maryland [1941]) revealed that the crystals of ettringite, when formed from an unsaturated solution of Ca(OH)2, are long columns or needles, which show a great crystal pressure. Since then, there have appeared reports suggesting its use for reducing dry shrinkage of concrete and for introducing prestress into concrete, and under the stimulus thereof, active development studies have been made into utilisation of the expansion effect, the starting point being use in expansive cement.

Thus, ettringite is useful as an expansion-producing ingredient in cement, and its use is roughly divided into two categories. One is shrinkage compensating concrete for reducing drying shrinkage, and the other is in chemically prestressed concrete for introducing prestress into a structure by taking advantage of the expansive power of ettringite. The former concrete is mainly intended for preventing cracking and used in making water tanks, reservoirs, dams, buildings, bridges and so on, while the latter is mostly intended for increasing cracking resistance or cracking load of factory products such as pipes, precast floor boards, concrete sheet piles, box culverts and composite concrete/steel pipes. What is important for an expansive admixture usable for these various purposes is that in hydration, ettringite or its monosulfate does not precipitate in the liquid phase but is formed on the surface of the solid grain, which is the basis for hydration. It is necessary that, as shown in the scanning electron micrograph of Figure 1, ettringite crystals should grow on the surface of solid grains, whereas the form of the solid grains remains the same as before hydration. It has been thought that this ettringite is the source of expansive power in the cement hydration process. Such uses are almost all of the hitherto known uses of ettringite, and it may be said that ettringite is known only for these uses.

Recently there has been a tendency to synthesize ettringite in liquid phase, isolate the same and search for possible uses thereof as a material for industry, in addition to its use as an expansive ingredient for cement as mentioned above. According to one typical research report by Kondo et al found in Abstracts of Papers, 1973 Annual Meeting of Ceramic Society of Japan, page 106 and in Seramikkusu (Ceramics, a monthly journal), volume 8, No. 10, pages 67 to 73 (1973), ettringite obtainable following preliminary synthesis of C3A from such reagents as Ca(OH)2, Al2(SO4)3.18H2O, CaSO4 and Al203 and boehmite (aAl2O3.H2O) consists of needlelike crystals with a size of 2 llm. The percentage of voids and compressive strength after pressure molding under a pressure of 500 to 600 kg/cm2 are not more than 0.07 and not less than 800 kg/cm, respectively, and a long period of pressing or gradual drying turns the ettringite into an almost completely compacted and translucent hardened body. These procedures may be regarded as sintering and hot pressing at an ordinary temperature. However, in the case of monosulfate and satin white (granular crystal material having a size of 0.1 ijm, said to have a chemical composition of the same kind as ettringite, and used as coating material), such a high strength hardened body cannot be obtained. According to another report by Sugi et al reported in Technical Reports of Osaka Cement Co. Ltd., No. 40 pages 23 to 28 (1977), experiments were carried out in the synthesis of ettringite using finely pulverized blast furnace granulated slag (chemical analysis 36.8% SiO2, 19.1% Awl203, 41.3% CaO) and gypsum (gypsum dihydrate) originating from exhaust gas desulfurization in a slag/gypsum ratio of from 1 to 1.5 with so-called accelerators such as a 10 to 15% aqueous solution of Portland cement, Ca(OH)2, KOH or NaOH in an amount about ten times the quantity of the starting materials. After the mixture was stirred for 20 to 24 hours, synthetic ettringite with a purity of about 60% was obtained. This ettringite consists of needlelike or prismatic crystals 20 to 30 llm in length and 2 to 5 llm in diameter.

Although the above reports show that synthetic ettringite crystals have a length of 2 to 30 ,um and, when long, they are prismatic with a relatively large diameter, synthetic ettringite, for reasons related to its crystalline form, aspect ratio and so forth, has so far not found any practical use.

Methods for manufacturing a-hemihydrate gypsum from sulfur oxides present in exhaust gases, and the resulting gypsum whisker composites, are featured in Patent Specifications Nos. 1547422 and 1547424, of which the inventors are three of the present applicants, and there is also disclosed therein a method of producing II-anhydrate gypsum needles, namely gypsum whiskers useful in industry, from needle crystals of a-hemihydrate gypsum as a starting material. Since the said gypsum whiskers are useful as short fibres for reinforcing purposes because of their large aspect ratios, we have tried to prepare highly pure needlelike synthetic ettringite having an aspect ratio much higher than before, and we have succeeded in this effort.

According to the present invention, there is provided a composite product comprising ettringite crystal whiskers having an aspect ratio of 30 to 50, mixed with and dispersed in at least one matrix or binder material consisting of fibrous pulp, a rein, gypsum or cement.

The ettringite whiskers we have produced not only can be us 3d in the same way as gypsum whiskers, but also can be expected to greatly increase the self-extinguishing ability of materials with which such ettringite whiskers are admixed owing to the presence of a large amount of water of crystallization in ettringite (C3A.3CaSO4.32H2O). The ettringite whiskers will also be advantageous in paper-making where ettringite is mixed with pulp, owing to its solubility being lower than that of gypsum whiskers by a factor of ten (solubility of ettringite in one liter of water at 25"C being 0.215 gram for CaSo4, 0.043 gram for CaO and 0.035 gram for Al2O-,).

The desired ettringite-based composite materials are prepared by admixing highly pure and high aspect ratio ettringite whiskers, which may be synthesized in accordance with the method to be described later, with a material (matrix) which can serve as a base for the composite material, such as base material for paper making, resin, gypsum, cement, fibrous material for reinforcement purposes and light-weight aggregate.

The carrying of the invention into practice will now be described by way of examples and with reference to the accompanying drawings in which: Figure 1 is a photomicrograph of ettringite growing on the surface of a solid grain as a result of hydration of an expansive cement admixture.

Figure 2 is a photomicrograph of ettringite produced according to a conventional method of synthesis, Figures 3 to 5 are photomicrographs (300 magnifications) of paper sheets prepared by admixing ettringite whiskers produced by the method of synthesis disclosed herein with a base material for paper-making, Figure 6 is a photomicrograph (500 magnifications) of ettringite whiskers produced by the method of synthesis disclosed herein, and Figure 7 is a photomicrograph (500 magnifications) of gypsum whiskers.

The ettringite whiskers used are the product of the synthetic method to be described later and have unique characteristics.

Example 1 Hand-made paper sheets were made by using 50 to 90% by weight of ettringite whiskers and 50 to 10% by weight chemical pulp from soft wood as binding material according to JIS P 8209 'Method of preparing hand-sheets for pulp-testing'. The sheets (Figures 3 to 5) were soft and flexible, smooth to the touch, and showed good printability. When compared with sheets made by using gypsum whiskers, the above sheets showed far better selfextinguishing ability. The entangled state of the ettringite whiskers and the pulp is seen in Figures 3 to 5. Sheet mixtures including both gypsum whiskers and ettringite whiskers in appropriate proportions also give good results.

Example 2 A slurry prepared by adding water to 80 to 90 parts by weight of ettringite whiskers, 5 to 20 parts by weight of Portland cement, 3 to 5 parts by weight of chemical pulp from soft wood and 0 to 5 parts by weight of gypsum was poured into a mold internally covered with a flannel and dehydration and hardening of the sheet were effected under a slight pressure.

The sheet having a thickness of 10 mm showed a bending strength of 25 to 60 kg/cm2, a bulk density of 1.0 to 1.7 g/cm3 and strong fire-resisting and self-extinguishing properties.

Example 3 60 to 80 parts by weight of a mixture of ettringite whiskers, cement and gypsum in the proportions of Example 2, and 20 to 40 parts by weight of aggregate (i.e. perlite or fine light-weight aggregate) was formed into moldings by a wet process under pressure, and the formed bodies were cured in an autoclave. The moldings having a thickness of 10 mm showed a bending strength of 10 to 30 kg/cm2 and a bulk density of 0.5 to 0.8 g/cm3.

Example 4 Ettringite whiskers were dispersed in air, and sprayed with a resin (e.g. phenolic or epoxy resin) diluted with an appropriate solvent, and made into mat-like moldings, which were then cured at a temperature not higher than 100 C. The mats were light or of a low density (0.3 to 0.5 g/cm3) and showed strong self-extinguishing ability.

The ettringite whiskers obtainable by the method of synthesis to be described later, which show no diffused reflection under a polarization microscope, contain a large amount of water of crystallization and show how solubility in water and other characteristics such that, when admixed with various kinds of base materials, they provide ettringite composites that can be formed by paper-making, sheet-making and spinning techniques. Industrial applications include fire-resisting and/or self-extinguishing fillers, reinforcing fibrous materials, and a wide number of uses to which the electrical characteristics and heat-resisting property of ettringite are applicable. Application of ettringite to new industrial uses will be accelerated by our development that has made it possible to synthesize for the first time beautiful needlelike ettringite whiskers with a high aspect ratio, like gypsum whiskers, at low cost by a mass production method.

The method of synthesizing ettringite whiskers which may be used in practising the invention will now be described.

Any of the ettringites obtainable in accordance with one of the following methods may be used. The first method of synthesizing ettringite whiskers is characterised by preparing a slurry by adding about 10 or more parts by weight of water to one part by weight of a mixture of starting materials, namely lime, aluminous material such as aluminous cement, and calcium sulfite or a mixture of gypsum with calcium sulfite, effecting oxidation by blowing air into and thereby stirring the slurry at a temperature not exceeding 100"C, preferably at 700 to 100"C, and an air pressure of 2 to 4 kg/cm2, and dehydrating and drying the slurry so treated.

An example of this method for synthesizing ettringite whiskers is as follows: Example 5 Calcium sulfite (CaSO3.-H2O) 6 g Aluminous cement 10 g Calcium hydroxide (Ca(OH)2) 1 g Water 500 ml The above materials were placed in a pressure vessel and air was blow into the slurry at a temperature of 80"C and at an air pressure of 2 kg/cm2. After about 7 hours, the reaction product was filtered off and dried in a desiccator.

X-ray diffraction analysis revealed that the product was ettringite of a very high purity.

Figure 6 is a photomicrograph of these ettringite crystal whiskers obtained by scanning electron photomicrography (500 magnifications). This ettringite shows beautiful needlelike crystals having aspect ratios of 30 to 50. In the conditions of this example, but at an oxidation temperature lower than 0 C, ettringite whiskers were not formed; at a temperature below 70"C the time required for the synthesis was prolonged, and at a temperature above 100"C ettringite whiskers could not be obtained. Figure 7 is a scanning electronmicrograph (500 magnifications) of gypsum whiskers for comparison. They are needlelike crystals with aspect ratios approximately the same as those of the ettringite whiskers.

The second method of synthesizing ettringite whiskers employs a catalyst which accelerates the oxidation and crystal growth in the step of oxidation/stirring mentioned for the first method. The catalyst is a metal salt such as nickel sulfate, iron sulfate or aluminium sulfate and, when used in an amount of about 0.1 to 1.0% by weight, it is possible to decrease the synthesis time (time required for whisker production) considerably. The following is an example of this method of ettringite whisker synthesis.

Example 6 Following the procedure of Example 5, but adding 0.1 gram of nickel sulfate (NiSO4.7H2O), ettringite whiskers quite identical to those of Example 5 were produced, the time required for the synthesis or production being 2 hours instead of 7 hours as in Example 5. The same effect of decreasing the reaction time was observed when a metal salt other than nickel sulfate, such as iron sulfate or aluminium sulfate was used. If the amount of the metal salt is less than 0.1% by weight, based on the starting materials for the synthesis, the effect of decreasing the reaction time will be little. If the amount is more than 1.0% by weight, the reaction product tends to colour. In Example 7, cases will be described where the calcium sulfite used in Examples 5 and 6 is partly replaced by gypsum, and aluminous cement is replaced by other aluminous materials.

Example 7 Replacing the aluminous cement in the mixes of Examples 5 and 6 with each of the aluminous materials given in Table 1 in turn and proceeding in accordance with the above mentioned, there is obtained the same quality of ettringite whiskers as in Examples 5 and 6.

In these cases, when lime, prior to reacting, is mixed with the aluminous material and the mixture pulverized, the reaction proceeds more rapidly.

TABLE 1 Chemical composition of aluminous materials used CaO Al203 SiO2 Others Blast furnace slag (pulverized) 39.45 14.05 31.05 MgO 4.3 Product of firing sludges from paper industry 11.29 42.88 15.81 Mn 8.04 Waste product obtained from aluminium industry 1.12 55.80 2.28 S04 6.81 A further possible method of synthesizing ettringite whiskers is a combination of one of the above-mentioned methods of synthesizing ettringite whiskers with the technique concerning the direct production of a-hemihydrate gypsum from sulfur oxides in exhaust gases disclosed in Patent Specification No. 1547422. Thus, ettringite whiskers can be synthesized by adding lime, aluminous cement, or another aluminous material (preferably one having the composition C3A(3CaO.A1,03)) and a metal salt to a calcium sulfite slurry prepared by absorption of sulfur oxides in exhaust gases into a lime slurry, adjusting the slurry concentration to 5 to 20%, effecting oxidation and stirring by blowing air into the slurry at a temperatue of 70 to 100"C and a pressure of 2 to 4 kg/cm , and dehydrating the slurry so treated.

The ettringite whiskers resulting from the above-mentioned methods of synthesizing ettringite are always in the form of beautiful needlelike crystals with a high aspect ratio, contain a large amount of water of crystallization and are difficult to dissolve in water as can be seen from the results of an experiment shown in Table 2. Thus it is possible to provide materials for industrial uses that are incombustible, self-extinguishing, and possess favourable electrical and heat-resisting properties, by admixing the ettringite whiskers with a matrix material, such as base material for paper making, resin, gypsum, cement, reinforcing fibrous material or light-weight aggregate as mentioned above.

TABLE 2 mg/100 cc Ca2+ S042- Aye3+ Ettringite of the invention 11.8 18.4 3.9 Gypsum whiskers 80.5 189.0 Ettringite in literature 9.3 15.2 1.9 Remarks: The results shown in Table 2 were obtained by shaking a mixture of 3.5 grams of ettringite whiskers produced by the method described herein and 500 ml of water at 25"C in a testing shaker at a rate of 100 times every minute for 5 hours.

Gypsum whiskers (calcined at 600"C) were used for comparison. The solubility data found in the literature for ettringite is also cited.

Reference is directed to our companion Patent Specification No. 10324/78 (Serial No.

1587578) in which similar subject matter is disclosed and the method of sythesising ettringite whiskers is claimed.

WHAT WE CLAIM IS: 1. A composite product comprising ettringite crystal whiskers having an aspect ratio of 30 to 50, mixed with and dispersed in at least one matrix or binder material consisting of fibrous pulp, a resin, gypsum or cement.

2. A product as claimed in claim 1, also containing a reinforcing fibrous material or light-weight aggregate.

3. A product as claimed in claim 1 or claim 2, in the form of a sheet.

4. A product as claimed in claim 3, in the form of a paper sheet.

5. A product as claimed in claim 1 or claim 2, which has been formed by spinning.

6. A composite product according to claim 1 and substantially according to any one of Examples 1 to 4 herein.

7. A composite product according to any one of the preceding claims, wherein the ettringite crystal whiskers are the product of a method according to any one of Examples 5 to 7 herein.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   a matrix material, such as base material for paper making, resin, gypsum, cement, reinforcing fibrous material or light-weight aggregate as mentioned above.

   TABLE 2 mg/100 cc Ca2+ S042- Aye3+ Ettringite of the invention 11.8 18.4 3.9 Gypsum whiskers 80.5 189.0 Ettringite in literature 9.3 15.2 1.9 Remarks: The results shown in Table 2 were obtained by shaking a mixture of 3.5 grams of ettringite whiskers produced by the method described herein and 500 ml of water at 25"C in a testing shaker at a rate of 100 times every minute for 5 hours.

   Gypsum whiskers (calcined at 600"C) were used for comparison. The solubility data found in the literature for ettringite is also cited.

   Reference is directed to our companion Patent Specification No. 10324/78 (Serial No.

   1587578) in which similar subject matter is disclosed and the method of sythesising ettringite whiskers is claimed.

   WHAT WE CLAIM IS: 1. A composite product comprising ettringite crystal whiskers having an aspect ratio of 30 to 50, mixed with and dispersed in at least one matrix or binder material consisting of fibrous pulp, a resin, gypsum or cement.
2. 2. A product as claimed in claim 1, also containing a reinforcing fibrous material or light-weight aggregate.
3. 3. A product as claimed in claim 1 or claim 2, in the form of a sheet.
4. 4. A product as claimed in claim 3, in the form of a paper sheet.
5. 5. A product as claimed in claim 1 or claim 2, which has been formed by spinning.
6. 6. A composite product according to claim 1 and substantially according to any one of Examples 1 to 4 herein.
7. 7. A composite product according to any one of the preceding claims, wherein the ettringite crystal whiskers are the product of a method according to any one of Examples 5 to 7 herein.