GB1587578A

GB1587578A - Method of synthesizing ettringite

Info

Publication number: GB1587578A
Application number: GB1032478A
Authority: GB
Original assignee: HASHIZUME G
Current assignee: HASHIZUME G
Priority date: 1977-03-15
Filing date: 1978-03-15
Publication date: 1981-04-08
Also published as: JPS53137093A; DE2811268A1; JPS5760288B2; DE2811268C2

Description

(54) METHOD OF SYNTHESIZING ETTRINGITE (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 the 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 a method of synthesizing ettringite.

Ettringite is a sulfate double salt with calcium aluminate and has the formula C3A.3CaSO4.

32H2O (throughout the specification the following cement chemistry notation will be used in accordance with the general practice in the field of cement chemistry: C = CaO, A = Awl203, S = SO3 and H = H2O). It forms in the initial stage of hydration of Portland cement, and once was thought to be a substance harmful 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) reported that the crystals of ettringite, when formed from an unsaturated solution of Ca(OH)2, are long prismatic or needlelike crystals, which show a great crystalline pressure. Since then, there have appeared reports suggesting its use for decreasing the drying shrinkage of concrete and for producing chemically prestressed concrete, and development studies have been made with the view to utilising the expansion effect, the starting point being its use in expansive cement.

Thus, ettringite is effective as an expanding ingredient for cement, and its use is roughly divided into two categories, one in shrinkage-compensating concrete for reducing drying shrinkage, and the other in chemically prestressed concrete for introducing prestress into a structure by taking advantage of the expansive power of ettringite. The former concrete is used in making water tanks, reservoirs, dams, buildings, pavements, slugs for bridges and so on, while the latter is mostly intended for increasing the cracking resistance or cracking load of concrete factory products such as pipes, precast floor boards, concrete sheet piles, box culverts and composite steel/concrete pipes.What is important for an expanding ingredient usable for these various purposes is that, in hydration, ettringite (or monosulfate) does not precipitate in the liquid phase but is formed on the surface of the solid grains. It is necessary that, as shown in the scanning electronmicrograph 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. The expanding admixture utilizes this crystal growth on the solid grains of the ettringite formed in the cement hydration process. Such uses are substantially all of the hitherto known uses of ettringite.

Recently, there has been a tendency to synthesize ettringite in the liquid phase, isolate the same and search for possible uses thereof as a material for industry, in addition to the use as an expansion producing ingredient for cement as mentioned above. However, this is still being studied.

According to one typical research report by R. 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 is obtainable following preliminary synthesis of C3A from such reagents as Ca(OH)2, Al2(SO4)3, CaSO4 and A1203 and boehmite (a-A1203. H20), it consists of needlelike crystals with a size of 2 CLm. the percentage of void and the compressive strength after pressing under 500 to 600 kg/cm2 are not more than 0.07 and not less than 800 kg/cm2, respectively, and a long period of pressing or gradual drying turns the ettringite into an almost completely compacted and translucent hardened body.These processes may be regarded as sintering or hot pressing at ordinary temperatures. It should also be remarked that monosulfate satin white granules having a size of 0.1 clam, and said to have a chemical composition of the same kind as ettringite, are used as a paper coating material; however, a high strength hardened body cannot be obtained from such granules.

According to another report by Sugi et al. reported in Technical Report of Osaka Cement Co., Ltd., No. 40, pages 23 to 28 (1977), experiments were carried out on the synthesis of ettringite using finely pulverized blast furnace granulated slag (chemical analysis 3.68% SiO2, 19.1%Al203, 41.3% CaO) and gypsum (gypsum dihydrate) produced from a desulfurization process, with a slag/gypsum ratio of 1 to 1.5 and addition of a so-called accelerator such as Portland cement, Ca(OH)2, KOH or NaOH in an amount of 10 to 15%, in an aqueous solution about ten times the quantity of the starting materials. After the mixture is stirred for 20 to 24 hours, synthetic ettringite with a purity of about 60% is obtained. This ettringite consists of needlelike or primsmatic crystals 20 to 30 ,um in length and 2 to 5,u m 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 crystals, for reasons related to their crystalline form, aspect ratio and so forth, have not hitherto found any satisfactorily practical use.

The present inventors have already invented a method of producing a-hemihydrate gypsum from sulfur oxides in exhaust gases, and a gypsum whisker composite, (Patent Specifications Nos. 1547422 and 1547424') and have disclosed a method of producing II-anhydrate gypsum needles, that is to say gypsum whiskers useful in industry, from needle crystals of a-hemihydrate gypsum as a starting material. In view of the fact that these gypsum whiskers are useful as short fibers for reinforcing purposes because of their large aspect ratios, the inventors have tried to prepare highly pure needle-like synthetic ettringite whiskers having an aspect ratio much higher than before, and have indeed found a method of preparing the desired ettringite whiskers.

According to the invention, there is provided a method of synthesizing ettringite crystals whiskers which comprises: preparing a slurry by mixing together at least 10 parts by weight of water to one part by weight of a starting material mixture comprising a) a component to provide CaO b) aluminous material to provide Al2O3 c) calcium sulphite, or a mixture of gypsum and calcium sulphite, to provide CaSO4 effecting oxidation by blowing air into, and thereby also agitating, the slurry at a temperature in the range of 70"C to 1 000C and an air pressure in the range of 2 to 4 Kg/cm2, and dewatering the thus oxidized slurry to separate the ettringite crystal whiskers.

The invention further provides a method of synthesizing ettringite crystals whiskers comprising: adding lime to provide CaO, gypsum to provide CaSO4, an aluminous material to provide Al203, and a metal salt. to a calcium sulphite slurry prepared by a process of desulphurization of exhaust gases.

adjusting the slurry concentration to 5 to 20% by weight, effecting oxidation and agitating by blowing air into the slurry at a temperature between 70"C and 100"C, and with an air pressure of 2 to 4Kg/cm2, and dewatering the thus oxidized slurry to separate ettringite crystal whiskers.

The invention results in ettringite whiskers which not only can be used in the same way as gypsum whiskers, but also increases 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 produced in accordance with the invention are also advantageous in paper-making, when the ettringite whiskers are mixed with the pulp, owing to the solubility of ettringite being lower than that of gypsum whiskers by a factor of 10 (solubility of the 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 Awl203) .

The carrying of the invention into practice will now be described in more detail with reference to the accompanying micrographs 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 commercially available ettringite, Figure 3 is a photomicrograph (500 magnifications) of ettringite whiskers prepared by the method of the invention, and Figure 4 is a photomicrograph (500 magnifications) of gypsum whiskers.

The first method of synthesizing ettringite whiskers is characterized as follows. A slurry is prepared by adding about 10 or more parts by weight of water to one part by weight of a mixture of starting materials, comprising a 'lime' component to provide CaO, an aluminous cement or other aluminous component to provide Al203 and a component or component mixture to provide CaSO4 . The 'lime' component may be Ca(OH)2 but CaCO3 can also be used. The CaSO4 component may be calcium sulphite, which will oxidise to the sulphate, or a mixture of calcium sulphite and gypsum. Then by blowing air into the slurry and thereby stirring the slurry, at a temperature in the range 70O to 1000C, with an air pressure of 2 to 4 Kg/cm2, and dewatering and drying the slurry so treated, ettringite whiskers are obtained.

An example of this method of synthesizing ettringite whiskers is as follows: Example I Calcium sulphite (CaSO3.H2O) 6 g Aluminous cement lOg Calcium hydroxide [Ca(OH)2] 1 g Water 500 ml The above materials were placed in a pressure vessel and air was blown into the slurry at a temperature of 80"C and at an air pressure of 2 kg/cm2. After 7 hours, the reaction product was dewatered by filtering and dried in a desiccator.

X-ray diffraction analysis revealed that the product was ettringite whiskers of a very high purity. Figure 3 is a photomicrograph of these ettringite whiskers obtained by scanning electron micrography (500 magnifications). These ettringite whiskers are beautiful needlelike crystals having aspect ratios of 30 to 50. In the conditions of this example, but with an oxidation temperature lower than 0 C, ettringite was not formed, at a temperature below 70"C the time required for the synthesis was prolonged, and at a temperature above 100"C ettringite was not obtained. Figure 4 is a scanning electron micrograph (500 magnifications) of gypsum whiskers for comparison. They are needle 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 0.1 to 1.0% by weight, it is possible to reduce the synthesis time (time required for production) considerably. The following is an example of this method of ettringite whisker synthesis.

Example 2 Following the procedure of Example 1, but adding 0.1 gram of nickel sulfate (NiS04.7H20), ettringite whiskers quite identical to those of Example 1 were produced, the time required for the synthesis or production being 2 hours instead of the 7 hours in Example 1. The same effect of reducing the reaction time was observed when a metal salt other than nickel sulfate was used, such as iron sulfate or aluminium sulfate.

In Example 3. cases will be described where the calcium sulfite used in Examples 1 and 2 is partly replaced by gypsum, and the aluminous cement is replaced by other aluminous materials.

Example 3 Replacing the aluminous cement in the batches of Examples 1 and 2 with aluminous materials given in Table 1 and proceeding in accordance with the above mentioned method, there was obtained the same quality of ettringite whiskers as in Examples 1 and 2. In these cases, when the lime, prior to reacting, was mixed with the aluminous material and the mixture pulverized before the reaction, the reaction proceeded more rapidly.

Table 1 Chemical composition of aluminous materials used ( 0)

AlzO I SiOz Blast furnace granulated 14.05 31.0 slag (pulverized) 39.45 42.88 1 15.81 Mn 8.04 Firing sludges obtained from paper industry 11.29 Waste product obtained from aluminium industry 1.12 55.80 j 2.28 S046.81 This invention also provides a further method of synthesizing ettringite whiskers, which is a combination of one of the above-mentioned methods of synthesizing ettringite whiskers with a technique involving the direct production of a-hemihydrate gypsum from sulfur oxides in exhaust gases as described in patent specification No. 1547422. Thus, ettringite whiskers can be synthesized by adding lime, an aluminous cement or other aluminous material (preferably one having the composition C3A (3CaO.Al203)) and a metal salt to a calcium sulfite slurry or a sulfur oxide absorption liquid from a desulfurization process, adjusting the slurry concentration to 5 to 20% by weight, effecting oxidation and stirring by blowing air into the slurry at a temperature of 70 to 100"C at an air pressure of 2 to 4 kg/cm2, and dewatering the slurry so treated.

The calcium sulfite slurry or sulfur oxide absorption liquid may be one prepared by the technical processes that are disclosed in patent specifications Nos. 1547422 and 1547424, and any of the following may be used: a calcium sulfite slurry resulting from bringing an exhaust gas cooled to a temperature of about 60"C and containing sulfur oxides into contact with a solution or slurry of lime or calcium carbonate thereby allowing the sulfur oxides to be absorbed; a calcium sulfite slurry from a process called the "ammonia-lime process", wherein the sulfur oxides are absorbed in an ammonia solution and them the resulting acid ammonium sulfate or ammonium sulfite is reacted with lime by adding a lime slurry, ammonia being recovered thereby; a calcium sulfite slurry from a so-called "soda-lime process", wherein the sulfur oxides are absorbed in a caustic soda solution and the resulting acid sodium sulfate or sodium sulfite is treated with a lime slurry, caustic soda being recovered thereby; a calcium sulfite slurry from a so-called "dilute sulfuric acid process"; absorption liquids resulting from absorption of sulfur oxides in diluted sulfuric acid, ammonium sulfate solution, borax solution or the like in dilute sulfuric acid, ammonium sulfate or borax process. In this way, a desulfurization step can be introduced into or combined with the present method.

It is advantageous to synthesize a composition C3A in advance of the ettringite whisker synthesis. Since calcium hydroxide is said to be an accelerator of crystal growth, combinations of that compound with a composition such as C12A7 or CA are also advantageous.

Strictly, there is found, in the ettringite whiskers of Example 1, for instance, a small amount of amorphous impurities between the meedlelike crystals. These impurities, however, can be removed by physical means. Thus, considering that such beautiful needlelike crystals as can be seen in Figure 3 are obtainable only in a period of time as short as about 2 hours, the method of the invention is a considerable step forward in the art. In addition, since the present method employs calcium sulfite as one of the starting materials, it is a great advantage that the synthesis can be carried out directly following an exhaust gas desulfurization step.

Direct recovery of a-hemihydrate gypsum in a lime-gypsum method, sodium sulfite method or other lime method for desulfurization of gases, includes a step of oxidation, or steps of oxidation and double decomposition, and the same can be said of the present method of synthesizing ettringite whiskers. In the production of o-hemihydrate gypsum, the conditions are such that the pH is 3 to 5 and the temperature is 120O to 1300C, so that the inside wall of the oxidation tower, for example, should be built or lined accordingly. The method of synthesizing ettringite whiskers according to the invention does not need any special provision in this respect, because the pH in this case is 12 to 13 and the temperature lies around 80"C.

Moreover, in the production of a-hemihydrate gypsum, it is necessary, after the reaction, to conduct the separation, such as filtration, and the drying rapidly at a temperature higher than 90"C, and care should be taken even thereafter to prevent the product hydrating.

However, there are no such requirements in the production of ettringite whiskers, and any of the conventional methods of recovering gypsum dihydrate is applicable thereto. Further, in the production of gypsum whiskers. it is necessary that needlelike a-hemihydrate gypsum crystals are converted into insoluble anhydrous gypsum by calcination, whereas in the present case ettringite whiskers are chemically stable and consequently stabilization by calcination is not required.

The ettringite whiskers produced by the method of the invention have a lower solubility than gypsum whiskers, as shown in Table 2, so that uses thereof in compositions for paper-making. spinning or sheet-making can be expected.

Table 2 mg/ 100 cc

Ca2+ S042- Al3+ Ettringite of theinvention 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 of the present invention 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 6000C) were used for comparison. The solubility data found in the literature for ettringite have also been given.

Thus, in accordance with the invention, it is possible to produce highly pure needlelike ettringite crystals with high aspect ratios. In the case where the ettringite whisker synthesis is combined with an exhaust gas desulfurization process, the production cost should not be much higher than that in the case of gypsum whisker production even if the synthesis is carried out from the preparation of C3A. If aluminous cement or a material such as shown in Table 1 is used as the source of alumina, high production of ettringite whiskers at low cost will be possible. The production of these ettringite whiskers as an industrial material may help in solving the current problem of oversupply of gypsum.

WHAT WE CLAIM IS: 1. A method of synthesizing ettringite crystal whiskers which comprises: preparing a slurry by mixing together at least 10 parts by weight of water to one part by weight of a starting matering mixture comprising a a component to provide CaO b aluminous material to provide Al203 c) calcium sulphite, or a mixture of gypsum and calcium sulphite, to provide Cay04 effecting oxidation by blowing air into, and thereby also agitating, the slurry at a tempera ture in the range of 70"C to 100C and an air pressure in the range of 2 to 4Kg/cm2, and dewatering the thus oxidized slurry to separate the ettringite crystal whiskers.

2. A method as claimed in Claim 1, in which a metal salt which is a catalyst for oxidation and crystal growth is used in an amount from 0.1 to 1.0% by weight based on the weight of starting materials.

3. A method as claimed in Claim 2, in which the metal salt is nickel sulphate, iron sulphate or aluminium sulphate.

4. A method of synthesizing ettringite crystal whiskers comprising: adding lime to provide CaO, gypsum to provide CaSO4, an aluminous material to provide Awl203, and a metal salt, to a calcium sulphite slurry prepared by a process of desulphurization of exhaust gases, adjusting the slurry concentration to 5 to 20 % by weight, effecting oxidation and agitating by blowing air into the slurry at a temperature between 70"C and 100 C, and with an air pressure of 2 to 4 Kg/cm2, and dewatering the thus oxidized slurry to separate ettringite crystal whiskers.

5. A method as claimed in any preceding claim in which the aluminous material has the formula C3A(3CaO.Al203).

6. A method of synthesizing ettringite crystal whiskers substantially as herein described with reference to any one of the Examples.

7. Ettringite crystal whiskers prepared by a method as claimed in any preceding claim.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. combined with an exhaust gas desulfurization process, the production cost should not be much higher than that in the case of gypsum whisker production even if the synthesis is carried out from the preparation of C3A. If aluminous cement or a material such as shown in Table 1 is used as the source of alumina, high production of ettringite whiskers at low cost will be possible. The production of these ettringite whiskers as an industrial material may help in solving the current problem of oversupply of gypsum. WHAT WE CLAIM IS:

1. A method of synthesizing ettringite crystal whiskers which comprises: preparing a slurry by mixing together at least 10 parts by weight of water to one part by weight of a starting matering mixture comprising a a component to provide CaO b aluminous material to provide Al203 c) calcium sulphite, or a mixture of gypsum and calcium sulphite, to provide Cay04 effecting oxidation by blowing air into, and thereby also agitating, the slurry at a tempera ture in the range of 70"C to 100C and an air pressure in the range of 2 to 4Kg/cm2, and dewatering the thus oxidized slurry to separate the ettringite crystal whiskers.