Process for producing articles from cement being reinforced with fibers

The invention relates to a process for the production of fiber-reinforced cement articles and in particular to a process for the production of slabs, wherein the cement does not contain asbestos fibers.

Prior art

On the one hand, a method for producing ashest fiber-reinforced cement boards using mixtures is well known. cement and asbestos fibers diluted with water, usually at a water / cement ratio of about 10; asbestos fibers are used as both processing and reinforcing fibers.

On the other hand, it is known that the use of asbestos fibers, even as a construction material, is prohibited in some countries for hygienic and environmental reasons ·

For this reason, great efforts are being made to replace asbestos in construction materials and to develop an equivalent material free of asbestos fibers.

Various solutions to this problem are described in British Patent Specifications Nos. 1,605,004, 2,028,331, 2,033,317 and European Patent Application Publication No. 81,309,557. In all these cases, it is a material which contains essentially special cellulose fibers as processing and reinforcing materials. folder. The use of such fibers ensures that the product can be pressed and cured in an autoclave in the plastic state by the action of saturated steam at a temperature of about 180 ° C and a pressure of about 1 MPa.

Italian Patent Application No. 67194 -A / 80 describes a process for the production of a hydraulically curable fiber-reinforced material, the fibers as such being present as filter or processing fibers in an amount of 2 to 20% by volume and as reinforcing fibers in an amount of 1 to 10% by volume. Both types of fibers are mixed during pretreatment, thereby increasing the dispersion in the suspension. In this treatment, at least one component, the salt, precipitates in the fibers or fibers, the fibers being contacted first with the first salt solution and then with the second salt solution.

The method is laborious and difficult and in practice the following is used:

- filter or processing fibers, which allow the use of traditional machines for the production of asbestos-reinforced cement articles. fibers, and

- reinforcing fibers, which provide the physical and mechanical properties required of the finished product.

As processing fibers, cellulose, waste paper, wool, silk, polypropylene, mineral wool and kaolin wool fibers are used as reinforcing fibers, and steel, carbon, glass, polypropylene, polyvinyl alcohol, polyester, polyamide and polyacrylic fibers are used as reinforcing fibers.

In each case, these fibers require specific chemical pretreatment and the addition of a flocculating agent capable of improving the retention of solids for their dispersion in the mixture.

In practice, to achieve a positive result, the percentage of processing and reinforcing fibers in the dry mixture is generally high with respect to the content of asbestos fibers for such products, although this is not the case with the Italian patent.

In this case, the percentage of processing fibers is 7 to 8% by weight and the reinforcing fibers are 2 to 3% by weight, which corresponds to 14 to 16% by volume and 4 to 6% by weight.

It is recalled that the specific gravity of these fibers is half that of asbestos fibers and the initial dry mixture contains them. about 11% by weight. These alternative fibers therefore provide a product which, due to its large volume, has a very low density (approximately 1.2 to 1.3 g / cm 3)³ compared to 1.6 to g / cm³ in the case of cement reinforced with asbestos fibers /, so that the flexural strength is low after immersion of the finished product in water, taking into account European standards.

It is therefore necessary to develop a material that can be processed in a plastic state by pressing at a pressure of 20 to 25 MPa to achieve a density of 1.5 to 1.6 g / cm³.

The methods known from the prior art do not give completely positive and reliable results, in particular due to the fact that cellulose fibers (especially in large quantities) adversely affect the durability of the product and cause the boards to shrink and crack, especially in very hot and dry weather.

Fiber-reinforced cement boards, which can be considered to be durable to some extent, are boards which have a minimum percentage of cellulose fibers in the mixture such that they have good retention of cement particles in the production of the boards.

- the specific weight of the boards is approximately 1,4 g / cm, so that they show good frost resistance,

the content in the mixture of primary components is in sufficient contrast to the coagulation caused by the cellulose in the mixture, and

- the boards are thermerated in conditioning chambers with controlled temperature and humidity.

The present invention seeks to solve the problem of producing fiber-reinforced and asbestos-free cementitious materials in a new manner and in accordance with the above requirements and using a new mixture of processing and reinforcing fibers.

The essence of the invention

The essence of the process for the production of fiber-reinforced and asbestos-free cementitious articles according to the invention consists in preparing a mixture containing 2 to 5% by weight of cellulose fibers, adding 5 to 10% by weight of micro-silicon / sepiolite or kaolin powder or airborne. fly ash, then 1 to 3% by weight of reinforcing fibers, 5 to 10% by weight of agents are added, which cause a stress contrast from the expansion of the finished product, then more than necessary Portland dementia 325 and a flocculator in water are added, the product is sfi: shaped and heat treated and conditioned in a temperature controlled chamber.

The invention therefore in fact relates to a method of manufacturing articles similar to asbestos-fiber-reinforced articles, which, however, do not have the drawbacks, defects, limitations and inflexible methods of manufacture known from the prior art, and preferably such articles do not have to be manufactured

From or with pre-processing, pressing and autoclaving ·

The low percentage of cellulose fibers improves the durability of the finished products and reduces the risk of cracks and fissures due to shrinkage or thermal expansion. Weight content. 2 to 5% and preferably 3 to 4% are generally considered to be the most suitable »,_F

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The fibers are subjected to a pre-mechanical treatment, so that the opening of the fibers themselves is achieved in a defibration refiner in which cellulose solutions in water are treated to a yield of 4 to 5% until a value of 50 to 70 ° Schopper Riegier is reached.

This value of Schopper Riegler is considered important, as it achieves good retention of cement particles in the Hatschek forming machine.

The addition of micro-silicon, sepiolite or kaolin powder or fly ash in an amount of 5 to 10% by weight, preferably 7 to 8%, provides a layered damping material with good plasticity in shaping the product, which is necessary for shaping, especially in corrugated sheets. good adhesion between the different layers of the product, thus preventing delamination of the product, which sometimes occurs in multi-layer products.

Polyvinyl alcohol or polyacrylic fibers with a length of about 6 mm and in an amount of 1 to 3, preferably 1.5 to 2.5% by weight, are preferably used as reinforcing fibers.

Wolastonite and / or mica in the above percentages are used. preferably used as contrast agents. In fact, these powders in the form of an acidifying component form a micro-reinforcement inside the final mass, which prevents any micro-leakage in the finished products.

The Portland cement used may contain siliceous substances, and the addition of a flocculant in water further improves the filtration properties of the formulated mixture.

Finally, the necessary thermal conditioning of the finished products is carried out in a temperature-controlled chamber in the first part 30 to 4-0 ° C, in the second part 50 to 60 ° C and in the final part 70 to 80 ° C.

The plates are left in the chamber for about 10 hours in a high humidity environment, so that curing is achieved in the best possible way and reliable and long-term loadable products are obtained.

Industrial applicability

Fiber-reinforced and asbestos-free construction boards.