NO761314L - - Google Patents

Description

Procedure in the manufacture of

products containing hydraulic

binders

The invention relates to a method for pressing

at high pressure to produce products containing hydraulic binders, preferably cement, with a firmness that significantly exceeds what has previously been possible.

Hydraulic binders are produced by heating the starting material more or less strongly, and the hardening . then happens when water is absorbed. When producing e.g. portland cement, the material mixture is heated to temperatures of around 1<1>+00°C and cooled very quickly after the firing is complete so that the valuable, hydraulically reactive reaction products are "frozen".

The minerals produced by this production method have a very high content of energy, which is partly released when the Portland cement is stirred out with water. When reacting with water, hydrates are formed with a lower energy content, and the excess energy that is then released gives rise to a more or less strong heat development in the body.

With conventional shaping of concrete, it is necessary

that the concrete mass has a more or less plastic consistency,

and at least 30 -<*>+0% water is then required, calculated by the cemerite weight.

In some cases, higher water-cement numbers are required so that

a malleable mass must be obtained. In order to completely hydrate, for example, Portland cement, approx. 10% water is included, calculated by the weight of the cement. This corresponds to a water-cement number, i.e., of 0.<*>+0.

In that all necessary water is added during the mixing

the reaction between the cement minerals and the water will lead to a strong increase in temperature in the concrete mass. It is above all the calcium aluminate's reaction with added gypsum as under it

the first phase of the cement minerals' hydration contributes to this heat generation. Later, reactions between water and other clinker minerals will also contribute to the increase in temperature to which the body is exposed. The increase in temperature will lead to the body's firmness being reduced as tensile stresses in the body's interior will arise as a result of the thermal shrinkage that occurs when the body's temperature is lowered. These tensile stresses'

can give rise to microcracks, which, together with residual stresses, will reduce the firmness of the body.

Also in cases where the water cement number exceeds 0,>+0,. a reduction of the firmness will apply.

If the water-cement number is too high, excess added water will give rise to pores in the cement gel, which will lead to a loss of firmness.

It is common knowledge that you should keep the water cement ratio as low as possible. According to a method for producing, for example, slabs of concrete, the water cement number, by lowering the temperature of the binder to below the freezing point of water and then mixing the cement with powdered ice, is lowered to approx. 0.20. The shaping then takes place by pressing in a heated press, whereby the ice melts under the influence of heat and pressure. In this method, a pressing pressure of approx. 20 bar is used.

The effect of the pore volume on the firmness of the cement paste, i.e. a mixture of water and cement, has from a long time ago interested researchers. As early as 1937, so-called compacts were produced by pressing cement paste in order to try to reduce the pores in the cement gel. A pressure of approx. '2.5 kbar and achieved a compressive strength of 180 MPa (L'Hermite & Valera). In 1970 a Cement and Concrete Association study was published in which C.D.•Lawrence had studied certain properties of compacts .of

.cement paste. Lawrence produced bodies from cement without the addition of water. After the pressing, the bodies were allowed to

react with water. as they were stored in water until testing..

Later, A. Bajza and D.M. Roy, G.R. Gouda and

A. Bobrowsky studied properties of compacts made from wood.

pressing, of cement paste by. press up to approx. 8 kbar.

The results obtained indicate that a completely pore-free cement gel has a compressive strength of approx. 500 - 700 MPa.. As a comparison, it can be mentioned that cement has a compressive strength of 50 - 60 MPa in standard testing.

The present invention is based on the well-known fact that the water-cement number in a concrete must be kept as low as is practically possible during forming. To

■ reason for the invention also lies in the per se known method of shaping bodies from cement paste by pressing at high pressure.

Attempts have been made to reinforce thin plates ■

of concrete, which is formed under pressure. Concrete itself has relatively poor tensile and impact strength. One method to increase this is to reinforce the concrete object with fibres,

e.g. steel fibers. Unfortunately, it has been shown that the pressing of e.g. plates occur, damage, i.a. caused by the fibers trying to regain some of their original shape during the pressure generation, and the cement paste then had too little cohesion to counteract this. The result has been that they have not been successful in producing fibre-reinforced products by pressing.

However, it has proved possible, by a combination of a high pressure and a pre-hydration of the cement at a low water-cement number, to produce fibre-reinforced panels which are not damaged by the pressure relief. By prehydration is meant here a method whereby the cement minerals have been allowed to partially react with mixed water before the shaping of the products begins.

In the manufacture of products according to the invention, water and cement are mixed with a water-cement number that does not exceed 0.15, preferably a water-cement number that does not exceed 0.10. The cement-water mixture is mixed until the added water is evenly distributed in the almost dry mass. The pre-hydration can take place at room temperature for a time of up to ^-8 hours, but can also be accelerated by the addition of accelerators, e.g. sodium carbonate, or by applying heat. In the event that heat is supplied, the temperature is kept at 100°C, preferably at hO - 70°C.

After pre-hydration is finished, the press mass is mixed, after the cement-water mixture has been sieved to remove any lumps that could cause damage to it. f finished pressed product.. The particles are then filled into the mold and a pressure of up to 10 kbar, preferably 3 - 5 kbar, is applied. The pressing takes place following a pressing curve with a pressure increase up to the working pressure for 30 - 2^-0 seconds, the working pressure is then kept constant for a time of up to 900 ■ seconds, preferably up to 300 seconds, after which the pressure is lowered to atmospheric pressure for a time in up to 2^-0 seconds. In the manufacture of certain products, it has proved advantageous to apply pressure for a period of 3'0 - 2<*>+0 secondsbg thereafter, after the top pressure has been achieved. immediately to

lower the pressure again for a time of up to 2^-0 seconds.

After pressing, the products are stored in a moist storage room with a relative humidity of 90 -100$, or immersed in water in the case of smaller details.

According to the invention, conventional building elements can thus be produced, but also more complicated objects can be produced.

The pressing of the products can take place partly in a conventional manner by applying the pressing pressure uniaxially, but it is also possible, within the scope of the invention, to use isostatic presses.

When manufacturing products from sheets or panels, these can be coated with sheet metal, reinforced or unreinforced plastic sheets, veneers, etc.

The invention is further illustrated by the following examples.

Example 1

Cylindrical specimens were made of pure cement with a water cement number of 0.10 and 0.15. CaCNO^^ was used as a setting time accelerator, which was added in an amount of 5% of the cement weight. The same time, 120 seconds, was used for the pressure increase and the pressure decrease, and the peak pressure of 5 kbar was maintained for 1,00 seconds. After press forming, the test bodies were stored in water for 7 days, after which they were tested.

The result as a function of the water cement number was:

Experiments were also carried out with an extra finely ground cement at a water cement ratio of 0.10, whereby the compressive strength became 3^° MPa.

Example 2

Cylindrical specimens were prepared from pure cement

with the water cement ratio of 0.10 and-Na2C0^ as setting time accelerator.

In the experiment, the same pressure curve as in example 1 was used,

but the peak pressure was varied.

The result was:

Example

A cement-water mixture with a water-cement number of 0.07 was prehydrated for 5)16 and 2h hours. The mixture, which. appeared to be a dry powder, was charged immediately for the molding into cylindrical shaped bodies. The same times for pressure boiling and pressure lowering as in example 1 were used while the peak pressure was varied, as was also the time for peak pressure and hydration time. The table below shows the results and the variables.

Example h

A sample with a water cement number of 0.10 was pressed after heating to 0°C at a pressure of 5 kbar for 300 seconds. After 7 dbgns of water storage, the specimen had a compressive strength of 190 MPa.

Example 5

A cement mixture with the water cement number of 0.10 was pressed with. a peak pressure of 5 kbar for 300 seconds. The sample was then stored in water and compressive strength tests were carried out after 1, 7, 28 and 91 days.

The result was:

The cylindrical test bodies that were used for the above samples had a diameter of<*>+0 mm and a height of h0 mm.

Experiments have also been carried out with plates.■

Example 6

Reinforced and unreinforced plates measuring 25 x ^-0 cm were pressed from pre-hydrated cement. The cement mixture used had a water-cement number of 0.10.'Boy tensile strength

and impact resistance were tested in part on unreinforced and partly on reinforced plates. The buoyant tensile strength for unreinforced plates was 27.8

and the corresponding value for reinforced plates was 28.9 MPa. The impact strength for unreinforced plates was 1.90 KNm/m<2>, while the reinforced plates had an impact strength of 1.5.90' KNm/m 2.. •

The invention provides products that have approx. 10 times higher compressive strength than conventionally produced concrete, and the impact resistance is approx. 1<*>+ times higher than with unreinforced concrete.

By coating one or more surfaces with metal, tinplate, veneer, plastic or others during manufacture. materials, boards are available that can be used both indoors and outdoors without further surface treatment.

In order to increase the tensile strength, the products can be reinforced during production with net reinforcement, which is then adapted to the size of the object.

Claims (3)

1. Procedure for producing pro by press forming ducts containing' hydraulic binders, k a r a k-, t e r i s e r t by a prehydration of the binder i a time of up to V8 hours. and a high pressure of 0.5 to 10 kbar, preferably 3-5 kbar j whereby, the pressing takes place, in accordance with a curve so that the pressing pressure is applied for a time of 30 - 2^0 seconds, held constant for a time of up to 900 seconds and relieved for a time of up to 2k0 seconds. 2. Procedure according to claim 1, characterized by . that the products are reinforced with fibres, preferably steel fibres, or provided with net reinforcement. 3. Method according to claim 1 or 2, characterized in that the prehydration is accelerated by additives and/or heat supply. <!> +. Procedure according to requirements 1 - 3? characterized by the fact that one or more sides of the product' at the front • the position is coated with metal; such as tin plates or steel foils, plastic material or cellulose material.