GB2085866A - Hardening bodies made from a mixture of building materials with mineral components - Google Patents

Abstract

These bodies, which have been rendered porous and have been compressed, are hardened by heat treatment with high frequency electro- magnetic radiation e.g. microwaves.

Description

SPECIFICATION A method for hardening bodies of a mixture of building materials with mineral components The inventionies of a mixture of building materials with mineral components in which the bodies are subjected to heat treatment with the use of high frequency electromagnetic waves.

Blocks made from a mixture of mineral components, such as cement, lime, gypsum and from a suitable amount of added aggregate of sand, pebbles, stone chips, expanded clay aggregate, and similar material, with water by casting, ramming, spraying, pressing and pumping can be hardened either undisturbed or the hardening can be speeded up by heat treatment in which heated formwork, hot steam or warm air, which flow past the bodies are used, or the bodies may be irradiated by means of electric heaters or put into heating chambers. Very often materials pressed into bodies are hardened also in an autoclave at increased steam pressure and increased temperature, while pressures of about 16 bar and temperatures of about 200"C are used.

All these methods of speeding up hardening are lengthy and use too much energy. In addition the mentioned heat treatment has the disadvantage that the heat penetrates only slowly from the outside into the inside of the bodies. As a consequence the outer regions of the bodies are hardened while the core of the bodies contains still unhardened materials which gradually hardens only subsequently. This causes tensions between the outer regions and the core of the bodies which may cause either immediately or subsequently damage to the bodies. In addition, in view of the long time needed for hardening, correspondently large storage space for storing the not yet hardened bodies must be available because stacking in the open is possible only after sufficient hardening.

It is also known to use high frequency alternating current to speed up hardening of concrete. Forthis treatment concrete has been situated between two plates which act as capacitor plates and are connected to a high frequency generator. The components of the concrete are in this high frequency alternating field exposed to a dipole polarization and are caused to vibrate which increases the temperature. This increase in temperature acts as the so-called direct heating in which the electrical energy is inside the concrete block to be hardened directly converted into heat which causes hardening of the block. Because the hardening takes place similtaneously on all parts of the block, i.e. inside the blocks as well as in its outer regions, tensions, mentioned in connection with the conventional heat treatment acting from outside, are avoided.

It was shown, however, that with a high frequency heating of concrete blocks the structure of the already compacted concrete is disturbed by the fast heating. This structural disturbances may have their origin in the pressure of the water vapour increased by the direct heating inside the block. Attempts were made to reverse these structural disturbances by additional vibration of the blocks, i.e. by acting on the blocks mechanically, but this represents not only an additional step but also a provision which may make the strength of the hardened block question able.

The aim of the present invention is to devise a method for hardening bodies of a mixture of build ing materials, in which, after being treated by high frequency electromagnetic waves, the bodies have, without further treatment, both a high initial strength and also the desired final strength.

This is achieved according to the invention in that the bodies obtain during their shaping a porous structure and, before they are treated by high frequency electromagnetic waves, they are subjected to compression.

To enable equalization of the water vapour press ure formed during the hardening inside the concrete body, without damage to the structure thereof, there is made during the manufacture of the bodies to ensure that they have open pores. Through these pores which are provided in the body and are open so as to communicate with the outside of the body, can flow out the water vapour or steam from therein until balance is obtained with the vapour pressure in the ambient atmosphere.

The porisity of the bodies is achieved in a manner known per se in that the proportion of the binder in a mineral mixture poor in sand is kept at such level that even at optimum compression cavities remain in the bodies. Such mixtures for the manufacture of concrete bodies are part of the state of common practice.

No absolute values can be given for the porosity of the bodies. This depends on various parameters, for instance shape and size of the respective body. It is, however, within the technological competence of any person skilled in the art to establish by simple experiment the minimum porisity needed to ensure the necessary water vapour perviousness. The following examples give sufficient indication for the variations of the components of the concrete mixturves to obtain sufficient porosity of the bodies.

The maximum porosity is naturally limited by the need to provide sufficient contact areas between the mineral substances in the body to enable hardening of the body by high frequency treatment.

Experiments showed that provision of sufficient contact areas inside the structure of the formed body for the formation of a hardened body by treatment with electromagnetic waves is of paramount import ance. If these contact areas are too small the bodies fall to pieces after completed hardening either on their own or when they are stressed for the first time.

It was found that by compression, preferably by -pressing, ramming and/or vibration or by analogical treatment, of the bodies before the high frequency treatment is achieved that the reaction areas are so strong that they provide, during hardening, suffi cient bond for the structure.

It was found that for carrying out a method according to the invention particularly building material mixtures are suitable which contain as a natural mineral fly-ash. Surprisingly, bodies which contained as a mineral component nearly exclusive ly fly-ash and small amount of binder of 1-15% by weight of cement and/or lime could be so hardened by microwave treatment that wall bricks, artifical aggregate and similar building materials may be manufactured in this way.

But even mixtures of building materials which contain as mineral components sand and/or aggregate of coarser grain and also as binder cement and/or lime, may be pressed into bodies which have a porosity suitable for carrying out the method.

Hardening of bodies by a method according to the invention opens the possibility of making use of waste materials deposited on waste dumps, e.g.

filter dust from power stations, with little use of energy for the manufacture of building elements.

These building elements have, for instance, the advantage that they are particularly light. This advantage can be achieved by the use of the method according to the invention with small expenses.

It was found that irradiation of the body may be made even in individual stages, while for instance preliminary storing of the formed mixture of building materials before the beginning of the irradiation and the introduction of intermissions between each two successive irradiation stages provides the best results. Such intermittent high frequency treatment may easily be accepted because it was found that the periods needed for treatment are exceptionally short and are of the order of minutes.

It was further found that the consumption of energy during the method according to the invention is much lower that that during known heat treatment. Due to the short hardening times no large area working halls are needed because material hardened according to the invention can be immediately stacked and be stored in the open.

A method according to the invention relates to the manufacture of bodies of all kinds. This includes not only bricks, blocks and form pieces, but also granules, made of mixtures of building materials, which are used as artifical aggregate for mixtures of building materials with mineral or organic binders.

Such granules are treated and thereby hardened by electromagnetic waves. This means that mixtures of building materials, preferably with the use of flue-ash, may be used in a simpie way, to form granules.

It is pointed out that mixtures of building materials, which are bought into a formwork to form a compact body, are not suitable for each treatment with irradiation by microwaves because the formed body has not the necessary porosity and suffers therefore from the known structural changes during high frequency treatment because the formed water vapour cannot escape in sufficient quantity.

Details of the invention will be apparent from the following description with reference to several examples.

Example 1 Mixture of building materials comprising 2310 g of sand used for concrete of granulation 0/4 mm, 1080 g of Portland cement 45 F and 212 g of water is mixed with the addition of a commercially available accelerator in the amount of 1% (related to the proportion of binder contained in the mixture) and with the addition of a commercially available superliquefier, in the amount of 0.5% (related to the proportion of cement in the mixture) and pressed to form a body. From this mixture is obtained a porous and sand-concrete body. After preliminary storing lasting for 30 minutes, the body is subjected to irradiation by microwaves at 0.022 kWh/kg. After intermediate storing without irradiation, lasting about 15 minutes, the body is again irradiated by microwaves at 0.022 kWh/kg.

On completion of the second irradiation the body is firm and hardened to such an extent that it may be stacked for transport and storing.

Example2 The amount of 1200 g of sand concrete of granulation 0/4 mm, of the composition according to Example 1 is mixed with 600 g of lime chips of granulation 2/4 mm and 264 g of lime chips of granulation 4/8 mm. The addition of an accelerator and superliquefier corresponds to that of Example 1.

On completion of the mixing the mixture of building materials is pressed to a porous body. The result is a porous concrete body. As soon as it is removed from the mould the body is subjected to microwave treatment at 0.03 kWh/kg. Towards the end of the treatment and after cooling of the body the latter is hardened to such an extent that it may be stacked, transported and stored.

Example 3 A mixture of 1250 g of flue ash stored in the open, the humidity contents of which is about 18%, of 250 g of fine white lime, of 2.5 g of soda, of 2.5 g of flux (liquefier) and 150 g of 10% solution of water-glass are pressed to form a porous body and then irradiated by microwaves as described in Example 1.

The hardened body has a dry bulk density of 1220 kg/m3 and a compression strength of 6.4 N/mm2, measured after 14 days.

This shows that the addition of soda and waterglass brings about sufficient strength even attem- peratures of about 1000 C.

Example 4 Porous bodies of a mixture of 1200 g of dust from an electrostatic filter having a water content of about 220 g, of 100 g of fine white lime, of 100 g Portland cement PZ 35F, of 2.05 g of soda, of 2.5 g of flux (liquefier) and of 200 g of 10% solution of waterglass were pressed and hardened. The dry bulk density of the body was 1000 kg/m3 and its compression strength was 4.5 N/mm2 measured after 14 days.

The compositions of the building material mix turesdescribed in Examples 1 can, in addition, contain also other substances, such as sand, pebbles, stones, pieces of bricks, expanded clay and slag of all kinds. Even then the hardness of the porous bodies is sufficient when high frequency is used.

Claims (11)

1. A method for hardening bodies of a mixture of building materials with mineral components, in which the bodies are subjected to heat treatment with the use of high frequency electromagnetic waves, wherein the bodies obtain during their shaping a porous structure and before they are treated by the high frequency electromagnetic waves they are subjected to compression.

2. A method according to claim 1 wherein the shaping of the mixture of building materials is made by ramming pressing and/or vibration.

3. A method according to claim 1 or 2 wherein the high frequency treatment consists of one or more irradiation steps and, after being shaped, the bodies are, before the beginning of the first irradiation step, stored for 15 to 45 minutes.

4. A method according to claim 3 wherein the bodies are stored for 30 minutes.

5. A method according to any one of claims 1 to 4 wherein the bodies are between the individual irradiation steps stored, without being exposed to radiation, for 10 to 30 minutes.

6. A method according to claim 5 wherein the bodies are stored for 15 minutes.

7. A method according to any one of claims 1 to 6 wherein the energy spent during every irradiation step is 0.01 to 1.0 kWh/kg.

8. A method according to any one of claims 1 to 7 wherein the bodies are put into microwave ovens for hardening.

9. A method according to any one of claims 1 to 8 wherein the bodies are formed into granules and hardened.

10. A method according to any one of claims 1 to 9 wherein the mixture of building materials contains as a mineral flue ash in the amount of 80-100%.

11. A method of hardening bodies of a mixture of building materials with mineral components, substantially as herein described with reference to the Examples.