Method and apparatus for microwave aided setting of settable compositions

The present invention generally concerns a method for setting of settable water containing compositions by application of energy in the form of microwaves. According to another aspect the invention concerns an apparatus for use in the same connection.

Background

Use of settable compositions occurs in a number of industrial applications. For example within land-based construction settable compositions are used either as stand-alone products or as components of products, like containers of different types, more or less fire resistant, as foundations for houses or other constructions like bridges or the like or as layers if such constructions to improve properties like compressive strength, fire resistance, or the like. In such settable compositions there may optionally be reinforcing materials such as reinforcing steel rods, or fibres of different sizes and types.

A number of different properties need to be combined in settable compositions, depending on their area of use. It may be strength in the form of compressive strength and tensile strength, ability to endure variations in temperature without crack formations, ability to resist different chemicals such as salts and acids, density, and fire resistance. The setting rate, i.e. the time the settable composition needs to obtain its strength, is often important as well as its lifetime up to required replacement in the relevant environment. The industrially most used settable compositions are hydraulic cement compositions that set by a reaction with water (hydration).

Typically there is a certain conflict between the different properties. If a short setting time is required this may be obtained by adding materials that in a short or longer term negatively affects other properties of the compositions such as compressive strength, tensile strength and/ or expected lifetime. Tests have been conducted under varying conditions to improve the properties of the end product, but so far one has not found any method that is generally successful and does not introduce new disadvantages.

Attempts have been made to accelerate the rate of setting of the settable compositions by means of microwaves. Use of microwaves leads to an accelerated setting process and thereby a more extensive heat development. The heat development leads i.a. to development of a vapour pressure within the composition that involves formation of bubbles therein like during boiling, which results in a significant pore formation in the material, especially at its surface, which is highly unwanted since its properties is thereby deteriorated. In addition the development of strong heat, typically a temperature above about 60 ⁰ C, leads to crack formation in the set concrete which is thus not suited for construction purposes.

Objectives

It is the object of the present invention to provide a method and an apparatus for conducting setting of a settable composition in a manner which is generally suitable, which very rapidly yields a set, ready for use product, which does not negatively influence on other properties of the product and which results in a product with a very long expected lifetime.

It is furthermore an object to achieve the objects mentioned by means which are applicable for industrial purposes, i.e. means that are useful also in a large scale.

The invention in general The above mentioned objects have proven possible to realize through the method according to the present invention, comprising setting of settable compositions containing water thorugh application of energy in the form of microwaves, in which the settable composition is placed in a substantially isolated chamber and that the static pressure in the chamber at the start of the setting or at an early phase thereof is reduced to a pressure lower than atmospheric pressure and that the vapour pressure that develops in the composition during the setting reaction is compensated by a pressure increase, counterpressure, in the chamber, whereby an exchange of water molecules may take place at the surface of the composition without deterioration of the composition in the form of pore formation. According to another aspect the present invention concerns an apparatus for use during setting of a settable composition containing water by application of energy in the form of microwaves. Said apparatus includes a substantially closed chamber as well as means for controlling the chamber pressure and at least one microwave emitting source.

Preferred embodiments are disclosed by the dependent claims. Preferred embodiments

An important element of the invention is the pressure reduction affected in the chamber and in the composition before the setting process has developed substantial heat. It is therefore preferred that the pressure reduction is conducted to a level lower than 0.5 bars, more preferably lower than 0.25 bars and most preferred to a pressure lower than 0.05 bars.

Although the present invention may be useful for different types of settable compositions it is very useful for hydraulic cement compositions which represent a preferred type of composition also due to their extensive industrial use. The composition can also or in stead be based on other settable components such as micronized aplite. Settable compositions based in part on micronized aplite are described in Norwegian patent

application No. 2005 2035. When aplite is used dominantly or solely as binding agent in the composition, the composition is typically exposed to a strong acid to initiate the setting process. In such cases the setting process is not a hydraulic one but there will still be moisture present in the composition that will contribute to a vapour pressure during the setting and the temperature increase thereby associated. Hydrochloric acid or other inorganic or organic acids that do not influence negatively on other components in the composition can be used.

The lower the proportion of hydraulic cement used and the higher the proportion of aplite or the like, the higher the temperature is allowable during the setting process without damaging the resulting product. It is thus preferred that an aplite content that allows a setting temperature higher than 60 ⁰ C during setting to thereby shorten the setting time. With a pure or substantially pure aplite mass temperatures as high as at least 100 ⁰ C are allowable.

Additives per se known to add to settable masses can also be added to the settable masses according to present invention, e.g. materials to adjust the specific density of the composition. Same materials or other materials can also contain metals or metal oxides or other particles that makes the composition more receptive to the energy in microwaves.

For some embodiments it is preferred that the settable compositions used also comprises a reactive slag that can contribute to the binding of the composition during setting, in particular when substantial amounts of oxygen is present in contact with the composition. It is thus preferred that when reactive slag is used the pressure increase in the chamber is at least partially caused by adding an oxygen containing gas.

By the method according to the invention it is possible even with compositions based on hydraulic cement, to obtain setting of products with sufficient strength in a matter of very short time, such as within a period shorter than 2 hours, typically shorter than 1 hour and, depending of the composition ingredients and volume, the obtainable pressure reduction in the first phase, and the maximum allowable temperature increase for the composition, a setting time down to as short a period as e.g. 10 to 15 seconds. Setting of a flowing composition to a ready-to-use product in such a short time has earlier not been feasible for most of such compositions and therefore represents something of novelty and high industrial importance.

The mainly closed chamber can typically be a chamber of solid walls and with a size adapted to the product or products in question. Another variant that is particularly well suited for small products involves use of a chamber with flexible walls in which the pressure builds up during the setting process in line with the build-up of vapour pressure in

the settable composition. The chamber has in this embodiment the form and function of a strong balloon.

For most purposes it will be more preferred and more easy to control the pressure by use of solid walls which is the case for the second aspect of the present invention. With manometers and one or more pumps which are capable of adjusting the pressure to high levels as well as low levels, one can obtain an optimal control of the conditions in the chamber at any given point in time. Based on calculations and partially based on empirical data, a pre-programmed progress of pressure variations, providing optimal conditions, may be set up for a defined product (of a certain size and composition). The point in time for initiating injection of gas and the rate at which the gas is injected, can then be controlled without supervision by persons skilled in the art.

The apparatus which is subject to the second aspect of the invention is in principle simple. It mainly comprises a substantially closed chamber, at least one microwave emitting source and means to control pressure. Preferably there are means to manually monitor the pressure, but for an industrial apparatus it is more important to include manometers that are adapted to automatically monitor the pressure development in the chamber. The means for control the pressure typically comprise at least one vacuum pump and at least one compressor.

In chambers for large products there should be microwave emitting sources for every meter extension in each direction. For oblong products with a quite limited cross-sectional area it will be sufficient having microwave emitting sources arranged linearly along the products length axis.

The settable compositions can be poured into moulds of silicone or the like having an inner shape that corresponds to the desired shape of the product in question to ensure that the composition maintains the desired shape until it is set.

It is preferred that the means to control the chamber pressure is controlled automatically by a processor that is pre-programmed with the relevant information and which consecutively receives data of the actual conditions in the chamber.

It is obvious that equipment of this type be provided with tight doors etc to ensure that the microwaves not can escape to the environment as well as means to monitor any leakage of microwaves and in case to trigger an alarm. Such means are, however, not part of the present invention and therefore not described in any further detail here.

The areas of use mentioned in this description are exemplifying and not exhaustive. The present invention is not limited to specific areas of use but is generally applicable.