GB2068930A - Producing calcium silicate bricks or cement - Google Patents

Abstract

Bricks and cement primarily composed of calcium silicate are made by mixing finely grained limestone comprising finely crystalline CaCO3 intimately mixed with extremely fine cryptocrystalline and amorphous silica and variable amounts of vitreous aluminosilicate materials, abundant for example in the Lefkara and Pakha formations in Cyprus, with lime and water so that reactions are saturated. Curing takes place at low temperatures and pressures so that in a warm climate, natural heat is all that is necessary.

Description

SPECIFICATION Process for producing calcium silicate bricks or cement This invention relates to a process for making calcium silicate bricks and cement.

The conventional process for producing calcium silicate bricks is to mix properly graded silica sand with 5-1 0% lime and water, then strongly press the mixture into the required shape and heat it in a moist condition at a pressure of about 14 atmospheres in an autoclave for six to ten hours.

The process resuits in the formation of a compound known as hydrated calcium silicate (Tobermorite). The surface of the silica sand grains react with the lime, and the finished brick consists mostly of unchanged sand bound by the hydrated calcium silicate cementing material. Very fine grains of silica sand react in their entirety with the line. The cementing material is very hard and strong and is completely formed during the curing period in the autoclave.

The increasing fuel cost involved in producing bricks in this manner is making the process relatively expensive when compared with the manufacture of other types of bricks, and this relative expense is increased when the process is operated on a small scale since the capital investment required is high.

Other types of bricks can, of course, be produced and may be preferred where there is a readily available local source of starting material.

The most common type of brick produced is a ceramic one. These are produced by mixing clays with water, extruding them in perforated form, drying them, and subsequently firing them at 950-11 000C. This process again requires relatively high capital investment, mainly for the firing kilns, and involves excessive handling and high fuel consumption. Furthermore the presence of such minerals as montmorillonite in some otherwise suitable clays causes a high degree of shrinkage and cracking during drying and firing which adds to the cost of the process.

Calcium silicate cements such as Portland cement also require high temperatures not merely to produce lime but also to cause the lime to react with siliceous material.

It is an object of the present invention to provide a process for making calcium silicate bricks and cement that is, in certain situations, more economical than the conventional processes. The process relies on the use of certain types of rocks with particular physical and chemical characteristics. These rocks are sedimentary in origin and probably formed in deep sea water in contrast to the unsuitable reef limestones formed in shallow water. They consist of very finely crystalline calcium carbonate intimateiy mixed with extremely fine cryptocrystalline and amorphous silica and small and variable amounts of vitreous aluminosilicate materials. The calcium carbonate and silica are not in a recrystallised form, for example not as in metamorphic limestones such as marble, or limestones where the silica is recrystallised as quartz.Chemical analysis indicates that these rocks have a calcium carbonate content in the range 20 to 84%; silica content, as SO2, is between 10 and 70%. The remainder is Al2O3, MgO Fe2O3 and minor amounts of alkalis.

The rock is very hard when the CaCO2 content is low whereas when it is over 50% the rock is very soft and friable. This type of rock is, for example, abundant in Cyprus where it is shown on geological maps as "Chalks-cherts-marls-sha leslimestones" of the Lefkara and Pakhna formations extending in age from Upper Cretaceous to Middle Miocene. These rocks will be referred to below as 'rocks of the type specified'.

Accordingly, the present invention provides a method of making calcium silicate bricks comprising the steps of forming a mixture of particles of rock of the type specified with lime and water in such proportions that the reactions between the lime and the constituents of the rock are substantially saturated, shaping the mixture, and curing in humid conditions at a relatively low temperature.

The curing temperature may be produced by the natural heat of the sun, preferably by enclosing the bricks to be cured in a transparent enclosure in which a green house effect obtains.

The invention also provides a method of making calcium silicate cement using rock of the type specified mixed with lime at normal temperatures and pressure.

In one embodiment of the invention building bricks are made from rock of the type specified requiring little artificial heating. The rock is finely crushed to particles that are less than 5 mms in size and then mixed with lime and water. The optimum proportions of these constituents depends on the compositions of the particular rock used.

Typically these proportions are: Rocks of the type specified 92% Lime 5% Cement 3% The principle on which the proportions are ascertained is that sufficient lime should be added to saturate all possible reactions between the lime and the reactive constituents of the rock. To achieve the maximum possible reaction it is essential to bring the milk of lime into contact with as much as possible of the silica and vitreous aluminosilicate material present in the rock. This can be done by increasing the fluidity of the milk of lime by applying pressure or elevated temperature, for example, by kneading the mixture through an extruder or by moulding the mixture in a press, i.e., as the brick shapes are being formed.

To give the formed bricks a high initial strength a small amount of portland cement can be added to the mixture.

Having been formed by extrusion or pressing the bricks then need to be cured. It is in this procedure that considerable advantages accrue from the present invention. Curing may be accomplished in a simple type of green house formed as a relatively long tunnel of polyethylene.

The bricks are driven on pallets slowly through the tunnel where elevated temperature and humidity prevail. It has been found that in a warm climate, i.e., 40--600C, 8 hour curing under these conditions followed by about 4 days in the open is sufficient to make bricks suitable for use in building.

When the invention is used to make cement the mixture is dried and without further steps being needed. The advantage of the method in cement manufacture is that the silica and aluminosilicate material in the rocks combines with the lime at ordinary temperature and pressure, it furthermore has the reactive elements of the rock intimately mixed with finely-grained inert material (calcium carbonate).

Claims (7)

1. A method of making calcium silicate bricks comprising the steps of:- forming a mixture of particles of rock of the type specified with lime and water in such proportions that the reactions between the lime and the constituents of the rock are substantially saturated; shaping the mixture; and curing in humid conditions at a relatively low temperature.

2. A method as claimed in claim 1 wherein the curing temperature is mainly or wholly provided by the heat of the sun.

3. A method as claimed in claim 2 wherein the bricks to be cured are located in a transparent enclosure and exposed to sunlight, the transparent material having radiation-transmitting properties such that the interior of the enclosure becomes heated to a temperature above that of the air temperature outside the enclosure.

4. A method as claimed in claim 3 wherein the transparent enclosure comprises a tunnel made from polyethylene through which the bricks are transported.

5. A method as claimed in any one of claims 1 to 4 wherein a small amount of cement is added to the mixture ot increase its initial strength.

6. A method of making calcium silicate building cement comprising mixing rock of the type specified with lime and water in such proportions that the reactions between the lime and the constituents are substantially saturated; the mixture being carried out at a temperature and pressure in ranges relatively close to normal air temperatures and atmospheric pressure respectively.

7. Building cement made by a method as claimed in claim 6.