GB2218412A - Lightweight aggregates - Google Patents

Abstract

A lightweight aggregate is prepared by mixing bloatable clay with fly ash or equivalent argillaceous material in controlled proportions. Then the mixture is fired in a rotary kiln to produce an aggregate in the density range 700 to 1900 kg/m<3>.

Description

Title Lightweight Aggregates Field of the invention This invention relates to a lightweight aggregate and to a method of production thereof.

Background to the invention In many countries, the U.K. included, there are no known naturally occurring reserves of commercially exploitable lightweight aggregates. Lower density aggregates such as pumice, scoria etc., are imported, for example for use in the manufacture of lightweight masonry blocks for building to improve the thermal efficiency. The high costs of imported lightweight aggregates make the local manufacture of low density aggregates into a commercial proposition.

Aggregates based on bloatable clays, pelletized slag and sintered pulverised-fuel ashes are the most commonly manufactured in the U.K.

The incorporation of lightweight aggregates in structural reinforced concrete is particularly useful for tall or larger structures such as bridges where foundation costs are high. Further significant increases in efficiency and lower overall building costs can be achieved by using aggregates of specific density lower than that of natural aggregates (about 2600 kg/m3). For example, in order to reduce the density of natural aggregate concrete from about 2350 kg/m3 to about 1750 kg/m3, a coarse aggregate of about 1300 kg/m3 specific density is required. It is possible with this specific density of aggregate to produce structural concrete with strengths in excess of 50 N/mm2 at 28 days.

The following details are relevant to existing techniques for the commercial production of lightweight aggregates.

In relation to the use of clay, it is known that certain geological deposits of clays expand when they become pyroplastic. Most expandable clays, when they reach the optimum temperature for bloating, expand rapidly two to three times their normal volume in five minutes. The bloating temperature is normally in excess of 1050"C and lower than 12500C.

Kiln control, speed, volume and temperature are critical to successful bloating, because for a particular clay if the temperature is exceeded by 25"C, the expanded pellets can change into a thick liquid, adhering to the equipment, causing serious conglomeration.

Lightweight expanded clay aggregate (UK registered trade name LECA) is less suitable for structural concrete than dense and stronger pellets from clay which, however, are more costly per unit volume to produce. Thus LECA is mainly used in lightweight concrete masonry for internal walling to reduce thermal losses. Some is used in structural grades of concrete but due to its low density the strengths obtainable are usually restricted to less than 25 N/mm2 at 28 days.

In relation to fuel ashes, it is known that pulverizedfuel ashes (fly ash) can be pelletized with small amounts of water and/ or coal or coal shale and pyro-sintered to form granules (UK registered trade name LYTAG). This aggregate is conventionally produced with a specific density of about 1800 to 2000 kg/m2. However, the grading and granulometry of the fly ash raw material is critical to successful formation of the green pellet. While coarse fly ashes (+ 50% retained at 45 um) have the lowest specific density, they are difficult to pelletize.

Moreover, the production of fly ash to form a cementitious component to BS 3892 Part 1 : 1982 requires that the amount retained at 45 um should not exceed 12 1/2%. On the other hand, "run of station" fly ashes from power stations have a typical sieve residue 30 to 40% retained at 45 um size, so that an elutriation is typically used as a means of classifying selected "run of station" fly ash into fines, having less than 10% retained at 45 um, and coarse, having more than 50% retained at 45 um. Although the coarse fly ash is the major proportion of the split, there has been to date very limited industrial application for this grade of material, and it is usually either returned to the electricity utility for disposal or contractors are hired for its removal and subsequent dumping.

Fly ash has been found to possess some unusual physical properties:i) as both coarseness and unburnt fuel increases the specific density decreases; ii) as the exposure temperature in the furnace increases, the shape of the particle becomes increasingly spherical; iii) coarse fly ashes from bituminous coals invariably contain a greater proportion (0.25 to 1.0% by mass) of carbonaceous material (coke) and boiler slag grits than finer fly ashes.

It is for these reasons that the benefits, particularly of the spherical shape of the finer fly ashes, to reduce the water content of concrete, cannot be derived from the coarser fraction by grinding this material to the required fineness.

Thus LYTAG, the sintered fly ash aggregate currently commercially available in the U.K., is used principally for structural grades of concrete because strengths in excess of 70 N/mm2 can be obtained at 28 days. Although the density of the concrete approaches 2000 kg/m3, this is nevertheless a density reduction of about 15% on natural aggregate concrete.

Due to pellet forming requirements, LYTAG is conventionally manufactured on a sinter strand and not in a rotary kiln, which would be a more thermally efficient method of manufacture. It has thus been proposed to enable the manufacture of a fly ash aggregate in a rotary kiln, that it should be combined with a proportion, say about 30 to 40%, of suitable brick making clay. However, the resultant aggregate would have a specific density of about 2000 kg/m3, so that the applications for this aggregate would be very limited.

It is also relevant that brick making techniques often use so-called "grog" to improve the burnability of bricks during firing. The "grog", which is a pre-burnt vesicular lightweight porous material, containing some carbonaceous material, is blended with the brick clay to enhance combustion and enable escape of combustion products during firing, without damage to the fusion structure of the brick. The technique also increases brick production.

The invention According to one aspect of the present invention, there is provided a method of manufacturing a lightweight aggregate according to which a bloatable clay is mixed with fly ash or equivalent pulverised arillaceous material having 15 to 85 per cent retained at 45 um, the bloatable clay in the mixture being in the range 20 to 80 per cent by weight, and the firing temperature of the mixture, when fired in a rotary kiln, is controlled to produce a lightweight aggregate in the specific density range 700 to 1900 kg/m3.

The invention, in a second aspect, relates to a lightweight aggregate produced by the afore-described method.

The method in accordance with the invention is believed to achieve the desired advantageous result because the bloatable clay (as distinct from brick making clay), when bloating, ceramically bonds the particles of fly ash or equivalent arillaceous material together, which when cool provides a continuous film of vitrified product that not only increases strength but also reduces water permeability without reducing porosity.

The use of fly ash not only ventilates the internal structure of the composite particulate so that heating across its section is more effective, but the presence of 4 to 12% "coke" in the pulverised-fuel ash is an effective method of dispersing coke which adds to the controlled combustability of the composite particle during firing.

If the required amount of "coke" is not present in the pulverised-fuel ash, extra "coke" would have to be added.

Description of embodiment A preferred method in accordance with the invention has three stages for manufacture of the expanded aggregate.

First, appropriate de-stoned bloatable clay is brought to a suitable consistency with water in a primary pug mill, before adding the selected proportion of coarse fly ash.

This mixture then passes for homogenisation to the second pug mill in preparation for extrusion through die-casts of 5 to 12 mm diameter which are cut into pellet lengths of 8 to 12 mm. The actual size prepared pellet before firing depends on the required sizing of the finished product.

This in turn depends upon the amount of expansion required and the strength of the pellet as a result of firing.

The second stage is performed in a rotary dryer and preheater. This is a revolving, inclined, semi-lined 2.5 to 3.Om diameter long cylinder which, as it revolves, rounds the damp elongated cylindrical pellets into small spheres 5 to 11 mm diameter, drying them to a pre-calcinating temperature of 400 to 5000C. At this temperature the organic materials associated with bloating are not burnt out. In the third stage, the pre-heated particulate passes either via an insulated holding silo or direct into the rotary furnace or kiln for expansion or bloating.

This kiln is similar to a conventional refractory lined kiln for producing Portland cement clinker. The principal requirement is that the refractory lining is sustained at the temperature required for bloating, i.e.

between 1000 and 1200"C, by firing either with pulverized coal (the ash being insufflated into the product), or fuel oil or gas, or mixtures of fuel.

Firing and temperature control is critical for 1008 clay products, heating times being limited because pyroplastic clay, if overheated, changes into a viscous liquid. The temperature range over which the particle starts to expand to the point of pyroplasticity is usually limited to within 250C, and the particulate having reached that critical optimum temperature, has then to be "frozen" quickly into a solid state by cooling.

The present invention uses coarse fly ash to permit the expansion of suitable bloatable clay to be controlled over a much wider range of temperature, as "bloating" commences gradually from about 9000C continuously through-to 1200 C, thus avoiding the critical optimum temperature of bloating. The invention thus permits manufacture of a range of controlled specific density aggregates, which has not previously been possible with either clay or fly ash separately.

Claims (6)

Claims

1. A method of manufacturing a lightweight aggregate according to which a bloatable clay is mixed with fly ash or equivalent pulverised arillaceous material having 15 to 85 per cent retained at 45 um, the bloatable clay in the mixture being in the range 20 to 80 per cent by weight, and the firing temperature of the mixture, when fired in a rotary kiln, is controlled to produce a lightweight aggregate in the specific density range 700 to 1900 kg/m3.

2. A method as claimed in claim 1, according to which, in a first manufacturing stage, de-stoned bloatable clay is ground to a predetermined consistency with water, before adding a preselected proportion of fly ash thereto.

3. A method as claimed in claim 2, according to which, in the first manufacturing stage, after mixing the clay and fly ash, the mixture is homogenised prior to extrusion into pellets.

4. A method as claimed in claim 3, according to which, in a second manufacturing stage, the pellets are shaped into particulate spheres of 5 to 11 mm diameter, simultaneously being dried to a pre-heating temperature in the range 400 to 500 degrees C.

5. A method as claimed in claim 4, according to which, in a third manufacturing stage, the pre-heated particulate is kiln bloated at a temperature in the range 1000 to 1200 degrees C.

6. A lightweight aggregate produced by the method of any of claims 1 to 5.