WO2023094926A1

WO2023094926A1 - An aggregate

Info

Publication number: WO2023094926A1
Application number: PCT/IB2022/060833
Authority: WO
Inventors: Stuart Grant HOFMEYR; Elizabeth Paulina KEARSLEY
Original assignee: University Of Pretoria
Priority date: 2021-11-23
Filing date: 2022-11-10
Publication date: 2023-06-01

Abstract

The invention provides an aggregate and a method of manufacturing an aggregate which is suitable for use in concrete. The aggregate includes as a primary constituent, fly ash. The method includes mixing particulate fly ash and a binder to form a dry mixture. The dry mixture 5 is formed into aggregate elements in a granulator after which the aggregate elements are hardened by heat treatment.

Description

AN AGGREGATE

This invention relates to concrete. More particularly, it relates to an aggregate and a method of manufacturing an aggregate for concrete. It further relates to a cementitious material.

Fly Ash (FA) is a by-product of pulverised coal combustion in thermal coal-fired power plants. At present, only about 25% of fly ash generated globally and 8% of the total fly ash produced in South Africa is utilised while the remainder, which the Inventors estimate to be about 37 Mt/annum, is a waste product which is disposed of in ash dams and landfills.

It is an object of this invention to provide means which the Inventors believe will at least ameliorate this problem or provide a useful alternative.

According to one aspect of the invention there is provided an aggregate for concrete, a major component of the aggregate being fly ash.

According to another aspect of the invention there is provided a method of manufacturing an aggregate which is suitable for use in concrete which method includes the steps of mixing particles of fly ash with a binder, dividing the mixture into aggregate elements of a desired configuration and hardening the aggregate elements.

Dividing the mixture into aggregate elements may include forming aggregate elements into the desired size and/or shape.

The binder may be in the form of a clay. In one embodiment of the invention the binder may be Kaolin clay.

The method may include mixing particles of fly ash and binder to form a dry mixture.

The dry mixture may comprise between 70 - 100% wt. of fly ash.

The dry mixture may comprise between 0 - 30% wt. binder. The method may include adding water to the dry mixture either after the solid components, i.e., the particles of fly ash and binder, have been mixed or simultaneously with the mixing of the solid components. Typically, approximately 20% of the mass of solids is added as water.

The mixture may be divided into aggregate elements or pellets by a disc granulation process. During granulation, the aggregate elements or pellets are formed without external compaction forces. Granulation, or pelletisation, can be described as the bridging of moist particles using cohesive and tumbling forces. Granulation uses tumbling forces, as well as capillary forces, to join the particles of the powdered constituents. In order for this to occur, a bonding agent in the form of a liquid is required. Water is generally used as a bonding agent, but other materials, such as waterglass, can also be used.

The aggregate elements may be hardened by heat treatment. In one embodiment of the invention, the heat treatment may include a drying step followed by a sintering step.

The drying step my include heating the aggregate elements to a temperature of about 100°C for about 60 minutes.

The sintering step may include heating the aggregate elements to a temperature between 900°C and 1300°C. In a preferred embodiment, the sintering step may include heating the elements to a temperature of about 1200°C for about 60 minutes.

The hardened aggregate elements may have an apparent density of between 1200 kg/m³ to 2000 kg/m³ More particularly the hardened aggregate elements may have an apparent density of about 1600kg/m³.

Due to the high-water absorption of the manufactured lightweight aggregates, the apparent density of the aggregate elements were determined by use of a 'wax density test'. The test method followed was in accordance with ASTM C914 - 09 (2015) and is often used in laboratories to determine the apparent density of soils and permeable materials. This method is derived from the definition of apparent density which involves the mass of the material per unit volume of both solid portion of material plus impermeable pores.

The hardened aggregate elements may have a bulk density of between 600 kg/m³ to 1200 kg/m³ More particularly the hardened aggregate elements may have a bulk density of about 990kg/m³. The bulk density is to be understood as the mass of the many particles of the material divided by the total volume they occupy.

The fly ash may be a low calcium fly ash.

The fly ash may be a Class F fly ash as defined in ASTM C618-19 or similar.

The fly ash may have a particle size of 1 to 100 micrometer.

The fly ash may have a relative density between 2.1 and 2.35.

The fly ash may have a Blaine surface area between 250 and 700 m²/kg.

The clay may be in the form of kaolin clay. In particular, the clay may consist of at least 79.6% kaolinite.

The hardened aggregate elements may have a 24-hour water absorption of 12% wt.

The hardened aggregate elements may be reactive in an alkaline environment.

The invention extends to an aggregate which is manufactured in accordance with the above method.

According to another aspect of the invention there is provided a cementitious material which includes a mixture of cement and an aggregate of the type described above. The invention accordingly provides a practical use for fly ash which otherwise would be regarded as a waste product.

The invention is now described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings:

Figure 1 shows a functional block diagram which illustrates a method of manufacturing an aggregate suitable for use in concrete in accordance with the invention;

Figure 2 shows a side view of a disc granulation apparatus used in the manufacture of an aggregate in accordance with the invention; and

Figure 3 shows a front view of the apparatus of Figure .

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

In the block diagram of Figure 1, reference numeral 10 refers generally to a process or method for the manufacture of an aggregate which is suitable for use in concrete in accordance with the invention. A first step, generally indicated by reference numeral 12 includes the mixing of particulate fly ash and a binder in the form of clay to form a dry mixture. The dry mixture is homogenized, typically by mixing in a container which, in laboratory tests conducted by the Inventors, was in the form of a bucket, and placed in a granulator as indicated at 14.

In the granulator discrete elements of a desired size are formed from the mixture.

To this end and referring now to Figures 2 and 3 of the drawings, reference numeral 14 refers to an experimental disc granulation apparatus which was used by the Inventors to manufacture aggregate elements for testing. The disc granulation apparatus 14 was made using an aluminium pot 22, with inside diameter of 350 mm and height of 140 mm. A hole was drilled into the centre of a base 24 of the pot 22 through which an axle 26 was attached. The pot 22 and axle 26 were attached to a motor 28 and then clamped into position using a table mounted vice 30 as seen in Figure 2. The vice 30 setup was used as an inexpensive way to adjust the angle of the base 24 which formed the disk for disk granulation. The motor 28 was set at a constant rotational speed of 50 rpm. The angle of the disc was adjusted during initial testing to a point where the dry material began to tumble and fall over itself which was essential for effective granulation to occur. An angle of 35° from the horizontal was found to be optimal for this condition to occur. A scraper 32, as seen in Figure 3, was also added as it prevented material from clumping on the side of the disc as well as increased the particle growth rate.

Raw materials were used in the form of powders. The raw material constituents were first weighed and mixed thoroughly in 25 I buckets. The materials were then added to the granulator using a scoop and were spread by means of a shaking motion. While material was added, water was sprayed in small amounts onto the surface using a spray bottle. As soon as all the powdered material had formed into small spherical pellets more material could be added. The surfaces of the small pellets were then sprayed with water to aid in pellet growth. It is important for the correct water content to be added to the raw powdered materials. Too much water resulted in the pellets sticking together during granulation thus forming large clumps. This would stop the process and require these clumps to be broken up manually. It was found that a water content of 20% (by mass) allowed for effective granulation to occur without pellets forming large agglomerations. The addition of powdered raw materials and water continued until the disc was filled with spherical green pellets, i.e., pellets which had not yet been subjected to any heat treatment. At this stage pellets were removed from the disc and sieved through a 4.75 mm sieve. The coarse pellets, i.e., those that did not pass through the sieve, were then stored to be taken to the oven for drying and sintering. The green pellets passing through the 4.75 mm sieve were returned to the disc granulator for further granulation and pellet growth. This method of particle separation was adopted due to the small size and simplicity of the apparatus used in the experimental phase. Automation of the granulation process may allow for continuous separation and removal of particles of the desired size.

As indicated at box 16 (Figure 1), the discrete elements are placed in a kiln or furnace. The elements are then hardened by heat treatment as indicated at box 18. The heat treatment includes a first drying step. This involves heating the aggregate elements to a temperature of about 100°C for about 60 minutes. Subsequently the dried aggregate elements are sintered at a temperature of about 1200°C for about 60 minutes. This hardens the aggregate elements.

The hardened aggregate elements are then allowed to cool and are packaged and transported to a desired location as indicated by box 20.

Aggregates of various fly ash to clay ratios were produced and tested for certain physical properties.

In tests conducted by the Inventors, a 24-hour water absorption test was conducted on each batch of aggregates in which aggregate samples were weighed to an accuracy of 0.01 g and then placed in a bucket of water for 24 hours. The aggregates were then removed from the water and wiped dry using a non-absorbent cloth such that they were saturated surface-dry. The wet aggregate was then weighed to the nearest 0.01 g. The water absorption is taken as the ratio of the mass of water retained by the aggregates to the oven-dry mass of the aggregate and it is expressed as a percentage. ASTM C 1761/C 176 M (2017) states that LWA for internal curing in concrete shall have a water absorption no less than 5%. This provides one with a lower bound for the target water absorption. An upper bound is provided by Neville (2011) who stated that a water absorption below 15% is required for good quality aggregate for use in structural concrete. In the embodiment tested, the hardened aggregate elements had a 24-hour water absorption of 12% wt.

The manufactured aggregates were tested for potential reactivity in a high alkaline environment, similar to what is found in concrete. The presence of new crystal phases in the aggregates after exposure to calcium hydroxide, detected in the XRD and visible in SEM images, shows that the aggregates are reactive in this environment. The Inventors believe that this reactivity would result in increased concrete strength and stiffness caused by the effect of the aggregate-paste interfacial transition zone.

From this, the composition which produced the most desirable qualities was mass produced for further testing in concrete. The final aggregate used was produced with the following composition: 80% fly ash and 20% kaolin clay (by mass of solids). It was found that 20% water (by mass of solids) was used during the disc granulation process in order to produce the discrete aggregate 'green' elements (before heat treating). The aggregate elements or pellets produced had a grading which conformed to 19 mm coarse aggregate as according to ASTM C33/C33M (2018), with little variation, 24-hour water absorption of 12%, aggregate apparent density of 1600 kg/m³, bulk density of 990 kg/m³, Aggregate Crushing Value (ACV) of 24.4%, 10% Fines Aggregate Crushing Test (FACT) of 185 kN and Aggregate Impact Value (AIV) of 25%.

The aggregate strength was classified using typical aggregate strength characterization methods. The tests included the ACV, AIV and 10% FACT. These tests provide knowledge about the relative strength between different types of aggregates.

When compared with the prior art, based on the test results the Inventors believe that the manufactured aggregates are significantly stronger than other weaker aggregates deemed to be suitable for use in concrete elements.

In particular, Alexander and Mindess (2005) state that aggregates with 10% FACT values below 50 kN are unsuitable for concrete. Addis (1994) recommends loads of no less than 110 kN and 70 kN for concretes subjected to abrasion and no abrasion, respectively. Neville (2011) cites the British Standard BS: 1992 as prescribing a minimum 10% FACT value of 150 kN for use in heavy duty floors, 100 kN for concrete wearing surfaces, and 50 kN for use in other concretes. Based on the above test results, the 10% FACT values of 185 kN for aggregate elements produced in accordance with the invention, is more than sufficient for use in concrete application. The 10% FACT results obtained for the aggregates also prove that they are acceptable for use in high strength concrete (concrete with strengths in excess of 50 MPa) as they exceed the 150 kN target recommended (Alexander & Mindess, 2005).

The AIV of 25% for the test samples is less than the maximum of 45% required for use in concrete as suggested by BS 882:1992 according to Alexander and Mindess (2005).

The results obtained from all three strength tests conducted on the manufactured aggregates confirm that it satisfies all requirements for application in concrete specimens for normal strength, and even high strength, concrete.

Based on tests conducted by the inventors, the following conclusions were reached with regards to the final aggregates produced:

• Lightweight aggregates, with the optimum aggregate properties for use in structural concrete, were produced using unclassified fly ash with 20% kaolin (by mass) as a binder. Green pellets are produced using disc granulation in a disc with inside diameter of 350 mm, and operated at an angle of 35° from the horizontal and a rotational speed of 50 rpm. An optimum water content of 20% (by mass) was found which allowed for effective granulation to occur. Aggregates were hardened by achieving liquid phase sintering at 1200°C for 1 hour.

• The aggregates produced using the optimised manufacturing procedure and composition were lightweight with an apparent density of 1600 kg/m³ and bulk density of 990 kg/m³. The low apparent density of the manufactured aggregates was found to be attributed to a high void ratio (0.54) which resulted in a 24-hour water absorption of 12% (by mass). The degree of saturation after the 24-hour water absorption was determined to be 0.55. This shows that aggregates are not truly saturated at this state, and many small voids remain empty, or filled with air, at this time. • The manufactured aggregates had an ACV of 24.4%, 10% FACT of 185 kN and AIV of 25%. Based on these results, and the values considered acceptable from literature, the aggregates produced are significantly strong and acceptable for use in high strength concrete.

• During sintering, the raw materials go through a phase of liquid sintering whereby new phases of mullite, cristobalite and plagioclase are formed. It was found that mullite was formed as a result of the interaction between the fly ash and kaolin raw materials during sintering which contributed to the high strength of the aggregates. The amorphous content of the raw fly ash material decreases during the sintering process for the formation of new crystalline phases.

• The aggregates were found to be reactive in an alkaline environment. This was evident by the new crystal phases of hydrocalumite, gypsum and calcite detected in XRD analysis. SEM imagery confirmed aggregate reactivity by the presence of the new crystals on the surface of the LWA as well as gel aluminosilicate reaction products after exposure to an alkaline environment.

The Inventors believe that concrete aggregates in accordance with the invention will be more environmentally friendly than conventional rock aggregate since it reduces the need to mine aggregates. Further, it utilises fly ash which otherwise would be regarded as waste material. In addition, it has a lighter weight than conventional aggregates thereby reducing transport costs and reduce the size of structural foundations required.

Further, the Inventors believe that the aggregate in accordance with the invention will permit concretes to be cast having a lighter weight, improved thermal insulation properties and a higher strength to weight ratio than when conventional stone aggregates are used.

The Inventors further believe that the aggregate will be particularly useful in fire resistant concrete and aesthetic concrete.

Claims

1. A method of manufacturing an aggregate which is suitable for use in concrete which method includes the steps of mixing particles of fly ash with a binder, dividing the mixture into aggregate elements of a desired configuration and hardening the aggregate elements.

2. The method of claim 1, in which dividing the mixture into aggregate elements includes forming aggregate elements into the desired size and/or shape.

3. The method of claim 1 or claim 2, in which the binder is in the form of a clay.

4. The method of any one of claims 1 to 3, in which the binder comprises Kaolin clay.

5. The method of any one of claims 1 to 4, in which the binder consists of at least 79.6% kaolinite.

6. The method of any one of claims 1 to 5, which includes mixing particles of fly ash and binder to form a dry mixture.

7. The method of claim 6, in which the dry mixture comprises between 70 - 100% wt. of fly ash.

8. The method of claim 6 or claim 7, in which the dry mixture comprises between 0 - 30% wt. binder.

9. The method of any one of claims 6 to 8, which includes adding water to the dry mixture either after the particles of fly ash and binder have been mixed or simultaneously with the mixing of the particles of fly ash and binder.

10. The method of claim 9, in which approximately 20% of the mass of the dry mixture is added as water.

11. The method of any one of claims I to 10, in which the mixture is divided into aggregate elements or pellets by a disc granulation process.

12. The method of claim 11 in which, during granulation, the aggregate elements or pellets are formed without external compaction forces.

13. The method of any one of claims 1 to 12, in which hardening the aggregate elements is by heat treatment.

14. The method as claimed in claim 13, in which the heat treatment includes a drying step followed by a sintering step.

15. The method of claim 14, in which the drying step includes heating the aggregate elements to a temperature of about 100°C for about 60 minutes.

16. The method of claim 14 or claim 15, in which the sintering step includes heating the aggregate elements to a temperature between 900°C and 1300°C.

17. The method of any one of claims 14 to 16, in which the sintering step includes heating the elements to a temperature of about 1200°C for about 60 minutes.

18. The method of any one of claims 1 to 17, in which the hardened aggregate elements have an apparent density of between 1200 kg/m³ to 2000 kg/m³ .

19. The method of claim 18, in which the hardened aggregate elements have an apparent density of about 1600kg/m³.

20. The method of any one of claims 1 to 19, in which the hardened aggregate elements have a bulk density of between 600 kg/m³ to 1200 kg/m³.

21. The method of claim 20, in which the hardened aggregate elements have a bulk density of about 990kg/m³.

22. The method of any one of claims 1 to 21, in which the fly ash is a low calcium fly ash.

23. The method of any one of claims 1 to 22, in which the fly ash is a Class F fly ash as defined in ASTM C618-19 or similar.

24. The method of any one of claims 1 to 23, in which the fly ash has a particle size of 1 to 100 micrometer.

25. The method of any one of claims 1 to 24, in which the fly ash has a relative density between 2.1 and 2.35.

26. The method of any one of claims 1 to 25, in which the fly ash has a Blaine surface area between 250 and 700 m²/kg.

27. The method of any one of claims 1 to 26, in which the hardened aggregate elements have a 24-hour water absorption of 12% wt.

28. The method of any one of claims 1 to 27, in which the hardened aggregate elements are reactive in an alkaline environment.

29. An aggregate which is suitable for use in concrete which is manufactured in accordance with the method of any one of claims 1 to 28.

30. An aggregate for concrete, a major component of the aggregate being fly ash.

31. The aggregate of claim 30, which has an apparent density of between 1200 kg/m³ to 2000 kg/m³, more particularly the hardened aggregate elements have an apparent density of about 1600kg/m³.

32. The aggregate of claim 30 or claim 31 which has a bulk density of between 600 kg/m³ to 1200 kg/m³, more particularly the aggregate may has a bulk density of about 990 kg/m³.

33. The aggregate of any one of claims 30 to 32, in which the fly ash is a low calcium fly ash.

34. The aggregate of any one of claims 30 to 33, in which the fly ash is a Class F fly ash as defined in ASTM C618-19 or similar.

35. The aggregate of any one of claims 30 to 34, in which the fly ash has a particle size of 1 to 100 micrometer.

36. The aggregate of any one of claims 30 to 35, in which the fly ash has a relative density between 2.1 and 2.35.

37. The aggregate of any one of claims 30 to 36, in which the fly ash has a Blaine surface area between 250 and 700 m²/kg.

38. The aggregate of any one of claims 30 to 37 which includes a binder such as clay with which the fly ash is mixed.

39. The aggregsate of any one of claims 30 to 38 which has a 24-hour water absorption of 12% wt.

40. The aggregate of any one of claims 30 to 39, which is hardened reactive in an alkaline environment.

41. A cementitious material which includes a mixture of cement and an aggregate as claimed in claim 29 or claim 30.